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				5	<title>The LLVM Target-Independent Code Generator</title>
				6	<link rel="stylesheet" href="llvm.css" type="text/css">
				7	</head>
				8	<body>
				9
				10	<div class="doc_title">
				11	The LLVM Target-Independent Code Generator
				12	</div>
				13
				14	<ol>
				15	<li><a href="#introduction">Introduction</a>
				16	<ul>
				17	<li><a href="#required">Required components in the code generator</a></li>
				18	<li><a href="#high-level-design">The high-level design of the code generator</a></li>
				19	<li><a href="#tablegen">Using TableGen for target description</a></li>
				20	</ul>
				21	</li>
				22	<li><a href="#targetdesc">Target description classes</a>
				23	<ul>
				24	<li><a href="#targetmachine">The <tt>TargetMachine</tt> class</a></li>
				25	<li><a href="#targetdata">The <tt>TargetData</tt> class</a></li>
				26	<li><a href="#mregisterinfo">The <tt>MRegisterInfo</tt> class</a></li>
				27	<li><a href="#targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a></li>
				28	<li><a href="#targetframeinfo">The <tt>TargetFrameInfo</tt> class</a></li>
				29	<li><a href="#targetjitinfo">The <tt>TargetJITInfo</tt> class</a></li>
				30	</ul>
				31	</li>
				32	<li><a href="#codegendesc">Machine code description classes</a>
Chris Lattner	ec94f80	2004-06-04 00:16:02 +0000	[diff] [blame]	33	<ul>
				34	<li><a href="#machineinstr">The <tt>MachineInstr</tt> class</a></li>
				35	</ul>
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	36	</li>
				37	<li><a href="#codegenalgs">Target-independent code generation algorithms</a>
				38	</li>
				39	<li><a href="#targetimpls">Target description implementations</a>
				40	<ul>
				41	<li><a href="#x86">The X86 backend</a></li>
Chris Lattner	10d6800	2004-06-01 17:18:11 +0000	[diff] [blame]	42	</ul>
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	43	</li>
				44
				45	</ol>
				46
				47	<div class="doc_author">
				48	<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
				49	</div>
				50
Chris Lattner	10d6800	2004-06-01 17:18:11 +0000	[diff] [blame]	51	<div class="doc_warning">
				52	<p>Warning: This is a work in progress.</p>
				53	</div>
				54
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	55	<!-- *********************************************************************** -->
				56	<div class="doc_section">
				57	<a name="introduction">Introduction</a>
				58	</div>
				59	<!-- *********************************************************************** -->
				60
				61	<div class="doc_text">
				62
				63	<p>The LLVM target-independent code generator is a framework that provides a
				64	suite of reusable components for translating the LLVM internal representation to
				65	the machine code for a specified target -- either in assembly form (suitable for
				66	a static compiler) or in binary machine code format (usable for a JIT compiler).
Chris Lattner	ec94f80	2004-06-04 00:16:02 +0000	[diff] [blame]	67	The LLVM target-independent code generator consists of five main components:</p>
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	68
				69	<ol>
				70	<li><a href="#targetdesc">Abstract target description</a> interfaces which
Reid Spencer	bdbcb8a	2004-06-05 14:39:24 +0000	[diff] [blame]	71	capture important properties about various aspects of the machine, independently
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	72	of how they will be used. These interfaces are defined in
				73	<tt>include/llvm/Target/</tt>.</li>
				74
				75	<li>Classes used to represent the <a href="#codegendesc">machine code</a> being
Reid Spencer	bdbcb8a	2004-06-05 14:39:24 +0000	[diff] [blame]	76	generated for a target. These classes are intended to be abstract enough to
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	77	represent the machine code for <i>any</i> target machine. These classes are
				78	defined in <tt>include/llvm/CodeGen/</tt>.</li>
				79
				80	<li><a href="#codegenalgs">Target-independent algorithms</a> used to implement
				81	various phases of native code generation (register allocation, scheduling, stack
				82	frame representation, etc). This code lives in <tt>lib/CodeGen/</tt>.</li>
				83
				84	<li><a href="#targetimpls">Implementations of the abstract target description
				85	interfaces</a> for particular targets. These machine descriptions make use of
				86	the components provided by LLVM, and can optionally provide custom
				87	target-specific passes, to build complete code generators for a specific target.
				88	Target descriptions live in <tt>lib/Target/</tt>.</li>
				89
Chris Lattner	ec94f80	2004-06-04 00:16:02 +0000	[diff] [blame]	90	<li><a href="#jit">The target-independent JIT components</a>. The LLVM JIT is
				91	completely target independent (it uses the <tt>TargetJITInfo</tt> structure to
				92	interface for target-specific issues. The code for the target-independent
				93	JIT lives in <tt>lib/ExecutionEngine/JIT</tt>.</li>
				94
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	95	</ol>
				96
				97	<p>
				98	Depending on which part of the code generator you are interested in working on,
				99	different pieces of this will be useful to you. In any case, you should be
				100	familiar with the <a href="#targetdesc">target description</a> and <a
				101	href="#codegendesc">machine code representation</a> classes. If you want to add
Reid Spencer	bdbcb8a	2004-06-05 14:39:24 +0000	[diff] [blame]	102	a backend for a new target, you will need to <a href="#targetimpls">implement the
				103	target description</a> classes for your new target and understand the <a
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	104	href="LangRef.html">LLVM code representation</a>. If you are interested in
				105	implementing a new <a href="#codegenalgs">code generation algorithm</a>, it
				106	should only depend on the target-description and machine code representation
				107	classes, ensuring that it is portable.
				108	</p>
				109
				110	</div>
				111
				112	<!-- ======================================================================= -->
				113	<div class="doc_subsection">
				114	<a name="required">Required components in the code generator</a>
				115	</div>
				116
				117	<div class="doc_text">
				118
				119	<p>The two pieces of the LLVM code generator are the high-level interface to the
				120	code generator and the set of reusable components that can be used to build
				121	target-specific backends. The two most important interfaces (<a
				122	href="#targetmachine"><tt>TargetMachine</tt></a> and <a
				123	href="#targetdata"><tt>TargetData</tt></a> classes) are the only ones that are
				124	required to be defined for a backend to fit into the LLVM system, but the others
				125	must be defined if the reusable code generator components are going to be
				126	used.</p>
				127
				128	<p>This design has two important implications. The first is that LLVM can
				129	support completely non-traditional code generation targets. For example, the C
				130	backend does not require register allocation, instruction selection, or any of
				131	the other standard components provided by the system. As such, it only
				132	implements these two interfaces, and does its own thing. Another example of a
				133	code generator like this is a (purely hypothetical) backend that converts LLVM
				134	to the GCC RTL form and uses GCC to emit machine code for a target.</p>
				135
Reid Spencer	bdbcb8a	2004-06-05 14:39:24 +0000	[diff] [blame]	136	<p>This design also implies that it is possible to design and
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	137	implement radically different code generators in the LLVM system that do not
				138	make use of any of the built-in components. Doing so is not recommended at all,
				139	but could be required for radically different targets that do not fit into the
				140	LLVM machine description model: programmable FPGAs for example.</p>
Chris Lattner	900bf8c	2004-06-02 07:06:06 +0000	[diff] [blame]	141
				142	<p><b>Important Note:</b> For historical reasons, the LLVM SparcV9 code
				143	generator uses almost entirely different code paths than described in this
				144	document. For this reason, there are some deprecated interfaces (such as
				145	<tt>TargetRegInfo</tt> and <tt>TargetSchedInfo</tt>), which are only used by the
				146	V9 backend and should not be used by any other targets. Also, all code in the
				147	<tt>lib/Target/SparcV9</tt> directory and subdirectories should be considered
				148	deprecated, and should not be used as the basis for future code generator work.
Misha Brukman	f3709d6	2004-06-03 16:55:57 +0000	[diff] [blame]	149	The SparcV9 backend is slowly being merged into the rest of the
				150	target-independent code generators, but this is a low-priority process with no
Chris Lattner	900bf8c	2004-06-02 07:06:06 +0000	[diff] [blame]	151	predictable completion date.</p>
				152
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	153	</div>
				154
				155	<!-- ======================================================================= -->
				156	<div class="doc_subsection">
Chris Lattner	10d6800	2004-06-01 17:18:11 +0000	[diff] [blame]	157	<a name="high-level-design">The high-level design of the code generator</a>
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	158	</div>
				159
				160	<div class="doc_text">
				161
				162	<p>The LLVM target-indendent code generator is designed to support efficient and
				163	quality code generation for standard register-based microprocessors. Code
				164	generation in this model is divided into the following stages:</p>
				165
				166	<ol>
Reid Spencer	bdbcb8a	2004-06-05 14:39:24 +0000	[diff] [blame]	167	<li><b>Instruction Selection</b> - Determining an efficient implementation of the
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	168	input LLVM code in the target instruction set. This stage produces the initial
Reid Spencer	bdbcb8a	2004-06-05 14:39:24 +0000	[diff] [blame]	169	code for the program in the target instruction set, then makes use of virtual
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	170	registers in SSA form and physical registers that represent any required
				171	register assignments due to target constraints or calling conventions.</li>
				172
				173	<li><b>SSA-based Machine Code Optimizations</b> - This (optional) stage consists
				174	of a series of machine-code optimizations that operate on the SSA-form produced
				175	by the instruction selector. Optimizations like modulo-scheduling, normal
				176	scheduling, or peephole optimization work here.</li>
				177
				178	<li><b>Register Allocation</b> - The target code is transformed from an infinite
				179	virtual register file in SSA form to the concrete register file used by the
				180	target. This phase introduces spill code and eliminates all virtual register
				181	references from the program.</li>
				182
				183	<li><b>Prolog/Epilog Code Insertion</b> - Once the machine code has been
				184	generated for the function and the amount of stack space required is known (used
				185	for LLVM alloca's and spill slots), the prolog and epilog code for the function
				186	can be inserted and "abstract stack location references" can be eliminated.
				187	This stage is responsible for implementing optimizations like frame-pointer
				188	elimination and stack packing.</li>
				189
				190	<li><b>Late Machine Code Optimizations</b> - Optimizations that operate on
				191	"final" machine code can go here, such as spill code scheduling and peephole
				192	optimizations.</li>
				193
Reid Spencer	bdbcb8a	2004-06-05 14:39:24 +0000	[diff] [blame]	194	<li><b>Code Emission</b> - The final stage actually outputs the code for
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	195	the current function, either in the target assembler format or in machine
				196	code.</li>
				197
				198	</ol>
				199
				200	<p>
				201	The code generator is based on the assumption that the instruction selector will
				202	use an optimal pattern matching selector to create high-quality sequences of
Reid Spencer	bdbcb8a	2004-06-05 14:39:24 +0000	[diff] [blame]	203	native instructions. Alternative code generator designs based on pattern
				204	expansion and
				205	aggressive iterative peephole optimization are much slower. This design
				206	permits efficient compilation (important for JIT environments) and
				207	aggressive optimization (used when generating code offline) by allowing
				208	components of varying levels of sophisication to be used for any step of
				209	compilation.</p>
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	210
				211	<p>
				212	In addition to these stages, target implementations can insert arbitrary
				213	target-specific passes into the flow. For example, the X86 target uses a
				214	special pass to handle the 80x87 floating point stack architecture. Other
				215	targets with unusual requirements can be supported with custom passes as needed.
				216	</p>
				217
				218	</div>
				219
				220
				221	<!-- ======================================================================= -->
				222	<div class="doc_subsection">
Chris Lattner	10d6800	2004-06-01 17:18:11 +0000	[diff] [blame]	223	<a name="tablegen">Using TableGen for target description</a>
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	224	</div>
				225
				226	<div class="doc_text">
				227
Chris Lattner	5489e93	2004-06-01 18:35:00 +0000	[diff] [blame]	228	<p>The target description classes require a detailed description of the target
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	229	architecture. These target descriptions often have a large amount of common
				230	information (e.g., an add instruction is almost identical to a sub instruction).
				231	In order to allow the maximum amount of commonality to be factored out, the LLVM
				232	code generator uses the <a href="TableGenFundamentals.html">TableGen</a> tool to
Chris Lattner	5489e93	2004-06-01 18:35:00 +0000	[diff] [blame]	233	describe big chunks of the target machine, which allows the use of domain- and
				234	target-specific abstractions to reduce the amount of repetition.
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	235	</p>
				236
				237	</div>
				238
				239	<!-- *********************************************************************** -->
				240	<div class="doc_section">
				241	<a name="targetdesc">Target description classes</a>
				242	</div>
				243	<!-- *********************************************************************** -->
				244
				245	<div class="doc_text">
				246
				247	<p>The LLVM target description classes (which are located in the
				248	<tt>include/llvm/Target</tt> directory) provide an abstract description of the
				249	target machine, independent of any particular client. These classes are
				250	designed to capture the <i>abstract</i> properties of the target (such as what
				251	instruction and registers it has), and do not incorporate any particular pieces
				252	of code generation algorithms (these interfaces do not take interference graphs
				253	as inputs or other algorithm-specific data structures).</p>
				254
				255	<p>All of the target description classes (except the <tt><a
				256	href="#targetdata">TargetData</a></tt> class) are designed to be subclassed by
				257	the concrete target implementation, and have virtual methods implemented. To
Reid Spencer	bdbcb8a	2004-06-05 14:39:24 +0000	[diff] [blame]	258	get to these implementations, the <tt><a
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	259	href="#targetmachine">TargetMachine</a></tt> class provides accessors that
				260	should be implemented by the target.</p>
				261
				262	</div>
				263
				264	<!-- ======================================================================= -->
				265	<div class="doc_subsection">
				266	<a name="targetmachine">The <tt>TargetMachine</tt> class</a>
				267	</div>
				268
				269	<div class="doc_text">
				270
				271	<p>The <tt>TargetMachine</tt> class provides virtual methods that are used to
				272	access the target-specific implementations of the various target description
				273	classes (with the <tt>getInstrInfo</tt>, <tt>getRegisterInfo</tt>,
Reid Spencer	bdbcb8a	2004-06-05 14:39:24 +0000	[diff] [blame]	274	<tt>getFrameInfo</tt>, ... methods). This class is designed to be specialized by
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	275	a concrete target implementation (e.g., <tt>X86TargetMachine</tt>) which
				276	implements the various virtual methods. The only required target description
				277	class is the <a href="#targetdata"><tt>TargetData</tt></a> class, but if the
				278	code generator components are to be used, the other interfaces should be
				279	implemented as well.</p>
				280
				281	</div>
				282
				283
				284	<!-- ======================================================================= -->
				285	<div class="doc_subsection">
				286	<a name="targetdata">The <tt>TargetData</tt> class</a>
				287	</div>
				288
				289	<div class="doc_text">
				290
				291	<p>The <tt>TargetData</tt> class is the only required target description class,
				292	and it is the only class that is not extensible (it cannot be derived from). It
				293	specifies information about how the target lays out memory for structures, the
				294	alignment requirements for various data types, the size of pointers in the
				295	target, and whether the target is little- or big-endian.</p>
				296
				297	</div>
				298
				299
				300	<!-- ======================================================================= -->
				301	<div class="doc_subsection">
Chris Lattner	10d6800	2004-06-01 17:18:11 +0000	[diff] [blame]	302	<a name="mregisterinfo">The <tt>MRegisterInfo</tt> class</a>
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	303	</div>
				304
				305	<div class="doc_text">
				306
				307	<p>The <tt>MRegisterInfo</tt> class (which will eventually be renamed to
				308	<tt>TargetRegisterInfo</tt>) is used to describe the register file of the
				309	target and any interactions between the registers.</p>
				310
				311	<p>Registers in the code generator are represented in the code generator by
				312	unsigned numbers. Physical registers (those that actually exist in the target
				313	description) are unique small numbers, and virtual registers are generally
				314	large.</p>
				315
				316	<p>Each register in the processor description has an associated
				317	<tt>MRegisterDesc</tt> entry, which provides a textual name for the register
				318	(used for assembly output and debugging dumps), a set of aliases (used to
				319	indicate that one register overlaps with another), and some flag bits.
				320	</p>
				321
				322	<p>In addition to the per-register description, the <tt>MRegisterInfo</tt> class
				323	exposes a set of processor specific register classes (instances of the
				324	<tt>TargetRegisterClass</tt> class). Each register class contains sets of
				325	registers that have the same properties (for example, they are all 32-bit
				326	integer registers). Each SSA virtual register created by the instruction
				327	selector has an associated register class. When the register allocator runs, it
				328	replaces virtual registers with a physical register in the set.</p>
				329
				330	<p>
				331	The target-specific implementations of these classes is auto-generated from a <a
				332	href="TableGenFundamentals.html">TableGen</a> description of the register file.
				333	</p>
				334
				335	</div>
				336
				337	<!-- ======================================================================= -->
				338	<div class="doc_subsection">
Chris Lattner	10d6800	2004-06-01 17:18:11 +0000	[diff] [blame]	339	<a name="targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a>
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	340	</div>
				341
				342	<!-- ======================================================================= -->
				343	<div class="doc_subsection">
Chris Lattner	10d6800	2004-06-01 17:18:11 +0000	[diff] [blame]	344	<a name="targetframeinfo">The <tt>TargetFrameInfo</tt> class</a>
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	345	</div>
				346
				347	<!-- ======================================================================= -->
				348	<div class="doc_subsection">
Chris Lattner	10d6800	2004-06-01 17:18:11 +0000	[diff] [blame]	349	<a name="targetjitinfo">The <tt>TargetJITInfo</tt> class</a>
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	350	</div>
				351
				352	<!-- *********************************************************************** -->
				353	<div class="doc_section">
				354	<a name="codegendesc">Machine code description classes</a>
				355	</div>
				356	<!-- *********************************************************************** -->
				357
Chris Lattner	ec94f80	2004-06-04 00:16:02 +0000	[diff] [blame]	358	<div class="doc_text">
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	359
Chris Lattner	ec94f80	2004-06-04 00:16:02 +0000	[diff] [blame]	360	<p>
				361	At the high-level, LLVM code is translated to a machine specific representation
				362	formed out of MachineFunction, MachineBasicBlock, and <a
				363	href="#machineinstr"><tt>MachineInstr</tt></a> instances
				364	(defined in include/llvm/CodeGen). This representation is completely target
				365	agnostic, representing instructions in their most abstract form: an opcode and a
				366	series of operands. This representation is designed to support both SSA
				367	representation for machine code, as well as a register allocated, non-SSA form.
				368	</p>
				369
				370	</div>
				371
				372	<!-- ======================================================================= -->
				373	<div class="doc_subsection">
				374	<a name="machineinstr">The <tt>MachineInstr</tt> class</a>
				375	</div>
				376
				377	<div class="doc_text">
				378
				379	<p>Target machine instructions are represented as instances of the
				380	<tt>MachineInstr</tt> class. This class is an extremely abstract way of
				381	representing machine instructions. In particular, all it keeps track of is
				382	an opcode number and some number of operands.</p>
				383
				384	<p>The opcode number is an simple unsigned number that only has meaning to a
				385	specific backend. All of the instructions for a target should be defined in
				386	the <tt>*InstrInfo.td</tt> file for the target, and the opcode enum values
				387	are autogenerated from this description. The <tt>MachineInstr</tt> class does
				388	not have any information about how to intepret the instruction (i.e., what the
				389	semantics of the instruction are): for that you must refer to the
				390	<tt><a href="#targetinstrinfo">TargetInstrInfo</a></tt> class.</p>
				391
				392	<p>The operands of a machine instruction can be of several different types:
				393	they can be a register reference, constant integer, basic block reference, etc.
				394	In addition, a machine operand should be marked as a def or a use of the value
				395	(though only registers are allowed to be defs).</p>
				396
				397	<p>By convention, the LLVM code generator orders instruction operands so that
				398	all register definitions come before the register uses, even on architectures
				399	that are normally printed in other orders. For example, the sparc add
				400	instruction: "<tt>add %i1, %i2, %i3</tt>" adds the "%i1", and "%i2" registers
				401	and stores the result into the "%i3" register. In the LLVM code generator,
				402	the operands should be stored as "<tt>%i3, %i1, %i2</tt>": with the destination
				403	first.</p>
				404
				405	<p>Keeping destination operands at the beginning of the operand list has several
				406	advantages. In particular, the debugging printer will print the instruction
				407	like this:</p>
				408
				409	<pre>
				410	%r3 = add %i1, %i2
				411	</pre>
				412
				413	<p>If the first operand is a def, and it is also easier to <a
				414	href="#buildmi">create instructions</a> whose only def is the first
				415	operand.</p>
				416
				417	</div>
				418
				419	<!-- _______________________________________________________________________ -->
				420	<div class="doc_subsubsection">
				421	<a name="buildmi">Using the <tt>MachineInstrBuilder.h</tt> functions</a>
				422	</div>
				423
				424	<div class="doc_text">
				425
				426	<p>Machine instructions are created by using the <tt>BuildMI</tt> functions,
				427	located in the <tt>include/llvm/CodeGen/MachineInstrBuilder.h</tt> file. The
				428	<tt>BuildMI</tt> functions make it easy to build arbitrary machine
				429	instructions. Usage of the <tt>BuildMI</tt> functions look like this:
				430	</p>
				431
				432	<pre>
				433	// Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42')
				434	// instruction. The '1' specifies how many operands will be added.
				435	MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42);
				436
				437	// Create the same instr, but insert it at the end of a basic block.
				438	MachineBasicBlock &MBB = ...
				439	BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42);
				440
				441	// Create the same instr, but insert it before a specified iterator point.
				442	MachineBasicBlock::iterator MBBI = ...
				443	BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42);
				444
				445	// Create a 'cmp Reg, 0' instruction, no destination reg.
				446	MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0);
				447	// Create an 'sahf' instruction which takes no operands and stores nothing.
				448	MI = BuildMI(X86::SAHF, 0);
				449
				450	// Create a self looping branch instruction.
				451	BuildMI(MBB, X86::JNE, 1).addMBB(&MBB);
				452	</pre>
				453
				454	<p>
				455	The key thing to remember with the <tt>BuildMI</tt> functions is that you have
				456	to specify the number of operands that the machine instruction will take
				457	(allowing efficient memory allocation). Also, if operands default to be uses
				458	of values, not definitions. If you need to add a definition operand (other
				459	than the optional destination register), you must explicitly mark it as such.
				460	</p>
				461
				462	</div>
				463
				464	<!-- _______________________________________________________________________ -->
				465	<div class="doc_subsubsection">
				466	<a name="fixedregs">Fixed (aka preassigned) registers</a>
				467	</div>
				468
				469	<div class="doc_text">
				470
				471	<p>One important issue that the code generator needs to be aware of is the
				472	presence of fixed registers. In particular, there are often places in the
				473	instruction stream where the register allocator <em>must</em> arrange for a
				474	particular value to be in a particular register. This can occur due to
				475	limitations in the instruction set (e.g., the X86 can only do a 32-bit divide
				476	with the <tt>EAX</tt>/<tt>EDX</tt> registers), or external factors like calling
				477	conventions. In any case, the instruction selector should emit code that
				478	copies a virtual register into or out of a physical register when needed.</p>
				479
				480	<p>For example, consider this simple LLVM example:</p>
				481
				482	<pre>
				483	int %test(int %X, int %Y) {
				484	%Z = div int %X, %Y
				485	ret int %Z
				486	}
				487	</pre>
				488
				489	<p>The X86 instruction selector produces this machine code for the div
				490	and ret (use
				491	"<tt>llc X.bc -march=x86 -print-machineinstrs</tt>" to get this):</p>
				492
				493	<pre>
				494	;; Start of div
				495	%EAX = mov %reg1024 ;; Copy X (in reg1024) into EAX
				496	%reg1027 = sar %reg1024, 31
				497	%EDX = mov %reg1027 ;; Sign extend X into EDX
				498	idiv %reg1025 ;; Divide by Y (in reg1025)
				499	%reg1026 = mov %EAX ;; Read the result (Z) out of EAX
				500
				501	;; Start of ret
				502	%EAX = mov %reg1026 ;; 32-bit return value goes in EAX
				503	ret
				504	</pre>
				505
				506	<p>By the end of code generation, the register allocator has coallesced
				507	the registers and deleted the resultant identity moves, producing the
				508	following code:</p>
				509
				510	<pre>
				511	;; X is in EAX, Y is in ECX
				512	mov %EAX, %EDX
				513	sar %EDX, 31
				514	idiv %ECX
				515	ret
				516	</pre>
				517
				518	<p>This approach is extremely general (if it can handle the X86 architecture,
				519	it can handle anything!) and allows all of the target specific
				520	knowledge about the instruction stream to be isolated in the instruction
				521	selector. Note that physical registers should have a short lifetime for good
				522	code generation, and all physical registers are assumed dead on entry and
				523	exit of basic blocks (before register allocation). Thus if you need a value
				524	to be live across basic block boundaries, it <em>must</em> live in a virtual
				525	register.</p>
				526
				527	</div>
				528
				529	<!-- _______________________________________________________________________ -->
				530	<div class="doc_subsubsection">
				531	<a name="ssa">Machine code SSA form</a>
				532	</div>
				533
				534	<div class="doc_text">
				535
				536	<p><tt>MachineInstr</tt>'s are initially instruction selected in SSA-form, and
				537	are maintained in SSA-form until register allocation happens. For the most
				538	part, this is trivially simple since LLVM is already in SSA form: LLVM PHI nodes
				539	become machine code PHI nodes, and virtual registers are only allowed to have a
				540	single definition.</p>
				541
				542	<p>After register allocation, machine code is no longer in SSA-form, as there
				543	are no virtual registers left in the code.</p>
				544
				545	</div>
				546
				547	<!-- *********************************************************************** -->
				548	<div class="doc_section">
				549	<a name="targetimpls">Target description implementations</a>
				550	</div>
				551	<!-- *********************************************************************** -->
				552
				553	<div class="doc_text">
				554
				555	<p>This section of the document explains any features or design decisions that
				556	are specific to the code generator for a particular target.</p>
				557
				558	</div>
				559
				560
				561	<!-- ======================================================================= -->
				562	<div class="doc_subsection">
				563	<a name="x86">The X86 backend</a>
				564	</div>
				565
				566	<div class="doc_text">
				567
				568	<p>
				569	The X86 code generator lives in the <tt>lib/Target/X86</tt> directory. This
				570	code generator currently targets a generic P6-like processor. As such, it
				571	produces a few P6-and-above instructions (like conditional moves), but it does
				572	not make use of newer features like MMX or SSE. In the future, the X86 backend
				573	will have subtarget support added for specific processor families and
				574	implementations.</p>
				575
				576	</div>
				577
				578	<!-- _______________________________________________________________________ -->
				579	<div class="doc_subsubsection">
				580	<a name="x86_memory">Representing X86 addressing modes in MachineInstrs</a>
				581	</div>
				582
				583	<div class="doc_text">
				584
				585	<p>
				586	The x86 has a very, uhm, flexible, way of accessing memory. It is capable of
				587	forming memory addresses of the following expression directly in integer
				588	instructions (which use ModR/M addressing):</p>
				589
				590	<pre>
				591	Base+[1,2,4,8]*IndexReg+Disp32
				592	</pre>
				593
				594	<p>Wow, that's crazy. In order to represent this, LLVM tracks no less that 4
				595	operands for each memory operand of this form. This means that the "load" form
				596	of 'mov' has the following "Operands" in this order:</p>
				597
				598	<pre>
				599	Index: 0 \| 1 2 3 4
				600	Meaning: DestReg, \| BaseReg, Scale, IndexReg, Displacement
				601	OperandTy: VirtReg, \| VirtReg, UnsImm, VirtReg, SignExtImm
				602	</pre>
				603
				604	<p>Stores and all other instructions treat the four memory operands in the same
				605	way, in the same order.</p>
Chris Lattner	ec94f80	2004-06-04 00:16:02 +0000	[diff] [blame]	606
				607	</div>
				608
				609	<!-- _______________________________________________________________________ -->
				610	<div class="doc_subsubsection">
				611	<a name="x86_names">Instruction naming</a>
				612	</div>
				613
				614	<div class="doc_text">
				615
				616	<p>
				617	An instruction name consists of the base name, a default operand size
				618	followed by a character per operand with an optional special size. For
				619	example:</p>
				620
				621	<p>
				622	<tt>ADD8rr</tt> -> add, 8-bit register, 8-bit register<br>
				623	<tt>IMUL16rmi</tt> -> imul, 16-bit register, 16-bit memory, 16-bit immediate<br>
				624	<tt>IMUL16rmi8</tt> -> imul, 16-bit register, 16-bit memory, 8-bit immediate<br>
				625	<tt>MOVSX32rm16</tt> -> movsx, 32-bit register, 16-bit memory
				626	</p>
				627
				628	</div>
Chris Lattner	ce52b7e	2004-06-01 06:48:00 +0000	[diff] [blame]	629
				630	<!-- *********************************************************************** -->
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				638	<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
				639	<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a><br>
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