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Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]6 <title>The LLVM Target-Independent Code Generator</title>
7 <link rel="stylesheet" href="llvm.css" type="text/css">
8</head>
9<body>
10
11<div class="doc_title">
12 The LLVM Target-Independent Code Generator
13</div>
14
15<ol>
16 <li><a href="#introduction">Introduction</a>
17 <ul>
18 <li><a href="#required">Required components in the code generator</a></li>
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]19 <li><a href="#high-level-design">The high-level design of the code
20 generator</a></li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]21 <li><a href="#tablegen">Using TableGen for target description</a></li>
22 </ul>
23 </li>
24 <li><a href="#targetdesc">Target description classes</a>
25 <ul>
26 <li><a href="#targetmachine">The <tt>TargetMachine</tt> class</a></li>
27 <li><a href="#targetdata">The <tt>TargetData</tt> class</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]28 <li><a href="#targetlowering">The <tt>TargetLowering</tt> class</a></li>
Dan Gohman6f0d0242008-02-10 18:45:23 +0000[diff] [blame]29 <li><a href="#targetregisterinfo">The <tt>TargetRegisterInfo</tt> class</a></li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]30 <li><a href="#targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a></li>
31 <li><a href="#targetframeinfo">The <tt>TargetFrameInfo</tt> class</a></li>
Chris Lattner47adebb2005-10-16 17:06:07 +0000[diff] [blame]32 <li><a href="#targetsubtarget">The <tt>TargetSubtarget</tt> class</a></li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]33 <li><a href="#targetjitinfo">The <tt>TargetJITInfo</tt> class</a></li>
34 </ul>
35 </li>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]36 <li><a href="#codegendesc">The "Machine" Code Generator classes</a>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]37 <ul>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]38 <li><a href="#machineinstr">The <tt>MachineInstr</tt> class</a></li>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]39 <li><a href="#machinebasicblock">The <tt>MachineBasicBlock</tt>
40 class</a></li>
41 <li><a href="#machinefunction">The <tt>MachineFunction</tt> class</a></li>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]42 </ul>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]43 </li>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]44 <li><a href="#mc">The "MC" Layer</a>
45 <ul>
46 <li><a href="#mcstreamer">The <tt>MCStreamer</tt> API</a></li>
47 <li><a href="#mccontext">The <tt>MCContext</tt> class</a>
48 <li><a href="#mcsymbol">The <tt>MCSymbol</tt> class</a></li>
49 <li><a href="#mcsection">The <tt>MCSection</tt> class</a></li>
50 <li><a href="#mcinst">The <tt>MCInst</tt> class</a></li>
51 </ul>
52 </li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]53 <li><a href="#codegenalgs">Target-independent code generation algorithms</a>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]54 <ul>
55 <li><a href="#instselect">Instruction Selection</a>
56 <ul>
57 <li><a href="#selectiondag_intro">Introduction to SelectionDAGs</a></li>
58 <li><a href="#selectiondag_process">SelectionDAG Code Generation
59 Process</a></li>
60 <li><a href="#selectiondag_build">Initial SelectionDAG
61 Construction</a></li>
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]62 <li><a href="#selectiondag_legalize_types">SelectionDAG LegalizeTypes Phase</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]63 <li><a href="#selectiondag_legalize">SelectionDAG Legalize Phase</a></li>
64 <li><a href="#selectiondag_optimize">SelectionDAG Optimization
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]65 Phase: the DAG Combiner</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]66 <li><a href="#selectiondag_select">SelectionDAG Select Phase</a></li>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]67 <li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]68 Phase</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]69 <li><a href="#selectiondag_future">Future directions for the
70 SelectionDAG</a></li>
71 </ul></li>
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]72 <li><a href="#liveintervals">Live Intervals</a>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]73 <ul>
74 <li><a href="#livevariable_analysis">Live Variable Analysis</a></li>
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]75 <li><a href="#liveintervals_analysis">Live Intervals Analysis</a></li>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]76 </ul></li>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]77 <li><a href="#regalloc">Register Allocation</a>
78 <ul>
79 <li><a href="#regAlloc_represent">How registers are represented in
80 LLVM</a></li>
81 <li><a href="#regAlloc_howTo">Mapping virtual registers to physical
82 registers</a></li>
83 <li><a href="#regAlloc_twoAddr">Handling two address instructions</a></li>
84 <li><a href="#regAlloc_ssaDecon">The SSA deconstruction phase</a></li>
85 <li><a href="#regAlloc_fold">Instruction folding</a></li>
86 <li><a href="#regAlloc_builtIn">Built in register allocators</a></li>
87 </ul></li>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]88 <li><a href="#codeemit">Code Emission</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]89 </ul>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]90 </li>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]91 <li><a href="#nativeassembler">Implementing a Native Assembler</a></li>
92
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]93 <li><a href="#targetimpls">Target-specific Implementation Notes</a>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]94 <ul>
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]95 <li><a href="#targetfeatures">Target Feature Matrix</a></li>
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]96 <li><a href="#tailcallopt">Tail call optimization</a></li>
Evan Chengdc444e92010-03-08 21:05:02 +0000[diff] [blame]97 <li><a href="#sibcallopt">Sibling call optimization</a></li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]98 <li><a href="#x86">The X86 backend</a></li>
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]99 <li><a href="#ppc">The PowerPC backend</a>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]100 <ul>
101 <li><a href="#ppc_abi">LLVM PowerPC ABI</a></li>
102 <li><a href="#ppc_frame">Frame Layout</a></li>
103 <li><a href="#ppc_prolog">Prolog/Epilog</a></li>
104 <li><a href="#ppc_dynamic">Dynamic Allocation</a></li>
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]105 </ul></li>
106 </ul></li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]107
108</ol>
109
110<div class="doc_author">
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]111 <p>Written by the LLVM Team.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]112</div>
113
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]114<div class="doc_warning">
115 <p>Warning: This is a work in progress.</p>
116</div>
117
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]118<!-- *********************************************************************** -->
119<div class="doc_section">
120 <a name="introduction">Introduction</a>
121</div>
122<!-- *********************************************************************** -->
123
124<div class="doc_text">
125
126<p>The LLVM target-independent code generator is a framework that provides a
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]127 suite of reusable components for translating the LLVM internal representation
128 to the machine code for a specified target&mdash;either in assembly form
129 (suitable for a static compiler) or in binary machine code format (usable for
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]130 a JIT compiler). The LLVM target-independent code generator consists of six
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]131 main components:</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]132
133<ol>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]134 <li><a href="#targetdesc">Abstract target description</a> interfaces which
135 capture important properties about various aspects of the machine,
136 independently of how they will be used. These interfaces are defined in
137 <tt>include/llvm/Target/</tt>.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]138
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]139 <li>Classes used to represent the <a href="#codegendesc">code being
140 generated</a> for a target. These classes are intended to be abstract
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]141 enough to represent the machine code for <i>any</i> target machine. These
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]142 classes are defined in <tt>include/llvm/CodeGen/</tt>. At this level,
143 concepts like "constant pool entries" and "jump tables" are explicitly
144 exposed.</li>
145
146 <li>Classes and algorithms used to represent code as the object file level,
147 the <a href="#mc">MC Layer</a>. These classes represent assembly level
148 constructs like labels, sections, and instructions. At this level,
149 concepts like "constant pool entries" and "jump tables" don't exist.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]150
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]151 <li><a href="#codegenalgs">Target-independent algorithms</a> used to implement
152 various phases of native code generation (register allocation, scheduling,
153 stack frame representation, etc). This code lives
154 in <tt>lib/CodeGen/</tt>.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]155
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]156 <li><a href="#targetimpls">Implementations of the abstract target description
157 interfaces</a> for particular targets. These machine descriptions make
158 use of the components provided by LLVM, and can optionally provide custom
159 target-specific passes, to build complete code generators for a specific
160 target. Target descriptions live in <tt>lib/Target/</tt>.</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="#jit">The target-independent JIT components</a>. The LLVM JIT is
163 completely target independent (it uses the <tt>TargetJITInfo</tt>
164 structure to interface for target-specific issues. The code for the
165 target-independent JIT lives in <tt>lib/ExecutionEngine/JIT</tt>.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]166</ol>
167
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]168<p>Depending on which part of the code generator you are interested in working
169 on, different pieces of this will be useful to you. In any case, you should
170 be familiar with the <a href="#targetdesc">target description</a>
171 and <a href="#codegendesc">machine code representation</a> classes. If you
172 want to add a backend for a new target, you will need
173 to <a href="#targetimpls">implement the target description</a> classes for
174 your new target and understand the <a href="LangRef.html">LLVM code
175 representation</a>. If you are interested in implementing a
176 new <a href="#codegenalgs">code generation algorithm</a>, it should only
177 depend on the target-description and machine code representation classes,
178 ensuring that it is portable.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]179
180</div>
181
182<!-- ======================================================================= -->
183<div class="doc_subsection">
184 <a name="required">Required components in the code generator</a>
185</div>
186
187<div class="doc_text">
188
189<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]190 code generator and the set of reusable components that can be used to build
191 target-specific backends. The two most important interfaces
192 (<a href="#targetmachine"><tt>TargetMachine</tt></a>
193 and <a href="#targetdata"><tt>TargetData</tt></a>) are the only ones that are
194 required to be defined for a backend to fit into the LLVM system, but the
195 others must be defined if the reusable code generator components are going to
196 be used.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]197
198<p>This design has two important implications. The first is that LLVM can
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]199 support completely non-traditional code generation targets. For example, the
200 C backend does not require register allocation, instruction selection, or any
201 of the other standard components provided by the system. As such, it only
202 implements these two interfaces, and does its own thing. Another example of
203 a code generator like this is a (purely hypothetical) backend that converts
204 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]205
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]206<p>This design also implies that it is possible to design and implement
207 radically different code generators in the LLVM system that do not make use
208 of any of the built-in components. Doing so is not recommended at all, but
209 could be required for radically different targets that do not fit into the
210 LLVM machine description model: FPGAs for example.</p>
Chris Lattner900bf8c2004-06-02 07:06:06 +0000[diff] [blame]211
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]212</div>
213
214<!-- ======================================================================= -->
215<div class="doc_subsection">
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]216 <a name="high-level-design">The high-level design of the code generator</a>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]217</div>
218
219<div class="doc_text">
220
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]221<p>The LLVM target-independent code generator is designed to support efficient
222 and quality code generation for standard register-based microprocessors.
223 Code generation in this model is divided into the following stages:</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]224
225<ol>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]226 <li><b><a href="#instselect">Instruction Selection</a></b> &mdash; This phase
227 determines an efficient way to express the input LLVM code in the target
228 instruction set. This stage produces the initial code for the program in
229 the target instruction set, then makes use of virtual registers in SSA
230 form and physical registers that represent any required register
231 assignments due to target constraints or calling conventions. This step
232 turns the LLVM code into a DAG of target instructions.</li>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]233
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]234 <li><b><a href="#selectiondag_sched">Scheduling and Formation</a></b> &mdash;
235 This phase takes the DAG of target instructions produced by the
236 instruction selection phase, determines an ordering of the instructions,
237 then emits the instructions
238 as <tt><a href="#machineinstr">MachineInstr</a></tt>s with that ordering.
239 Note that we describe this in the <a href="#instselect">instruction
240 selection section</a> because it operates on
241 a <a href="#selectiondag_intro">SelectionDAG</a>.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]242
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]243 <li><b><a href="#ssamco">SSA-based Machine Code Optimizations</a></b> &mdash;
244 This optional stage consists of a series of machine-code optimizations
245 that operate on the SSA-form produced by the instruction selector.
246 Optimizations like modulo-scheduling or peephole optimization work
247 here.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]248
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]249 <li><b><a href="#regalloc">Register Allocation</a></b> &mdash; The target code
250 is transformed from an infinite virtual register file in SSA form to the
251 concrete register file used by the target. This phase introduces spill
252 code and eliminates all virtual register references from the program.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]253
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]254 <li><b><a href="#proepicode">Prolog/Epilog Code Insertion</a></b> &mdash; Once
255 the machine code has been generated for the function and the amount of
256 stack space required is known (used for LLVM alloca's and spill slots),
257 the prolog and epilog code for the function can be inserted and "abstract
258 stack location references" can be eliminated. This stage is responsible
259 for implementing optimizations like frame-pointer elimination and stack
260 packing.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]261
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]262 <li><b><a href="#latemco">Late Machine Code Optimizations</a></b> &mdash;
263 Optimizations that operate on "final" machine code can go here, such as
264 spill code scheduling and peephole optimizations.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]265
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]266 <li><b><a href="#codeemit">Code Emission</a></b> &mdash; The final stage
267 actually puts out the code for the current function, either in the target
268 assembler format or in machine code.</li>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]269</ol>
270
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]271<p>The code generator is based on the assumption that the instruction selector
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]272 will use an optimal pattern matching selector to create high-quality
273 sequences of native instructions. Alternative code generator designs based
274 on pattern expansion and aggressive iterative peephole optimization are much
275 slower. This design permits efficient compilation (important for JIT
276 environments) and aggressive optimization (used when generating code offline)
277 by allowing components of varying levels of sophistication to be used for any
278 step of compilation.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]279
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]280<p>In addition to these stages, target implementations can insert arbitrary
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]281 target-specific passes into the flow. For example, the X86 target uses a
282 special pass to handle the 80x87 floating point stack architecture. Other
283 targets with unusual requirements can be supported with custom passes as
284 needed.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]285
286</div>
287
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]288<!-- ======================================================================= -->
289<div class="doc_subsection">
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]290 <a name="tablegen">Using TableGen for target description</a>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]291</div>
292
293<div class="doc_text">
294
Chris Lattner5489e932004-06-01 18:35:00 +0000[diff] [blame]295<p>The target description classes require a detailed description of the target
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]296 architecture. These target descriptions often have a large amount of common
297 information (e.g., an <tt>add</tt> instruction is almost identical to a
298 <tt>sub</tt> instruction). In order to allow the maximum amount of
299 commonality to be factored out, the LLVM code generator uses
300 the <a href="TableGenFundamentals.html">TableGen</a> tool to describe big
301 chunks of the target machine, which allows the use of domain-specific and
302 target-specific abstractions to reduce the amount of repetition.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]303
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]304<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]305 of the target description to the <tt>.td</tt> form. Doing so gives us a
306 number of advantages. The most important is that it makes it easier to port
307 LLVM because it reduces the amount of C++ code that has to be written, and
308 the surface area of the code generator that needs to be understood before
309 someone can get something working. Second, it makes it easier to change
310 things. In particular, if tables and other things are all emitted
311 by <tt>tblgen</tt>, we only need a change in one place (<tt>tblgen</tt>) to
312 update all of the targets to a new interface.</p>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]313
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]314</div>
315
316<!-- *********************************************************************** -->
317<div class="doc_section">
318 <a name="targetdesc">Target description classes</a>
319</div>
320<!-- *********************************************************************** -->
321
322<div class="doc_text">
323
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]324<p>The LLVM target description classes (located in the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]325 <tt>include/llvm/Target</tt> directory) provide an abstract description of
326 the target machine independent of any particular client. These classes are
327 designed to capture the <i>abstract</i> properties of the target (such as the
328 instructions and registers it has), and do not incorporate any particular
329 pieces of code generation algorithms.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]330
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]331<p>All of the target description classes (except the
332 <tt><a href="#targetdata">TargetData</a></tt> class) are designed to be
333 subclassed by the concrete target implementation, and have virtual methods
334 implemented. To get to these implementations, the
335 <tt><a href="#targetmachine">TargetMachine</a></tt> class provides accessors
336 that should be implemented by the target.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]337
338</div>
339
340<!-- ======================================================================= -->
341<div class="doc_subsection">
342 <a name="targetmachine">The <tt>TargetMachine</tt> class</a>
343</div>
344
345<div class="doc_text">
346
347<p>The <tt>TargetMachine</tt> class provides virtual methods that are used to
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]348 access the target-specific implementations of the various target description
349 classes via the <tt>get*Info</tt> methods (<tt>getInstrInfo</tt>,
350 <tt>getRegisterInfo</tt>, <tt>getFrameInfo</tt>, etc.). This class is
351 designed to be specialized by a concrete target implementation
352 (e.g., <tt>X86TargetMachine</tt>) which implements the various virtual
353 methods. The only required target description class is
354 the <a href="#targetdata"><tt>TargetData</tt></a> class, but if the code
355 generator components are to be used, the other interfaces should be
356 implemented as well.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]357
358</div>
359
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]360<!-- ======================================================================= -->
361<div class="doc_subsection">
362 <a name="targetdata">The <tt>TargetData</tt> class</a>
363</div>
364
365<div class="doc_text">
366
367<p>The <tt>TargetData</tt> class is the only required target description class,
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]368 and it is the only class that is not extensible (you cannot derived a new
369 class from it). <tt>TargetData</tt> specifies information about how the
370 target lays out memory for structures, the alignment requirements for various
371 data types, the size of pointers in the target, and whether the target is
372 little-endian or big-endian.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]373
374</div>
375
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]376<!-- ======================================================================= -->
377<div class="doc_subsection">
378 <a name="targetlowering">The <tt>TargetLowering</tt> class</a>
379</div>
380
381<div class="doc_text">
382
383<p>The <tt>TargetLowering</tt> class is used by SelectionDAG based instruction
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]384 selectors primarily to describe how LLVM code should be lowered to
385 SelectionDAG operations. Among other things, this class indicates:</p>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]386
387<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]388 <li>an initial register class to use for various <tt>ValueType</tt>s,</li>
389
390 <li>which operations are natively supported by the target machine,</li>
391
392 <li>the return type of <tt>setcc</tt> operations,</li>
393
394 <li>the type to use for shift amounts, and</li>
395
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]396 <li>various high-level characteristics, like whether it is profitable to turn
397 division by a constant into a multiplication sequence</li>
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]398</ul>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]399
400</div>
401
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]402<!-- ======================================================================= -->
403<div class="doc_subsection">
Dan Gohman6f0d0242008-02-10 18:45:23 +0000[diff] [blame]404 <a name="targetregisterinfo">The <tt>TargetRegisterInfo</tt> class</a>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]405</div>
406
407<div class="doc_text">
408
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]409<p>The <tt>TargetRegisterInfo</tt> class is used to describe the register file
410 of the target and any interactions between the registers.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]411
412<p>Registers in the code generator are represented in the code generator by
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]413 unsigned integers. Physical registers (those that actually exist in the
414 target description) are unique small numbers, and virtual registers are
415 generally large. Note that register #0 is reserved as a flag value.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]416
417<p>Each register in the processor description has an associated
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]418 <tt>TargetRegisterDesc</tt> entry, which provides a textual name for the
419 register (used for assembly output and debugging dumps) and a set of aliases
420 (used to indicate whether one register overlaps with another).</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]421
Dan Gohman6f0d0242008-02-10 18:45:23 +0000[diff] [blame]422<p>In addition to the per-register description, the <tt>TargetRegisterInfo</tt>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]423 class exposes a set of processor specific register classes (instances of the
424 <tt>TargetRegisterClass</tt> class). Each register class contains sets of
425 registers that have the same properties (for example, they are all 32-bit
426 integer registers). Each SSA virtual register created by the instruction
427 selector has an associated register class. When the register allocator runs,
428 it replaces virtual registers with a physical register in the set.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]429
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]430<p>The target-specific implementations of these classes is auto-generated from
431 a <a href="TableGenFundamentals.html">TableGen</a> description of the
432 register file.</p>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]433
434</div>
435
436<!-- ======================================================================= -->
437<div class="doc_subsection">
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]438 <a name="targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]439</div>
440
Reid Spencer627cd002005-07-19 01:36:35 +0000[diff] [blame]441<div class="doc_text">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]442
443<p>The <tt>TargetInstrInfo</tt> class is used to describe the machine
444 instructions supported by the target. It is essentially an array of
445 <tt>TargetInstrDescriptor</tt> objects, each of which describes one
446 instruction the target supports. Descriptors define things like the mnemonic
447 for the opcode, the number of operands, the list of implicit register uses
448 and defs, whether the instruction has certain target-independent properties
449 (accesses memory, is commutable, etc), and holds any target-specific
450 flags.</p>
451
Reid Spencer627cd002005-07-19 01:36:35 +0000[diff] [blame]452</div>
453
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]454<!-- ======================================================================= -->
455<div class="doc_subsection">
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]456 <a name="targetframeinfo">The <tt>TargetFrameInfo</tt> class</a>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]457</div>
458
Reid Spencer627cd002005-07-19 01:36:35 +0000[diff] [blame]459<div class="doc_text">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]460
461<p>The <tt>TargetFrameInfo</tt> class is used to provide information about the
462 stack frame layout of the target. It holds the direction of stack growth, the
463 known stack alignment on entry to each function, and the offset to the local
464 area. The offset to the local area is the offset from the stack pointer on
465 function entry to the first location where function data (local variables,
466 spill locations) can be stored.</p>
467
Reid Spencer627cd002005-07-19 01:36:35 +0000[diff] [blame]468</div>
Chris Lattner47adebb2005-10-16 17:06:07 +0000[diff] [blame]469
470<!-- ======================================================================= -->
471<div class="doc_subsection">
472 <a name="targetsubtarget">The <tt>TargetSubtarget</tt> class</a>
473</div>
474
475<div class="doc_text">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]476
477<p>The <tt>TargetSubtarget</tt> class is used to provide information about the
478 specific chip set being targeted. A sub-target informs code generation of
479 which instructions are supported, instruction latencies and instruction
480 execution itinerary; i.e., which processing units are used, in what order,
481 and for how long.</p>
482
Chris Lattner47adebb2005-10-16 17:06:07 +0000[diff] [blame]483</div>
484
485
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]486<!-- ======================================================================= -->
487<div class="doc_subsection">
Chris Lattner10d68002004-06-01 17:18:11 +0000[diff] [blame]488 <a name="targetjitinfo">The <tt>TargetJITInfo</tt> class</a>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]489</div>
490
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]491<div class="doc_text">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]492
493<p>The <tt>TargetJITInfo</tt> class exposes an abstract interface used by the
494 Just-In-Time code generator to perform target-specific activities, such as
495 emitting stubs. If a <tt>TargetMachine</tt> supports JIT code generation, it
496 should provide one of these objects through the <tt>getJITInfo</tt>
497 method.</p>
498
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]499</div>
500
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]501<!-- *********************************************************************** -->
502<div class="doc_section">
503 <a name="codegendesc">Machine code description classes</a>
504</div>
505<!-- *********************************************************************** -->
506
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]507<div class="doc_text">
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]508
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]509<p>At the high-level, LLVM code is translated to a machine specific
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]510 representation formed out of
511 <a href="#machinefunction"><tt>MachineFunction</tt></a>,
512 <a href="#machinebasicblock"><tt>MachineBasicBlock</tt></a>,
513 and <a href="#machineinstr"><tt>MachineInstr</tt></a> instances (defined
514 in <tt>include/llvm/CodeGen</tt>). This representation is completely target
515 agnostic, representing instructions in their most abstract form: an opcode
516 and a series of operands. This representation is designed to support both an
517 SSA representation for machine code, as well as a register allocated, non-SSA
518 form.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]519
520</div>
521
522<!-- ======================================================================= -->
523<div class="doc_subsection">
524 <a name="machineinstr">The <tt>MachineInstr</tt> class</a>
525</div>
526
527<div class="doc_text">
528
529<p>Target machine instructions are represented as instances of the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]530 <tt>MachineInstr</tt> class. This class is an extremely abstract way of
531 representing machine instructions. In particular, it only keeps track of an
532 opcode number and a set of operands.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]533
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]534<p>The opcode number is a simple unsigned integer that only has meaning to a
535 specific backend. All of the instructions for a target should be defined in
536 the <tt>*InstrInfo.td</tt> file for the target. The opcode enum values are
537 auto-generated from this description. The <tt>MachineInstr</tt> class does
538 not have any information about how to interpret the instruction (i.e., what
539 the semantics of the instruction are); for that you must refer to the
540 <tt><a href="#targetinstrinfo">TargetInstrInfo</a></tt> class.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]541
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]542<p>The operands of a machine instruction can be of several different types: a
543 register reference, a constant integer, a basic block reference, etc. In
544 addition, a machine operand should be marked as a def or a use of the value
545 (though only registers are allowed to be defs).</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]546
547<p>By convention, the LLVM code generator orders instruction operands so that
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]548 all register definitions come before the register uses, even on architectures
549 that are normally printed in other orders. For example, the SPARC add
550 instruction: "<tt>add %i1, %i2, %i3</tt>" adds the "%i1", and "%i2" registers
551 and stores the result into the "%i3" register. In the LLVM code generator,
552 the operands should be stored as "<tt>%i3, %i1, %i2</tt>": with the
553 destination first.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]554
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]555<p>Keeping destination (definition) operands at the beginning of the operand
556 list has several advantages. In particular, the debugging printer will print
557 the instruction like this:</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]558
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]559<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]560<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]561%r3 = add %i1, %i2
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]562</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]563</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]564
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]565<p>Also if the first operand is a def, it is easier to <a href="#buildmi">create
566 instructions</a> whose only def is the first operand.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]567
568</div>
569
570<!-- _______________________________________________________________________ -->
571<div class="doc_subsubsection">
572 <a name="buildmi">Using the <tt>MachineInstrBuilder.h</tt> functions</a>
573</div>
574
575<div class="doc_text">
576
577<p>Machine instructions are created by using the <tt>BuildMI</tt> functions,
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]578 located in the <tt>include/llvm/CodeGen/MachineInstrBuilder.h</tt> file. The
579 <tt>BuildMI</tt> functions make it easy to build arbitrary machine
580 instructions. Usage of the <tt>BuildMI</tt> functions look like this:</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]581
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]582<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]583<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]584// Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42')
585// instruction. The '1' specifies how many operands will be added.
586MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42);
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]587
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]588// Create the same instr, but insert it at the end of a basic block.
589MachineBasicBlock &amp;MBB = ...
590BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42);
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]591
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]592// Create the same instr, but insert it before a specified iterator point.
593MachineBasicBlock::iterator MBBI = ...
594BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42);
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]595
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]596// Create a 'cmp Reg, 0' instruction, no destination reg.
597MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0);
598// Create an 'sahf' instruction which takes no operands and stores nothing.
599MI = BuildMI(X86::SAHF, 0);
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]600
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]601// Create a self looping branch instruction.
602BuildMI(MBB, X86::JNE, 1).addMBB(&amp;MBB);
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]603</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]604</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]605
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]606<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]607 have to specify the number of operands that the machine instruction will
608 take. This allows for efficient memory allocation. You also need to specify
609 if operands default to be uses of values, not definitions. If you need to
610 add a definition operand (other than the optional destination register), you
611 must explicitly mark it as such:</p>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]612
613<div class="doc_code">
614<pre>
Bill Wendling587daed2009-05-13 21:33:08 +0000[diff] [blame]615MI.addReg(Reg, RegState::Define);
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]616</pre>
617</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]618
619</div>
620
621<!-- _______________________________________________________________________ -->
622<div class="doc_subsubsection">
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]623 <a name="fixedregs">Fixed (preassigned) registers</a>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]624</div>
625
626<div class="doc_text">
627
628<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]629 presence of fixed registers. In particular, there are often places in the
630 instruction stream where the register allocator <em>must</em> arrange for a
631 particular value to be in a particular register. This can occur due to
632 limitations of the instruction set (e.g., the X86 can only do a 32-bit divide
633 with the <tt>EAX</tt>/<tt>EDX</tt> registers), or external factors like
634 calling conventions. In any case, the instruction selector should emit code
635 that copies a virtual register into or out of a physical register when
636 needed.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]637
638<p>For example, consider this simple LLVM example:</p>
639
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]640<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]641<pre>
Matthijs Kooijman61399af2008-06-04 15:46:35 +0000[diff] [blame]642define i32 @test(i32 %X, i32 %Y) {
643 %Z = udiv i32 %X, %Y
644 ret i32 %Z
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]645}
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]646</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]647</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]648
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]649<p>The X86 instruction selector produces this machine code for the <tt>div</tt>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]650 and <tt>ret</tt> (use "<tt>llc X.bc -march=x86 -print-machineinstrs</tt>" to
651 get this):</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]652
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]653<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]654<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]655;; Start of div
656%EAX = mov %reg1024 ;; Copy X (in reg1024) into EAX
657%reg1027 = sar %reg1024, 31
658%EDX = mov %reg1027 ;; Sign extend X into EDX
659idiv %reg1025 ;; Divide by Y (in reg1025)
660%reg1026 = mov %EAX ;; Read the result (Z) out of EAX
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]661
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]662;; Start of ret
663%EAX = mov %reg1026 ;; 32-bit return value goes in EAX
664ret
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]665</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]666</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]667
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]668<p>By the end of code generation, the register allocator has coalesced the
669 registers and deleted the resultant identity moves producing the following
670 code:</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]671
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]672<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]673<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]674;; X is in EAX, Y is in ECX
675mov %EAX, %EDX
676sar %EDX, 31
677idiv %ECX
678ret
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]679</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]680</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]681
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]682<p>This approach is extremely general (if it can handle the X86 architecture, it
683 can handle anything!) and allows all of the target specific knowledge about
684 the instruction stream to be isolated in the instruction selector. Note that
685 physical registers should have a short lifetime for good code generation, and
686 all physical registers are assumed dead on entry to and exit from basic
687 blocks (before register allocation). Thus, if you need a value to be live
688 across basic block boundaries, it <em>must</em> live in a virtual
689 register.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]690
691</div>
692
693<!-- _______________________________________________________________________ -->
694<div class="doc_subsubsection">
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]695 <a name="ssa">Machine code in SSA form</a>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]696</div>
697
698<div class="doc_text">
699
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]700<p><tt>MachineInstr</tt>'s are initially selected in SSA-form, and are
701 maintained in SSA-form until register allocation happens. For the most part,
702 this is trivially simple since LLVM is already in SSA form; LLVM PHI nodes
703 become machine code PHI nodes, and virtual registers are only allowed to have
704 a single definition.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]705
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]706<p>After register allocation, machine code is no longer in SSA-form because
707 there are no virtual registers left in the code.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]708
709</div>
710
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]711<!-- ======================================================================= -->
712<div class="doc_subsection">
713 <a name="machinebasicblock">The <tt>MachineBasicBlock</tt> class</a>
714</div>
715
716<div class="doc_text">
717
718<p>The <tt>MachineBasicBlock</tt> class contains a list of machine instructions
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]719 (<tt><a href="#machineinstr">MachineInstr</a></tt> instances). It roughly
720 corresponds to the LLVM code input to the instruction selector, but there can
721 be a one-to-many mapping (i.e. one LLVM basic block can map to multiple
722 machine basic blocks). The <tt>MachineBasicBlock</tt> class has a
723 "<tt>getBasicBlock</tt>" method, which returns the LLVM basic block that it
724 comes from.</p>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]725
726</div>
727
728<!-- ======================================================================= -->
729<div class="doc_subsection">
730 <a name="machinefunction">The <tt>MachineFunction</tt> class</a>
731</div>
732
733<div class="doc_text">
734
735<p>The <tt>MachineFunction</tt> class contains a list of machine basic blocks
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]736 (<tt><a href="#machinebasicblock">MachineBasicBlock</a></tt> instances). It
737 corresponds one-to-one with the LLVM function input to the instruction
738 selector. In addition to a list of basic blocks,
739 the <tt>MachineFunction</tt> contains a a <tt>MachineConstantPool</tt>,
740 a <tt>MachineFrameInfo</tt>, a <tt>MachineFunctionInfo</tt>, and a
741 <tt>MachineRegisterInfo</tt>. See
742 <tt>include/llvm/CodeGen/MachineFunction.h</tt> for more information.</p>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]743
744</div>
745
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]746
747<!-- *********************************************************************** -->
748<div class="doc_section">
749 <a name="mc">The "MC" Layer</a>
750</div>
751<!-- *********************************************************************** -->
752
753<div class="doc_text">
754
755<p>
756The MC Layer is used to represent and process code at the raw machine code
757level, devoid of "high level" information like "constant pools", "jump tables",
758"global variables" or anything like that. At this level, LLVM handles things
759like label names, machine instructions, and sections in the object file. The
760code in this layer is used for a number of important purposes: the tail end of
761the code generator uses it to write a .s or .o file, and it is also used by the
762llvm-mc tool to implement standalone machine codeassemblers and disassemblers.
763</p>
764
765<p>
766This section describes some of the important classes. There are also a number
767of important subsystems that interact at this layer, they are described later
768in this manual.
769</p>
770
771</div>
772
773
774<!-- ======================================================================= -->
775<div class="doc_subsection">
776 <a name="mcstreamer">The <tt>MCStreamer</tt> API</a>
777</div>
778
779<div class="doc_text">
780
781<p>
782MCStreamer is best thought of as an assembler API. It is an abstract API which
783is <em>implemented</em> in different ways (e.g. to output a .s file, output an
784ELF .o file, etc) but whose API correspond directly to what you see in a .s
785file. MCStreamer has one method per directive, such as EmitLabel,
786EmitSymbolAttribute, SwitchSection, EmitValue (for .byte, .word), etc, which
787directly correspond to assembly level directives. It also has an
788EmitInstruction method, which is used to output an MCInst to the streamer.
789</p>
790
791<p>
792This API is most important for two clients: the llvm-mc stand-alone assembler is
793effectively a parser that parses a line, then invokes a method on MCStreamer. In
794the code generator, the <a href="#codeemit">Code Emission</a> phase of the code
795generator lowers higher level LLVM IR and Machine* constructs down to the MC
796layer, emitting directives through MCStreamer.</p>
797
798<p>
799On the implementation side of MCStreamer, there are two major implementations:
800one for writing out a .s file (MCAsmStreamer), and one for writing out a .o
801file (MCObjectStreamer). MCAsmStreamer is a straight-forward implementation
802that prints out a directive for each method (e.g. EmitValue -&gt; .byte), but
803MCObjectStreamer implements a full assembler.
804</p>
805
806</div>
807
808<!-- ======================================================================= -->
809<div class="doc_subsection">
810 <a name="mccontext">The <tt>MCContext</tt> class</a>
811</div>
812
813<div class="doc_text">
814
815<p>
816The MCContext class is the owner of a variety of uniqued data structures at the
817MC layer, including symbols, sections, etc. As such, this is the class that you
818interact with to create symbols and sections. This class can not be subclassed.
819</p>
820
821</div>
822
823<!-- ======================================================================= -->
824<div class="doc_subsection">
825 <a name="mcsymbol">The <tt>MCSymbol</tt> class</a>
826</div>
827
828<div class="doc_text">
829
830<p>
831The MCSymbol class represents a symbol (aka label) in the assembly file. There
832are two interesting kinds of symbols: assembler temporary symbols, and normal
833symbols. Assembler temporary symbols are used and processed by the assembler
834but are discarded when the object file is produced. The distinction is usually
835represented by adding a prefix to the label, for example "L" labels are
836assembler temporary labels in MachO.
837</p>
838
839<p>MCSymbols are created by MCContext and uniqued there. This means that
840MCSymbols can be compared for pointer equivalence to find out if they are the
841same symbol. Note that pointer inequality does not guarantee the labels will
842end up at different addresses though. It's perfectly legal to output something
843like this to the .s file:<p>
844
845<pre>
846 foo:
847 bar:
848 .byte 4
849</pre>
850
851<p>In this case, both the foo and bar symbols will have the same address.</p>
852
853</div>
854
855<!-- ======================================================================= -->
856<div class="doc_subsection">
857 <a name="mcsection">The <tt>MCSection</tt> class</a>
858</div>
859
860<div class="doc_text">
861
862<p>
863The MCSection class represents an object-file specific section. It is subclassed
864by object file specific implementations (e.g. <tt>MCSectionMachO</tt>,
865<tt>MCSectionCOFF</tt>, <tt>MCSectionELF</tt>) and these are created and uniqued
866by MCContext. The MCStreamer has a notion of the current section, which can be
867changed with the SwitchToSection method (which corresponds to a ".section"
868directive in a .s file).
869</p>
870
871</div>
872
873<!-- ======================================================================= -->
874<div class="doc_subsection">
875 <a name="mcinst">The <tt>MCInst</tt> class</a></li>
876</div>
877
878<div class="doc_text">
879
880<p>
881The MCInst class is a target-independent representation of an instruction. It
882is a simple class (much more so than <a href="#machineinstr">MachineInstr</a>)
883that holds a target-specific opcode and a vector of MCOperands. MCOperand, in
884turn, is a simple discriminated union of three cases: 1) a simple immediate,
8852) a target register ID, 3) a symbolic expression (e.g. "Lfoo-Lbar+42") as an
886MCExpr.
887</p>
888
889<p>MCInst is the common currency used to represent machine instructions at the
890MC layer. It is the type used by the instruction encoder, the instruction
891printer, and the type generated by the assembly parser and disassembler.
892</p>
893
894</div>
895
896
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]897<!-- *********************************************************************** -->
898<div class="doc_section">
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]899 <a name="codegenalgs">Target-independent code generation algorithms</a>
900</div>
901<!-- *********************************************************************** -->
902
903<div class="doc_text">
904
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]905<p>This section documents the phases described in the
906 <a href="#high-level-design">high-level design of the code generator</a>.
907 It explains how they work and some of the rationale behind their design.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]908
909</div>
910
911<!-- ======================================================================= -->
912<div class="doc_subsection">
913 <a name="instselect">Instruction Selection</a>
914</div>
915
916<div class="doc_text">
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]917
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]918<p>Instruction Selection is the process of translating LLVM code presented to
919 the code generator into target-specific machine instructions. There are
920 several well-known ways to do this in the literature. LLVM uses a
921 SelectionDAG based instruction selector.</p>
922
923<p>Portions of the DAG instruction selector are generated from the target
924 description (<tt>*.td</tt>) files. Our goal is for the entire instruction
925 selector to be generated from these <tt>.td</tt> files, though currently
926 there are still things that require custom C++ code.</p>
927
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]928</div>
929
930<!-- _______________________________________________________________________ -->
931<div class="doc_subsubsection">
932 <a name="selectiondag_intro">Introduction to SelectionDAGs</a>
933</div>
934
935<div class="doc_text">
936
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]937<p>The SelectionDAG provides an abstraction for code representation in a way
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]938 that is amenable to instruction selection using automatic techniques
939 (e.g. dynamic-programming based optimal pattern matching selectors). It is
940 also well-suited to other phases of code generation; in particular,
941 instruction scheduling (SelectionDAG's are very close to scheduling DAGs
942 post-selection). Additionally, the SelectionDAG provides a host
943 representation where a large variety of very-low-level (but
944 target-independent) <a href="#selectiondag_optimize">optimizations</a> may be
945 performed; ones which require extensive information about the instructions
946 efficiently supported by the target.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]947
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]948<p>The SelectionDAG is a Directed-Acyclic-Graph whose nodes are instances of the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]949 <tt>SDNode</tt> class. The primary payload of the <tt>SDNode</tt> is its
950 operation code (Opcode) that indicates what operation the node performs and
951 the operands to the operation. The various operation node types are
952 described at the top of the <tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt>
953 file.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]954
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]955<p>Although most operations define a single value, each node in the graph may
956 define multiple values. For example, a combined div/rem operation will
957 define both the dividend and the remainder. Many other situations require
958 multiple values as well. Each node also has some number of operands, which
959 are edges to the node defining the used value. Because nodes may define
960 multiple values, edges are represented by instances of the <tt>SDValue</tt>
961 class, which is a <tt>&lt;SDNode, unsigned&gt;</tt> pair, indicating the node
962 and result value being used, respectively. Each value produced by
963 an <tt>SDNode</tt> has an associated <tt>MVT</tt> (Machine Value Type)
964 indicating what the type of the value is.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]965
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]966<p>SelectionDAGs contain two different kinds of values: those that represent
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]967 data flow and those that represent control flow dependencies. Data values
968 are simple edges with an integer or floating point value type. Control edges
969 are represented as "chain" edges which are of type <tt>MVT::Other</tt>.
970 These edges provide an ordering between nodes that have side effects (such as
971 loads, stores, calls, returns, etc). All nodes that have side effects should
972 take a token chain as input and produce a new one as output. By convention,
973 token chain inputs are always operand #0, and chain results are always the
974 last value produced by an operation.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]975
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]976<p>A SelectionDAG has designated "Entry" and "Root" nodes. The Entry node is
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]977 always a marker node with an Opcode of <tt>ISD::EntryToken</tt>. The Root
978 node is the final side-effecting node in the token chain. For example, in a
979 single basic block function it would be the return node.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]980
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]981<p>One important concept for SelectionDAGs is the notion of a "legal" vs.
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]982 "illegal" DAG. A legal DAG for a target is one that only uses supported
983 operations and supported types. On a 32-bit PowerPC, for example, a DAG with
984 a value of type i1, i8, i16, or i64 would be illegal, as would a DAG that
985 uses a SREM or UREM operation. The
986 <a href="#selectinodag_legalize_types">legalize types</a> and
987 <a href="#selectiondag_legalize">legalize operations</a> phases are
988 responsible for turning an illegal DAG into a legal DAG.</p>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]989
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]990</div>
991
992<!-- _______________________________________________________________________ -->
993<div class="doc_subsubsection">
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]994 <a name="selectiondag_process">SelectionDAG Instruction Selection Process</a>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]995</div>
996
997<div class="doc_text">
998
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]999<p>SelectionDAG-based instruction selection consists of the following steps:</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1000
1001<ol>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1002 <li><a href="#selectiondag_build">Build initial DAG</a> &mdash; This stage
1003 performs a simple translation from the input LLVM code to an illegal
1004 SelectionDAG.</li>
1005
1006 <li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> &mdash; This
1007 stage performs simple optimizations on the SelectionDAG to simplify it,
1008 and recognize meta instructions (like rotates
1009 and <tt>div</tt>/<tt>rem</tt> pairs) for targets that support these meta
1010 operations. This makes the resultant code more efficient and
1011 the <a href="#selectiondag_select">select instructions from DAG</a> phase
1012 (below) simpler.</li>
1013
1014 <li><a href="#selectiondag_legalize_types">Legalize SelectionDAG Types</a>
1015 &mdash; This stage transforms SelectionDAG nodes to eliminate any types
1016 that are unsupported on the target.</li>
1017
1018 <li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> &mdash; The
1019 SelectionDAG optimizer is run to clean up redundancies exposed by type
1020 legalization.</li>
1021
1022 <li><a href="#selectiondag_legalize">Legalize SelectionDAG Types</a> &mdash;
1023 This stage transforms SelectionDAG nodes to eliminate any types that are
1024 unsupported on the target.</li>
1025
1026 <li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> &mdash; The
1027 SelectionDAG optimizer is run to eliminate inefficiencies introduced by
1028 operation legalization.</li>
1029
1030 <li><a href="#selectiondag_select">Select instructions from DAG</a> &mdash;
1031 Finally, the target instruction selector matches the DAG operations to
1032 target instructions. This process translates the target-independent input
1033 DAG into another DAG of target instructions.</li>
1034
1035 <li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation</a>
1036 &mdash; The last phase assigns a linear order to the instructions in the
1037 target-instruction DAG and emits them into the MachineFunction being
1038 compiled. This step uses traditional prepass scheduling techniques.</li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1039</ol>
1040
1041<p>After all of these steps are complete, the SelectionDAG is destroyed and the
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1042 rest of the code generation passes are run.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1043
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1044<p>One great way to visualize what is going on here is to take advantage of a
1045 few LLC command line options. The following options pop up a window
1046 displaying the SelectionDAG at specific times (if you only get errors printed
1047 to the console while using this, you probably
1048 <a href="ProgrammersManual.html#ViewGraph">need to configure your system</a>
1049 to add support for it).</p>
Dan Gohman8c9c55f2008-09-10 22:23:41 +0000[diff] [blame]1050
1051<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1052 <li><tt>-view-dag-combine1-dags</tt> displays the DAG after being built,
1053 before the first optimization pass.</li>
1054
1055 <li><tt>-view-legalize-dags</tt> displays the DAG before Legalization.</li>
1056
1057 <li><tt>-view-dag-combine2-dags</tt> displays the DAG before the second
1058 optimization pass.</li>
1059
1060 <li><tt>-view-isel-dags</tt> displays the DAG before the Select phase.</li>
1061
1062 <li><tt>-view-sched-dags</tt> displays the DAG before Scheduling.</li>
Dan Gohman8c9c55f2008-09-10 22:23:41 +0000[diff] [blame]1063</ul>
1064
1065<p>The <tt>-view-sunit-dags</tt> displays the Scheduler's dependency graph.
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1066 This graph is based on the final SelectionDAG, with nodes that must be
1067 scheduled together bundled into a single scheduling-unit node, and with
1068 immediate operands and other nodes that aren't relevant for scheduling
1069 omitted.</p>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1070
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1071</div>
1072
1073<!-- _______________________________________________________________________ -->
1074<div class="doc_subsubsection">
1075 <a name="selectiondag_build">Initial SelectionDAG Construction</a>
1076</div>
1077
1078<div class="doc_text">
1079
Bill Wendling16448772006-08-28 03:04:05 +0000[diff] [blame]1080<p>The initial SelectionDAG is na&iuml;vely peephole expanded from the LLVM
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1081 input by the <tt>SelectionDAGLowering</tt> class in the
1082 <tt>lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp</tt> file. The intent of
1083 this pass is to expose as much low-level, target-specific details to the
1084 SelectionDAG as possible. This pass is mostly hard-coded (e.g. an
1085 LLVM <tt>add</tt> turns into an <tt>SDNode add</tt> while a
1086 <tt>getelementptr</tt> is expanded into the obvious arithmetic). This pass
1087 requires target-specific hooks to lower calls, returns, varargs, etc. For
1088 these features, the <tt><a href="#targetlowering">TargetLowering</a></tt>
1089 interface is used.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1090
1091</div>
1092
1093<!-- _______________________________________________________________________ -->
1094<div class="doc_subsubsection">
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1095 <a name="selectiondag_legalize_types">SelectionDAG LegalizeTypes Phase</a>
1096</div>
1097
1098<div class="doc_text">
1099
1100<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]1101 that are natively supported by the target.</p>
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1102
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1103<p>There are two main ways of converting values of unsupported scalar types to
1104 values of supported types: converting small types to larger types
1105 ("promoting"), and breaking up large integer types into smaller ones
1106 ("expanding"). For example, a target might require that all f32 values are
1107 promoted to f64 and that all i1/i8/i16 values are promoted to i32. The same
1108 target might require that all i64 values be expanded into pairs of i32
1109 values. These changes can insert sign and zero extensions as needed to make
1110 sure that the final code has the same behavior as the input.</p>
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1111
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1112<p>There are two main ways of converting values of unsupported vector types to
1113 value of supported types: splitting vector types, multiple times if
1114 necessary, until a legal type is found, and extending vector types by adding
1115 elements to the end to round them out to legal types ("widening"). If a
1116 vector gets split all the way down to single-element parts with no supported
1117 vector type being found, the elements are converted to scalars
1118 ("scalarizing").</p>
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1119
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1120<p>A target implementation tells the legalizer which types are supported (and
1121 which register class to use for them) by calling the
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1122 <tt>addRegisterClass</tt> method in its TargetLowering constructor.</p>
1123
1124</div>
1125
1126<!-- _______________________________________________________________________ -->
1127<div class="doc_subsubsection">
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1128 <a name="selectiondag_legalize">SelectionDAG Legalize Phase</a>
1129</div>
1130
1131<div class="doc_text">
1132
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1133<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]1134 operations that are natively supported by the target.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1135
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1136<p>Targets often have weird constraints, such as not supporting every operation
1137 on every supported datatype (e.g. X86 does not support byte conditional moves
1138 and PowerPC does not support sign-extending loads from a 16-bit memory
1139 location). Legalize takes care of this by open-coding another sequence of
1140 operations to emulate the operation ("expansion"), by promoting one type to a
1141 larger type that supports the operation ("promotion"), or by using a
1142 target-specific hook to implement the legalization ("custom").</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1143
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1144<p>A target implementation tells the legalizer which operations are not
1145 supported (and which of the above three actions to take) by calling the
1146 <tt>setOperationAction</tt> method in its <tt>TargetLowering</tt>
1147 constructor.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1148
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]1149<p>Prior to the existence of the Legalize passes, we required that every target
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1150 <a href="#selectiondag_optimize">selector</a> supported and handled every
1151 operator and type even if they are not natively supported. The introduction
1152 of the Legalize phases allows all of the canonicalization patterns to be
1153 shared across targets, and makes it very easy to optimize the canonicalized
1154 code because it is still in the form of a DAG.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1155
1156</div>
1157
1158<!-- _______________________________________________________________________ -->
1159<div class="doc_subsubsection">
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]1160 <a name="selectiondag_optimize">SelectionDAG Optimization Phase: the DAG
1161 Combiner</a>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1162</div>
1163
1164<div class="doc_text">
1165
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1166<p>The SelectionDAG optimization phase is run multiple times for code
1167 generation, immediately after the DAG is built and once after each
1168 legalization. The first run of the pass allows the initial code to be
1169 cleaned up (e.g. performing optimizations that depend on knowing that the
1170 operators have restricted type inputs). Subsequent runs of the pass clean up
1171 the messy code generated by the Legalize passes, which allows Legalize to be
1172 very simple (it can focus on making code legal instead of focusing on
1173 generating <em>good</em> and legal code).</p>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1174
1175<p>One important class of optimizations performed is optimizing inserted sign
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1176 and zero extension instructions. We currently use ad-hoc techniques, but
1177 could move to more rigorous techniques in the future. Here are some good
1178 papers on the subject:</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1179
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1180<p>"<a href="http://www.eecs.harvard.edu/~nr/pubs/widen-abstract.html">Widening
1181 integer arithmetic</a>"<br>
1182 Kevin Redwine and Norman Ramsey<br>
1183 International Conference on Compiler Construction (CC) 2004</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://portal.acm.org/citation.cfm?doid=512529.512552">Effective
1186 sign extension elimination</a>"<br>
1187 Motohiro Kawahito, Hideaki Komatsu, and Toshio Nakatani<br>
1188 Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language Design
1189 and Implementation.</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1190
1191</div>
1192
1193<!-- _______________________________________________________________________ -->
1194<div class="doc_subsubsection">
1195 <a name="selectiondag_select">SelectionDAG Select Phase</a>
1196</div>
1197
1198<div class="doc_text">
1199
1200<p>The Select phase is the bulk of the target-specific code for instruction
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1201 selection. This phase takes a legal SelectionDAG as input, pattern matches
1202 the instructions supported by the target to this DAG, and produces a new DAG
1203 of target code. For example, consider the following LLVM fragment:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1204
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1205<div class="doc_code">
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1206<pre>
Dan Gohmana9445e12010-03-02 01:11:08 +0000[diff] [blame]1207%t1 = fadd float %W, %X
1208%t2 = fmul float %t1, %Y
1209%t3 = fadd float %t2, %Z
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1210</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1211</div>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1212
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1213<p>This LLVM code corresponds to a SelectionDAG that looks basically like
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1214 this:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1215
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1216<div class="doc_code">
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1217<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1218(fadd:f32 (fmul:f32 (fadd:f32 W, X), Y), Z)
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1219</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1220</div>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1221
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1222<p>If a target supports floating point multiply-and-add (FMA) operations, one of
1223 the adds can be merged with the multiply. On the PowerPC, for example, the
1224 output of the instruction selector might look like this DAG:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1225
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1226<div class="doc_code">
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1227<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1228(FMADDS (FADDS W, X), Y, Z)
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1229</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1230</div>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1231
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1232<p>The <tt>FMADDS</tt> instruction is a ternary instruction that multiplies its
1233first two operands and adds the third (as single-precision floating-point
1234numbers). The <tt>FADDS</tt> instruction is a simple binary single-precision
1235add instruction. To perform this pattern match, the PowerPC backend includes
1236the following instruction definitions:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1237
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1238<div class="doc_code">
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1239<pre>
1240def FMADDS : AForm_1&lt;59, 29,
1241 (ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRC, F4RC:$FRB),
1242 "fmadds $FRT, $FRA, $FRC, $FRB",
1243 [<b>(set F4RC:$FRT, (fadd (fmul F4RC:$FRA, F4RC:$FRC),
1244 F4RC:$FRB))</b>]&gt;;
1245def FADDS : AForm_2&lt;59, 21,
1246 (ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRB),
1247 "fadds $FRT, $FRA, $FRB",
1248 [<b>(set F4RC:$FRT, (fadd F4RC:$FRA, F4RC:$FRB))</b>]&gt;;
1249</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1250</div>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1251
1252<p>The portion of the instruction definition in bold indicates the pattern used
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1253 to match the instruction. The DAG operators
1254 (like <tt>fmul</tt>/<tt>fadd</tt>) are defined in
Dan Gohman6a4824c2010-03-25 00:03:04 +0000[diff] [blame]1255 the <tt>include/llvm/Target/TargetSelectionDAG.td</tt> file. "
1256 <tt>F4RC</tt>" is the register class of the input and result values.</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1257
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1258<p>The TableGen DAG instruction selector generator reads the instruction
1259 patterns in the <tt>.td</tt> file and automatically builds parts of the
1260 pattern matching code for your target. It has the following strengths:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1261
1262<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1263 <li>At compiler-compiler time, it analyzes your instruction patterns and tells
1264 you if your patterns make sense or not.</li>
1265
1266 <li>It can handle arbitrary constraints on operands for the pattern match. In
1267 particular, it is straight-forward to say things like "match any immediate
1268 that is a 13-bit sign-extended value". For examples, see the
1269 <tt>immSExt16</tt> and related <tt>tblgen</tt> classes in the PowerPC
1270 backend.</li>
1271
1272 <li>It knows several important identities for the patterns defined. For
1273 example, it knows that addition is commutative, so it allows the
1274 <tt>FMADDS</tt> pattern above to match "<tt>(fadd X, (fmul Y, Z))</tt>" as
1275 well as "<tt>(fadd (fmul X, Y), Z)</tt>", without the target author having
1276 to specially handle this case.</li>
1277
1278 <li>It has a full-featured type-inferencing system. In particular, you should
1279 rarely have to explicitly tell the system what type parts of your patterns
1280 are. In the <tt>FMADDS</tt> case above, we didn't have to tell
1281 <tt>tblgen</tt> that all of the nodes in the pattern are of type 'f32'.
1282 It was able to infer and propagate this knowledge from the fact that
1283 <tt>F4RC</tt> has type 'f32'.</li>
1284
1285 <li>Targets can define their own (and rely on built-in) "pattern fragments".
1286 Pattern fragments are chunks of reusable patterns that get inlined into
1287 your patterns during compiler-compiler time. For example, the integer
1288 "<tt>(not x)</tt>" operation is actually defined as a pattern fragment
1289 that expands as "<tt>(xor x, -1)</tt>", since the SelectionDAG does not
1290 have a native '<tt>not</tt>' operation. Targets can define their own
1291 short-hand fragments as they see fit. See the definition of
1292 '<tt>not</tt>' and '<tt>ineg</tt>' for examples.</li>
1293
1294 <li>In addition to instructions, targets can specify arbitrary patterns that
1295 map to one or more instructions using the 'Pat' class. For example, the
1296 PowerPC has no way to load an arbitrary integer immediate into a register
1297 in one instruction. To tell tblgen how to do this, it defines:
1298 <br>
1299 <br>
1300<div class="doc_code">
1301<pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1302// Arbitrary immediate support. Implement in terms of LIS/ORI.
1303def : Pat&lt;(i32 imm:$imm),
1304 (ORI (LIS (HI16 imm:$imm)), (LO16 imm:$imm))&gt;;
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1305</pre>
1306</div>
1307 <br>
1308 If none of the single-instruction patterns for loading an immediate into a
1309 register match, this will be used. This rule says "match an arbitrary i32
1310 immediate, turning it into an <tt>ORI</tt> ('or a 16-bit immediate') and
1311 an <tt>LIS</tt> ('load 16-bit immediate, where the immediate is shifted to
1312 the left 16 bits') instruction". To make this work, the
1313 <tt>LO16</tt>/<tt>HI16</tt> node transformations are used to manipulate
1314 the input immediate (in this case, take the high or low 16-bits of the
1315 immediate).</li>
1316
1317 <li>While the system does automate a lot, it still allows you to write custom
1318 C++ code to match special cases if there is something that is hard to
1319 express.</li>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1320</ul>
1321
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]1322<p>While it has many strengths, the system currently has some limitations,
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1323 primarily because it is a work in progress and is not yet finished:</p>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1324
1325<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1326 <li>Overall, there is no way to define or match SelectionDAG nodes that define
Dan Gohmane370c802009-04-22 15:55:31 +0000[diff] [blame]1327 multiple values (e.g. <tt>SMUL_LOHI</tt>, <tt>LOAD</tt>, <tt>CALL</tt>,
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1328 etc). This is the biggest reason that you currently still <em>have
1329 to</em> write custom C++ code for your instruction selector.</li>
1330
1331 <li>There is no great way to support matching complex addressing modes yet.
1332 In the future, we will extend pattern fragments to allow them to define
1333 multiple values (e.g. the four operands of the <a href="#x86_memory">X86
1334 addressing mode</a>, which are currently matched with custom C++ code).
1335 In addition, we'll extend fragments so that a fragment can match multiple
1336 different patterns.</li>
1337
1338 <li>We don't automatically infer flags like isStore/isLoad yet.</li>
1339
1340 <li>We don't automatically generate the set of supported registers and
1341 operations for the <a href="#selectiondag_legalize">Legalizer</a>
1342 yet.</li>
1343
1344 <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]1345</ul>
Chris Lattner7a025c82005-10-16 20:02:19 +0000[diff] [blame]1346
1347<p>Despite these limitations, the instruction selector generator is still quite
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1348 useful for most of the binary and logical operations in typical instruction
1349 sets. If you run into any problems or can't figure out how to do something,
1350 please let Chris know!</p>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1351
1352</div>
1353
1354<!-- _______________________________________________________________________ -->
1355<div class="doc_subsubsection">
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1356 <a name="selectiondag_sched">SelectionDAG Scheduling and Formation Phase</a>
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]1357</div>
1358
1359<div class="doc_text">
1360
1361<p>The scheduling phase takes the DAG of target instructions from the selection
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1362 phase and assigns an order. The scheduler can pick an order depending on
1363 various constraints of the machines (i.e. order for minimal register pressure
1364 or try to cover instruction latencies). Once an order is established, the
1365 DAG is converted to a list
1366 of <tt><a href="#machineinstr">MachineInstr</a></tt>s and the SelectionDAG is
1367 destroyed.</p>
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]1368
Jeff Cohen0b81cda2005-10-24 16:54:55 +0000[diff] [blame]1369<p>Note that this phase is logically separate from the instruction selection
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1370 phase, but is tied to it closely in the code because it operates on
1371 SelectionDAGs.</p>
Chris Lattnerc38959f2005-10-17 03:09:31 +0000[diff] [blame]1372
Chris Lattnere35d3bb2005-10-16 00:36:38 +0000[diff] [blame]1373</div>
1374
1375<!-- _______________________________________________________________________ -->
1376<div class="doc_subsubsection">
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1377 <a name="selectiondag_future">Future directions for the SelectionDAG</a>
1378</div>
1379
1380<div class="doc_text">
1381
1382<ol>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1383 <li>Optional function-at-a-time selection.</li>
1384
1385 <li>Auto-generate entire selector from <tt>.td</tt> file.</li>
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1386</ol>
1387
1388</div>
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1389
1390<!-- ======================================================================= -->
1391<div class="doc_subsection">
1392 <a name="ssamco">SSA-based Machine Code Optimizations</a>
1393</div>
1394<div class="doc_text"><p>To Be Written</p></div>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1395
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1396<!-- ======================================================================= -->
1397<div class="doc_subsection">
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]1398 <a name="liveintervals">Live Intervals</a>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1399</div>
1400
1401<div class="doc_text">
1402
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]1403<p>Live Intervals are the ranges (intervals) where a variable is <i>live</i>.
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1404 They are used by some <a href="#regalloc">register allocator</a> passes to
1405 determine if two or more virtual registers which require the same physical
1406 register are live at the same point in the program (i.e., they conflict).
1407 When this situation occurs, one virtual register must be <i>spilled</i>.</p>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1408
1409</div>
1410
1411<!-- _______________________________________________________________________ -->
1412<div class="doc_subsubsection">
1413 <a name="livevariable_analysis">Live Variable Analysis</a>
1414</div>
1415
1416<div class="doc_text">
1417
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1418<p>The first step in determining the live intervals of variables is to calculate
1419 the set of registers that are immediately dead after the instruction (i.e.,
1420 the instruction calculates the value, but it is never used) and the set of
1421 registers that are used by the instruction, but are never used after the
1422 instruction (i.e., they are killed). Live variable information is computed
1423 for each <i>virtual</i> register and <i>register allocatable</i> physical
1424 register in the function. This is done in a very efficient manner because it
1425 uses SSA to sparsely compute lifetime information for virtual registers
1426 (which are in SSA form) and only has to track physical registers within a
1427 block. Before register allocation, LLVM can assume that physical registers
1428 are only live within a single basic block. This allows it to do a single,
1429 local analysis to resolve physical register lifetimes within each basic
1430 block. If a physical register is not register allocatable (e.g., a stack
1431 pointer or condition codes), it is not tracked.</p>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1432
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1433<p>Physical registers may be live in to or out of a function. Live in values are
1434 typically arguments in registers. Live out values are typically return values
1435 in registers. Live in values are marked as such, and are given a dummy
1436 "defining" instruction during live intervals analysis. If the last basic
1437 block of a function is a <tt>return</tt>, then it's marked as using all live
1438 out values in the function.</p>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1439
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1440<p><tt>PHI</tt> nodes need to be handled specially, because the calculation of
1441 the live variable information from a depth first traversal of the CFG of the
1442 function won't guarantee that a virtual register used by the <tt>PHI</tt>
1443 node is defined before it's used. When a <tt>PHI</tt> node is encountered,
1444 only the definition is handled, because the uses will be handled in other
1445 basic blocks.</p>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1446
1447<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]1448 assignment at the end of the current basic block and traverse the successor
1449 basic blocks. If a successor basic block has a <tt>PHI</tt> node and one of
1450 the <tt>PHI</tt> node's operands is coming from the current basic block, then
1451 the variable is marked as <i>alive</i> within the current basic block and all
1452 of its predecessor basic blocks, until the basic block with the defining
1453 instruction is encountered.</p>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1454
1455</div>
1456
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]1457<!-- _______________________________________________________________________ -->
1458<div class="doc_subsubsection">
1459 <a name="liveintervals_analysis">Live Intervals Analysis</a>
1460</div>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1461
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]1462<div class="doc_text">
Bill Wendling3cd5ca62006-10-11 06:30:10 +0000[diff] [blame]1463
Bill Wendling82e2eea2006-10-11 18:00:22 +0000[diff] [blame]1464<p>We now have the information available to perform the live intervals analysis
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1465 and build the live intervals themselves. We start off by numbering the basic
1466 blocks and machine instructions. We then handle the "live-in" values. These
1467 are in physical registers, so the physical register is assumed to be killed
1468 by the end of the basic block. Live intervals for virtual registers are
1469 computed for some ordering of the machine instructions <tt>[1, N]</tt>. A
1470 live interval is an interval <tt>[i, j)</tt>, where <tt>1 &lt;= i &lt;= j
1471 &lt; N</tt>, for which a variable is live.</p>
Bill Wendling3cd5ca62006-10-11 06:30:10 +0000[diff] [blame]1472
Bill Wendling82e2eea2006-10-11 18:00:22 +0000[diff] [blame]1473<p><i><b>More to come...</b></i></p>
1474
Bill Wendling3fc488d2006-09-06 18:42:41 +0000[diff] [blame]1475</div>
Bill Wendling2f87a882006-09-04 23:35:52 +0000[diff] [blame]1476
1477<!-- ======================================================================= -->
1478<div class="doc_subsection">
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1479 <a name="regalloc">Register Allocation</a>
1480</div>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1481
1482<div class="doc_text">
1483
Bill Wendling3cd5ca62006-10-11 06:30:10 +0000[diff] [blame]1484<p>The <i>Register Allocation problem</i> consists in mapping a program
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1485 <i>P<sub>v</sub></i>, that can use an unbounded number of virtual registers,
1486 to a program <i>P<sub>p</sub></i> that contains a finite (possibly small)
1487 number of physical registers. Each target architecture has a different number
1488 of physical registers. If the number of physical registers is not enough to
1489 accommodate all the virtual registers, some of them will have to be mapped
1490 into memory. These virtuals are called <i>spilled virtuals</i>.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1491
1492</div>
1493
1494<!-- _______________________________________________________________________ -->
1495
1496<div class="doc_subsubsection">
1497 <a name="regAlloc_represent">How registers are represented in LLVM</a>
1498</div>
1499
1500<div class="doc_text">
1501
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1502<p>In LLVM, physical registers are denoted by integer numbers that normally
1503 range from 1 to 1023. To see how this numbering is defined for a particular
1504 architecture, you can read the <tt>GenRegisterNames.inc</tt> file for that
1505 architecture. For instance, by
1506 inspecting <tt>lib/Target/X86/X86GenRegisterNames.inc</tt> we see that the
1507 32-bit register <tt>EAX</tt> is denoted by 15, and the MMX register
1508 <tt>MM0</tt> is mapped to 48.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1509
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1510<p>Some architectures contain registers that share the same physical location. A
1511 notable example is the X86 platform. For instance, in the X86 architecture,
1512 the registers <tt>EAX</tt>, <tt>AX</tt> and <tt>AL</tt> share the first eight
1513 bits. These physical registers are marked as <i>aliased</i> in LLVM. Given a
1514 particular architecture, you can check which registers are aliased by
1515 inspecting its <tt>RegisterInfo.td</tt> file. Moreover, the method
1516 <tt>TargetRegisterInfo::getAliasSet(p_reg)</tt> returns an array containing
1517 all the physical registers aliased to the register <tt>p_reg</tt>.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1518
1519<p>Physical registers, in LLVM, are grouped in <i>Register Classes</i>.
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1520 Elements in the same register class are functionally equivalent, and can be
1521 interchangeably used. Each virtual register can only be mapped to physical
1522 registers of a particular class. For instance, in the X86 architecture, some
1523 virtuals can only be allocated to 8 bit registers. A register class is
1524 described by <tt>TargetRegisterClass</tt> objects. To discover if a virtual
1525 register is compatible with a given physical, this code can be used:</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1526
1527<div class="doc_code">
1528<pre>
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]1529bool RegMapping_Fer::compatible_class(MachineFunction &amp;mf,
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1530 unsigned v_reg,
1531 unsigned p_reg) {
Dan Gohman6f0d0242008-02-10 18:45:23 +0000[diff] [blame]1532 assert(TargetRegisterInfo::isPhysicalRegister(p_reg) &amp;&amp;
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1533 "Target register must be physical");
Chris Lattner534bcfb2007-12-31 04:16:08 +0000[diff] [blame]1534 const TargetRegisterClass *trc = mf.getRegInfo().getRegClass(v_reg);
1535 return trc-&gt;contains(p_reg);
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1536}
1537</pre>
1538</div>
1539
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1540<p>Sometimes, mostly for debugging purposes, it is useful to change the number
1541 of physical registers available in the target architecture. This must be done
1542 statically, inside the <tt>TargetRegsterInfo.td</tt> file. Just <tt>grep</tt>
1543 for <tt>RegisterClass</tt>, the last parameter of which is a list of
1544 registers. Just commenting some out is one simple way to avoid them being
1545 used. A more polite way is to explicitly exclude some registers from
Dan Gohmand2cb3d22009-07-24 00:30:09 +0000[diff] [blame]1546 the <i>allocation order</i>. See the definition of the <tt>GR8</tt> register
1547 class in <tt>lib/Target/X86/X86RegisterInfo.td</tt> for an example of this.
1548 </p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1549
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1550<p>Virtual registers are also denoted by integer numbers. Contrary to physical
1551 registers, different virtual registers never share the same number. The
1552 smallest virtual register is normally assigned the number 1024. This may
1553 change, so, in order to know which is the first virtual register, you should
1554 access <tt>TargetRegisterInfo::FirstVirtualRegister</tt>. Any register whose
1555 number is greater than or equal
1556 to <tt>TargetRegisterInfo::FirstVirtualRegister</tt> is considered a virtual
1557 register. Whereas physical registers are statically defined in
1558 a <tt>TargetRegisterInfo.td</tt> file and cannot be created by the
1559 application developer, that is not the case with virtual registers. In order
1560 to create new virtual registers, use the
1561 method <tt>MachineRegisterInfo::createVirtualRegister()</tt>. This method
1562 will return a virtual register with the highest code.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1563
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1564<p>Before register allocation, the operands of an instruction are mostly virtual
1565 registers, although physical registers may also be used. In order to check if
1566 a given machine operand is a register, use the boolean
1567 function <tt>MachineOperand::isRegister()</tt>. To obtain the integer code of
1568 a register, use <tt>MachineOperand::getReg()</tt>. An instruction may define
1569 or use a register. For instance, <tt>ADD reg:1026 := reg:1025 reg:1024</tt>
1570 defines the registers 1024, and uses registers 1025 and 1026. Given a
1571 register operand, the method <tt>MachineOperand::isUse()</tt> informs if that
1572 register is being used by the instruction. The
1573 method <tt>MachineOperand::isDef()</tt> informs if that registers is being
1574 defined.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1575
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1576<p>We will call physical registers present in the LLVM bitcode before register
1577 allocation <i>pre-colored registers</i>. Pre-colored registers are used in
1578 many different situations, for instance, to pass parameters of functions
1579 calls, and to store results of particular instructions. There are two types
1580 of pre-colored registers: the ones <i>implicitly</i> defined, and
1581 those <i>explicitly</i> defined. Explicitly defined registers are normal
1582 operands, and can be accessed
1583 with <tt>MachineInstr::getOperand(int)::getReg()</tt>. In order to check
1584 which registers are implicitly defined by an instruction, use
1585 the <tt>TargetInstrInfo::get(opcode)::ImplicitDefs</tt>,
1586 where <tt>opcode</tt> is the opcode of the target instruction. One important
1587 difference between explicit and implicit physical registers is that the
1588 latter are defined statically for each instruction, whereas the former may
1589 vary depending on the program being compiled. For example, an instruction
1590 that represents a function call will always implicitly define or use the same
1591 set of physical registers. To read the registers implicitly used by an
1592 instruction,
1593 use <tt>TargetInstrInfo::get(opcode)::ImplicitUses</tt>. Pre-colored
1594 registers impose constraints on any register allocation algorithm. The
Bob Wilson04738682010-04-09 18:39:54 +0000[diff] [blame]1595 register allocator must make sure that none of them are overwritten by
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1596 the values of virtual registers while still alive.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1597
1598</div>
1599
1600<!-- _______________________________________________________________________ -->
1601
1602<div class="doc_subsubsection">
1603 <a name="regAlloc_howTo">Mapping virtual registers to physical registers</a>
1604</div>
1605
1606<div class="doc_text">
1607
1608<p>There are two ways to map virtual registers to physical registers (or to
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1609 memory slots). The first way, that we will call <i>direct mapping</i>, is
1610 based on the use of methods of the classes <tt>TargetRegisterInfo</tt>,
1611 and <tt>MachineOperand</tt>. The second way, that we will call <i>indirect
1612 mapping</i>, relies on the <tt>VirtRegMap</tt> class in order to insert loads
1613 and stores sending and getting values to and from memory.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1614
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1615<p>The direct mapping provides more flexibility to the developer of the register
1616 allocator; however, it is more error prone, and demands more implementation
1617 work. Basically, the programmer will have to specify where load and store
1618 instructions should be inserted in the target function being compiled in
1619 order to get and store values in memory. To assign a physical register to a
1620 virtual register present in a given operand,
1621 use <tt>MachineOperand::setReg(p_reg)</tt>. To insert a store instruction,
Jakob Stoklund Olesen297907f2010-08-31 22:01:07 +0000[diff] [blame]1622 use <tt>TargetInstrInfo::storeRegToStackSlot(...)</tt>, and to insert a
1623 load instruction, use <tt>TargetInstrInfo::loadRegFromStackSlot</tt>.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1624
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1625<p>The indirect mapping shields the application developer from the complexities
1626 of inserting load and store instructions. In order to map a virtual register
1627 to a physical one, use <tt>VirtRegMap::assignVirt2Phys(vreg, preg)</tt>. In
1628 order to map a certain virtual register to memory,
1629 use <tt>VirtRegMap::assignVirt2StackSlot(vreg)</tt>. This method will return
1630 the stack slot where <tt>vreg</tt>'s value will be located. If it is
1631 necessary to map another virtual register to the same stack slot,
1632 use <tt>VirtRegMap::assignVirt2StackSlot(vreg, stack_location)</tt>. One
1633 important point to consider when using the indirect mapping, is that even if
1634 a virtual register is mapped to memory, it still needs to be mapped to a
1635 physical register. This physical register is the location where the virtual
1636 register is supposed to be found before being stored or after being
1637 reloaded.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1638
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1639<p>If the indirect strategy is used, after all the virtual registers have been
1640 mapped to physical registers or stack slots, it is necessary to use a spiller
1641 object to place load and store instructions in the code. Every virtual that
1642 has been mapped to a stack slot will be stored to memory after been defined
1643 and will be loaded before being used. The implementation of the spiller tries
1644 to recycle load/store instructions, avoiding unnecessary instructions. For an
1645 example of how to invoke the spiller,
1646 see <tt>RegAllocLinearScan::runOnMachineFunction</tt>
1647 in <tt>lib/CodeGen/RegAllocLinearScan.cpp</tt>.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1648
1649</div>
1650
1651<!-- _______________________________________________________________________ -->
1652<div class="doc_subsubsection">
1653 <a name="regAlloc_twoAddr">Handling two address instructions</a>
1654</div>
1655
1656<div class="doc_text">
1657
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1658<p>With very rare exceptions (e.g., function calls), the LLVM machine code
1659 instructions are three address instructions. That is, each instruction is
1660 expected to define at most one register, and to use at most two registers.
1661 However, some architectures use two address instructions. In this case, the
1662 defined register is also one of the used register. For instance, an
1663 instruction such as <tt>ADD %EAX, %EBX</tt>, in X86 is actually equivalent
1664 to <tt>%EAX = %EAX + %EBX</tt>.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1665
1666<p>In order to produce correct code, LLVM must convert three address
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1667 instructions that represent two address instructions into true two address
1668 instructions. LLVM provides the pass <tt>TwoAddressInstructionPass</tt> for
1669 this specific purpose. It must be run before register allocation takes
1670 place. After its execution, the resulting code may no longer be in SSA
1671 form. This happens, for instance, in situations where an instruction such
1672 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]1673
1674<div class="doc_code">
1675<pre>
1676%a = MOVE %b
Dan Gohman03e58572008-06-13 17:55:57 +0000[diff] [blame]1677%a = ADD %a %c
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1678</pre>
1679</div>
1680
1681<p>Notice that, internally, the second instruction is represented as
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1682 <tt>ADD %a[def/use] %c</tt>. I.e., the register operand <tt>%a</tt> is both
1683 used and defined by the instruction.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1684
1685</div>
1686
1687<!-- _______________________________________________________________________ -->
1688<div class="doc_subsubsection">
1689 <a name="regAlloc_ssaDecon">The SSA deconstruction phase</a>
1690</div>
1691
1692<div class="doc_text">
1693
1694<p>An important transformation that happens during register allocation is called
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1695 the <i>SSA Deconstruction Phase</i>. The SSA form simplifies many analyses
1696 that are performed on the control flow graph of programs. However,
1697 traditional instruction sets do not implement PHI instructions. Thus, in
1698 order to generate executable code, compilers must replace PHI instructions
1699 with other instructions that preserve their semantics.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1700
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1701<p>There are many ways in which PHI instructions can safely be removed from the
1702 target code. The most traditional PHI deconstruction algorithm replaces PHI
1703 instructions with copy instructions. That is the strategy adopted by
1704 LLVM. The SSA deconstruction algorithm is implemented
1705 in <tt>lib/CodeGen/PHIElimination.cpp</tt>. In order to invoke this pass, the
1706 identifier <tt>PHIEliminationID</tt> must be marked as required in the code
1707 of the register allocator.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1708
1709</div>
1710
1711<!-- _______________________________________________________________________ -->
1712<div class="doc_subsubsection">
1713 <a name="regAlloc_fold">Instruction folding</a>
1714</div>
1715
1716<div class="doc_text">
1717
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1718<p><i>Instruction folding</i> is an optimization performed during register
1719 allocation that removes unnecessary copy instructions. For instance, a
1720 sequence of instructions such as:</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1721
1722<div class="doc_code">
1723<pre>
1724%EBX = LOAD %mem_address
1725%EAX = COPY %EBX
1726</pre>
1727</div>
1728
Dan Gohmana7ab2bf2008-11-24 16:35:31 +0000[diff] [blame]1729<p>can be safely substituted by the single instruction:</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1730
1731<div class="doc_code">
1732<pre>
1733%EAX = LOAD %mem_address
1734</pre>
1735</div>
1736
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1737<p>Instructions can be folded with
1738 the <tt>TargetRegisterInfo::foldMemoryOperand(...)</tt> method. Care must be
1739 taken when folding instructions; a folded instruction can be quite different
1740 from the original
1741 instruction. See <tt>LiveIntervals::addIntervalsForSpills</tt>
1742 in <tt>lib/CodeGen/LiveIntervalAnalysis.cpp</tt> for an example of its
1743 use.</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1744
1745</div>
1746
1747<!-- _______________________________________________________________________ -->
1748
1749<div class="doc_subsubsection">
1750 <a name="regAlloc_builtIn">Built in register allocators</a>
1751</div>
1752
1753<div class="doc_text">
1754
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1755<p>The LLVM infrastructure provides the application developer with three
1756 different register allocators:</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1757
1758<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1759 <li><i>Linear Scan</i> &mdash; <i>The default allocator</i>. This is the
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1760 well-know linear scan register allocator. Whereas the
1761 <i>Simple</i> and <i>Local</i> algorithms use a direct mapping
1762 implementation technique, the <i>Linear Scan</i> implementation
1763 uses a spiller in order to place load and stores.</li>
Jakob Stoklund Olesen8a3eab92010-06-15 21:58:33 +0000[diff] [blame]1764
1765 <li><i>Fast</i> &mdash; This register allocator is the default for debug
1766 builds. It allocates registers on a basic block level, attempting to keep
1767 values in registers and reusing registers as appropriate.</li>
1768
1769 <li><i>PBQP</i> &mdash; A Partitioned Boolean Quadratic Programming (PBQP)
1770 based register allocator. This allocator works by constructing a PBQP
1771 problem representing the register allocation problem under consideration,
1772 solving this using a PBQP solver, and mapping the solution back to a
1773 register assignment.</li>
1774
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1775</ul>
1776
1777<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]1778 command line option <tt>-regalloc=...</tt>:</p>
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1779
1780<div class="doc_code">
1781<pre>
Dan Gohman0cabaa52009-08-25 15:54:01 +0000[diff] [blame]1782$ llc -regalloc=linearscan file.bc -o ln.s;
Jakob Stoklund Olesen8a3eab92010-06-15 21:58:33 +0000[diff] [blame]1783$ llc -regalloc=fast file.bc -o fa.s;
1784$ llc -regalloc=pbqp file.bc -o pbqp.s;
Bill Wendlinga396ee82006-09-01 21:46:00 +0000[diff] [blame]1785</pre>
1786</div>
1787
1788</div>
1789
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1790<!-- ======================================================================= -->
1791<div class="doc_subsection">
1792 <a name="proepicode">Prolog/Epilog Code Insertion</a>
1793</div>
1794<div class="doc_text"><p>To Be Written</p></div>
1795<!-- ======================================================================= -->
1796<div class="doc_subsection">
1797 <a name="latemco">Late Machine Code Optimizations</a>
1798</div>
1799<div class="doc_text"><p>To Be Written</p></div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1800
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1801<!-- ======================================================================= -->
1802<div class="doc_subsection">
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1803 <a name="codeemit">Code Emission</a>
Reid Spencerad1f0cd2005-04-24 20:56:18 +0000[diff] [blame]1804</div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1805
1806<div class="doc_text">
1807
1808<p>The code emission step of code generation is responsible for lowering from
1809the code generator abstractions (like <a
1810href="#machinefunction">MachineFunction</a>, <a
1811href="#machineinstr">MachineInstr</a>, etc) down
1812to the abstractions used by the MC layer (<a href="#mcinst">MCInst</a>,
1813<a href="#mcstreamer">MCStreamer</a>, etc). This is
1814done with a combination of several different classes: the (misnamed)
1815target-independent AsmPrinter class, target-specific subclasses of AsmPrinter
1816(such as SparcAsmPrinter), and the TargetLoweringObjectFile class.</p>
1817
1818<p>Since the MC layer works at the level of abstraction of object files, it
1819doesn't have a notion of functions, global variables etc. Instead, it thinks
1820about labels, directives, and instructions. A key class used at this time is
1821the MCStreamer class. This is an abstract API that is implemented in different
1822ways (e.g. to output a .s file, output an ELF .o file, etc) that is effectively
1823an "assembler API". MCStreamer has one method per directive, such as EmitLabel,
1824EmitSymbolAttribute, SwitchSection, etc, which directly correspond to assembly
1825level directives.
1826</p>
1827
1828<p>If you are interested in implementing a code generator for a target, there
1829are three important things that you have to implement for your target:</p>
1830
1831<ol>
1832<li>First, you need a subclass of AsmPrinter for your target. This class
1833implements the general lowering process converting MachineFunction's into MC
1834label constructs. The AsmPrinter base class provides a number of useful methods
1835and routines, and also allows you to override the lowering process in some
1836important ways. You should get much of the lowering for free if you are
1837implementing an ELF, COFF, or MachO target, because the TargetLoweringObjectFile
1838class implements much of the common logic.</li>
1839
1840<li>Second, you need to implement an instruction printer for your target. The
1841instruction printer takes an <a href="#mcinst">MCInst</a> and renders it to a
1842raw_ostream as text. Most of this is automatically generated from the .td file
1843(when you specify something like "<tt>add $dst, $src1, $src2</tt>" in the
1844instructions), but you need to implement routines to print operands.</li>
1845
1846<li>Third, you need to implement code that lowers a <a
1847href="#machineinstr">MachineInstr</a> to an MCInst, usually implemented in
1848"&lt;target&gt;MCInstLower.cpp". This lowering process is often target
1849specific, and is responsible for turning jump table entries, constant pool
1850indices, global variable addresses, etc into MCLabels as appropriate. This
1851translation layer is also responsible for expanding pseudo ops used by the code
1852generator into the actual machine instructions they correspond to. The MCInsts
1853that are generated by this are fed into the instruction printer or the encoder.
1854</li>
1855
1856</ol>
1857
1858<p>Finally, at your choosing, you can also implement an subclass of
1859MCCodeEmitter which lowers MCInst's into machine code bytes and relocations.
1860This is important if you want to support direct .o file emission, or would like
1861to implement an assembler for your target.</p>
1862
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1863</div>
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1864
1865
Chris Lattner22481f22010-09-21 04:03:39 +0000[diff] [blame]1866<!-- *********************************************************************** -->
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1867<div class="doc_section">
1868 <a name="nativeassembler">Implementing a Native Assembler</a>
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1869</div>
Chris Lattner22481f22010-09-21 04:03:39 +0000[diff] [blame]1870<!-- *********************************************************************** -->
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1871
1872<div class="doc_text">
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1873
Chris Lattner22481f22010-09-21 04:03:39 +0000[diff] [blame]1874<p>Though you're probably reading this because you want to write or maintain a
1875compiler backend, LLVM also fully supports building a native assemblers too.
1876We've tried hard to automate the generation of the assembler from the .td files
1877(in particular the instruction syntax and encodings), which means that a large
1878part of the manual and repetitive data entry can be factored and shared with the
1879compiler.</p>
1880
1881
Chris Lattnere1b83452010-09-11 23:02:10 +0000[diff] [blame]1882
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1883</div>
1884
1885
Chris Lattner22481f22010-09-21 04:03:39 +0000[diff] [blame]1886<!-- ======================================================================= -->
1887<div class="doc_subsection">
1888 <a name="proepicode">Prolog/Epilog Code Insertion</a>
1889</div>
1890<div class="doc_text"><p>To Be Written</p></div>
1891
1892
1893
1894
Chris Lattneraa5bcb52005-01-28 17:22:53 +0000[diff] [blame]1895<!-- *********************************************************************** -->
1896<div class="doc_section">
Chris Lattner32e89f22005-10-16 18:31:08 +0000[diff] [blame]1897 <a name="targetimpls">Target-specific Implementation Notes</a>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]1898</div>
1899<!-- *********************************************************************** -->
1900
1901<div class="doc_text">
1902
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]1903<p>This section of the document explains features or design decisions that are
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]1904 specific to the code generator for a particular target. First we start
1905 with a table that summarizes what features are supported by each target.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]1906
1907</div>
1908
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]1909<!-- ======================================================================= -->
1910<div class="doc_subsection">
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]1911 <a name="targetfeatures">Target Feature Matrix</a>
1912</div>
1913
1914<style type="text/css">
1915 .unknown { background-color: #C0C0C0; text-align: center; }
1916 .unknown:before { content: "?" }
1917 .no { background-color: #C11B17 }
1918 .no:before { content: "N" }
1919 .partial { background-color: #F88017 }
1920 .yes { background-color: #00FF00; }
1921 .yes:before { content: "Y" }
1922</style>
1923
1924
1925<div class="doc_text">
1926
1927<p>Note that this table does not include the C backend or Cpp backends, since
1928they do not use the target independent code generator infrastructure. It also
1929doesn't list features that are not supported fully by any target yet. It
1930considers a feature to be supported if at least one subtarget supports it. A
1931feature being supported means that it is useful and works for most cases, it
1932does not indicate that there are zero known bugs in the implementation. Here
1933is the key:</p>
1934
1935
1936<table border="1" cellspacing="0">
1937 <tr>
1938 <th>Unknown</th>
1939 <th>No support</th>
1940 <th>Partial Support</th>
1941 <th>Complete Support</th>
1942 </tr>
1943 <tr>
1944 <td class="unknown"></td>
1945 <td class="no"></td>
1946 <td class="partial"></td>
1947 <td class="yes"></td>
1948 </tr>
1949</table>
1950
1951<p>Here is the table:</p>
1952
1953<table width="689" border="1" cellspacing="0">
1954<tr><td></td>
1955<td colspan="13" align="center" bgcolor="#ffffcc">Target</td>
1956</tr>
1957 <tr>
1958 <th>Feature</th>
1959 <th>ARM</th>
1960 <th>Alpha</th>
1961 <th>Blackfin</th>
1962 <th>CellSPU</th>
1963 <th>MBlaze</th>
1964 <th>MSP430</th>
1965 <th>Mips</th>
1966 <th>PTX</th>
1967 <th>PowerPC</th>
1968 <th>Sparc</th>
1969 <th>SystemZ</th>
1970 <th>X86</th>
1971 <th>XCore</th>
1972 </tr>
1973
1974<tr>
1975 <td><a href="#feat_reliable">is generally reliable</a></td>
1976 <td class="yes"></td> <!-- ARM -->
1977 <td class="unknown"></td> <!-- Alpha -->
Jakob Stoklund Olesen4e136122010-10-24 20:04:05 +0000[diff] [blame]1978 <td class="no"></td> <!-- Blackfin -->
Kalle Raiskila94cc4fe2010-10-25 08:57:30 +0000[diff] [blame]1979 <td class="no"></td> <!-- CellSPU -->
Wesley Peckc6a45242010-10-24 18:50:12 +0000[diff] [blame]1980 <td class="no"></td> <!-- MBlaze -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]1981 <td class="unknown"></td> <!-- MSP430 -->
1982 <td class="unknown"></td> <!-- Mips -->
1983 <td class="no"></td> <!-- PTX -->
1984 <td class="yes"></td> <!-- PowerPC -->
1985 <td class="yes"></td> <!-- Sparc -->
1986 <td class="unknown"></td> <!-- SystemZ -->
1987 <td class="yes"></td> <!-- X86 -->
1988 <td class="unknown"></td> <!-- XCore -->
1989</tr>
1990
1991<tr>
1992 <td><a href="#feat_asmparser">assembly parser</a></td>
1993 <td class="no"></td> <!-- ARM -->
1994 <td class="no"></td> <!-- Alpha -->
1995 <td class="no"></td> <!-- Blackfin -->
1996 <td class="no"></td> <!-- CellSPU -->
1997 <td class="no"></td> <!-- MBlaze -->
1998 <td class="no"></td> <!-- MSP430 -->
1999 <td class="no"></td> <!-- Mips -->
2000 <td class="no"></td> <!-- PTX -->
2001 <td class="no"></td> <!-- PowerPC -->
2002 <td class="no"></td> <!-- Sparc -->
2003 <td class="no"></td> <!-- SystemZ -->
2004 <td class="yes"></td> <!-- X86 -->
2005 <td class="no"></td> <!-- XCore -->
2006</tr>
2007
2008<tr>
2009 <td><a href="#feat_disassembler">disassembler</a></td>
2010 <td class="yes"></td> <!-- ARM -->
2011 <td class="no"></td> <!-- Alpha -->
2012 <td class="no"></td> <!-- Blackfin -->
2013 <td class="no"></td> <!-- CellSPU -->
2014 <td class="no"></td> <!-- MBlaze -->
2015 <td class="no"></td> <!-- MSP430 -->
2016 <td class="no"></td> <!-- Mips -->
2017 <td class="no"></td> <!-- PTX -->
2018 <td class="no"></td> <!-- PowerPC -->
2019 <td class="no"></td> <!-- Sparc -->
2020 <td class="no"></td> <!-- SystemZ -->
2021 <td class="yes"></td> <!-- X86 -->
2022 <td class="no"></td> <!-- XCore -->
2023</tr>
2024
2025<tr>
2026 <td><a href="#feat_inlineasm">inline asm</a></td>
2027 <td class="yes"></td> <!-- ARM -->
2028 <td class="unknown"></td> <!-- Alpha -->
Jakob Stoklund Olesen4e136122010-10-24 20:04:05 +0000[diff] [blame]2029 <td class="yes"></td> <!-- Blackfin -->
Kalle Raiskila94cc4fe2010-10-25 08:57:30 +0000[diff] [blame]2030 <td class="no"></td> <!-- CellSPU -->
Wesley Peckc6a45242010-10-24 18:50:12 +0000[diff] [blame]2031 <td class="no"></td> <!-- MBlaze -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2032 <td class="unknown"></td> <!-- MSP430 -->
2033 <td class="unknown"></td> <!-- Mips -->
2034 <td class="unknown"></td> <!-- PTX -->
2035 <td class="yes"></td> <!-- PowerPC -->
2036 <td class="unknown"></td> <!-- Sparc -->
2037 <td class="unknown"></td> <!-- SystemZ -->
2038 <td class="yes"><a href="#feat_inlineasm_x86">*</a></td> <!-- X86 -->
2039 <td class="unknown"></td> <!-- XCore -->
2040</tr>
2041
2042<tr>
2043 <td><a href="#feat_jit">jit</a></td>
2044 <td class="partial"><a href="#feat_jit_arm">*</a></td> <!-- ARM -->
2045 <td class="unknown"></td> <!-- Alpha -->
Jakob Stoklund Olesen4e136122010-10-24 20:04:05 +0000[diff] [blame]2046 <td class="no"></td> <!-- Blackfin -->
Kalle Raiskila94cc4fe2010-10-25 08:57:30 +0000[diff] [blame]2047 <td class="no"></td> <!-- CellSPU -->
Wesley Peckc6a45242010-10-24 18:50:12 +0000[diff] [blame]2048 <td class="no"></td> <!-- MBlaze -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2049 <td class="unknown"></td> <!-- MSP430 -->
2050 <td class="unknown"></td> <!-- Mips -->
2051 <td class="unknown"></td> <!-- PTX -->
2052 <td class="yes"></td> <!-- PowerPC -->
2053 <td class="unknown"></td> <!-- Sparc -->
2054 <td class="unknown"></td> <!-- SystemZ -->
2055 <td class="yes"></td> <!-- X86 -->
2056 <td class="unknown"></td> <!-- XCore -->
2057</tr>
2058
2059<tr>
2060 <td><a href="#feat_objectwrite">.o&nbsp;file writing</a></td>
2061 <td class="no"></td> <!-- ARM -->
2062 <td class="no"></td> <!-- Alpha -->
2063 <td class="no"></td> <!-- Blackfin -->
2064 <td class="no"></td> <!-- CellSPU -->
2065 <td class="no"></td> <!-- MBlaze -->
2066 <td class="no"></td> <!-- MSP430 -->
2067 <td class="no"></td> <!-- Mips -->
2068 <td class="no"></td> <!-- PTX -->
2069 <td class="no"></td> <!-- PowerPC -->
2070 <td class="no"></td> <!-- Sparc -->
2071 <td class="no"></td> <!-- SystemZ -->
2072 <td class="yes"></td> <!-- X86 -->
2073 <td class="no"></td> <!-- XCore -->
2074</tr>
2075
2076<tr>
2077 <td><a href="#feat_tailcall">tail calls</a></td>
2078 <td class="yes"></td> <!-- ARM -->
2079 <td class="unknown"></td> <!-- Alpha -->
Jakob Stoklund Olesen4e136122010-10-24 20:04:05 +0000[diff] [blame]2080 <td class="no"></td> <!-- Blackfin -->
Kalle Raiskila94cc4fe2010-10-25 08:57:30 +0000[diff] [blame]2081 <td class="no"></td> <!-- CellSPU -->
Wesley Peckc6a45242010-10-24 18:50:12 +0000[diff] [blame]2082 <td class="no"></td> <!-- MBlaze -->
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2083 <td class="unknown"></td> <!-- MSP430 -->
2084 <td class="unknown"></td> <!-- Mips -->
2085 <td class="unknown"></td> <!-- PTX -->
2086 <td class="yes"></td> <!-- PowerPC -->
2087 <td class="unknown"></td> <!-- Sparc -->
2088 <td class="unknown"></td> <!-- SystemZ -->
2089 <td class="yes"></td> <!-- X86 -->
2090 <td class="unknown"></td> <!-- XCore -->
2091</tr>
2092
2093
2094</table>
2095
2096</div>
2097
2098<!-- _______________________________________________________________________ -->
2099<div class="doc_subsubsection" id="feat_reliable">Is Generally Reliable</div>
2100
2101<div class="doc_text">
2102<p>This box indicates whether the target is considered to be production quality.
2103This indicates that the target has been used as a static compiler to
2104compile large amounts of code by a variety of different people and is in
2105continuous use.</p>
2106</div>
2107
2108<!-- _______________________________________________________________________ -->
2109<div class="doc_subsubsection" id="feat_asmparser">Assembly Parser</div>
2110
2111<div class="doc_text">
2112<p>This box indicates whether the target supports parsing target specific .s
2113files by implementing the MCAsmParser interface. This is required for llvm-mc
2114to be able to act as a native assembler and is required for inline assembly
2115support in the native .o file writer.</p>
2116
2117</div>
2118
2119
2120<!-- _______________________________________________________________________ -->
2121<div class="doc_subsubsection" id="feat_disassembler">Disassembler</div>
2122
2123<div class="doc_text">
2124<p>This box indicates whether the target supports the MCDisassembler API for
2125disassembling machine opcode bytes into MCInst's.</p>
2126
2127</div>
2128
2129<!-- _______________________________________________________________________ -->
2130<div class="doc_subsubsection" id="feat_inlineasm">Inline Asm</div>
2131
2132<div class="doc_text">
2133<p>This box indicates whether the target supports most popular inline assembly
2134constraints and modifiers.</p>
2135
2136<p id="feat_inlineasm_x86">X86 lacks reliable support for inline assembly
2137constraints relating to the X86 floating point stack.</p>
2138
2139</div>
2140
2141<!-- _______________________________________________________________________ -->
2142<div class="doc_subsubsection" id="feat_jit">JIT Support</div>
2143
2144<div class="doc_text">
2145<p>This box indicates whether the target supports the JIT compiler through
2146the ExecutionEngine interface.</p>
2147
Chris Lattner6fb99552010-10-24 16:24:22 +0000[diff] [blame]2148<p id="feat_jit_arm">The ARM backend has basic support for integer code
Chris Lattner68de6022010-10-24 16:18:00 +0000[diff] [blame]2149in ARM codegen mode, but lacks NEON and full Thumb support.</p>
2150
2151</div>
2152
2153<!-- _______________________________________________________________________ -->
2154<div class="doc_subsubsection" id="feat_objectwrite">.o File Writing</div>
2155
2156<div class="doc_text">
2157
2158<p>This box indicates whether the target supports writing .o files (e.g. MachO,
2159ELF, and/or COFF) files directly from the target. Note that the target also
2160must include an assembly parser and general inline assembly support for full
2161inline assembly support in the .o writer.</p>
2162
2163</div>
2164
2165<!-- _______________________________________________________________________ -->
2166<div class="doc_subsubsection" id="feat_tailcall">Tail Calls</div>
2167
2168<div class="doc_text">
2169
2170<p>This box indicates whether the target supports guaranteed tail calls. These
2171are calls marked "<a href="LangRef.html#i_call">tail</a>" and use the fastcc
2172calling convention. Please see the <a href="#tailcallopt">tail call section
2173more more details</a>.</p>
2174
2175</div>
2176
2177
2178
2179
2180<!-- ======================================================================= -->
2181<div class="doc_subsection">
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]2182 <a name="tailcallopt">Tail call optimization</a>
2183</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2184
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]2185<div class="doc_text">
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]2186
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2187<p>Tail call optimization, callee reusing the stack of the caller, is currently
2188 supported on x86/x86-64 and PowerPC. It is performed if:</p>
2189
2190<ul>
Chris Lattner29689432010-03-11 00:22:57 +0000[diff] [blame]2191 <li>Caller and callee have the calling convention <tt>fastcc</tt> or
2192 <tt>cc 10</tt> (GHC call convention).</li>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2193
2194 <li>The call is a tail call - in tail position (ret immediately follows call
2195 and ret uses value of call or is void).</li>
2196
2197 <li>Option <tt>-tailcallopt</tt> is enabled.</li>
2198
2199 <li>Platform specific constraints are met.</li>
2200</ul>
2201
2202<p>x86/x86-64 constraints:</p>
2203
2204<ul>
2205 <li>No variable argument lists are used.</li>
2206
2207 <li>On x86-64 when generating GOT/PIC code only module-local calls (visibility
2208 = hidden or protected) are supported.</li>
2209</ul>
2210
2211<p>PowerPC constraints:</p>
2212
2213<ul>
2214 <li>No variable argument lists are used.</li>
2215
2216 <li>No byval parameters are used.</li>
2217
2218 <li>On ppc32/64 GOT/PIC only module-local calls (visibility = hidden or protected) are supported.</li>
2219</ul>
2220
2221<p>Example:</p>
2222
2223<p>Call as <tt>llc -tailcallopt test.ll</tt>.</p>
2224
2225<div class="doc_code">
2226<pre>
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]2227declare fastcc i32 @tailcallee(i32 inreg %a1, i32 inreg %a2, i32 %a3, i32 %a4)
2228
2229define fastcc i32 @tailcaller(i32 %in1, i32 %in2) {
2230 %l1 = add i32 %in1, %in2
2231 %tmp = tail call fastcc i32 @tailcallee(i32 %in1 inreg, i32 %in2 inreg, i32 %in1, i32 %l1)
2232 ret i32 %tmp
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2233}
2234</pre>
2235</div>
2236
2237<p>Implications of <tt>-tailcallopt</tt>:</p>
2238
2239<p>To support tail call optimization in situations where the callee has more
2240 arguments than the caller a 'callee pops arguments' convention is used. This
2241 currently causes each <tt>fastcc</tt> call that is not tail call optimized
2242 (because one or more of above constraints are not met) to be followed by a
2243 readjustment of the stack. So performance might be worse in such cases.</p>
2244
Arnold Schwaighofer9097d142008-05-14 09:17:12 +0000[diff] [blame]2245</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2246<!-- ======================================================================= -->
2247<div class="doc_subsection">
Evan Chengdc444e92010-03-08 21:05:02 +0000[diff] [blame]2248 <a name="sibcallopt">Sibling call optimization</a>
2249</div>
2250
2251<div class="doc_text">
2252
2253<p>Sibling call optimization is a restricted form of tail call optimization.
2254 Unlike tail call optimization described in the previous section, it can be
2255 performed automatically on any tail calls when <tt>-tailcallopt</tt> option
2256 is not specified.</p>
2257
2258<p>Sibling call optimization is currently performed on x86/x86-64 when the
2259 following constraints are met:</p>
2260
2261<ul>
2262 <li>Caller and callee have the same calling convention. It can be either
2263 <tt>c</tt> or <tt>fastcc</tt>.
2264
2265 <li>The call is a tail call - in tail position (ret immediately follows call
2266 and ret uses value of call or is void).</li>
2267
2268 <li>Caller and callee have matching return type or the callee result is not
2269 used.
2270
2271 <li>If any of the callee arguments are being passed in stack, they must be
2272 available in caller's own incoming argument stack and the frame offsets
2273 must be the same.
2274</ul>
2275
2276<p>Example:</p>
2277<div class="doc_code">
2278<pre>
2279declare i32 @bar(i32, i32)
2280
2281define i32 @foo(i32 %a, i32 %b, i32 %c) {
2282entry:
2283 %0 = tail call i32 @bar(i32 %a, i32 %b)
2284 ret i32 %0
2285}
2286</pre>
2287</div>
2288
2289</div>
2290<!-- ======================================================================= -->
2291<div class="doc_subsection">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2292 <a name="x86">The X86 backend</a>
2293</div>
2294
2295<div class="doc_text">
2296
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]2297<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]2298 code generator is capable of targeting a variety of x86-32 and x86-64
2299 processors, and includes support for ISA extensions such as MMX and SSE.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2300
2301</div>
2302
2303<!-- _______________________________________________________________________ -->
2304<div class="doc_subsubsection">
Nate Begeman34509842009-01-26 02:54:45 +0000[diff] [blame]2305 <a name="x86_tt">X86 Target Triples supported</a>
Chris Lattner9b988be2005-07-12 00:20:49 +0000[diff] [blame]2306</div>
2307
2308<div class="doc_text">
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]2309
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2310<p>The following are the known target triples that are supported by the X86
2311 backend. This is not an exhaustive list, and it would be useful to add those
2312 that people test.</p>
Chris Lattner9b988be2005-07-12 00:20:49 +0000[diff] [blame]2313
2314<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2315 <li><b>i686-pc-linux-gnu</b> &mdash; Linux</li>
2316
2317 <li><b>i386-unknown-freebsd5.3</b> &mdash; FreeBSD 5.3</li>
2318
2319 <li><b>i686-pc-cygwin</b> &mdash; Cygwin on Win32</li>
2320
2321 <li><b>i686-pc-mingw32</b> &mdash; MingW on Win32</li>
2322
2323 <li><b>i386-pc-mingw32msvc</b> &mdash; MingW crosscompiler on Linux</li>
2324
2325 <li><b>i686-apple-darwin*</b> &mdash; Apple Darwin on X86</li>
Torok Edwinc457b652009-06-15 12:17:44 +0000[diff] [blame]2326
2327 <li><b>x86_64-unknown-linux-gnu</b> &mdash; Linux</li>
Chris Lattner9b988be2005-07-12 00:20:49 +0000[diff] [blame]2328</ul>
2329
2330</div>
2331
2332<!-- _______________________________________________________________________ -->
2333<div class="doc_subsubsection">
Anton Korobeynikovbcb97702006-09-17 20:25:45 +0000[diff] [blame]2334 <a name="x86_cc">X86 Calling Conventions supported</a>
2335</div>
2336
2337
2338<div class="doc_text">
2339
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]2340<p>The following target-specific calling conventions are known to backend:</p>
Anton Korobeynikovbcb97702006-09-17 20:25:45 +0000[diff] [blame]2341
2342<ul>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2343 <li><b>x86_StdCall</b> &mdash; stdcall calling convention seen on Microsoft
2344 Windows platform (CC ID = 64).</li>
2345
2346 <li><b>x86_FastCall</b> &mdash; fastcall calling convention seen on Microsoft
2347 Windows platform (CC ID = 65).</li>
Anton Korobeynikovbcb97702006-09-17 20:25:45 +0000[diff] [blame]2348</ul>
2349
2350</div>
2351
2352<!-- _______________________________________________________________________ -->
2353<div class="doc_subsubsection">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2354 <a name="x86_memory">Representing X86 addressing modes in MachineInstrs</a>
2355</div>
2356
2357<div class="doc_text">
2358
Misha Brukman600df452005-02-17 22:22:24 +0000[diff] [blame]2359<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]2360 forming memory addresses of the following expression directly in integer
2361 instructions (which use ModR/M addressing):</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2362
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]2363<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2364<pre>
Chris Lattnerb91227d2009-10-10 21:30:55 +0000[diff] [blame]2365SegmentReg: Base + [1,2,4,8] * IndexReg + Disp32
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2366</pre>
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]2367</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2368
Chris Lattnerb91227d2009-10-10 21:30:55 +0000[diff] [blame]2369<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]2370 memory operand of this form. This means that the "load" form of
2371 '<tt>mov</tt>' has the following <tt>MachineOperand</tt>s in this order:</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2372
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2373<div class="doc_code">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2374<pre>
Chris Lattnerb91227d2009-10-10 21:30:55 +0000[diff] [blame]2375Index: 0 | 1 2 3 4 5
2376Meaning: DestReg, | BaseReg, Scale, IndexReg, Displacement Segment
2377OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm PhysReg
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2378</pre>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2379</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2380
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2381<p>Stores, and all other instructions, treat the four memory operands in the
Chris Lattnerb91227d2009-10-10 21:30:55 +0000[diff] [blame]2382 same way and in the same order. If the segment register is unspecified
2383 (regno = 0), then no segment override is generated. "Lea" operations do not
2384 have a segment register specified, so they only have 4 operands for their
2385 memory reference.</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2386
2387</div>
2388
2389<!-- _______________________________________________________________________ -->
2390<div class="doc_subsubsection">
Nate Begeman34509842009-01-26 02:54:45 +0000[diff] [blame]2391 <a name="x86_memory">X86 address spaces supported</a>
2392</div>
2393
2394<div class="doc_text">
2395
Dan Gohmand26795a2009-05-05 20:48:47 +0000[diff] [blame]2396<p>x86 has an experimental feature which provides
2397 the ability to perform loads and stores to different address spaces
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2398 via the x86 segment registers. A segment override prefix byte on an
2399 instruction causes the instruction's memory access to go to the specified
2400 segment. LLVM address space 0 is the default address space, which includes
2401 the stack, and any unqualified memory accesses in a program. Address spaces
2402 1-255 are currently reserved for user-defined code. The GS-segment is
Chris Lattner1777d0c2009-05-05 18:52:19 +0000[diff] [blame]2403 represented by address space 256, while the FS-segment is represented by
2404 address space 257. Other x86 segments have yet to be allocated address space
2405 numbers.</p>
Nate Begeman34509842009-01-26 02:54:45 +0000[diff] [blame]2406
Dan Gohmand26795a2009-05-05 20:48:47 +0000[diff] [blame]2407<p>While these address spaces may seem similar to TLS via the
2408 <tt>thread_local</tt> keyword, and often use the same underlying hardware,
2409 there are some fundamental differences.</p>
2410
2411<p>The <tt>thread_local</tt> keyword applies to global variables and
2412 specifies that they are to be allocated in thread-local memory. There are
2413 no type qualifiers involved, and these variables can be pointed to with
2414 normal pointers and accessed with normal loads and stores.
2415 The <tt>thread_local</tt> keyword is target-independent at the LLVM IR
2416 level (though LLVM doesn't yet have implementations of it for some
2417 configurations).<p>
2418
2419<p>Special address spaces, in contrast, apply to static types. Every
2420 load and store has a particular address space in its address operand type,
2421 and this is what determines which address space is accessed.
2422 LLVM ignores these special address space qualifiers on global variables,
2423 and does not provide a way to directly allocate storage in them.
2424 At the LLVM IR level, the behavior of these special address spaces depends
2425 in part on the underlying OS or runtime environment, and they are specific
2426 to x86 (and LLVM doesn't yet handle them correctly in some cases).</p>
2427
2428<p>Some operating systems and runtime environments use (or may in the future
2429 use) the FS/GS-segment registers for various low-level purposes, so care
2430 should be taken when considering them.</p>
Nate Begeman34509842009-01-26 02:54:45 +0000[diff] [blame]2431
2432</div>
2433
2434<!-- _______________________________________________________________________ -->
2435<div class="doc_subsubsection">
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2436 <a name="x86_names">Instruction naming</a>
2437</div>
2438
2439<div class="doc_text">
2440
Bill Wendling91e10c42006-08-28 02:26:32 +0000[diff] [blame]2441<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]2442 a character per operand with an optional special size. For example:</p>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2443
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2444<div class="doc_code">
2445<pre>
2446ADD8rr -&gt; add, 8-bit register, 8-bit register
2447IMUL16rmi -&gt; imul, 16-bit register, 16-bit memory, 16-bit immediate
2448IMUL16rmi8 -&gt; imul, 16-bit register, 16-bit memory, 8-bit immediate
2449MOVSX32rm16 -&gt; movsx, 32-bit register, 16-bit memory
2450</pre>
2451</div>
Chris Lattnerec94f802004-06-04 00:16:02 +0000[diff] [blame]2452
2453</div>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]2454
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2455<!-- ======================================================================= -->
2456<div class="doc_subsection">
2457 <a name="ppc">The PowerPC backend</a>
2458</div>
2459
2460<div class="doc_text">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2461
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2462<p>The PowerPC code generator lives in the lib/Target/PowerPC directory. The
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2463 code generation is retargetable to several variations or <i>subtargets</i> of
2464 the PowerPC ISA; including ppc32, ppc64 and altivec.</p>
2465
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2466</div>
2467
2468<!-- _______________________________________________________________________ -->
2469<div class="doc_subsubsection">
2470 <a name="ppc_abi">LLVM PowerPC ABI</a>
2471</div>
2472
2473<div class="doc_text">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2474
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2475<p>LLVM follows the AIX PowerPC ABI, with two deviations. LLVM uses a PC
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2476 relative (PIC) or static addressing for accessing global values, so no TOC
2477 (r2) is used. Second, r31 is used as a frame pointer to allow dynamic growth
2478 of a stack frame. LLVM takes advantage of having no TOC to provide space to
2479 save the frame pointer in the PowerPC linkage area of the caller frame.
2480 Other details of PowerPC ABI can be found at <a href=
2481 "http://developer.apple.com/documentation/DeveloperTools/Conceptual/LowLevelABI/Articles/32bitPowerPC.html"
2482 >PowerPC ABI.</a> Note: This link describes the 32 bit ABI. The 64 bit ABI
2483 is similar except space for GPRs are 8 bytes wide (not 4) and r13 is reserved
2484 for system use.</p>
2485
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2486</div>
2487
2488<!-- _______________________________________________________________________ -->
2489<div class="doc_subsubsection">
2490 <a name="ppc_frame">Frame Layout</a>
2491</div>
2492
2493<div class="doc_text">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2494
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2495<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]2496 function's invocation. Since the frame is fixed size, all references
2497 into the frame can be accessed via fixed offsets from the stack pointer. The
2498 exception to this is when dynamic alloca or variable sized arrays are
2499 present, then a base pointer (r31) is used as a proxy for the stack pointer
2500 and stack pointer is free to grow or shrink. A base pointer is also used if
2501 llvm-gcc is not passed the -fomit-frame-pointer flag. The stack pointer is
2502 always aligned to 16 bytes, so that space allocated for altivec vectors will
2503 be properly aligned.</p>
2504
Dan Gohman641b2792008-11-24 16:27:17 +0000[diff] [blame]2505<p>An invocation frame is laid out as follows (low memory at top);</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2506
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2507<table class="layout">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2508 <tr>
2509 <td>Linkage<br><br></td>
2510 </tr>
2511 <tr>
2512 <td>Parameter area<br><br></td>
2513 </tr>
2514 <tr>
2515 <td>Dynamic area<br><br></td>
2516 </tr>
2517 <tr>
2518 <td>Locals area<br><br></td>
2519 </tr>
2520 <tr>
2521 <td>Saved registers area<br><br></td>
2522 </tr>
2523 <tr style="border-style: none hidden none hidden;">
2524 <td><br></td>
2525 </tr>
2526 <tr>
2527 <td>Previous Frame<br><br></td>
2528 </tr>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2529</table>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2530
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2531<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]2532 to allocating its own frame. Only three entries are relevant to LLVM. The
2533 first entry is the previous stack pointer (sp), aka link. This allows
2534 probing tools like gdb or exception handlers to quickly scan the frames in
2535 the stack. A function epilog can also use the link to pop the frame from the
2536 stack. The third entry in the linkage area is used to save the return
2537 address from the lr register. Finally, as mentioned above, the last entry is
2538 used to save the previous frame pointer (r31.) The entries in the linkage
2539 area are the size of a GPR, thus the linkage area is 24 bytes long in 32 bit
2540 mode and 48 bytes in 64 bit mode.</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2541
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2542<p>32 bit linkage area</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2543
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2544<table class="layout">
2545 <tr>
2546 <td>0</td>
2547 <td>Saved SP (r1)</td>
2548 </tr>
2549 <tr>
2550 <td>4</td>
2551 <td>Saved CR</td>
2552 </tr>
2553 <tr>
2554 <td>8</td>
2555 <td>Saved LR</td>
2556 </tr>
2557 <tr>
2558 <td>12</td>
2559 <td>Reserved</td>
2560 </tr>
2561 <tr>
2562 <td>16</td>
2563 <td>Reserved</td>
2564 </tr>
2565 <tr>
2566 <td>20</td>
2567 <td>Saved FP (r31)</td>
2568 </tr>
2569</table>
2570
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2571<p>64 bit linkage area</p>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2572
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2573<table class="layout">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2574 <tr>
2575 <td>0</td>
2576 <td>Saved SP (r1)</td>
2577 </tr>
2578 <tr>
2579 <td>8</td>
2580 <td>Saved CR</td>
2581 </tr>
2582 <tr>
2583 <td>16</td>
2584 <td>Saved LR</td>
2585 </tr>
2586 <tr>
2587 <td>24</td>
2588 <td>Reserved</td>
2589 </tr>
2590 <tr>
2591 <td>32</td>
2592 <td>Reserved</td>
2593 </tr>
2594 <tr>
2595 <td>40</td>
2596 <td>Saved FP (r31)</td>
2597 </tr>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2598</table>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2599
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2600<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]2601 function. Following the PowerPC ABI, the first few arguments are actually
2602 passed in registers, with the space in the parameter area unused. However,
2603 if there are not enough registers or the callee is a thunk or vararg
2604 function, these register arguments can be spilled into the parameter area.
2605 Thus, the parameter area must be large enough to store all the parameters for
2606 the largest call sequence made by the caller. The size must also be
2607 minimally large enough to spill registers r3-r10. This allows callees blind
2608 to the call signature, such as thunks and vararg functions, enough space to
2609 cache the argument registers. Therefore, the parameter area is minimally 32
2610 bytes (64 bytes in 64 bit mode.) Also note that since the parameter area is
2611 a fixed offset from the top of the frame, that a callee can access its spilt
2612 arguments using fixed offsets from the stack pointer (or base pointer.)</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2613
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2614<p>Combining the information about the linkage, parameter areas and alignment. A
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2615 stack frame is minimally 64 bytes in 32 bit mode and 128 bytes in 64 bit
2616 mode.</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2617
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2618<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]2619 alloca then space is added to the stack, the linkage and parameter areas are
2620 shifted to top of stack, and the new space is available immediately below the
2621 linkage and parameter areas. The cost of shifting the linkage and parameter
2622 areas is minor since only the link value needs to be copied. The link value
2623 can be easily fetched by adding the original frame size to the base pointer.
2624 Note that allocations in the dynamic space need to observe 16 byte
2625 alignment.</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2626
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2627<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]2628 variables.</p>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2629
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2630<p>The <i>saved registers area</i> is where the llvm compiler spills callee
2631 saved registers on entry to the callee.</p>
2632
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2633</div>
2634
2635<!-- _______________________________________________________________________ -->
2636<div class="doc_subsubsection">
2637 <a name="ppc_prolog">Prolog/Epilog</a>
2638</div>
2639
2640<div class="doc_text">
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2641
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2642<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]2643 the following exceptions. Callee saved registers are spilled after the frame
2644 is created. This allows the llvm epilog/prolog support to be common with
2645 other targets. The base pointer callee saved register r31 is saved in the
2646 TOC slot of linkage area. This simplifies allocation of space for the base
2647 pointer and makes it convenient to locate programatically and during
2648 debugging.</p>
2649
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2650</div>
2651
2652<!-- _______________________________________________________________________ -->
2653<div class="doc_subsubsection">
2654 <a name="ppc_dynamic">Dynamic Allocation</a>
2655</div>
2656
2657<div class="doc_text">
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2658
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]2659<p><i>TODO - More to come.</i></p>
Bill Wendling80118802009-04-15 02:12:37 +0000[diff] [blame]2660
Jim Laskeyb744c252006-12-15 10:40:48 +0000[diff] [blame]2661</div>
Jim Laskey762b6cb2006-12-14 17:19:50 +0000[diff] [blame]2662
2663
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]2664<!-- *********************************************************************** -->
2665<hr>
2666<address>
2667 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
Misha Brukman44408702008-12-11 17:34:48 +0000[diff] [blame]2668 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]2671
2672 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]2673 <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
Chris Lattnerce52b7e2004-06-01 06:48:00 +0000[diff] [blame]2674 Last modified: $Date$
2675</address>
2676
2677</body>
2678</html>