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Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]6 <title>The LLVM Target-Independent Code Generator</title>
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21
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]22<h1>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]23 The LLVM Target-Independent Code Generator
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]24</h1>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]25
26<ol>
27 <li><a href="#introduction">Introduction</a>
28 <ul>
29 <li><a href="#required">Required components in the code generator</a></li>
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]30 <li><a href="#high-level-design">The high-level design of the code
31 generator</a></li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]32 <li><a href="#tablegen">Using TableGen for target description</a></li>
33 </ul>
34 </li>
35 <li><a href="#targetdesc">Target description classes</a>
36 <ul>
37 <li><a href="#targetmachine">The <tt>TargetMachine</tt> class</a></li>
38 <li><a href="#targetdata">The <tt>TargetData</tt> class</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]39 <li><a href="#targetlowering">The <tt>TargetLowering</tt> class</a></li>
Dan Gohman6f0d0242008-02-10 18:45:23 +0000[diff] [blame]40 <li><a href="#targetregisterinfo">The <tt>TargetRegisterInfo</tt> class</a></li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]41 <li><a href="#targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a></li>
42 <li><a href="#targetframeinfo">The <tt>TargetFrameInfo</tt> class</a></li>
Chris Lattner47adebb2005-10-16 17:06:07 +0000[diff] [blame]43 <li><a href="#targetsubtarget">The <tt>TargetSubtarget</tt> class</a></li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]44 <li><a href="#targetjitinfo">The <tt>TargetJITInfo</tt> class</a></li>
45 </ul>
46 </li>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]47 <li><a href="#codegendesc">The "Machine" Code Generator classes</a>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]48 <ul>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]49 <li><a href="#machineinstr">The <tt>MachineInstr</tt> class</a></li>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]50 <li><a href="#machinebasicblock">The <tt>MachineBasicBlock</tt>
51 class</a></li>
52 <li><a href="#machinefunction">The <tt>MachineFunction</tt> class</a></li>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]53 </ul>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]54 </li>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]55 <li><a href="#mc">The "MC" Layer</a>
56 <ul>
57 <li><a href="#mcstreamer">The <tt>MCStreamer</tt> API</a></li>
58 <li><a href="#mccontext">The <tt>MCContext</tt> class</a>
59 <li><a href="#mcsymbol">The <tt>MCSymbol</tt> class</a></li>
60 <li><a href="#mcsection">The <tt>MCSection</tt> class</a></li>
61 <li><a href="#mcinst">The <tt>MCInst</tt> class</a></li>
62 </ul>
63 </li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]64 <li><a href="#codegenalgs">Target-independent code generation algorithms</a>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]65 <ul>
66 <li><a href="#instselect">Instruction Selection</a>
67 <ul>
68 <li><a href="#selectiondag_intro">Introduction to SelectionDAGs</a></li>
69 <li><a href="#selectiondag_process">SelectionDAG Code Generation
70 Process</a></li>
71 <li><a href="#selectiondag_build">Initial SelectionDAG
72 Construction</a></li>
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]73 <li><a href="#selectiondag_legalize_types">SelectionDAG LegalizeTypes Phase</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]74 <li><a href="#selectiondag_legalize">SelectionDAG Legalize Phase</a></li>
75 <li><a href="#selectiondag_optimize">SelectionDAG Optimization
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]76 Phase: the DAG Combiner</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]77 <li><a href="#selectiondag_select">SelectionDAG Select Phase</a></li>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]78 <li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]79 Phase</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]80 <li><a href="#selectiondag_future">Future directions for the
81 SelectionDAG</a></li>
82 </ul></li>
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]83 <li><a href="#liveintervals">Live Intervals</a>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]84 <ul>
85 <li><a href="#livevariable_analysis">Live Variable Analysis</a></li>
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]86 <li><a href="#liveintervals_analysis">Live Intervals Analysis</a></li>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]87 </ul></li>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]88 <li><a href="#regalloc">Register Allocation</a>
89 <ul>
90 <li><a href="#regAlloc_represent">How registers are represented in
91 LLVM</a></li>
92 <li><a href="#regAlloc_howTo">Mapping virtual registers to physical
93 registers</a></li>
94 <li><a href="#regAlloc_twoAddr">Handling two address instructions</a></li>
95 <li><a href="#regAlloc_ssaDecon">The SSA deconstruction phase</a></li>
96 <li><a href="#regAlloc_fold">Instruction folding</a></li>
97 <li><a href="#regAlloc_builtIn">Built in register allocators</a></li>
98 </ul></li>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]99 <li><a href="#codeemit">Code Emission</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]100 </ul>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]101 </li>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]102 <li><a href="#nativeassembler">Implementing a Native Assembler</a></li>
103
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]104 <li><a href="#targetimpls">Target-specific Implementation Notes</a>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]105 <ul>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]106 <li><a href="#targetfeatures">Target Feature Matrix</a></li>
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]107 <li><a href="#tailcallopt">Tail call optimization</a></li>
Evan Chengdc444e92010-03-08 21:05:02 +0000[diff] [blame]108 <li><a href="#sibcallopt">Sibling call optimization</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]109 <li><a href="#x86">The X86 backend</a></li>
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]110 <li><a href="#ppc">The PowerPC backend</a>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]111 <ul>
112 <li><a href="#ppc_abi">LLVM PowerPC ABI</a></li>
113 <li><a href="#ppc_frame">Frame Layout</a></li>
114 <li><a href="#ppc_prolog">Prolog/Epilog</a></li>
115 <li><a href="#ppc_dynamic">Dynamic Allocation</a></li>
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]116 </ul></li>
117 </ul></li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]118
119</ol>
120
121<div class="doc_author">
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]122 <p>Written by the LLVM Team.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]123</div>
124
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]125<div class="doc_warning">
126 <p>Warning: This is a work in progress.</p>
127</div>
128
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]129<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]130<h2>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]131 <a name="introduction">Introduction</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]132</h2>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]133<!-- *********************************************************************** -->
134
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]135<div>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]136
137<p>The LLVM target-independent code generator is a framework that provides a
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]138 suite of reusable components for translating the LLVM internal representation
139 to the machine code for a specified target&mdash;either in assembly form
140 (suitable for a static compiler) or in binary machine code format (usable for
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]141 a JIT compiler). The LLVM target-independent code generator consists of six
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]142 main components:</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]143
144<ol>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]145 <li><a href="#targetdesc">Abstract target description</a> interfaces which
146 capture important properties about various aspects of the machine,
147 independently of how they will be used. These interfaces are defined in
148 <tt>include/llvm/Target/</tt>.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]149
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]150 <li>Classes used to represent the <a href="#codegendesc">code being
151 generated</a> for a target. These classes are intended to be abstract
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]152 enough to represent the machine code for <i>any</i> target machine. These
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]153 classes are defined in <tt>include/llvm/CodeGen/</tt>. At this level,
154 concepts like "constant pool entries" and "jump tables" are explicitly
155 exposed.</li>
156
157 <li>Classes and algorithms used to represent code as the object file level,
158 the <a href="#mc">MC Layer</a>. These classes represent assembly level
159 constructs like labels, sections, and instructions. At this level,
160 concepts like "constant pool entries" and "jump tables" don't exist.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]161
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]162 <li><a href="#codegenalgs">Target-independent algorithms</a> used to implement
163 various phases of native code generation (register allocation, scheduling,
164 stack frame representation, etc). This code lives
165 in <tt>lib/CodeGen/</tt>.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]166
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]167 <li><a href="#targetimpls">Implementations of the abstract target description
168 interfaces</a> for particular targets. These machine descriptions make
169 use of the components provided by LLVM, and can optionally provide custom
170 target-specific passes, to build complete code generators for a specific
171 target. Target descriptions live in <tt>lib/Target/</tt>.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]172
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]173 <li><a href="#jit">The target-independent JIT components</a>. The LLVM JIT is
174 completely target independent (it uses the <tt>TargetJITInfo</tt>
175 structure to interface for target-specific issues. The code for the
176 target-independent JIT lives in <tt>lib/ExecutionEngine/JIT</tt>.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]177</ol>
178
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]179<p>Depending on which part of the code generator you are interested in working
180 on, different pieces of this will be useful to you. In any case, you should
181 be familiar with the <a href="#targetdesc">target description</a>
182 and <a href="#codegendesc">machine code representation</a> classes. If you
183 want to add a backend for a new target, you will need
184 to <a href="#targetimpls">implement the target description</a> classes for
185 your new target and understand the <a href="LangRef.html">LLVM code
186 representation</a>. If you are interested in implementing a
187 new <a href="#codegenalgs">code generation algorithm</a>, it should only
188 depend on the target-description and machine code representation classes,
189 ensuring that it is portable.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]190
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]191<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]192<h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]193 <a name="required">Required components in the code generator</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]194</h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]195
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]196<div>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]197
198<p>The two pieces of the LLVM code generator are the high-level interface to the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]199 code generator and the set of reusable components that can be used to build
200 target-specific backends. The two most important interfaces
201 (<a href="#targetmachine"><tt>TargetMachine</tt></a>
202 and <a href="#targetdata"><tt>TargetData</tt></a>) are the only ones that are
203 required to be defined for a backend to fit into the LLVM system, but the
204 others must be defined if the reusable code generator components are going to
205 be used.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]206
207<p>This design has two important implications. The first is that LLVM can
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]208 support completely non-traditional code generation targets. For example, the
209 C backend does not require register allocation, instruction selection, or any
210 of the other standard components provided by the system. As such, it only
211 implements these two interfaces, and does its own thing. Another example of
212 a code generator like this is a (purely hypothetical) backend that converts
213 LLVM to the GCC RTL form and uses GCC to emit machine code for a target.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]214
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]215<p>This design also implies that it is possible to design and implement
216 radically different code generators in the LLVM system that do not make use
217 of any of the built-in components. Doing so is not recommended at all, but
218 could be required for radically different targets that do not fit into the
219 LLVM machine description model: FPGAs for example.</p>
Chris Lattner900bf8c2004-06-02 07:06:06 +0000[diff] [blame]220
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]221</div>
222
223<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]224<h3>
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]225 <a name="high-level-design">The high-level design of the code generator</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]226</h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]227
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]228<div>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]229
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]230<p>The LLVM target-independent code generator is designed to support efficient
231 and quality code generation for standard register-based microprocessors.
232 Code generation in this model is divided into the following stages:</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]233
234<ol>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]235 <li><b><a href="#instselect">Instruction Selection</a></b> &mdash; This phase
236 determines an efficient way to express the input LLVM code in the target
237 instruction set. This stage produces the initial code for the program in
238 the target instruction set, then makes use of virtual registers in SSA
239 form and physical registers that represent any required register
240 assignments due to target constraints or calling conventions. This step
241 turns the LLVM code into a DAG of target instructions.</li>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]242
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]243 <li><b><a href="#selectiondag_sched">Scheduling and Formation</a></b> &mdash;
244 This phase takes the DAG of target instructions produced by the
245 instruction selection phase, determines an ordering of the instructions,
246 then emits the instructions
247 as <tt><a href="#machineinstr">MachineInstr</a></tt>s with that ordering.
248 Note that we describe this in the <a href="#instselect">instruction
249 selection section</a> because it operates on
250 a <a href="#selectiondag_intro">SelectionDAG</a>.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]251
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]252 <li><b><a href="#ssamco">SSA-based Machine Code Optimizations</a></b> &mdash;
253 This optional stage consists of a series of machine-code optimizations
254 that operate on the SSA-form produced by the instruction selector.
255 Optimizations like modulo-scheduling or peephole optimization work
256 here.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]257
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]258 <li><b><a href="#regalloc">Register Allocation</a></b> &mdash; The target code
259 is transformed from an infinite virtual register file in SSA form to the
260 concrete register file used by the target. This phase introduces spill
261 code and eliminates all virtual register references from the program.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]262
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]263 <li><b><a href="#proepicode">Prolog/Epilog Code Insertion</a></b> &mdash; Once
264 the machine code has been generated for the function and the amount of
265 stack space required is known (used for LLVM alloca's and spill slots),
266 the prolog and epilog code for the function can be inserted and "abstract
267 stack location references" can be eliminated. This stage is responsible
268 for implementing optimizations like frame-pointer elimination and stack
269 packing.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]270
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]271 <li><b><a href="#latemco">Late Machine Code Optimizations</a></b> &mdash;
272 Optimizations that operate on "final" machine code can go here, such as
273 spill code scheduling and peephole optimizations.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]274
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]275 <li><b><a href="#codeemit">Code Emission</a></b> &mdash; The final stage
276 actually puts out the code for the current function, either in the target
277 assembler format or in machine code.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]278</ol>
279
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]280<p>The code generator is based on the assumption that the instruction selector
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]281 will use an optimal pattern matching selector to create high-quality
282 sequences of native instructions. Alternative code generator designs based
283 on pattern expansion and aggressive iterative peephole optimization are much
284 slower. This design permits efficient compilation (important for JIT
285 environments) and aggressive optimization (used when generating code offline)
286 by allowing components of varying levels of sophistication to be used for any
287 step of compilation.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]288
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]289<p>In addition to these stages, target implementations can insert arbitrary
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]290 target-specific passes into the flow. For example, the X86 target uses a
291 special pass to handle the 80x87 floating point stack architecture. Other
292 targets with unusual requirements can be supported with custom passes as
293 needed.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]294
295</div>
296
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]297<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]298<h3>
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]299 <a name="tablegen">Using TableGen for target description</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]300</h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]301
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]302<div>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]303
Chris Lattner5489e932004-06-01 18:35:00 +0000[diff] [blame]304<p>The target description classes require a detailed description of the target
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]305 architecture. These target descriptions often have a large amount of common
306 information (e.g., an <tt>add</tt> instruction is almost identical to a
307 <tt>sub</tt> instruction). In order to allow the maximum amount of
308 commonality to be factored out, the LLVM code generator uses
309 the <a href="TableGenFundamentals.html">TableGen</a> tool to describe big
310 chunks of the target machine, which allows the use of domain-specific and
311 target-specific abstractions to reduce the amount of repetition.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]312
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]313<p>As LLVM continues to be developed and refined, we plan to move more and more
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]314 of the target description to the <tt>.td</tt> form. Doing so gives us a
315 number of advantages. The most important is that it makes it easier to port
316 LLVM because it reduces the amount of C++ code that has to be written, and
317 the surface area of the code generator that needs to be understood before
318 someone can get something working. Second, it makes it easier to change
319 things. In particular, if tables and other things are all emitted
320 by <tt>tblgen</tt>, we only need a change in one place (<tt>tblgen</tt>) to
321 update all of the targets to a new interface.</p>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]322
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]323</div>
324
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]325</div>
326
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]327<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]328<h2>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]329 <a name="targetdesc">Target description classes</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]330</h2>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]331<!-- *********************************************************************** -->
332
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]333<div>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]334
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]335<p>The LLVM target description classes (located in the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]336 <tt>include/llvm/Target</tt> directory) provide an abstract description of
337 the target machine independent of any particular client. These classes are
338 designed to capture the <i>abstract</i> properties of the target (such as the
339 instructions and registers it has), and do not incorporate any particular
340 pieces of code generation algorithms.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]341
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]342<p>All of the target description classes (except the
343 <tt><a href="#targetdata">TargetData</a></tt> class) are designed to be
344 subclassed by the concrete target implementation, and have virtual methods
345 implemented. To get to these implementations, the
346 <tt><a href="#targetmachine">TargetMachine</a></tt> class provides accessors
347 that should be implemented by the target.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]348
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]349<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]350<h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]351 <a name="targetmachine">The <tt>TargetMachine</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]352</h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]353
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]354<div>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]355
356<p>The <tt>TargetMachine</tt> class provides virtual methods that are used to
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]357 access the target-specific implementations of the various target description
358 classes via the <tt>get*Info</tt> methods (<tt>getInstrInfo</tt>,
359 <tt>getRegisterInfo</tt>, <tt>getFrameInfo</tt>, etc.). This class is
360 designed to be specialized by a concrete target implementation
361 (e.g., <tt>X86TargetMachine</tt>) which implements the various virtual
362 methods. The only required target description class is
363 the <a href="#targetdata"><tt>TargetData</tt></a> class, but if the code
364 generator components are to be used, the other interfaces should be
365 implemented as well.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]366
367</div>
368
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]369<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]370<h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]371 <a name="targetdata">The <tt>TargetData</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]372</h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]373
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]374<div>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]375
376<p>The <tt>TargetData</tt> class is the only required target description class,
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]377 and it is the only class that is not extensible (you cannot derived a new
378 class from it). <tt>TargetData</tt> specifies information about how the
379 target lays out memory for structures, the alignment requirements for various
380 data types, the size of pointers in the target, and whether the target is
381 little-endian or big-endian.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]382
383</div>
384
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]385<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]386<h3>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]387 <a name="targetlowering">The <tt>TargetLowering</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]388</h3>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]389
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]390<div>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]391
392<p>The <tt>TargetLowering</tt> class is used by SelectionDAG based instruction
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]393 selectors primarily to describe how LLVM code should be lowered to
394 SelectionDAG operations. Among other things, this class indicates:</p>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]395
396<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]397 <li>an initial register class to use for various <tt>ValueType</tt>s,</li>
398
399 <li>which operations are natively supported by the target machine,</li>
400
401 <li>the return type of <tt>setcc</tt> operations,</li>
402
403 <li>the type to use for shift amounts, and</li>
404
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]405 <li>various high-level characteristics, like whether it is profitable to turn
406 division by a constant into a multiplication sequence</li>
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]407</ul>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]408
409</div>
410
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]411<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]412<h3>
Dan Gohman6f0d0242008-02-10 18:45:23 +0000[diff] [blame]413 <a name="targetregisterinfo">The <tt>TargetRegisterInfo</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]414</h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]415
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]416<div>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]417
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]418<p>The <tt>TargetRegisterInfo</tt> class is used to describe the register file
419 of the target and any interactions between the registers.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]420
421<p>Registers in the code generator are represented in the code generator by
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]422 unsigned integers. Physical registers (those that actually exist in the
423 target description) are unique small numbers, and virtual registers are
424 generally large. Note that register #0 is reserved as a flag value.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]425
426<p>Each register in the processor description has an associated
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]427 <tt>TargetRegisterDesc</tt> entry, which provides a textual name for the
428 register (used for assembly output and debugging dumps) and a set of aliases
429 (used to indicate whether one register overlaps with another).</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]430
Dan Gohman6f0d0242008-02-10 18:45:23 +0000[diff] [blame]431<p>In addition to the per-register description, the <tt>TargetRegisterInfo</tt>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]432 class exposes a set of processor specific register classes (instances of the
433 <tt>TargetRegisterClass</tt> class). Each register class contains sets of
434 registers that have the same properties (for example, they are all 32-bit
435 integer registers). Each SSA virtual register created by the instruction
436 selector has an associated register class. When the register allocator runs,
437 it replaces virtual registers with a physical register in the set.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]438
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]439<p>The target-specific implementations of these classes is auto-generated from
440 a <a href="TableGenFundamentals.html">TableGen</a> description of the
441 register file.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]442
443</div>
444
445<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]446<h3>
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]447 <a name="targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]448</h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]449
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]450<div>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]451
452<p>The <tt>TargetInstrInfo</tt> class is used to describe the machine
453 instructions supported by the target. It is essentially an array of
454 <tt>TargetInstrDescriptor</tt> objects, each of which describes one
455 instruction the target supports. Descriptors define things like the mnemonic
456 for the opcode, the number of operands, the list of implicit register uses
457 and defs, whether the instruction has certain target-independent properties
458 (accesses memory, is commutable, etc), and holds any target-specific
459 flags.</p>
460
Reid Spencer627cd002005-07-19 01:36:35 +0000[diff] [blame]461</div>
462
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]463<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]464<h3>
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]465 <a name="targetframeinfo">The <tt>TargetFrameInfo</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]466</h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]467
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]468<div>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]469
470<p>The <tt>TargetFrameInfo</tt> class is used to provide information about the
471 stack frame layout of the target. It holds the direction of stack growth, the
472 known stack alignment on entry to each function, and the offset to the local
473 area. The offset to the local area is the offset from the stack pointer on
474 function entry to the first location where function data (local variables,
475 spill locations) can be stored.</p>
476
Reid Spencer627cd002005-07-19 01:36:35 +0000[diff] [blame]477</div>
Chris Lattner47adebb2005-10-16 17:06:07 +0000[diff] [blame]478
479<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]480<h3>
Chris Lattner47adebb2005-10-16 17:06:07 +0000[diff] [blame]481 <a name="targetsubtarget">The <tt>TargetSubtarget</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]482</h3>
Chris Lattner47adebb2005-10-16 17:06:07 +0000[diff] [blame]483
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]484<div>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]485
486<p>The <tt>TargetSubtarget</tt> class is used to provide information about the
487 specific chip set being targeted. A sub-target informs code generation of
488 which instructions are supported, instruction latencies and instruction
489 execution itinerary; i.e., which processing units are used, in what order,
490 and for how long.</p>
491
Chris Lattner47adebb2005-10-16 17:06:07 +0000[diff] [blame]492</div>
493
494
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]495<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]496<h3>
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]497 <a name="targetjitinfo">The <tt>TargetJITInfo</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]498</h3>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]499
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]500<div>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]501
502<p>The <tt>TargetJITInfo</tt> class exposes an abstract interface used by the
503 Just-In-Time code generator to perform target-specific activities, such as
504 emitting stubs. If a <tt>TargetMachine</tt> supports JIT code generation, it
505 should provide one of these objects through the <tt>getJITInfo</tt>
506 method.</p>
507
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]508</div>
509
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]510</div>
511
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]512<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]513<h2>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]514 <a name="codegendesc">Machine code description classes</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]515</h2>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]516<!-- *********************************************************************** -->
517
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]518<div>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]519
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]520<p>At the high-level, LLVM code is translated to a machine specific
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]521 representation formed out of
522 <a href="#machinefunction"><tt>MachineFunction</tt></a>,
523 <a href="#machinebasicblock"><tt>MachineBasicBlock</tt></a>,
524 and <a href="#machineinstr"><tt>MachineInstr</tt></a> instances (defined
525 in <tt>include/llvm/CodeGen</tt>). This representation is completely target
526 agnostic, representing instructions in their most abstract form: an opcode
527 and a series of operands. This representation is designed to support both an
528 SSA representation for machine code, as well as a register allocated, non-SSA
529 form.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]530
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]531<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]532<h3>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]533 <a name="machineinstr">The <tt>MachineInstr</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]534</h3>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]535
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]536<div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]537
538<p>Target machine instructions are represented as instances of the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]539 <tt>MachineInstr</tt> class. This class is an extremely abstract way of
540 representing machine instructions. In particular, it only keeps track of an
541 opcode number and a set of operands.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]542
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]543<p>The opcode number is a simple unsigned integer that only has meaning to a
544 specific backend. All of the instructions for a target should be defined in
545 the <tt>*InstrInfo.td</tt> file for the target. The opcode enum values are
546 auto-generated from this description. The <tt>MachineInstr</tt> class does
547 not have any information about how to interpret the instruction (i.e., what
548 the semantics of the instruction are); for that you must refer to the
549 <tt><a href="#targetinstrinfo">TargetInstrInfo</a></tt> class.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]550
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]551<p>The operands of a machine instruction can be of several different types: a
552 register reference, a constant integer, a basic block reference, etc. In
553 addition, a machine operand should be marked as a def or a use of the value
554 (though only registers are allowed to be defs).</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]555
556<p>By convention, the LLVM code generator orders instruction operands so that
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]557 all register definitions come before the register uses, even on architectures
558 that are normally printed in other orders. For example, the SPARC add
559 instruction: "<tt>add %i1, %i2, %i3</tt>" adds the "%i1", and "%i2" registers
560 and stores the result into the "%i3" register. In the LLVM code generator,
561 the operands should be stored as "<tt>%i3, %i1, %i2</tt>": with the
562 destination first.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]563
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]564<p>Keeping destination (definition) operands at the beginning of the operand
565 list has several advantages. In particular, the debugging printer will print
566 the instruction like this:</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]567
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]568<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]569<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]570%r3 = add %i1, %i2
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]571</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]572</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]573
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]574<p>Also if the first operand is a def, it is easier to <a href="#buildmi">create
575 instructions</a> whose only def is the first operand.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]576
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]577<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]578<h4>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]579 <a name="buildmi">Using the <tt>MachineInstrBuilder.h</tt> functions</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]580</h4>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]581
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]582<div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]583
584<p>Machine instructions are created by using the <tt>BuildMI</tt> functions,
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]585 located in the <tt>include/llvm/CodeGen/MachineInstrBuilder.h</tt> file. The
586 <tt>BuildMI</tt> functions make it easy to build arbitrary machine
587 instructions. Usage of the <tt>BuildMI</tt> functions look like this:</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]588
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]589<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]590<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]591// Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42')
592// instruction. The '1' specifies how many operands will be added.
593MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42);
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]594
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]595// Create the same instr, but insert it at the end of a basic block.
596MachineBasicBlock &amp;MBB = ...
597BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42);
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]598
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]599// Create the same instr, but insert it before a specified iterator point.
600MachineBasicBlock::iterator MBBI = ...
601BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42);
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]602
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]603// Create a 'cmp Reg, 0' instruction, no destination reg.
604MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0);
605// Create an 'sahf' instruction which takes no operands and stores nothing.
606MI = BuildMI(X86::SAHF, 0);
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]607
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]608// Create a self looping branch instruction.
609BuildMI(MBB, X86::JNE, 1).addMBB(&amp;MBB);
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]610</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]611</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]612
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]613<p>The key thing to remember with the <tt>BuildMI</tt> functions is that you
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]614 have to specify the number of operands that the machine instruction will
615 take. This allows for efficient memory allocation. You also need to specify
616 if operands default to be uses of values, not definitions. If you need to
617 add a definition operand (other than the optional destination register), you
618 must explicitly mark it as such:</p>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]619
620<div class="doc_code">
621<pre>
Bill Wendling587daed2009-05-13 21:33:08 +0000[diff] [blame]622MI.addReg(Reg, RegState::Define);
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]623</pre>
624</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]625
626</div>
627
628<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]629<h4>
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]630 <a name="fixedregs">Fixed (preassigned) registers</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]631</h4>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]632
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]633<div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]634
635<p>One important issue that the code generator needs to be aware of is the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]636 presence of fixed registers. In particular, there are often places in the
637 instruction stream where the register allocator <em>must</em> arrange for a
638 particular value to be in a particular register. This can occur due to
639 limitations of the instruction set (e.g., the X86 can only do a 32-bit divide
640 with the <tt>EAX</tt>/<tt>EDX</tt> registers), or external factors like
641 calling conventions. In any case, the instruction selector should emit code
642 that copies a virtual register into or out of a physical register when
643 needed.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]644
645<p>For example, consider this simple LLVM example:</p>
646
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]647<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]648<pre>
Matthijs Kooijman61399af2008-06-04 15:46:35 +0000[diff] [blame]649define i32 @test(i32 %X, i32 %Y) {
650 %Z = udiv i32 %X, %Y
651 ret i32 %Z
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]652}
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]653</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]654</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]655
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]656<p>The X86 instruction selector produces this machine code for the <tt>div</tt>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]657 and <tt>ret</tt> (use "<tt>llc X.bc -march=x86 -print-machineinstrs</tt>" to
658 get this):</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]659
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]660<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]661<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]662;; Start of div
663%EAX = mov %reg1024 ;; Copy X (in reg1024) into EAX
664%reg1027 = sar %reg1024, 31
665%EDX = mov %reg1027 ;; Sign extend X into EDX
666idiv %reg1025 ;; Divide by Y (in reg1025)
667%reg1026 = mov %EAX ;; Read the result (Z) out of EAX
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]668
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]669;; Start of ret
670%EAX = mov %reg1026 ;; 32-bit return value goes in EAX
671ret
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]672</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]673</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]674
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]675<p>By the end of code generation, the register allocator has coalesced the
676 registers and deleted the resultant identity moves producing the following
677 code:</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]678
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]679<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]680<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]681;; X is in EAX, Y is in ECX
682mov %EAX, %EDX
683sar %EDX, 31
684idiv %ECX
685ret
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]686</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]687</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]688
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]689<p>This approach is extremely general (if it can handle the X86 architecture, it
690 can handle anything!) and allows all of the target specific knowledge about
691 the instruction stream to be isolated in the instruction selector. Note that
692 physical registers should have a short lifetime for good code generation, and
693 all physical registers are assumed dead on entry to and exit from basic
694 blocks (before register allocation). Thus, if you need a value to be live
695 across basic block boundaries, it <em>must</em> live in a virtual
696 register.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]697
698</div>
699
700<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]701<h4>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]702 <a name="ssa">Machine code in SSA form</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]703</h4>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]704
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]705<div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]706
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]707<p><tt>MachineInstr</tt>'s are initially selected in SSA-form, and are
708 maintained in SSA-form until register allocation happens. For the most part,
709 this is trivially simple since LLVM is already in SSA form; LLVM PHI nodes
710 become machine code PHI nodes, and virtual registers are only allowed to have
711 a single definition.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]712
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]713<p>After register allocation, machine code is no longer in SSA-form because
714 there are no virtual registers left in the code.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]715
716</div>
717
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]718</div>
719
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]720<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]721<h3>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]722 <a name="machinebasicblock">The <tt>MachineBasicBlock</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]723</h3>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]724
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]725<div>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]726
727<p>The <tt>MachineBasicBlock</tt> class contains a list of machine instructions
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]728 (<tt><a href="#machineinstr">MachineInstr</a></tt> instances). It roughly
729 corresponds to the LLVM code input to the instruction selector, but there can
730 be a one-to-many mapping (i.e. one LLVM basic block can map to multiple
731 machine basic blocks). The <tt>MachineBasicBlock</tt> class has a
732 "<tt>getBasicBlock</tt>" method, which returns the LLVM basic block that it
733 comes from.</p>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]734
735</div>
736
737<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]738<h3>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]739 <a name="machinefunction">The <tt>MachineFunction</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]740</h3>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]741
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]742<div>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]743
744<p>The <tt>MachineFunction</tt> class contains a list of machine basic blocks
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]745 (<tt><a href="#machinebasicblock">MachineBasicBlock</a></tt> instances). It
746 corresponds one-to-one with the LLVM function input to the instruction
747 selector. In addition to a list of basic blocks,
748 the <tt>MachineFunction</tt> contains a a <tt>MachineConstantPool</tt>,
749 a <tt>MachineFrameInfo</tt>, a <tt>MachineFunctionInfo</tt>, and a
750 <tt>MachineRegisterInfo</tt>. See
751 <tt>include/llvm/CodeGen/MachineFunction.h</tt> for more information.</p>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]752
753</div>
754
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]755</div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]756
757<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]758<h2>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]759 <a name="mc">The "MC" Layer</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]760</h2>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]761<!-- *********************************************************************** -->
762
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]763<div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]764
765<p>
766The MC Layer is used to represent and process code at the raw machine code
767level, devoid of "high level" information like "constant pools", "jump tables",
768"global variables" or anything like that. At this level, LLVM handles things
769like label names, machine instructions, and sections in the object file. The
770code in this layer is used for a number of important purposes: the tail end of
771the code generator uses it to write a .s or .o file, and it is also used by the
Jay Foadd61895a2011-04-13 13:03:56 +0000[diff] [blame]772llvm-mc tool to implement standalone machine code assemblers and disassemblers.
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]773</p>
774
775<p>
776This section describes some of the important classes. There are also a number
777of important subsystems that interact at this layer, they are described later
778in this manual.
779</p>
780
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]781<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]782<h3>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]783 <a name="mcstreamer">The <tt>MCStreamer</tt> API</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]784</h3>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]785
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]786<div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]787
788<p>
789MCStreamer is best thought of as an assembler API. It is an abstract API which
790is <em>implemented</em> in different ways (e.g. to output a .s file, output an
791ELF .o file, etc) but whose API correspond directly to what you see in a .s
792file. MCStreamer has one method per directive, such as EmitLabel,
793EmitSymbolAttribute, SwitchSection, EmitValue (for .byte, .word), etc, which
794directly correspond to assembly level directives. It also has an
795EmitInstruction method, which is used to output an MCInst to the streamer.
796</p>
797
798<p>
799This API is most important for two clients: the llvm-mc stand-alone assembler is
800effectively a parser that parses a line, then invokes a method on MCStreamer. In
801the code generator, the <a href="#codeemit">Code Emission</a> phase of the code
802generator lowers higher level LLVM IR and Machine* constructs down to the MC
803layer, emitting directives through MCStreamer.</p>
804
805<p>
806On the implementation side of MCStreamer, there are two major implementations:
807one for writing out a .s file (MCAsmStreamer), and one for writing out a .o
808file (MCObjectStreamer). MCAsmStreamer is a straight-forward implementation
809that prints out a directive for each method (e.g. EmitValue -&gt; .byte), but
810MCObjectStreamer implements a full assembler.
811</p>
812
813</div>
814
815<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]816<h3>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]817 <a name="mccontext">The <tt>MCContext</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]818</h3>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]819
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]820<div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]821
822<p>
823The MCContext class is the owner of a variety of uniqued data structures at the
824MC layer, including symbols, sections, etc. As such, this is the class that you
825interact with to create symbols and sections. This class can not be subclassed.
826</p>
827
828</div>
829
830<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]831<h3>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]832 <a name="mcsymbol">The <tt>MCSymbol</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]833</h3>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]834
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]835<div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]836
837<p>
838The MCSymbol class represents a symbol (aka label) in the assembly file. There
839are two interesting kinds of symbols: assembler temporary symbols, and normal
840symbols. Assembler temporary symbols are used and processed by the assembler
841but are discarded when the object file is produced. The distinction is usually
842represented by adding a prefix to the label, for example "L" labels are
843assembler temporary labels in MachO.
844</p>
845
846<p>MCSymbols are created by MCContext and uniqued there. This means that
847MCSymbols can be compared for pointer equivalence to find out if they are the
848same symbol. Note that pointer inequality does not guarantee the labels will
849end up at different addresses though. It's perfectly legal to output something
850like this to the .s file:<p>
851
852<pre>
853 foo:
854 bar:
855 .byte 4
856</pre>
857
858<p>In this case, both the foo and bar symbols will have the same address.</p>
859
860</div>
861
862<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]863<h3>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]864 <a name="mcsection">The <tt>MCSection</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]865</h3>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]866
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]867<div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]868
869<p>
870The MCSection class represents an object-file specific section. It is subclassed
871by object file specific implementations (e.g. <tt>MCSectionMachO</tt>,
872<tt>MCSectionCOFF</tt>, <tt>MCSectionELF</tt>) and these are created and uniqued
873by MCContext. The MCStreamer has a notion of the current section, which can be
874changed with the SwitchToSection method (which corresponds to a ".section"
875directive in a .s file).
876</p>
877
878</div>
879
880<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]881<h3>
Benjamin Kramer943beeb2010-10-30 21:07:28 +0000[diff] [blame]882 <a name="mcinst">The <tt>MCInst</tt> class</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]883</h3>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]884
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]885<div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]886
887<p>
888The MCInst class is a target-independent representation of an instruction. It
889is a simple class (much more so than <a href="#machineinstr">MachineInstr</a>)
890that holds a target-specific opcode and a vector of MCOperands. MCOperand, in
891turn, is a simple discriminated union of three cases: 1) a simple immediate,
8922) a target register ID, 3) a symbolic expression (e.g. "Lfoo-Lbar+42") as an
893MCExpr.
894</p>
895
896<p>MCInst is the common currency used to represent machine instructions at the
897MC layer. It is the type used by the instruction encoder, the instruction
898printer, and the type generated by the assembly parser and disassembler.
899</p>
900
901</div>
902
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]903</div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]904
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]905<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]906<h2>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]907 <a name="codegenalgs">Target-independent code generation algorithms</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]908</h2>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]909<!-- *********************************************************************** -->
910
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]911<div>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]912
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]913<p>This section documents the phases described in the
914 <a href="#high-level-design">high-level design of the code generator</a>.
915 It explains how they work and some of the rationale behind their design.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]916
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]917<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]918<h3>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]919 <a name="instselect">Instruction Selection</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]920</h3>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]921
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]922<div>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]923
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]924<p>Instruction Selection is the process of translating LLVM code presented to
925 the code generator into target-specific machine instructions. There are
926 several well-known ways to do this in the literature. LLVM uses a
927 SelectionDAG based instruction selector.</p>
928
929<p>Portions of the DAG instruction selector are generated from the target
930 description (<tt>*.td</tt>) files. Our goal is for the entire instruction
931 selector to be generated from these <tt>.td</tt> files, though currently
932 there are still things that require custom C++ code.</p>
933
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]934<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]935<h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]936 <a name="selectiondag_intro">Introduction to SelectionDAGs</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]937</h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]938
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]939<div>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]940
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]941<p>The SelectionDAG provides an abstraction for code representation in a way
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]942 that is amenable to instruction selection using automatic techniques
943 (e.g. dynamic-programming based optimal pattern matching selectors). It is
944 also well-suited to other phases of code generation; in particular,
945 instruction scheduling (SelectionDAG's are very close to scheduling DAGs
946 post-selection). Additionally, the SelectionDAG provides a host
947 representation where a large variety of very-low-level (but
948 target-independent) <a href="#selectiondag_optimize">optimizations</a> may be
949 performed; ones which require extensive information about the instructions
950 efficiently supported by the target.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]951
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]952<p>The SelectionDAG is a Directed-Acyclic-Graph whose nodes are instances of the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]953 <tt>SDNode</tt> class. The primary payload of the <tt>SDNode</tt> is its
954 operation code (Opcode) that indicates what operation the node performs and
955 the operands to the operation. The various operation node types are
956 described at the top of the <tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt>
957 file.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]958
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]959<p>Although most operations define a single value, each node in the graph may
960 define multiple values. For example, a combined div/rem operation will
961 define both the dividend and the remainder. Many other situations require
962 multiple values as well. Each node also has some number of operands, which
963 are edges to the node defining the used value. Because nodes may define
964 multiple values, edges are represented by instances of the <tt>SDValue</tt>
965 class, which is a <tt>&lt;SDNode, unsigned&gt;</tt> pair, indicating the node
966 and result value being used, respectively. Each value produced by
967 an <tt>SDNode</tt> has an associated <tt>MVT</tt> (Machine Value Type)
968 indicating what the type of the value is.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]969
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]970<p>SelectionDAGs contain two different kinds of values: those that represent
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]971 data flow and those that represent control flow dependencies. Data values
972 are simple edges with an integer or floating point value type. Control edges
973 are represented as "chain" edges which are of type <tt>MVT::Other</tt>.
974 These edges provide an ordering between nodes that have side effects (such as
975 loads, stores, calls, returns, etc). All nodes that have side effects should
976 take a token chain as input and produce a new one as output. By convention,
977 token chain inputs are always operand #0, and chain results are always the
978 last value produced by an operation.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]979
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]980<p>A SelectionDAG has designated "Entry" and "Root" nodes. The Entry node is
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]981 always a marker node with an Opcode of <tt>ISD::EntryToken</tt>. The Root
982 node is the final side-effecting node in the token chain. For example, in a
983 single basic block function it would be the return node.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]984
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]985<p>One important concept for SelectionDAGs is the notion of a "legal" vs.
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]986 "illegal" DAG. A legal DAG for a target is one that only uses supported
987 operations and supported types. On a 32-bit PowerPC, for example, a DAG with
988 a value of type i1, i8, i16, or i64 would be illegal, as would a DAG that
989 uses a SREM or UREM operation. The
990 <a href="#selectinodag_legalize_types">legalize types</a> and
991 <a href="#selectiondag_legalize">legalize operations</a> phases are
992 responsible for turning an illegal DAG into a legal DAG.</p>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]993
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]994</div>
995
996<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]997<h4>
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]998 <a name="selectiondag_process">SelectionDAG Instruction Selection Process</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]999</h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1000
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1001<div>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1002
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1003<p>SelectionDAG-based instruction selection consists of the following steps:</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1004
1005<ol>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1006 <li><a href="#selectiondag_build">Build initial DAG</a> &mdash; This stage
1007 performs a simple translation from the input LLVM code to an illegal
1008 SelectionDAG.</li>
1009
1010 <li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> &mdash; This
1011 stage performs simple optimizations on the SelectionDAG to simplify it,
1012 and recognize meta instructions (like rotates
1013 and <tt>div</tt>/<tt>rem</tt> pairs) for targets that support these meta
1014 operations. This makes the resultant code more efficient and
1015 the <a href="#selectiondag_select">select instructions from DAG</a> phase
1016 (below) simpler.</li>
1017
1018 <li><a href="#selectiondag_legalize_types">Legalize SelectionDAG Types</a>
1019 &mdash; This stage transforms SelectionDAG nodes to eliminate any types
1020 that are unsupported on the target.</li>
1021
1022 <li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> &mdash; The
1023 SelectionDAG optimizer is run to clean up redundancies exposed by type
1024 legalization.</li>
1025
Chris Lattner71388632010-12-12 02:42:57 +0000[diff] [blame]1026 <li><a href="#selectiondag_legalize">Legalize SelectionDAG Ops</a> &mdash;
Chris Lattner4c247f62010-12-13 00:17:12 +0000[diff] [blame]1027 This stage transforms SelectionDAG nodes to eliminate any operations
1028 that are unsupported on the target.</li>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1029
1030 <li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> &mdash; The
1031 SelectionDAG optimizer is run to eliminate inefficiencies introduced by
1032 operation legalization.</li>
1033
1034 <li><a href="#selectiondag_select">Select instructions from DAG</a> &mdash;
1035 Finally, the target instruction selector matches the DAG operations to
1036 target instructions. This process translates the target-independent input
1037 DAG into another DAG of target instructions.</li>
1038
1039 <li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation</a>
1040 &mdash; The last phase assigns a linear order to the instructions in the
1041 target-instruction DAG and emits them into the MachineFunction being
1042 compiled. This step uses traditional prepass scheduling techniques.</li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1043</ol>
1044
1045<p>After all of these steps are complete, the SelectionDAG is destroyed and the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1046 rest of the code generation passes are run.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1047
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1048<p>One great way to visualize what is going on here is to take advantage of a
1049 few LLC command line options. The following options pop up a window
1050 displaying the SelectionDAG at specific times (if you only get errors printed
1051 to the console while using this, you probably
1052 <a href="ProgrammersManual.html#ViewGraph">need to configure your system</a>
1053 to add support for it).</p>
Dan Gohman8c9c55f2008-09-10 22:23:41 +0000[diff] [blame]1054
1055<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1056 <li><tt>-view-dag-combine1-dags</tt> displays the DAG after being built,
1057 before the first optimization pass.</li>
1058
1059 <li><tt>-view-legalize-dags</tt> displays the DAG before Legalization.</li>
1060
1061 <li><tt>-view-dag-combine2-dags</tt> displays the DAG before the second
1062 optimization pass.</li>
1063
1064 <li><tt>-view-isel-dags</tt> displays the DAG before the Select phase.</li>
1065
1066 <li><tt>-view-sched-dags</tt> displays the DAG before Scheduling.</li>
Dan Gohman8c9c55f2008-09-10 22:23:41 +0000[diff] [blame]1067</ul>
1068
1069<p>The <tt>-view-sunit-dags</tt> displays the Scheduler's dependency graph.
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1070 This graph is based on the final SelectionDAG, with nodes that must be
1071 scheduled together bundled into a single scheduling-unit node, and with
1072 immediate operands and other nodes that aren't relevant for scheduling
1073 omitted.</p>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1074
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1075</div>
1076
1077<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1078<h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1079 <a name="selectiondag_build">Initial SelectionDAG Construction</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1080</h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1081
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1082<div>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1083
Bill Wendling16448772006-08-28 03:04:05 +0000[diff] [blame]1084<p>The initial SelectionDAG is na&iuml;vely peephole expanded from the LLVM
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1085 input by the <tt>SelectionDAGLowering</tt> class in the
1086 <tt>lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp</tt> file. The intent of
1087 this pass is to expose as much low-level, target-specific details to the
1088 SelectionDAG as possible. This pass is mostly hard-coded (e.g. an
1089 LLVM <tt>add</tt> turns into an <tt>SDNode add</tt> while a
1090 <tt>getelementptr</tt> is expanded into the obvious arithmetic). This pass
1091 requires target-specific hooks to lower calls, returns, varargs, etc. For
1092 these features, the <tt><a href="#targetlowering">TargetLowering</a></tt>
1093 interface is used.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1094
1095</div>
1096
1097<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1098<h4>
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1099 <a name="selectiondag_legalize_types">SelectionDAG LegalizeTypes Phase</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1100</h4>
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1101
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1102<div>
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1103
1104<p>The Legalize phase is in charge of converting a DAG to only use the types
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1105 that are natively supported by the target.</p>
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1106
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1107<p>There are two main ways of converting values of unsupported scalar types to
1108 values of supported types: converting small types to larger types
1109 ("promoting"), and breaking up large integer types into smaller ones
1110 ("expanding"). For example, a target might require that all f32 values are
1111 promoted to f64 and that all i1/i8/i16 values are promoted to i32. The same
1112 target might require that all i64 values be expanded into pairs of i32
1113 values. These changes can insert sign and zero extensions as needed to make
1114 sure that the final code has the same behavior as the input.</p>
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1115
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1116<p>There are two main ways of converting values of unsupported vector types to
1117 value of supported types: splitting vector types, multiple times if
1118 necessary, until a legal type is found, and extending vector types by adding
1119 elements to the end to round them out to legal types ("widening"). If a
1120 vector gets split all the way down to single-element parts with no supported
1121 vector type being found, the elements are converted to scalars
1122 ("scalarizing").</p>
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1123
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1124<p>A target implementation tells the legalizer which types are supported (and
1125 which register class to use for them) by calling the
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1126 <tt>addRegisterClass</tt> method in its TargetLowering constructor.</p>
1127
1128</div>
1129
1130<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1131<h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1132 <a name="selectiondag_legalize">SelectionDAG Legalize Phase</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1133</h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1134
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1135<div>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1136
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1137<p>The Legalize phase is in charge of converting a DAG to only use the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1138 operations that are natively supported by the target.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1139
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1140<p>Targets often have weird constraints, such as not supporting every operation
1141 on every supported datatype (e.g. X86 does not support byte conditional moves
1142 and PowerPC does not support sign-extending loads from a 16-bit memory
1143 location). Legalize takes care of this by open-coding another sequence of
1144 operations to emulate the operation ("expansion"), by promoting one type to a
1145 larger type that supports the operation ("promotion"), or by using a
1146 target-specific hook to implement the legalization ("custom").</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1147
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1148<p>A target implementation tells the legalizer which operations are not
1149 supported (and which of the above three actions to take) by calling the
1150 <tt>setOperationAction</tt> method in its <tt>TargetLowering</tt>
1151 constructor.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1152
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1153<p>Prior to the existence of the Legalize passes, we required that every target
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1154 <a href="#selectiondag_optimize">selector</a> supported and handled every
1155 operator and type even if they are not natively supported. The introduction
1156 of the Legalize phases allows all of the canonicalization patterns to be
1157 shared across targets, and makes it very easy to optimize the canonicalized
1158 code because it is still in the form of a DAG.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1159
1160</div>
1161
1162<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1163<h4>
1164 <a name="selectiondag_optimize">
1165 SelectionDAG Optimization Phase: the DAG Combiner
1166 </a>
1167</h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1168
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1169<div>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1170
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1171<p>The SelectionDAG optimization phase is run multiple times for code
1172 generation, immediately after the DAG is built and once after each
1173 legalization. The first run of the pass allows the initial code to be
1174 cleaned up (e.g. performing optimizations that depend on knowing that the
1175 operators have restricted type inputs). Subsequent runs of the pass clean up
1176 the messy code generated by the Legalize passes, which allows Legalize to be
1177 very simple (it can focus on making code legal instead of focusing on
1178 generating <em>good</em> and legal code).</p>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1179
1180<p>One important class of optimizations performed is optimizing inserted sign
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1181 and zero extension instructions. We currently use ad-hoc techniques, but
1182 could move to more rigorous techniques in the future. Here are some good
1183 papers on the subject:</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1184
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1185<p>"<a href="http://www.eecs.harvard.edu/~nr/pubs/widen-abstract.html">Widening
1186 integer arithmetic</a>"<br>
1187 Kevin Redwine and Norman Ramsey<br>
1188 International Conference on Compiler Construction (CC) 2004</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1189
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1190<p>"<a href="http://portal.acm.org/citation.cfm?doid=512529.512552">Effective
1191 sign extension elimination</a>"<br>
1192 Motohiro Kawahito, Hideaki Komatsu, and Toshio Nakatani<br>
1193 Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language Design
1194 and Implementation.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1195
1196</div>
1197
1198<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1199<h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1200 <a name="selectiondag_select">SelectionDAG Select Phase</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1201</h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1202
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1203<div>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1204
1205<p>The Select phase is the bulk of the target-specific code for instruction
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1206 selection. This phase takes a legal SelectionDAG as input, pattern matches
1207 the instructions supported by the target to this DAG, and produces a new DAG
1208 of target code. For example, consider the following LLVM fragment:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1209
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1210<div class="doc_code">
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1211<pre>
Dan Gohmana9445e12010-03-02 01:11:08 +0000[diff] [blame]1212%t1 = fadd float %W, %X
1213%t2 = fmul float %t1, %Y
1214%t3 = fadd float %t2, %Z
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1215</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1216</div>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1217
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1218<p>This LLVM code corresponds to a SelectionDAG that looks basically like
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1219 this:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1220
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1221<div class="doc_code">
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1222<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1223(fadd:f32 (fmul:f32 (fadd:f32 W, X), Y), Z)
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1224</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1225</div>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1226
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1227<p>If a target supports floating point multiply-and-add (FMA) operations, one of
1228 the adds can be merged with the multiply. On the PowerPC, for example, the
1229 output of the instruction selector might look like this DAG:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1230
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1231<div class="doc_code">
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1232<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1233(FMADDS (FADDS W, X), Y, Z)
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1234</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1235</div>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1236
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1237<p>The <tt>FMADDS</tt> instruction is a ternary instruction that multiplies its
1238first two operands and adds the third (as single-precision floating-point
1239numbers). The <tt>FADDS</tt> instruction is a simple binary single-precision
1240add instruction. To perform this pattern match, the PowerPC backend includes
1241the following instruction definitions:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1242
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1243<div class="doc_code">
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1244<pre>
1245def FMADDS : AForm_1&lt;59, 29,
1246 (ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRC, F4RC:$FRB),
1247 "fmadds $FRT, $FRA, $FRC, $FRB",
1248 [<b>(set F4RC:$FRT, (fadd (fmul F4RC:$FRA, F4RC:$FRC),
1249 F4RC:$FRB))</b>]&gt;;
1250def FADDS : AForm_2&lt;59, 21,
1251 (ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRB),
1252 "fadds $FRT, $FRA, $FRB",
1253 [<b>(set F4RC:$FRT, (fadd F4RC:$FRA, F4RC:$FRB))</b>]&gt;;
1254</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1255</div>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1256
1257<p>The portion of the instruction definition in bold indicates the pattern used
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1258 to match the instruction. The DAG operators
1259 (like <tt>fmul</tt>/<tt>fadd</tt>) are defined in
Dan Gohman6a4824c2010-03-25 00:03:04 +0000[diff] [blame]1260 the <tt>include/llvm/Target/TargetSelectionDAG.td</tt> file. "
1261 <tt>F4RC</tt>" is the register class of the input and result values.</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1262
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1263<p>The TableGen DAG instruction selector generator reads the instruction
1264 patterns in the <tt>.td</tt> file and automatically builds parts of the
1265 pattern matching code for your target. It has the following strengths:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1266
1267<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1268 <li>At compiler-compiler time, it analyzes your instruction patterns and tells
1269 you if your patterns make sense or not.</li>
1270
1271 <li>It can handle arbitrary constraints on operands for the pattern match. In
1272 particular, it is straight-forward to say things like "match any immediate
1273 that is a 13-bit sign-extended value". For examples, see the
1274 <tt>immSExt16</tt> and related <tt>tblgen</tt> classes in the PowerPC
1275 backend.</li>
1276
1277 <li>It knows several important identities for the patterns defined. For
1278 example, it knows that addition is commutative, so it allows the
1279 <tt>FMADDS</tt> pattern above to match "<tt>(fadd X, (fmul Y, Z))</tt>" as
1280 well as "<tt>(fadd (fmul X, Y), Z)</tt>", without the target author having
1281 to specially handle this case.</li>
1282
1283 <li>It has a full-featured type-inferencing system. In particular, you should
1284 rarely have to explicitly tell the system what type parts of your patterns
1285 are. In the <tt>FMADDS</tt> case above, we didn't have to tell
1286 <tt>tblgen</tt> that all of the nodes in the pattern are of type 'f32'.
1287 It was able to infer and propagate this knowledge from the fact that
1288 <tt>F4RC</tt> has type 'f32'.</li>
1289
1290 <li>Targets can define their own (and rely on built-in) "pattern fragments".
1291 Pattern fragments are chunks of reusable patterns that get inlined into
1292 your patterns during compiler-compiler time. For example, the integer
1293 "<tt>(not x)</tt>" operation is actually defined as a pattern fragment
1294 that expands as "<tt>(xor x, -1)</tt>", since the SelectionDAG does not
1295 have a native '<tt>not</tt>' operation. Targets can define their own
1296 short-hand fragments as they see fit. See the definition of
1297 '<tt>not</tt>' and '<tt>ineg</tt>' for examples.</li>
1298
1299 <li>In addition to instructions, targets can specify arbitrary patterns that
1300 map to one or more instructions using the 'Pat' class. For example, the
1301 PowerPC has no way to load an arbitrary integer immediate into a register
1302 in one instruction. To tell tblgen how to do this, it defines:
1303 <br>
1304 <br>
1305<div class="doc_code">
1306<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1307// Arbitrary immediate support. Implement in terms of LIS/ORI.
1308def : Pat&lt;(i32 imm:$imm),
1309 (ORI (LIS (HI16 imm:$imm)), (LO16 imm:$imm))&gt;;
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1310</pre>
1311</div>
1312 <br>
1313 If none of the single-instruction patterns for loading an immediate into a
1314 register match, this will be used. This rule says "match an arbitrary i32
1315 immediate, turning it into an <tt>ORI</tt> ('or a 16-bit immediate') and
1316 an <tt>LIS</tt> ('load 16-bit immediate, where the immediate is shifted to
1317 the left 16 bits') instruction". To make this work, the
1318 <tt>LO16</tt>/<tt>HI16</tt> node transformations are used to manipulate
1319 the input immediate (in this case, take the high or low 16-bits of the
1320 immediate).</li>
1321
1322 <li>While the system does automate a lot, it still allows you to write custom
1323 C++ code to match special cases if there is something that is hard to
1324 express.</li>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1325</ul>
1326
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1327<p>While it has many strengths, the system currently has some limitations,
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1328 primarily because it is a work in progress and is not yet finished:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1329
1330<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1331 <li>Overall, there is no way to define or match SelectionDAG nodes that define
Dan Gohmane370c802009-04-22 15:55:31 +0000[diff] [blame]1332 multiple values (e.g. <tt>SMUL_LOHI</tt>, <tt>LOAD</tt>, <tt>CALL</tt>,
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1333 etc). This is the biggest reason that you currently still <em>have
1334 to</em> write custom C++ code for your instruction selector.</li>
1335
1336 <li>There is no great way to support matching complex addressing modes yet.
1337 In the future, we will extend pattern fragments to allow them to define
1338 multiple values (e.g. the four operands of the <a href="#x86_memory">X86
1339 addressing mode</a>, which are currently matched with custom C++ code).
1340 In addition, we'll extend fragments so that a fragment can match multiple
1341 different patterns.</li>
1342
1343 <li>We don't automatically infer flags like isStore/isLoad yet.</li>
1344
1345 <li>We don't automatically generate the set of supported registers and
1346 operations for the <a href="#selectiondag_legalize">Legalizer</a>
1347 yet.</li>
1348
1349 <li>We don't have a way of tying in custom legalized nodes yet.</li>
Chris Lattner7d6915c2005-10-17 04:18:41 +0000[diff] [blame]1350</ul>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1351
1352<p>Despite these limitations, the instruction selector generator is still quite
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1353 useful for most of the binary and logical operations in typical instruction
1354 sets. If you run into any problems or can't figure out how to do something,
1355 please let Chris know!</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1356
1357</div>
1358
1359<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1360<h4>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1361 <a name="selectiondag_sched">SelectionDAG Scheduling and Formation Phase</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1362</h4>
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]1363
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1364<div>
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]1365
1366<p>The scheduling phase takes the DAG of target instructions from the selection
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1367 phase and assigns an order. The scheduler can pick an order depending on
1368 various constraints of the machines (i.e. order for minimal register pressure
1369 or try to cover instruction latencies). Once an order is established, the
1370 DAG is converted to a list
1371 of <tt><a href="#machineinstr">MachineInstr</a></tt>s and the SelectionDAG is
1372 destroyed.</p>
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]1373
Jeff Cohen0b81cda2005-10-24 16:54:55 +0000[diff] [blame]1374<p>Note that this phase is logically separate from the instruction selection
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1375 phase, but is tied to it closely in the code because it operates on
1376 SelectionDAGs.</p>
Chris Lattnerc38959f2005-10-17 03:09:31 +0000[diff] [blame]1377
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]1378</div>
1379
1380<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1381<h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1382 <a name="selectiondag_future">Future directions for the SelectionDAG</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1383</h4>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1384
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1385<div>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1386
1387<ol>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1388 <li>Optional function-at-a-time selection.</li>
1389
1390 <li>Auto-generate entire selector from <tt>.td</tt> file.</li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1391</ol>
1392
1393</div>
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1394
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1395</div>
1396
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1397<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1398<h3>
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1399 <a name="ssamco">SSA-based Machine Code Optimizations</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1400</h3>
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1401<div><p>To Be Written</p></div>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1402
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1403<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1404<h3>
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]1405 <a name="liveintervals">Live Intervals</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1406</h3>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1407
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1408<div>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1409
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]1410<p>Live Intervals are the ranges (intervals) where a variable is <i>live</i>.
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1411 They are used by some <a href="#regalloc">register allocator</a> passes to
1412 determine if two or more virtual registers which require the same physical
1413 register are live at the same point in the program (i.e., they conflict).
1414 When this situation occurs, one virtual register must be <i>spilled</i>.</p>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1415
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1416<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1417<h4>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1418 <a name="livevariable_analysis">Live Variable Analysis</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1419</h4>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1420
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1421<div>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1422
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1423<p>The first step in determining the live intervals of variables is to calculate
1424 the set of registers that are immediately dead after the instruction (i.e.,
1425 the instruction calculates the value, but it is never used) and the set of
1426 registers that are used by the instruction, but are never used after the
1427 instruction (i.e., they are killed). Live variable information is computed
1428 for each <i>virtual</i> register and <i>register allocatable</i> physical
1429 register in the function. This is done in a very efficient manner because it
1430 uses SSA to sparsely compute lifetime information for virtual registers
1431 (which are in SSA form) and only has to track physical registers within a
1432 block. Before register allocation, LLVM can assume that physical registers
1433 are only live within a single basic block. This allows it to do a single,
1434 local analysis to resolve physical register lifetimes within each basic
1435 block. If a physical register is not register allocatable (e.g., a stack
1436 pointer or condition codes), it is not tracked.</p>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1437
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1438<p>Physical registers may be live in to or out of a function. Live in values are
1439 typically arguments in registers. Live out values are typically return values
1440 in registers. Live in values are marked as such, and are given a dummy
1441 "defining" instruction during live intervals analysis. If the last basic
1442 block of a function is a <tt>return</tt>, then it's marked as using all live
1443 out values in the function.</p>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1444
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1445<p><tt>PHI</tt> nodes need to be handled specially, because the calculation of
1446 the live variable information from a depth first traversal of the CFG of the
1447 function won't guarantee that a virtual register used by the <tt>PHI</tt>
1448 node is defined before it's used. When a <tt>PHI</tt> node is encountered,
1449 only the definition is handled, because the uses will be handled in other
1450 basic blocks.</p>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1451
1452<p>For each <tt>PHI</tt> node of the current basic block, we simulate an
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1453 assignment at the end of the current basic block and traverse the successor
1454 basic blocks. If a successor basic block has a <tt>PHI</tt> node and one of
1455 the <tt>PHI</tt> node's operands is coming from the current basic block, then
1456 the variable is marked as <i>alive</i> within the current basic block and all
1457 of its predecessor basic blocks, until the basic block with the defining
1458 instruction is encountered.</p>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1459
1460</div>
1461
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]1462<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1463<h4>
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]1464 <a name="liveintervals_analysis">Live Intervals Analysis</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1465</h4>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1466
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1467<div>
Bill Wendling3cd5ca62006-10-11 06:30:10 +0000[diff] [blame]1468
Bill Wendling82e2eea2006-10-11 18:00:22 +0000[diff] [blame]1469<p>We now have the information available to perform the live intervals analysis
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1470 and build the live intervals themselves. We start off by numbering the basic
1471 blocks and machine instructions. We then handle the "live-in" values. These
1472 are in physical registers, so the physical register is assumed to be killed
1473 by the end of the basic block. Live intervals for virtual registers are
1474 computed for some ordering of the machine instructions <tt>[1, N]</tt>. A
1475 live interval is an interval <tt>[i, j)</tt>, where <tt>1 &lt;= i &lt;= j
1476 &lt; N</tt>, for which a variable is live.</p>
Bill Wendling3cd5ca62006-10-11 06:30:10 +0000[diff] [blame]1477
Bill Wendling82e2eea2006-10-11 18:00:22 +0000[diff] [blame]1478<p><i><b>More to come...</b></i></p>
1479
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]1480</div>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1481
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1482</div>
1483
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1484<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1485<h3>
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1486 <a name="regalloc">Register Allocation</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1487</h3>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1488
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1489<div>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1490
Bill Wendling3cd5ca62006-10-11 06:30:10 +0000[diff] [blame]1491<p>The <i>Register Allocation problem</i> consists in mapping a program
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1492 <i>P<sub>v</sub></i>, that can use an unbounded number of virtual registers,
1493 to a program <i>P<sub>p</sub></i> that contains a finite (possibly small)
1494 number of physical registers. Each target architecture has a different number
1495 of physical registers. If the number of physical registers is not enough to
1496 accommodate all the virtual registers, some of them will have to be mapped
1497 into memory. These virtuals are called <i>spilled virtuals</i>.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1498
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1499<!-- _______________________________________________________________________ -->
1500
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1501<h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1502 <a name="regAlloc_represent">How registers are represented in LLVM</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1503</h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1504
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1505<div>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1506
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1507<p>In LLVM, physical registers are denoted by integer numbers that normally
1508 range from 1 to 1023. To see how this numbering is defined for a particular
1509 architecture, you can read the <tt>GenRegisterNames.inc</tt> file for that
1510 architecture. For instance, by
1511 inspecting <tt>lib/Target/X86/X86GenRegisterNames.inc</tt> we see that the
1512 32-bit register <tt>EAX</tt> is denoted by 15, and the MMX register
1513 <tt>MM0</tt> is mapped to 48.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1514
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1515<p>Some architectures contain registers that share the same physical location. A
1516 notable example is the X86 platform. For instance, in the X86 architecture,
1517 the registers <tt>EAX</tt>, <tt>AX</tt> and <tt>AL</tt> share the first eight
1518 bits. These physical registers are marked as <i>aliased</i> in LLVM. Given a
1519 particular architecture, you can check which registers are aliased by
1520 inspecting its <tt>RegisterInfo.td</tt> file. Moreover, the method
1521 <tt>TargetRegisterInfo::getAliasSet(p_reg)</tt> returns an array containing
1522 all the physical registers aliased to the register <tt>p_reg</tt>.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1523
1524<p>Physical registers, in LLVM, are grouped in <i>Register Classes</i>.
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1525 Elements in the same register class are functionally equivalent, and can be
1526 interchangeably used. Each virtual register can only be mapped to physical
1527 registers of a particular class. For instance, in the X86 architecture, some
1528 virtuals can only be allocated to 8 bit registers. A register class is
1529 described by <tt>TargetRegisterClass</tt> objects. To discover if a virtual
1530 register is compatible with a given physical, this code can be used:</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1531
1532<div class="doc_code">
1533<pre>
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]1534bool RegMapping_Fer::compatible_class(MachineFunction &amp;mf,
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1535 unsigned v_reg,
1536 unsigned p_reg) {
Dan Gohman6f0d0242008-02-10 18:45:23 +0000[diff] [blame]1537 assert(TargetRegisterInfo::isPhysicalRegister(p_reg) &amp;&amp;
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1538 "Target register must be physical");
Chris Lattner534bcfb2007-12-31 04:16:08 +0000[diff] [blame]1539 const TargetRegisterClass *trc = mf.getRegInfo().getRegClass(v_reg);
1540 return trc-&gt;contains(p_reg);
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1541}
1542</pre>
1543</div>
1544
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1545<p>Sometimes, mostly for debugging purposes, it is useful to change the number
1546 of physical registers available in the target architecture. This must be done
1547 statically, inside the <tt>TargetRegsterInfo.td</tt> file. Just <tt>grep</tt>
1548 for <tt>RegisterClass</tt>, the last parameter of which is a list of
1549 registers. Just commenting some out is one simple way to avoid them being
1550 used. A more polite way is to explicitly exclude some registers from
Dan Gohmand2cb3d22009-07-24 00:30:09 +0000[diff] [blame]1551 the <i>allocation order</i>. See the definition of the <tt>GR8</tt> register
1552 class in <tt>lib/Target/X86/X86RegisterInfo.td</tt> for an example of this.
1553 </p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1554
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1555<p>Virtual registers are also denoted by integer numbers. Contrary to physical
Jakob Stoklund Olesen3ca21022011-01-08 23:10:59 +0000[diff] [blame]1556 registers, different virtual registers never share the same number. Whereas
1557 physical registers are statically defined in a <tt>TargetRegisterInfo.td</tt>
1558 file and cannot be created by the application developer, that is not the case
1559 with virtual registers. In order to create new virtual registers, use the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1560 method <tt>MachineRegisterInfo::createVirtualRegister()</tt>. This method
Jakob Stoklund Olesen3ca21022011-01-08 23:10:59 +0000[diff] [blame]1561 will return a new virtual register. Use an <tt>IndexedMap&lt;Foo,
1562 VirtReg2IndexFunctor&gt;</tt> to hold information per virtual register. If you
1563 need to enumerate all virtual registers, use the function
1564 <tt>TargetRegisterInfo::index2VirtReg()</tt> to find the virtual register
1565 numbers:</p>
1566
1567<div class="doc_code">
1568<pre>
1569 for (unsigned i = 0, e = MRI->getNumVirtRegs(); i != e; ++i) {
1570 unsigned VirtReg = TargetRegisterInfo::index2VirtReg(i);
1571 stuff(VirtReg);
1572 }
1573</pre>
1574</div>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1575
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1576<p>Before register allocation, the operands of an instruction are mostly virtual
1577 registers, although physical registers may also be used. In order to check if
1578 a given machine operand is a register, use the boolean
1579 function <tt>MachineOperand::isRegister()</tt>. To obtain the integer code of
1580 a register, use <tt>MachineOperand::getReg()</tt>. An instruction may define
1581 or use a register. For instance, <tt>ADD reg:1026 := reg:1025 reg:1024</tt>
1582 defines the registers 1024, and uses registers 1025 and 1026. Given a
1583 register operand, the method <tt>MachineOperand::isUse()</tt> informs if that
1584 register is being used by the instruction. The
1585 method <tt>MachineOperand::isDef()</tt> informs if that registers is being
1586 defined.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1587
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1588<p>We will call physical registers present in the LLVM bitcode before register
1589 allocation <i>pre-colored registers</i>. Pre-colored registers are used in
1590 many different situations, for instance, to pass parameters of functions
1591 calls, and to store results of particular instructions. There are two types
1592 of pre-colored registers: the ones <i>implicitly</i> defined, and
1593 those <i>explicitly</i> defined. Explicitly defined registers are normal
1594 operands, and can be accessed
1595 with <tt>MachineInstr::getOperand(int)::getReg()</tt>. In order to check
1596 which registers are implicitly defined by an instruction, use
1597 the <tt>TargetInstrInfo::get(opcode)::ImplicitDefs</tt>,
1598 where <tt>opcode</tt> is the opcode of the target instruction. One important
1599 difference between explicit and implicit physical registers is that the
1600 latter are defined statically for each instruction, whereas the former may
1601 vary depending on the program being compiled. For example, an instruction
1602 that represents a function call will always implicitly define or use the same
1603 set of physical registers. To read the registers implicitly used by an
1604 instruction,
1605 use <tt>TargetInstrInfo::get(opcode)::ImplicitUses</tt>. Pre-colored
1606 registers impose constraints on any register allocation algorithm. The
Bob Wilson04738682010-04-09 18:39:54 +0000[diff] [blame]1607 register allocator must make sure that none of them are overwritten by
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1608 the values of virtual registers while still alive.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1609
1610</div>
1611
1612<!-- _______________________________________________________________________ -->
1613
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1614<h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1615 <a name="regAlloc_howTo">Mapping virtual registers to physical registers</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1616</h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1617
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1618<div>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1619
1620<p>There are two ways to map virtual registers to physical registers (or to
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1621 memory slots). The first way, that we will call <i>direct mapping</i>, is
1622 based on the use of methods of the classes <tt>TargetRegisterInfo</tt>,
1623 and <tt>MachineOperand</tt>. The second way, that we will call <i>indirect
1624 mapping</i>, relies on the <tt>VirtRegMap</tt> class in order to insert loads
1625 and stores sending and getting values to and from memory.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1626
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1627<p>The direct mapping provides more flexibility to the developer of the register
1628 allocator; however, it is more error prone, and demands more implementation
1629 work. Basically, the programmer will have to specify where load and store
1630 instructions should be inserted in the target function being compiled in
1631 order to get and store values in memory. To assign a physical register to a
1632 virtual register present in a given operand,
1633 use <tt>MachineOperand::setReg(p_reg)</tt>. To insert a store instruction,
Jakob Stoklund Olesen297907f2010-08-31 22:01:07 +0000[diff] [blame]1634 use <tt>TargetInstrInfo::storeRegToStackSlot(...)</tt>, and to insert a
1635 load instruction, use <tt>TargetInstrInfo::loadRegFromStackSlot</tt>.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1636
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1637<p>The indirect mapping shields the application developer from the complexities
1638 of inserting load and store instructions. In order to map a virtual register
1639 to a physical one, use <tt>VirtRegMap::assignVirt2Phys(vreg, preg)</tt>. In
1640 order to map a certain virtual register to memory,
1641 use <tt>VirtRegMap::assignVirt2StackSlot(vreg)</tt>. This method will return
1642 the stack slot where <tt>vreg</tt>'s value will be located. If it is
1643 necessary to map another virtual register to the same stack slot,
1644 use <tt>VirtRegMap::assignVirt2StackSlot(vreg, stack_location)</tt>. One
1645 important point to consider when using the indirect mapping, is that even if
1646 a virtual register is mapped to memory, it still needs to be mapped to a
1647 physical register. This physical register is the location where the virtual
1648 register is supposed to be found before being stored or after being
1649 reloaded.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1650
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1651<p>If the indirect strategy is used, after all the virtual registers have been
1652 mapped to physical registers or stack slots, it is necessary to use a spiller
1653 object to place load and store instructions in the code. Every virtual that
1654 has been mapped to a stack slot will be stored to memory after been defined
1655 and will be loaded before being used. The implementation of the spiller tries
1656 to recycle load/store instructions, avoiding unnecessary instructions. For an
1657 example of how to invoke the spiller,
1658 see <tt>RegAllocLinearScan::runOnMachineFunction</tt>
1659 in <tt>lib/CodeGen/RegAllocLinearScan.cpp</tt>.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1660
1661</div>
1662
1663<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1664<h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1665 <a name="regAlloc_twoAddr">Handling two address instructions</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1666</h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1667
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1668<div>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1669
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1670<p>With very rare exceptions (e.g., function calls), the LLVM machine code
1671 instructions are three address instructions. That is, each instruction is
1672 expected to define at most one register, and to use at most two registers.
1673 However, some architectures use two address instructions. In this case, the
1674 defined register is also one of the used register. For instance, an
1675 instruction such as <tt>ADD %EAX, %EBX</tt>, in X86 is actually equivalent
1676 to <tt>%EAX = %EAX + %EBX</tt>.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1677
1678<p>In order to produce correct code, LLVM must convert three address
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1679 instructions that represent two address instructions into true two address
1680 instructions. LLVM provides the pass <tt>TwoAddressInstructionPass</tt> for
1681 this specific purpose. It must be run before register allocation takes
1682 place. After its execution, the resulting code may no longer be in SSA
1683 form. This happens, for instance, in situations where an instruction such
1684 as <tt>%a = ADD %b %c</tt> is converted to two instructions such as:</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1685
1686<div class="doc_code">
1687<pre>
1688%a = MOVE %b
Dan Gohman03e58572008-06-13 17:55:57 +0000[diff] [blame]1689%a = ADD %a %c
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1690</pre>
1691</div>
1692
1693<p>Notice that, internally, the second instruction is represented as
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1694 <tt>ADD %a[def/use] %c</tt>. I.e., the register operand <tt>%a</tt> is both
1695 used and defined by the instruction.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1696
1697</div>
1698
1699<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1700<h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1701 <a name="regAlloc_ssaDecon">The SSA deconstruction phase</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1702</h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1703
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1704<div>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1705
1706<p>An important transformation that happens during register allocation is called
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1707 the <i>SSA Deconstruction Phase</i>. The SSA form simplifies many analyses
1708 that are performed on the control flow graph of programs. However,
1709 traditional instruction sets do not implement PHI instructions. Thus, in
1710 order to generate executable code, compilers must replace PHI instructions
1711 with other instructions that preserve their semantics.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1712
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1713<p>There are many ways in which PHI instructions can safely be removed from the
1714 target code. The most traditional PHI deconstruction algorithm replaces PHI
1715 instructions with copy instructions. That is the strategy adopted by
1716 LLVM. The SSA deconstruction algorithm is implemented
1717 in <tt>lib/CodeGen/PHIElimination.cpp</tt>. In order to invoke this pass, the
1718 identifier <tt>PHIEliminationID</tt> must be marked as required in the code
1719 of the register allocator.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1720
1721</div>
1722
1723<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1724<h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1725 <a name="regAlloc_fold">Instruction folding</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1726</h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1727
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1728<div>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1729
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1730<p><i>Instruction folding</i> is an optimization performed during register
1731 allocation that removes unnecessary copy instructions. For instance, a
1732 sequence of instructions such as:</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1733
1734<div class="doc_code">
1735<pre>
1736%EBX = LOAD %mem_address
1737%EAX = COPY %EBX
1738</pre>
1739</div>
1740
Dan Gohmana7ab2bf2008-11-24 16:35:31 +0000[diff] [blame]1741<p>can be safely substituted by the single instruction:</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1742
1743<div class="doc_code">
1744<pre>
1745%EAX = LOAD %mem_address
1746</pre>
1747</div>
1748
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1749<p>Instructions can be folded with
1750 the <tt>TargetRegisterInfo::foldMemoryOperand(...)</tt> method. Care must be
1751 taken when folding instructions; a folded instruction can be quite different
1752 from the original
1753 instruction. See <tt>LiveIntervals::addIntervalsForSpills</tt>
1754 in <tt>lib/CodeGen/LiveIntervalAnalysis.cpp</tt> for an example of its
1755 use.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1756
1757</div>
1758
1759<!-- _______________________________________________________________________ -->
1760
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1761<h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1762 <a name="regAlloc_builtIn">Built in register allocators</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1763</h4>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1764
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1765<div>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1766
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1767<p>The LLVM infrastructure provides the application developer with three
1768 different register allocators:</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1769
1770<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1771 <li><i>Linear Scan</i> &mdash; <i>The default allocator</i>. This is the
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1772 well-know linear scan register allocator. Whereas the
1773 <i>Simple</i> and <i>Local</i> algorithms use a direct mapping
1774 implementation technique, the <i>Linear Scan</i> implementation
1775 uses a spiller in order to place load and stores.</li>
Jakob Stoklund Olesen8a3eab92010-06-15 21:58:33 +0000[diff] [blame]1776
1777 <li><i>Fast</i> &mdash; This register allocator is the default for debug
1778 builds. It allocates registers on a basic block level, attempting to keep
1779 values in registers and reusing registers as appropriate.</li>
1780
1781 <li><i>PBQP</i> &mdash; A Partitioned Boolean Quadratic Programming (PBQP)
1782 based register allocator. This allocator works by constructing a PBQP
1783 problem representing the register allocation problem under consideration,
1784 solving this using a PBQP solver, and mapping the solution back to a
1785 register assignment.</li>
1786
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1787</ul>
1788
1789<p>The type of register allocator used in <tt>llc</tt> can be chosen with the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1790 command line option <tt>-regalloc=...</tt>:</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1791
1792<div class="doc_code">
1793<pre>
Dan Gohman0cabaa52009-08-25 15:54:01 +0000[diff] [blame]1794$ llc -regalloc=linearscan file.bc -o ln.s;
Jakob Stoklund Olesen8a3eab92010-06-15 21:58:33 +0000[diff] [blame]1795$ llc -regalloc=fast file.bc -o fa.s;
1796$ llc -regalloc=pbqp file.bc -o pbqp.s;
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1797</pre>
1798</div>
1799
1800</div>
1801
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1802</div>
1803
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1804<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1805<h3>
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1806 <a name="proepicode">Prolog/Epilog Code Insertion</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1807</h3>
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1808<div><p>To Be Written</p></div>
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1809<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1810<h3>
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1811 <a name="latemco">Late Machine Code Optimizations</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1812</h3>
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1813<div><p>To Be Written</p></div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1814
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1815<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1816<h3>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1817 <a name="codeemit">Code Emission</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1818</h3>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1819
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1820<div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1821
1822<p>The code emission step of code generation is responsible for lowering from
1823the code generator abstractions (like <a
1824href="#machinefunction">MachineFunction</a>, <a
1825href="#machineinstr">MachineInstr</a>, etc) down
1826to the abstractions used by the MC layer (<a href="#mcinst">MCInst</a>,
1827<a href="#mcstreamer">MCStreamer</a>, etc). This is
1828done with a combination of several different classes: the (misnamed)
1829target-independent AsmPrinter class, target-specific subclasses of AsmPrinter
1830(such as SparcAsmPrinter), and the TargetLoweringObjectFile class.</p>
1831
1832<p>Since the MC layer works at the level of abstraction of object files, it
1833doesn't have a notion of functions, global variables etc. Instead, it thinks
1834about labels, directives, and instructions. A key class used at this time is
1835the MCStreamer class. This is an abstract API that is implemented in different
1836ways (e.g. to output a .s file, output an ELF .o file, etc) that is effectively
1837an "assembler API". MCStreamer has one method per directive, such as EmitLabel,
1838EmitSymbolAttribute, SwitchSection, etc, which directly correspond to assembly
1839level directives.
1840</p>
1841
1842<p>If you are interested in implementing a code generator for a target, there
1843are three important things that you have to implement for your target:</p>
1844
1845<ol>
1846<li>First, you need a subclass of AsmPrinter for your target. This class
1847implements the general lowering process converting MachineFunction's into MC
1848label constructs. The AsmPrinter base class provides a number of useful methods
1849and routines, and also allows you to override the lowering process in some
1850important ways. You should get much of the lowering for free if you are
1851implementing an ELF, COFF, or MachO target, because the TargetLoweringObjectFile
1852class implements much of the common logic.</li>
1853
1854<li>Second, you need to implement an instruction printer for your target. The
1855instruction printer takes an <a href="#mcinst">MCInst</a> and renders it to a
1856raw_ostream as text. Most of this is automatically generated from the .td file
1857(when you specify something like "<tt>add $dst, $src1, $src2</tt>" in the
1858instructions), but you need to implement routines to print operands.</li>
1859
1860<li>Third, you need to implement code that lowers a <a
1861href="#machineinstr">MachineInstr</a> to an MCInst, usually implemented in
1862"&lt;target&gt;MCInstLower.cpp". This lowering process is often target
1863specific, and is responsible for turning jump table entries, constant pool
1864indices, global variable addresses, etc into MCLabels as appropriate. This
1865translation layer is also responsible for expanding pseudo ops used by the code
1866generator into the actual machine instructions they correspond to. The MCInsts
1867that are generated by this are fed into the instruction printer or the encoder.
1868</li>
1869
1870</ol>
1871
1872<p>Finally, at your choosing, you can also implement an subclass of
1873MCCodeEmitter which lowers MCInst's into machine code bytes and relocations.
1874This is important if you want to support direct .o file emission, or would like
1875to implement an assembler for your target.</p>
1876
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1877</div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1878
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1879</div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1880
Chris Lattner22481f22010-09-21 04:03:39 +0000[diff] [blame]1881<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1882<h2>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1883 <a name="nativeassembler">Implementing a Native Assembler</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1884</h2>
Chris Lattner22481f22010-09-21 04:03:39 +0000[diff] [blame]1885<!-- *********************************************************************** -->
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1886
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1887<div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1888
Chris Lattner22481f22010-09-21 04:03:39 +0000[diff] [blame]1889<p>Though you're probably reading this because you want to write or maintain a
1890compiler backend, LLVM also fully supports building a native assemblers too.
1891We've tried hard to automate the generation of the assembler from the .td files
1892(in particular the instruction syntax and encodings), which means that a large
1893part of the manual and repetitive data entry can be factored and shared with the
1894compiler.</p>
1895
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]1896<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1897<h3 id="na_instparsing">Instruction Parsing</h3>
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]1898
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1899<div><p>To Be Written</p></div>
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]1900
1901
1902<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1903<h3 id="na_instaliases">
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]1904 Instruction Alias Processing
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1905</h3>
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]1906
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1907<div>
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]1908<p>Once the instruction is parsed, it enters the MatchInstructionImpl function.
1909The MatchInstructionImpl function performs alias processing and then does
1910actual matching.</p>
1911
Chris Lattner693173f2010-10-30 19:23:13 +0000[diff] [blame]1912<p>Alias processing is the phase that canonicalizes different lexical forms of
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]1913the same instructions down to one representation. There are several different
1914kinds of alias that are possible to implement and they are listed below in the
1915order that they are processed (which is in order from simplest/weakest to most
1916complex/powerful). Generally you want to use the first alias mechanism that
1917meets the needs of your instruction, because it will allow a more concise
1918description.</p>
1919
1920<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1921<h4>Mnemonic Aliases</h4>
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]1922
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1923<div>
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]1924
Chris Lattner8cf8bcc2010-10-30 19:47:49 +0000[diff] [blame]1925<p>The first phase of alias processing is simple instruction mnemonic
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]1926remapping for classes of instructions which are allowed with two different
Chris Lattner693173f2010-10-30 19:23:13 +0000[diff] [blame]1927mnemonics. This phase is a simple and unconditionally remapping from one input
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]1928mnemonic to one output mnemonic. It isn't possible for this form of alias to
1929look at the operands at all, so the remapping must apply for all forms of a
1930given mnemonic. Mnemonic aliases are defined simply, for example X86 has:
1931</p>
1932
1933<div class="doc_code">
1934<pre>
1935def : MnemonicAlias&lt;"cbw", "cbtw"&gt;;
1936def : MnemonicAlias&lt;"smovq", "movsq"&gt;;
1937def : MnemonicAlias&lt;"fldcww", "fldcw"&gt;;
1938def : MnemonicAlias&lt;"fucompi", "fucomip"&gt;;
1939def : MnemonicAlias&lt;"ud2a", "ud2"&gt;;
1940</pre>
1941</div>
1942
1943<p>... and many others. With a MnemonicAlias definition, the mnemonic is
Chris Lattner693173f2010-10-30 19:23:13 +0000[diff] [blame]1944remapped simply and directly. Though MnemonicAlias's can't look at any aspect
1945of the instruction (such as the operands) they can depend on global modes (the
1946same ones supported by the matcher), through a Requires clause:</p>
1947
1948<div class="doc_code">
1949<pre>
1950def : MnemonicAlias&lt;"pushf", "pushfq"&gt;, Requires&lt;[In64BitMode]&gt;;
1951def : MnemonicAlias&lt;"pushf", "pushfl"&gt;, Requires&lt;[In32BitMode]&gt;;
1952</pre>
1953</div>
1954
1955<p>In this example, the mnemonic gets mapped into different a new one depending
1956on the current instruction set.</p>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1957
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1958</div>
1959
Chris Lattnerc7a03fb2010-11-06 08:30:26 +0000[diff] [blame]1960<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]1961<h4>Instruction Aliases</h4>
Chris Lattnerc7a03fb2010-11-06 08:30:26 +0000[diff] [blame]1962
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]1963<div>
Chris Lattnerc7a03fb2010-11-06 08:30:26 +0000[diff] [blame]1964
1965<p>The most general phase of alias processing occurs while matching is
1966happening: it provides new forms for the matcher to match along with a specific
1967instruction to generate. An instruction alias has two parts: the string to
1968match and the instruction to generate. For example:
1969</p>
1970
1971<div class="doc_code">
1972<pre>
1973def : InstAlias&lt;"movsx $src, $dst", (MOVSX16rr8W GR16:$dst, GR8 :$src)&gt;;
1974def : InstAlias&lt;"movsx $src, $dst", (MOVSX16rm8W GR16:$dst, i8mem:$src)&gt;;
1975def : InstAlias&lt;"movsx $src, $dst", (MOVSX32rr8 GR32:$dst, GR8 :$src)&gt;;
1976def : InstAlias&lt;"movsx $src, $dst", (MOVSX32rr16 GR32:$dst, GR16 :$src)&gt;;
1977def : InstAlias&lt;"movsx $src, $dst", (MOVSX64rr8 GR64:$dst, GR8 :$src)&gt;;
1978def : InstAlias&lt;"movsx $src, $dst", (MOVSX64rr16 GR64:$dst, GR16 :$src)&gt;;
1979def : InstAlias&lt;"movsx $src, $dst", (MOVSX64rr32 GR64:$dst, GR32 :$src)&gt;;
1980</pre>
1981</div>
1982
1983<p>This shows a powerful example of the instruction aliases, matching the
1984same mnemonic in multiple different ways depending on what operands are present
1985in the assembly. The result of instruction aliases can include operands in a
1986different order than the destination instruction, and can use an input
1987multiple times, for example:</p>
1988
1989<div class="doc_code">
1990<pre>
1991def : InstAlias&lt;"clrb $reg", (XOR8rr GR8 :$reg, GR8 :$reg)&gt;;
1992def : InstAlias&lt;"clrw $reg", (XOR16rr GR16:$reg, GR16:$reg)&gt;;
1993def : InstAlias&lt;"clrl $reg", (XOR32rr GR32:$reg, GR32:$reg)&gt;;
1994def : InstAlias&lt;"clrq $reg", (XOR64rr GR64:$reg, GR64:$reg)&gt;;
1995</pre>
1996</div>
1997
1998<p>This example also shows that tied operands are only listed once. In the X86
1999backend, XOR8rr has two input GR8's and one output GR8 (where an input is tied
2000to the output). InstAliases take a flattened operand list without duplicates
Chris Lattner90fd7972010-11-06 19:57:21 +0000[diff] [blame]2001for tied operands. The result of an instruction alias can also use immediates
2002and fixed physical registers which are added as simple immediate operands in the
2003result, for example:</p>
Chris Lattner98c870f2010-11-06 19:25:43 +0000[diff] [blame]2004
2005<div class="doc_code">
2006<pre>
Chris Lattner90fd7972010-11-06 19:57:21 +0000[diff] [blame]2007// Fixed Immediate operand.
Chris Lattner98c870f2010-11-06 19:25:43 +0000[diff] [blame]2008def : InstAlias&lt;"aad", (AAD8i8 10)&gt;;
Chris Lattner90fd7972010-11-06 19:57:21 +0000[diff] [blame]2009
2010// Fixed register operand.
2011def : InstAlias&lt;"fcomi", (COM_FIr ST1)&gt;;
2012
2013// Simple alias.
2014def : InstAlias&lt;"fcomi $reg", (COM_FIr RST:$reg)&gt;;
Chris Lattner98c870f2010-11-06 19:25:43 +0000[diff] [blame]2015</pre>
2016</div>
2017
Chris Lattnerc7a03fb2010-11-06 08:30:26 +0000[diff] [blame]2018
2019<p>Instruction aliases can also have a Requires clause to make them
2020subtarget specific.</p>
2021
2022</div>
2023
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2024</div>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]2025
Chris Lattner22481f22010-09-21 04:03:39 +0000[diff] [blame]2026<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2027<h3 id="na_matching">Instruction Matching</h3>
Chris Lattner674c1dc2010-10-30 17:36:36 +0000[diff] [blame]2028
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2029<div><p>To Be Written</p></div>
Chris Lattner22481f22010-09-21 04:03:39 +0000[diff] [blame]2030
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2031</div>
Chris Lattner22481f22010-09-21 04:03:39 +0000[diff] [blame]2032
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]2033<!-- *********************************************************************** -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2034<h2>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]2035 <a name="targetimpls">Target-specific Implementation Notes</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2036</h2>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2037<!-- *********************************************************************** -->
2038
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2039<div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2040
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2041<p>This section of the document explains features or design decisions that are
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2042 specific to the code generator for a particular target. First we start
2043 with a table that summarizes what features are supported by each target.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2044
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]2045<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2046<h3>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2047 <a name="targetfeatures">Target Feature Matrix</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2048</h3>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2049
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2050<div>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2051
2052<p>Note that this table does not include the C backend or Cpp backends, since
2053they do not use the target independent code generator infrastructure. It also
2054doesn't list features that are not supported fully by any target yet. It
2055considers a feature to be supported if at least one subtarget supports it. A
2056feature being supported means that it is useful and works for most cases, it
2057does not indicate that there are zero known bugs in the implementation. Here
2058is the key:</p>
2059
2060
2061<table border="1" cellspacing="0">
2062 <tr>
2063 <th>Unknown</th>
2064 <th>No support</th>
2065 <th>Partial Support</th>
2066 <th>Complete Support</th>
2067 </tr>
2068 <tr>
2069 <td class="unknown"></td>
2070 <td class="no"></td>
2071 <td class="partial"></td>
2072 <td class="yes"></td>
2073 </tr>
2074</table>
2075
2076<p>Here is the table:</p>
2077
2078<table width="689" border="1" cellspacing="0">
2079<tr><td></td>
Benjamin Kramer943beeb2010-10-30 21:07:28 +0000[diff] [blame]2080<td colspan="13" align="center" style="background-color:#ffc">Target</td>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2081</tr>
2082 <tr>
2083 <th>Feature</th>
2084 <th>ARM</th>
2085 <th>Alpha</th>
2086 <th>Blackfin</th>
2087 <th>CellSPU</th>
2088 <th>MBlaze</th>
2089 <th>MSP430</th>
2090 <th>Mips</th>
2091 <th>PTX</th>
2092 <th>PowerPC</th>
2093 <th>Sparc</th>
2094 <th>SystemZ</th>
2095 <th>X86</th>
2096 <th>XCore</th>
2097 </tr>
2098
2099<tr>
2100 <td><a href="#feat_reliable">is generally reliable</a></td>
2101 <td class="yes"></td> <!-- ARM -->
2102 <td class="unknown"></td> <!-- Alpha -->
Jakob Stoklund Olesen4e136122010-10-24 20:04:05 +0000[diff] [blame]2103 <td class="no"></td> <!-- Blackfin -->
Kalle Raiskila94cc4fe2010-10-25 08:57:30 +0000[diff] [blame]2104 <td class="no"></td> <!-- CellSPU -->
Wesley Peckc6a45242010-10-24 18:50:12 +0000[diff] [blame]2105 <td class="no"></td> <!-- MBlaze -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2106 <td class="unknown"></td> <!-- MSP430 -->
Bruno Cardoso Lopes48461f62010-12-19 22:41:43 +0000[diff] [blame]2107 <td class="no"></td> <!-- Mips -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2108 <td class="no"></td> <!-- PTX -->
2109 <td class="yes"></td> <!-- PowerPC -->
2110 <td class="yes"></td> <!-- Sparc -->
2111 <td class="unknown"></td> <!-- SystemZ -->
2112 <td class="yes"></td> <!-- X86 -->
2113 <td class="unknown"></td> <!-- XCore -->
2114</tr>
2115
2116<tr>
2117 <td><a href="#feat_asmparser">assembly parser</a></td>
2118 <td class="no"></td> <!-- ARM -->
2119 <td class="no"></td> <!-- Alpha -->
2120 <td class="no"></td> <!-- Blackfin -->
2121 <td class="no"></td> <!-- CellSPU -->
Wesley Peckd5fe3ef2010-12-20 21:54:50 +0000[diff] [blame]2122 <td class="yes"></td> <!-- MBlaze -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2123 <td class="no"></td> <!-- MSP430 -->
2124 <td class="no"></td> <!-- Mips -->
2125 <td class="no"></td> <!-- PTX -->
2126 <td class="no"></td> <!-- PowerPC -->
2127 <td class="no"></td> <!-- Sparc -->
2128 <td class="no"></td> <!-- SystemZ -->
2129 <td class="yes"></td> <!-- X86 -->
2130 <td class="no"></td> <!-- XCore -->
2131</tr>
2132
2133<tr>
2134 <td><a href="#feat_disassembler">disassembler</a></td>
2135 <td class="yes"></td> <!-- ARM -->
2136 <td class="no"></td> <!-- Alpha -->
2137 <td class="no"></td> <!-- Blackfin -->
2138 <td class="no"></td> <!-- CellSPU -->
Wesley Peckd5fe3ef2010-12-20 21:54:50 +0000[diff] [blame]2139 <td class="yes"></td> <!-- MBlaze -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2140 <td class="no"></td> <!-- MSP430 -->
2141 <td class="no"></td> <!-- Mips -->
2142 <td class="no"></td> <!-- PTX -->
2143 <td class="no"></td> <!-- PowerPC -->
2144 <td class="no"></td> <!-- Sparc -->
2145 <td class="no"></td> <!-- SystemZ -->
2146 <td class="yes"></td> <!-- X86 -->
2147 <td class="no"></td> <!-- XCore -->
2148</tr>
2149
2150<tr>
2151 <td><a href="#feat_inlineasm">inline asm</a></td>
2152 <td class="yes"></td> <!-- ARM -->
2153 <td class="unknown"></td> <!-- Alpha -->
Jakob Stoklund Olesen4e136122010-10-24 20:04:05 +0000[diff] [blame]2154 <td class="yes"></td> <!-- Blackfin -->
Kalle Raiskila94cc4fe2010-10-25 08:57:30 +0000[diff] [blame]2155 <td class="no"></td> <!-- CellSPU -->
Wesley Peckd5fe3ef2010-12-20 21:54:50 +0000[diff] [blame]2156 <td class="yes"></td> <!-- MBlaze -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2157 <td class="unknown"></td> <!-- MSP430 -->
Bruno Cardoso Lopes48461f62010-12-19 22:41:43 +0000[diff] [blame]2158 <td class="no"></td> <!-- Mips -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2159 <td class="unknown"></td> <!-- PTX -->
2160 <td class="yes"></td> <!-- PowerPC -->
2161 <td class="unknown"></td> <!-- Sparc -->
2162 <td class="unknown"></td> <!-- SystemZ -->
2163 <td class="yes"><a href="#feat_inlineasm_x86">*</a></td> <!-- X86 -->
2164 <td class="unknown"></td> <!-- XCore -->
2165</tr>
2166
2167<tr>
2168 <td><a href="#feat_jit">jit</a></td>
2169 <td class="partial"><a href="#feat_jit_arm">*</a></td> <!-- ARM -->
Chris Lattnerac3031a2010-11-14 18:25:50 +0000[diff] [blame]2170 <td class="no"></td> <!-- Alpha -->
Jakob Stoklund Olesen4e136122010-10-24 20:04:05 +0000[diff] [blame]2171 <td class="no"></td> <!-- Blackfin -->
Kalle Raiskila94cc4fe2010-10-25 08:57:30 +0000[diff] [blame]2172 <td class="no"></td> <!-- CellSPU -->
Wesley Peckc6a45242010-10-24 18:50:12 +0000[diff] [blame]2173 <td class="no"></td> <!-- MBlaze -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2174 <td class="unknown"></td> <!-- MSP430 -->
Bruno Cardoso Lopes48461f62010-12-19 22:41:43 +0000[diff] [blame]2175 <td class="no"></td> <!-- Mips -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2176 <td class="unknown"></td> <!-- PTX -->
2177 <td class="yes"></td> <!-- PowerPC -->
2178 <td class="unknown"></td> <!-- Sparc -->
2179 <td class="unknown"></td> <!-- SystemZ -->
2180 <td class="yes"></td> <!-- X86 -->
2181 <td class="unknown"></td> <!-- XCore -->
2182</tr>
2183
2184<tr>
2185 <td><a href="#feat_objectwrite">.o&nbsp;file writing</a></td>
2186 <td class="no"></td> <!-- ARM -->
2187 <td class="no"></td> <!-- Alpha -->
2188 <td class="no"></td> <!-- Blackfin -->
2189 <td class="no"></td> <!-- CellSPU -->
Wesley Peckd5fe3ef2010-12-20 21:54:50 +0000[diff] [blame]2190 <td class="yes"></td> <!-- MBlaze -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2191 <td class="no"></td> <!-- MSP430 -->
2192 <td class="no"></td> <!-- Mips -->
2193 <td class="no"></td> <!-- PTX -->
2194 <td class="no"></td> <!-- PowerPC -->
2195 <td class="no"></td> <!-- Sparc -->
2196 <td class="no"></td> <!-- SystemZ -->
2197 <td class="yes"></td> <!-- X86 -->
2198 <td class="no"></td> <!-- XCore -->
2199</tr>
2200
2201<tr>
2202 <td><a href="#feat_tailcall">tail calls</a></td>
2203 <td class="yes"></td> <!-- ARM -->
2204 <td class="unknown"></td> <!-- Alpha -->
Jakob Stoklund Olesen4e136122010-10-24 20:04:05 +0000[diff] [blame]2205 <td class="no"></td> <!-- Blackfin -->
Kalle Raiskila94cc4fe2010-10-25 08:57:30 +0000[diff] [blame]2206 <td class="no"></td> <!-- CellSPU -->
Wesley Peckc6a45242010-10-24 18:50:12 +0000[diff] [blame]2207 <td class="no"></td> <!-- MBlaze -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2208 <td class="unknown"></td> <!-- MSP430 -->
Bruno Cardoso Lopes48461f62010-12-19 22:41:43 +0000[diff] [blame]2209 <td class="no"></td> <!-- Mips -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2210 <td class="unknown"></td> <!-- PTX -->
2211 <td class="yes"></td> <!-- PowerPC -->
2212 <td class="unknown"></td> <!-- Sparc -->
2213 <td class="unknown"></td> <!-- SystemZ -->
2214 <td class="yes"></td> <!-- X86 -->
2215 <td class="unknown"></td> <!-- XCore -->
2216</tr>
2217
2218
2219</table>
2220
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2221<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2222<h4 id="feat_reliable">Is Generally Reliable</h4>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2223
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2224<div>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2225<p>This box indicates whether the target is considered to be production quality.
2226This indicates that the target has been used as a static compiler to
2227compile large amounts of code by a variety of different people and is in
2228continuous use.</p>
2229</div>
2230
2231<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2232<h4 id="feat_asmparser">Assembly Parser</h4>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2233
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2234<div>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2235<p>This box indicates whether the target supports parsing target specific .s
2236files by implementing the MCAsmParser interface. This is required for llvm-mc
2237to be able to act as a native assembler and is required for inline assembly
2238support in the native .o file writer.</p>
2239
2240</div>
2241
2242
2243<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2244<h4 id="feat_disassembler">Disassembler</h4>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2245
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2246<div>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2247<p>This box indicates whether the target supports the MCDisassembler API for
2248disassembling machine opcode bytes into MCInst's.</p>
2249
2250</div>
2251
2252<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2253<h4 id="feat_inlineasm">Inline Asm</h4>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2254
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2255<div>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2256<p>This box indicates whether the target supports most popular inline assembly
2257constraints and modifiers.</p>
2258
2259<p id="feat_inlineasm_x86">X86 lacks reliable support for inline assembly
2260constraints relating to the X86 floating point stack.</p>
2261
2262</div>
2263
2264<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2265<h4 id="feat_jit">JIT Support</h4>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2266
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2267<div>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2268<p>This box indicates whether the target supports the JIT compiler through
2269the ExecutionEngine interface.</p>
2270
Chris Lattner6fb99552010-10-24 16:24:22 +0000[diff] [blame]2271<p id="feat_jit_arm">The ARM backend has basic support for integer code
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2272in ARM codegen mode, but lacks NEON and full Thumb support.</p>
2273
2274</div>
2275
2276<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2277<h4 id="feat_objectwrite">.o File Writing</h4>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2278
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2279<div>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2280
2281<p>This box indicates whether the target supports writing .o files (e.g. MachO,
2282ELF, and/or COFF) files directly from the target. Note that the target also
2283must include an assembly parser and general inline assembly support for full
2284inline assembly support in the .o writer.</p>
2285
Chris Lattner219ddf52010-10-28 02:22:02 +0000[diff] [blame]2286<p>Targets that don't support this feature can obviously still write out .o
2287files, they just rely on having an external assembler to translate from a .s
2288file to a .o file (as is the case for many C compilers).</p>
2289
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2290</div>
2291
2292<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2293<h4 id="feat_tailcall">Tail Calls</h4>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2294
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2295<div>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2296
2297<p>This box indicates whether the target supports guaranteed tail calls. These
2298are calls marked "<a href="LangRef.html#i_call">tail</a>" and use the fastcc
2299calling convention. Please see the <a href="#tailcallopt">tail call section
2300more more details</a>.</p>
2301
2302</div>
2303
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2304</div>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2305
2306<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2307<h3>
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]2308 <a name="tailcallopt">Tail call optimization</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2309</h3>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2310
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2311<div>
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]2312
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2313<p>Tail call optimization, callee reusing the stack of the caller, is currently
2314 supported on x86/x86-64 and PowerPC. It is performed if:</p>
2315
2316<ul>
Chris Lattner29689432010-03-11 00:22:57 +0000[diff] [blame]2317 <li>Caller and callee have the calling convention <tt>fastcc</tt> or
2318 <tt>cc 10</tt> (GHC call convention).</li>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2319
2320 <li>The call is a tail call - in tail position (ret immediately follows call
2321 and ret uses value of call or is void).</li>
2322
2323 <li>Option <tt>-tailcallopt</tt> is enabled.</li>
2324
2325 <li>Platform specific constraints are met.</li>
2326</ul>
2327
2328<p>x86/x86-64 constraints:</p>
2329
2330<ul>
2331 <li>No variable argument lists are used.</li>
2332
2333 <li>On x86-64 when generating GOT/PIC code only module-local calls (visibility
2334 = hidden or protected) are supported.</li>
2335</ul>
2336
2337<p>PowerPC constraints:</p>
2338
2339<ul>
2340 <li>No variable argument lists are used.</li>
2341
2342 <li>No byval parameters are used.</li>
2343
2344 <li>On ppc32/64 GOT/PIC only module-local calls (visibility = hidden or protected) are supported.</li>
2345</ul>
2346
2347<p>Example:</p>
2348
2349<p>Call as <tt>llc -tailcallopt test.ll</tt>.</p>
2350
2351<div class="doc_code">
2352<pre>
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]2353declare fastcc i32 @tailcallee(i32 inreg %a1, i32 inreg %a2, i32 %a3, i32 %a4)
2354
2355define fastcc i32 @tailcaller(i32 %in1, i32 %in2) {
2356 %l1 = add i32 %in1, %in2
2357 %tmp = tail call fastcc i32 @tailcallee(i32 %in1 inreg, i32 %in2 inreg, i32 %in1, i32 %l1)
2358 ret i32 %tmp
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2359}
2360</pre>
2361</div>
2362
2363<p>Implications of <tt>-tailcallopt</tt>:</p>
2364
2365<p>To support tail call optimization in situations where the callee has more
2366 arguments than the caller a 'callee pops arguments' convention is used. This
2367 currently causes each <tt>fastcc</tt> call that is not tail call optimized
2368 (because one or more of above constraints are not met) to be followed by a
2369 readjustment of the stack. So performance might be worse in such cases.</p>
2370
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]2371</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2372<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2373<h3>
Evan Chengdc444e92010-03-08 21:05:02 +0000[diff] [blame]2374 <a name="sibcallopt">Sibling call optimization</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2375</h3>
Evan Chengdc444e92010-03-08 21:05:02 +0000[diff] [blame]2376
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2377<div>
Evan Chengdc444e92010-03-08 21:05:02 +0000[diff] [blame]2378
2379<p>Sibling call optimization is a restricted form of tail call optimization.
2380 Unlike tail call optimization described in the previous section, it can be
2381 performed automatically on any tail calls when <tt>-tailcallopt</tt> option
2382 is not specified.</p>
2383
2384<p>Sibling call optimization is currently performed on x86/x86-64 when the
2385 following constraints are met:</p>
2386
2387<ul>
2388 <li>Caller and callee have the same calling convention. It can be either
2389 <tt>c</tt> or <tt>fastcc</tt>.
2390
2391 <li>The call is a tail call - in tail position (ret immediately follows call
2392 and ret uses value of call or is void).</li>
2393
2394 <li>Caller and callee have matching return type or the callee result is not
2395 used.
2396
2397 <li>If any of the callee arguments are being passed in stack, they must be
2398 available in caller's own incoming argument stack and the frame offsets
2399 must be the same.
2400</ul>
2401
2402<p>Example:</p>
2403<div class="doc_code">
2404<pre>
2405declare i32 @bar(i32, i32)
2406
2407define i32 @foo(i32 %a, i32 %b, i32 %c) {
2408entry:
2409 %0 = tail call i32 @bar(i32 %a, i32 %b)
2410 ret i32 %0
2411}
2412</pre>
2413</div>
2414
2415</div>
2416<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2417<h3>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2418 <a name="x86">The X86 backend</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2419</h3>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2420
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2421<div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2422
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]2423<p>The X86 code generator lives in the <tt>lib/Target/X86</tt> directory. This
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2424 code generator is capable of targeting a variety of x86-32 and x86-64
2425 processors, and includes support for ISA extensions such as MMX and SSE.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2426
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2427<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2428<h4>
Nate Begeman34509842009-01-26 02:54:45 +0000[diff] [blame]2429 <a name="x86_tt">X86 Target Triples supported</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2430</h4>
Chris Lattner9b988be2005-07-12 00:20:49 +0000[diff] [blame]2431
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2432<div>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]2433
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2434<p>The following are the known target triples that are supported by the X86
2435 backend. This is not an exhaustive list, and it would be useful to add those
2436 that people test.</p>
Chris Lattner9b988be2005-07-12 00:20:49 +0000[diff] [blame]2437
2438<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2439 <li><b>i686-pc-linux-gnu</b> &mdash; Linux</li>
2440
2441 <li><b>i386-unknown-freebsd5.3</b> &mdash; FreeBSD 5.3</li>
2442
2443 <li><b>i686-pc-cygwin</b> &mdash; Cygwin on Win32</li>
2444
2445 <li><b>i686-pc-mingw32</b> &mdash; MingW on Win32</li>
2446
2447 <li><b>i386-pc-mingw32msvc</b> &mdash; MingW crosscompiler on Linux</li>
2448
2449 <li><b>i686-apple-darwin*</b> &mdash; Apple Darwin on X86</li>
Torok Edwinc457b652009-06-15 12:17:44 +0000[diff] [blame]2450
2451 <li><b>x86_64-unknown-linux-gnu</b> &mdash; Linux</li>
Chris Lattner9b988be2005-07-12 00:20:49 +0000[diff] [blame]2452</ul>
2453
2454</div>
2455
2456<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2457<h4>
Anton Korobeynikovbcb97702006-09-17 20:25:45 +0000[diff] [blame]2458 <a name="x86_cc">X86 Calling Conventions supported</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2459</h4>
Anton Korobeynikovbcb97702006-09-17 20:25:45 +0000[diff] [blame]2460
2461
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2462<div>
Anton Korobeynikovbcb97702006-09-17 20:25:45 +0000[diff] [blame]2463
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]2464<p>The following target-specific calling conventions are known to backend:</p>
Anton Korobeynikovbcb97702006-09-17 20:25:45 +0000[diff] [blame]2465
2466<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2467 <li><b>x86_StdCall</b> &mdash; stdcall calling convention seen on Microsoft
2468 Windows platform (CC ID = 64).</li>
2469
2470 <li><b>x86_FastCall</b> &mdash; fastcall calling convention seen on Microsoft
2471 Windows platform (CC ID = 65).</li>
Anton Korobeynikovbcb97702006-09-17 20:25:45 +0000[diff] [blame]2472</ul>
2473
2474</div>
2475
2476<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2477<h4>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2478 <a name="x86_memory">Representing X86 addressing modes in MachineInstrs</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2479</h4>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2480
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2481<div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2482
Misha Brukman600df452005-02-17 22:22:24 +0000[diff] [blame]2483<p>The x86 has a very flexible way of accessing memory. It is capable of
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2484 forming memory addresses of the following expression directly in integer
2485 instructions (which use ModR/M addressing):</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2486
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]2487<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2488<pre>
Chris Lattnerb91227d2009-10-10 21:30:55 +0000[diff] [blame]2489SegmentReg: Base + [1,2,4,8] * IndexReg + Disp32
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2490</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]2491</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2492
Chris Lattnerb91227d2009-10-10 21:30:55 +0000[diff] [blame]2493<p>In order to represent this, LLVM tracks no less than 5 operands for each
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2494 memory operand of this form. This means that the "load" form of
2495 '<tt>mov</tt>' has the following <tt>MachineOperand</tt>s in this order:</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2496
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2497<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2498<pre>
Chris Lattnerb91227d2009-10-10 21:30:55 +0000[diff] [blame]2499Index: 0 | 1 2 3 4 5
2500Meaning: DestReg, | BaseReg, Scale, IndexReg, Displacement Segment
2501OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm PhysReg
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2502</pre>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2503</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2504
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2505<p>Stores, and all other instructions, treat the four memory operands in the
Chris Lattnerb91227d2009-10-10 21:30:55 +0000[diff] [blame]2506 same way and in the same order. If the segment register is unspecified
2507 (regno = 0), then no segment override is generated. "Lea" operations do not
2508 have a segment register specified, so they only have 4 operands for their
2509 memory reference.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2510
2511</div>
2512
2513<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2514<h4>
Nate Begeman34509842009-01-26 02:54:45 +0000[diff] [blame]2515 <a name="x86_memory">X86 address spaces supported</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2516</h4>
Nate Begeman34509842009-01-26 02:54:45 +0000[diff] [blame]2517
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2518<div>
Nate Begeman34509842009-01-26 02:54:45 +0000[diff] [blame]2519
Jay Foadcb88ec32011-04-06 07:55:30 +0000[diff] [blame]2520<p>x86 has a feature which provides
Dan Gohmand26795a2009-05-05 20:48:47 +0000[diff] [blame]2521 the ability to perform loads and stores to different address spaces
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2522 via the x86 segment registers. A segment override prefix byte on an
2523 instruction causes the instruction's memory access to go to the specified
2524 segment. LLVM address space 0 is the default address space, which includes
2525 the stack, and any unqualified memory accesses in a program. Address spaces
2526 1-255 are currently reserved for user-defined code. The GS-segment is
Chris Lattner1777d0c2009-05-05 18:52:19 +0000[diff] [blame]2527 represented by address space 256, while the FS-segment is represented by
2528 address space 257. Other x86 segments have yet to be allocated address space
2529 numbers.</p>
Nate Begeman34509842009-01-26 02:54:45 +0000[diff] [blame]2530
Dan Gohmand26795a2009-05-05 20:48:47 +0000[diff] [blame]2531<p>While these address spaces may seem similar to TLS via the
2532 <tt>thread_local</tt> keyword, and often use the same underlying hardware,
2533 there are some fundamental differences.</p>
2534
2535<p>The <tt>thread_local</tt> keyword applies to global variables and
2536 specifies that they are to be allocated in thread-local memory. There are
2537 no type qualifiers involved, and these variables can be pointed to with
2538 normal pointers and accessed with normal loads and stores.
2539 The <tt>thread_local</tt> keyword is target-independent at the LLVM IR
2540 level (though LLVM doesn't yet have implementations of it for some
2541 configurations).<p>
2542
2543<p>Special address spaces, in contrast, apply to static types. Every
2544 load and store has a particular address space in its address operand type,
2545 and this is what determines which address space is accessed.
2546 LLVM ignores these special address space qualifiers on global variables,
2547 and does not provide a way to directly allocate storage in them.
2548 At the LLVM IR level, the behavior of these special address spaces depends
2549 in part on the underlying OS or runtime environment, and they are specific
2550 to x86 (and LLVM doesn't yet handle them correctly in some cases).</p>
2551
2552<p>Some operating systems and runtime environments use (or may in the future
2553 use) the FS/GS-segment registers for various low-level purposes, so care
2554 should be taken when considering them.</p>
Nate Begeman34509842009-01-26 02:54:45 +0000[diff] [blame]2555
2556</div>
2557
2558<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2559<h4>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2560 <a name="x86_names">Instruction naming</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2561</h4>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2562
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2563<div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2564
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]2565<p>An instruction name consists of the base name, a default operand size, and a
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2566 a character per operand with an optional special size. For example:</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2567
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2568<div class="doc_code">
2569<pre>
2570ADD8rr -&gt; add, 8-bit register, 8-bit register
2571IMUL16rmi -&gt; imul, 16-bit register, 16-bit memory, 16-bit immediate
2572IMUL16rmi8 -&gt; imul, 16-bit register, 16-bit memory, 8-bit immediate
2573MOVSX32rm16 -&gt; movsx, 32-bit register, 16-bit memory
2574</pre>
2575</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2576
2577</div>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]2578
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2579</div>
2580
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2581<!-- ======================================================================= -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2582<h3>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2583 <a name="ppc">The PowerPC backend</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2584</h3>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2585
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2586<div>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2587
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2588<p>The PowerPC code generator lives in the lib/Target/PowerPC directory. The
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2589 code generation is retargetable to several variations or <i>subtargets</i> of
2590 the PowerPC ISA; including ppc32, ppc64 and altivec.</p>
2591
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2592<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2593<h4>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2594 <a name="ppc_abi">LLVM PowerPC ABI</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2595</h4>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2596
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2597<div>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2598
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2599<p>LLVM follows the AIX PowerPC ABI, with two deviations. LLVM uses a PC
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2600 relative (PIC) or static addressing for accessing global values, so no TOC
2601 (r2) is used. Second, r31 is used as a frame pointer to allow dynamic growth
2602 of a stack frame. LLVM takes advantage of having no TOC to provide space to
2603 save the frame pointer in the PowerPC linkage area of the caller frame.
2604 Other details of PowerPC ABI can be found at <a href=
2605 "http://developer.apple.com/documentation/DeveloperTools/Conceptual/LowLevelABI/Articles/32bitPowerPC.html"
2606 >PowerPC ABI.</a> Note: This link describes the 32 bit ABI. The 64 bit ABI
2607 is similar except space for GPRs are 8 bytes wide (not 4) and r13 is reserved
2608 for system use.</p>
2609
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2610</div>
2611
2612<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2613<h4>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2614 <a name="ppc_frame">Frame Layout</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2615</h4>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2616
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2617<div>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2618
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2619<p>The size of a PowerPC frame is usually fixed for the duration of a
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2620 function's invocation. Since the frame is fixed size, all references
2621 into the frame can be accessed via fixed offsets from the stack pointer. The
2622 exception to this is when dynamic alloca or variable sized arrays are
2623 present, then a base pointer (r31) is used as a proxy for the stack pointer
2624 and stack pointer is free to grow or shrink. A base pointer is also used if
2625 llvm-gcc is not passed the -fomit-frame-pointer flag. The stack pointer is
2626 always aligned to 16 bytes, so that space allocated for altivec vectors will
2627 be properly aligned.</p>
2628
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]2629<p>An invocation frame is laid out as follows (low memory at top);</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2630
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2631<table class="layout">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2632 <tr>
2633 <td>Linkage<br><br></td>
2634 </tr>
2635 <tr>
2636 <td>Parameter area<br><br></td>
2637 </tr>
2638 <tr>
2639 <td>Dynamic area<br><br></td>
2640 </tr>
2641 <tr>
2642 <td>Locals area<br><br></td>
2643 </tr>
2644 <tr>
2645 <td>Saved registers area<br><br></td>
2646 </tr>
2647 <tr style="border-style: none hidden none hidden;">
2648 <td><br></td>
2649 </tr>
2650 <tr>
2651 <td>Previous Frame<br><br></td>
2652 </tr>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2653</table>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2654
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2655<p>The <i>linkage</i> area is used by a callee to save special registers prior
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2656 to allocating its own frame. Only three entries are relevant to LLVM. The
2657 first entry is the previous stack pointer (sp), aka link. This allows
2658 probing tools like gdb or exception handlers to quickly scan the frames in
2659 the stack. A function epilog can also use the link to pop the frame from the
2660 stack. The third entry in the linkage area is used to save the return
2661 address from the lr register. Finally, as mentioned above, the last entry is
2662 used to save the previous frame pointer (r31.) The entries in the linkage
2663 area are the size of a GPR, thus the linkage area is 24 bytes long in 32 bit
2664 mode and 48 bytes in 64 bit mode.</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2665
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2666<p>32 bit linkage area</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2667
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2668<table class="layout">
2669 <tr>
2670 <td>0</td>
2671 <td>Saved SP (r1)</td>
2672 </tr>
2673 <tr>
2674 <td>4</td>
2675 <td>Saved CR</td>
2676 </tr>
2677 <tr>
2678 <td>8</td>
2679 <td>Saved LR</td>
2680 </tr>
2681 <tr>
2682 <td>12</td>
2683 <td>Reserved</td>
2684 </tr>
2685 <tr>
2686 <td>16</td>
2687 <td>Reserved</td>
2688 </tr>
2689 <tr>
2690 <td>20</td>
2691 <td>Saved FP (r31)</td>
2692 </tr>
2693</table>
2694
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2695<p>64 bit linkage area</p>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2696
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2697<table class="layout">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2698 <tr>
2699 <td>0</td>
2700 <td>Saved SP (r1)</td>
2701 </tr>
2702 <tr>
2703 <td>8</td>
2704 <td>Saved CR</td>
2705 </tr>
2706 <tr>
2707 <td>16</td>
2708 <td>Saved LR</td>
2709 </tr>
2710 <tr>
2711 <td>24</td>
2712 <td>Reserved</td>
2713 </tr>
2714 <tr>
2715 <td>32</td>
2716 <td>Reserved</td>
2717 </tr>
2718 <tr>
2719 <td>40</td>
2720 <td>Saved FP (r31)</td>
2721 </tr>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2722</table>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2723
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2724<p>The <i>parameter area</i> is used to store arguments being passed to a callee
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2725 function. Following the PowerPC ABI, the first few arguments are actually
2726 passed in registers, with the space in the parameter area unused. However,
2727 if there are not enough registers or the callee is a thunk or vararg
2728 function, these register arguments can be spilled into the parameter area.
2729 Thus, the parameter area must be large enough to store all the parameters for
2730 the largest call sequence made by the caller. The size must also be
2731 minimally large enough to spill registers r3-r10. This allows callees blind
2732 to the call signature, such as thunks and vararg functions, enough space to
2733 cache the argument registers. Therefore, the parameter area is minimally 32
2734 bytes (64 bytes in 64 bit mode.) Also note that since the parameter area is
2735 a fixed offset from the top of the frame, that a callee can access its spilt
2736 arguments using fixed offsets from the stack pointer (or base pointer.)</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2737
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2738<p>Combining the information about the linkage, parameter areas and alignment. A
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2739 stack frame is minimally 64 bytes in 32 bit mode and 128 bytes in 64 bit
2740 mode.</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2741
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2742<p>The <i>dynamic area</i> starts out as size zero. If a function uses dynamic
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2743 alloca then space is added to the stack, the linkage and parameter areas are
2744 shifted to top of stack, and the new space is available immediately below the
2745 linkage and parameter areas. The cost of shifting the linkage and parameter
2746 areas is minor since only the link value needs to be copied. The link value
2747 can be easily fetched by adding the original frame size to the base pointer.
2748 Note that allocations in the dynamic space need to observe 16 byte
2749 alignment.</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2750
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2751<p>The <i>locals area</i> is where the llvm compiler reserves space for local
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2752 variables.</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2753
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2754<p>The <i>saved registers area</i> is where the llvm compiler spills callee
2755 saved registers on entry to the callee.</p>
2756
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2757</div>
2758
2759<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2760<h4>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2761 <a name="ppc_prolog">Prolog/Epilog</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2762</h4>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2763
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2764<div>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2765
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2766<p>The llvm prolog and epilog are the same as described in the PowerPC ABI, with
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2767 the following exceptions. Callee saved registers are spilled after the frame
2768 is created. This allows the llvm epilog/prolog support to be common with
2769 other targets. The base pointer callee saved register r31 is saved in the
2770 TOC slot of linkage area. This simplifies allocation of space for the base
2771 pointer and makes it convenient to locate programatically and during
2772 debugging.</p>
2773
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2774</div>
2775
2776<!-- _______________________________________________________________________ -->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2777<h4>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2778 <a name="ppc_dynamic">Dynamic Allocation</a>
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]2779</h4>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2780
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2781<div>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2782
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]2783<p><i>TODO - More to come.</i></p>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2784
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]2785</div>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2786
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame^]2787</div>
2788
2789</div>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2790
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]2791<!-- *********************************************************************** -->
2792<hr>
2793<address>
2794 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]2795 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
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Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]2798
2799 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
NAKAMURA Takumib9a33632011-04-09 02:13:37 +0000[diff] [blame]2800 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]2801 Last modified: $Date$
2802</address>
2803
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