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5 <title>The LLVM Target-Independent Code Generator</title>
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10 <div class="doc_title">
11 The LLVM Target-Independent Code Generator
15 <li><a href="#introduction">Introduction</a>
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
20 <li><a href="#tablegen">Using TableGen for target description</a></li>
23 <li><a href="#targetdesc">Target description classes</a>
25 <li><a href="#targetmachine">The <tt>TargetMachine</tt> class</a></li>
26 <li><a href="#targetdata">The <tt>TargetData</tt> class</a></li>
27 <li><a href="#targetlowering">The <tt>TargetLowering</tt> class</a></li>
28 <li><a href="#mregisterinfo">The <tt>MRegisterInfo</tt> class</a></li>
29 <li><a href="#targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a></li>
30 <li><a href="#targetframeinfo">The <tt>TargetFrameInfo</tt> class</a></li>
31 <li><a href="#targetsubtarget">The <tt>TargetSubtarget</tt> class</a></li>
32 <li><a href="#targetjitinfo">The <tt>TargetJITInfo</tt> class</a></li>
35 <li><a href="#codegendesc">Machine code description classes</a>
37 <li><a href="#machineinstr">The <tt>MachineInstr</tt> class</a></li>
38 <li><a href="#machinebasicblock">The <tt>MachineBasicBlock</tt>
40 <li><a href="#machinefunction">The <tt>MachineFunction</tt> class</a></li>
43 <li><a href="#codegenalgs">Target-independent code generation algorithms</a>
45 <li><a href="#instselect">Instruction Selection</a>
47 <li><a href="#selectiondag_intro">Introduction to SelectionDAGs</a></li>
48 <li><a href="#selectiondag_process">SelectionDAG Code Generation
50 <li><a href="#selectiondag_build">Initial SelectionDAG
52 <li><a href="#selectiondag_legalize">SelectionDAG Legalize Phase</a></li>
53 <li><a href="#selectiondag_optimize">SelectionDAG Optimization
54 Phase: the DAG Combiner</a></li>
55 <li><a href="#selectiondag_select">SelectionDAG Select Phase</a></li>
56 <li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation
58 <li><a href="#selectiondag_future">Future directions for the
61 <li><a href="#codeemit">Code Emission</a>
63 <li><a href="#codeemit_asm">Generating Assembly Code</a></li>
64 <li><a href="#codeemit_bin">Generating Binary Machine Code</a></li>
68 <li><a href="#targetimpls">Target-specific Implementation Notes</a>
70 <li><a href="#x86">The X86 backend</a></li>
76 <div class="doc_author">
77 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
80 <div class="doc_warning">
81 <p>Warning: This is a work in progress.</p>
84 <!-- *********************************************************************** -->
85 <div class="doc_section">
86 <a name="introduction">Introduction</a>
88 <!-- *********************************************************************** -->
90 <div class="doc_text">
92 <p>The LLVM target-independent code generator is a framework that provides a
93 suite of reusable components for translating the LLVM internal representation to
94 the machine code for a specified target -- either in assembly form (suitable for
95 a static compiler) or in binary machine code format (usable for a JIT compiler).
96 The LLVM target-independent code generator consists of five main components:</p>
99 <li><a href="#targetdesc">Abstract target description</a> interfaces which
100 capture important properties about various aspects of the machine, independently
101 of how they will be used. These interfaces are defined in
102 <tt>include/llvm/Target/</tt>.</li>
104 <li>Classes used to represent the <a href="#codegendesc">machine code</a> being
105 generated for a target. These classes are intended to be abstract enough to
106 represent the machine code for <i>any</i> target machine. These classes are
107 defined in <tt>include/llvm/CodeGen/</tt>.</li>
109 <li><a href="#codegenalgs">Target-independent algorithms</a> used to implement
110 various phases of native code generation (register allocation, scheduling, stack
111 frame representation, etc). This code lives in <tt>lib/CodeGen/</tt>.</li>
113 <li><a href="#targetimpls">Implementations of the abstract target description
114 interfaces</a> for particular targets. These machine descriptions make use of
115 the components provided by LLVM, and can optionally provide custom
116 target-specific passes, to build complete code generators for a specific target.
117 Target descriptions live in <tt>lib/Target/</tt>.</li>
119 <li><a href="#jit">The target-independent JIT components</a>. The LLVM JIT is
120 completely target independent (it uses the <tt>TargetJITInfo</tt> structure to
121 interface for target-specific issues. The code for the target-independent
122 JIT lives in <tt>lib/ExecutionEngine/JIT</tt>.</li>
127 Depending on which part of the code generator you are interested in working on,
128 different pieces of this will be useful to you. In any case, you should be
129 familiar with the <a href="#targetdesc">target description</a> and <a
130 href="#codegendesc">machine code representation</a> classes. If you want to add
131 a backend for a new target, you will need to <a href="#targetimpls">implement the
132 target description</a> classes for your new target and understand the <a
133 href="LangRef.html">LLVM code representation</a>. If you are interested in
134 implementing a new <a href="#codegenalgs">code generation algorithm</a>, it
135 should only depend on the target-description and machine code representation
136 classes, ensuring that it is portable.
141 <!-- ======================================================================= -->
142 <div class="doc_subsection">
143 <a name="required">Required components in the code generator</a>
146 <div class="doc_text">
148 <p>The two pieces of the LLVM code generator are the high-level interface to the
149 code generator and the set of reusable components that can be used to build
150 target-specific backends. The two most important interfaces (<a
151 href="#targetmachine"><tt>TargetMachine</tt></a> and <a
152 href="#targetdata"><tt>TargetData</tt></a>) are the only ones that are
153 required to be defined for a backend to fit into the LLVM system, but the others
154 must be defined if the reusable code generator components are going to be
157 <p>This design has two important implications. The first is that LLVM can
158 support completely non-traditional code generation targets. For example, the C
159 backend does not require register allocation, instruction selection, or any of
160 the other standard components provided by the system. As such, it only
161 implements these two interfaces, and does its own thing. Another example of a
162 code generator like this is a (purely hypothetical) backend that converts LLVM
163 to the GCC RTL form and uses GCC to emit machine code for a target.</p>
165 <p>This design also implies that it is possible to design and
166 implement radically different code generators in the LLVM system that do not
167 make use of any of the built-in components. Doing so is not recommended at all,
168 but could be required for radically different targets that do not fit into the
169 LLVM machine description model: programmable FPGAs for example.</p>
171 <p><b>Important Note:</b> For historical reasons, the LLVM SparcV9 code
172 generator uses almost entirely different code paths than described in this
173 document. For this reason, there are some deprecated interfaces (such as
174 <tt>TargetSchedInfo</tt>), which are only used by the
175 V9 backend and should not be used by any other targets. Also, all code in the
176 <tt>lib/Target/SparcV9</tt> directory and subdirectories should be considered
177 deprecated, and should not be used as the basis for future code generator work.
178 The SparcV9 backend is slowly being merged into the rest of the
179 target-independent code generators, but this is a low-priority process with no
180 predictable completion date.</p>
184 <!-- ======================================================================= -->
185 <div class="doc_subsection">
186 <a name="high-level-design">The high-level design of the code generator</a>
189 <div class="doc_text">
191 <p>The LLVM target-independent code generator is designed to support efficient and
192 quality code generation for standard register-based microprocessors. Code
193 generation in this model is divided into the following stages:</p>
196 <li><b><a href="#instselect">Instruction Selection</a></b> - This phase
197 determines an efficient way to express the input LLVM code in the target
199 This stage produces the initial code for the program in the target instruction
200 set, then makes use of virtual registers in SSA form and physical registers that
201 represent any required register assignments due to target constraints or calling
202 conventions. This step turns the LLVM code into a DAG of target
205 <li><b><a href="#selectiondag_sched">Scheduling and Formation</a></b> - This
206 phase takes the DAG of target instructions produced by the instruction selection
207 phase, determines an ordering of the instructions, then emits the instructions
208 as <tt><a href="#machineinstr">MachineInstr</a></tt>s with that ordering.
211 <li><b><a href="#ssamco">SSA-based Machine Code Optimizations</a></b> - This
212 optional stage consists of a series of machine-code optimizations that
213 operate on the SSA-form produced by the instruction selector. Optimizations
214 like modulo-scheduling or peephole optimization work here.
217 <li><b><a href="#regalloc">Register Allocation</a></b> - The
218 target code is transformed from an infinite virtual register file in SSA form
219 to the concrete register file used by the target. This phase introduces spill
220 code and eliminates all virtual register references from the program.</li>
222 <li><b><a href="#proepicode">Prolog/Epilog Code Insertion</a></b> - Once the
223 machine code has been generated for the function and the amount of stack space
224 required is known (used for LLVM alloca's and spill slots), the prolog and
225 epilog code for the function can be inserted and "abstract stack location
226 references" can be eliminated. This stage is responsible for implementing
227 optimizations like frame-pointer elimination and stack packing.</li>
229 <li><b><a href="#latemco">Late Machine Code Optimizations</a></b> - Optimizations
230 that operate on "final" machine code can go here, such as spill code scheduling
231 and peephole optimizations.</li>
233 <li><b><a href="#codeemit">Code Emission</a></b> - The final stage actually
234 puts out the code for the current function, either in the target assembler
235 format or in machine code.</li>
240 The code generator is based on the assumption that the instruction selector will
241 use an optimal pattern matching selector to create high-quality sequences of
242 native instructions. Alternative code generator designs based on pattern
244 aggressive iterative peephole optimization are much slower. This design
245 permits efficient compilation (important for JIT environments) and
246 aggressive optimization (used when generating code offline) by allowing
247 components of varying levels of sophistication to be used for any step of
251 In addition to these stages, target implementations can insert arbitrary
252 target-specific passes into the flow. For example, the X86 target uses a
253 special pass to handle the 80x87 floating point stack architecture. Other
254 targets with unusual requirements can be supported with custom passes as needed.
260 <!-- ======================================================================= -->
261 <div class="doc_subsection">
262 <a name="tablegen">Using TableGen for target description</a>
265 <div class="doc_text">
267 <p>The target description classes require a detailed description of the target
268 architecture. These target descriptions often have a large amount of common
269 information (e.g., an <tt>add</tt> instruction is almost identical to a
270 <tt>sub</tt> instruction).
271 In order to allow the maximum amount of commonality to be factored out, the LLVM
272 code generator uses the <a href="TableGenFundamentals.html">TableGen</a> tool to
273 describe big chunks of the target machine, which allows the use of
274 domain-specific and target-specific abstractions to reduce the amount of
278 <p>As LLVM continues to be developed and refined, we plan to move more and more
279 of the target description to be in <tt>.td</tt> form. Doing so gives us a
280 number of advantages. The most important is that it makes it easier to port
281 LLVM, because it reduces the amount of C++ code that has to be written and the
282 surface area of the code generator that needs to be understood before someone
283 can get in an get something working. Second, it is also important to us because
284 it makes it easier to change things: in particular, if tables and other things
285 are all emitted by tblgen, we only need to change one place (tblgen) to update
286 all of the targets to a new interface.</p>
290 <!-- *********************************************************************** -->
291 <div class="doc_section">
292 <a name="targetdesc">Target description classes</a>
294 <!-- *********************************************************************** -->
296 <div class="doc_text">
298 <p>The LLVM target description classes (which are located in the
299 <tt>include/llvm/Target</tt> directory) provide an abstract description of the
300 target machine; independent of any particular client. These classes are
301 designed to capture the <i>abstract</i> properties of the target (such as the
302 instructions and registers it has), and do not incorporate any particular pieces
303 of code generation algorithms.</p>
305 <p>All of the target description classes (except the <tt><a
306 href="#targetdata">TargetData</a></tt> class) are designed to be subclassed by
307 the concrete target implementation, and have virtual methods implemented. To
308 get to these implementations, the <tt><a
309 href="#targetmachine">TargetMachine</a></tt> class provides accessors that
310 should be implemented by the target.</p>
314 <!-- ======================================================================= -->
315 <div class="doc_subsection">
316 <a name="targetmachine">The <tt>TargetMachine</tt> class</a>
319 <div class="doc_text">
321 <p>The <tt>TargetMachine</tt> class provides virtual methods that are used to
322 access the target-specific implementations of the various target description
323 classes via the <tt>get*Info</tt> methods (<tt>getInstrInfo</tt>,
324 <tt>getRegisterInfo</tt>, <tt>getFrameInfo</tt>, etc.). This class is
325 designed to be specialized by
326 a concrete target implementation (e.g., <tt>X86TargetMachine</tt>) which
327 implements the various virtual methods. The only required target description
328 class is the <a href="#targetdata"><tt>TargetData</tt></a> class, but if the
329 code generator components are to be used, the other interfaces should be
330 implemented as well.</p>
335 <!-- ======================================================================= -->
336 <div class="doc_subsection">
337 <a name="targetdata">The <tt>TargetData</tt> class</a>
340 <div class="doc_text">
342 <p>The <tt>TargetData</tt> class is the only required target description class,
343 and it is the only class that is not extensible (you cannot derived a new
344 class from it). <tt>TargetData</tt> specifies information about how the target
345 lays out memory for structures, the alignment requirements for various data
346 types, the size of pointers in the target, and whether the target is
347 little-endian or big-endian.</p>
351 <!-- ======================================================================= -->
352 <div class="doc_subsection">
353 <a name="targetlowering">The <tt>TargetLowering</tt> class</a>
356 <div class="doc_text">
358 <p>The <tt>TargetLowering</tt> class is used by SelectionDAG based instruction
359 selectors primarily to describe how LLVM code should be lowered to SelectionDAG
360 operations. Among other things, this class indicates:
361 <ul><li>an initial register class to use for various ValueTypes</li>
362 <li>which operations are natively supported by the target machine</li>
363 <li>the return type of setcc operations</li>
364 <li>the type to use for shift amounts</li>
365 <li>various high-level characteristics, like whether it is profitable to turn
366 division by a constant into a multiplication sequence</li>
371 <!-- ======================================================================= -->
372 <div class="doc_subsection">
373 <a name="mregisterinfo">The <tt>MRegisterInfo</tt> class</a>
376 <div class="doc_text">
378 <p>The <tt>MRegisterInfo</tt> class (which will eventually be renamed to
379 <tt>TargetRegisterInfo</tt>) is used to describe the register file of the
380 target and any interactions between the registers.</p>
382 <p>Registers in the code generator are represented in the code generator by
383 unsigned numbers. Physical registers (those that actually exist in the target
384 description) are unique small numbers, and virtual registers are generally
385 large. Note that register #0 is reserved as a flag value.</p>
387 <p>Each register in the processor description has an associated
388 <tt>TargetRegisterDesc</tt> entry, which provides a textual name for the register
389 (used for assembly output and debugging dumps) and a set of aliases (used to
390 indicate that one register overlaps with another).
393 <p>In addition to the per-register description, the <tt>MRegisterInfo</tt> class
394 exposes a set of processor specific register classes (instances of the
395 <tt>TargetRegisterClass</tt> class). Each register class contains sets of
396 registers that have the same properties (for example, they are all 32-bit
397 integer registers). Each SSA virtual register created by the instruction
398 selector has an associated register class. When the register allocator runs, it
399 replaces virtual registers with a physical register in the set.</p>
402 The target-specific implementations of these classes is auto-generated from a <a
403 href="TableGenFundamentals.html">TableGen</a> description of the register file.
408 <!-- ======================================================================= -->
409 <div class="doc_subsection">
410 <a name="targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a>
413 <div class="doc_text">
414 <p>The <tt>TargetInstrInfo</tt> class is used to describe the machine
415 instructions supported by the target. It is essentially an array of
416 <tt>TargetInstrDescriptor</tt> objects, each of which describes one
417 instruction the target supports. Descriptors define things like the mnemonic
418 for the opcode, the number of operands, the list of implicit register uses
419 and defs, whether the instruction has certain target-independent properties
420 (accesses memory, is commutable, etc), and holds any target-specific flags.</p>
423 <!-- ======================================================================= -->
424 <div class="doc_subsection">
425 <a name="targetframeinfo">The <tt>TargetFrameInfo</tt> class</a>
428 <div class="doc_text">
429 <p>The <tt>TargetFrameInfo</tt> class is used to provide information about the
430 stack frame layout of the target. It holds the direction of stack growth,
431 the known stack alignment on entry to each function, and the offset to the
432 locals area. The offset to the local area is the offset from the stack
433 pointer on function entry to the first location where function data (local
434 variables, spill locations) can be stored.</p>
437 <!-- ======================================================================= -->
438 <div class="doc_subsection">
439 <a name="targetsubtarget">The <tt>TargetSubtarget</tt> class</a>
442 <div class="doc_text">
449 <!-- ======================================================================= -->
450 <div class="doc_subsection">
451 <a name="targetjitinfo">The <tt>TargetJITInfo</tt> class</a>
454 <!-- *********************************************************************** -->
455 <div class="doc_section">
456 <a name="codegendesc">Machine code description classes</a>
458 <!-- *********************************************************************** -->
460 <div class="doc_text">
463 At the high-level, LLVM code is translated to a machine specific representation
464 formed out of <a href="#machinefunction">MachineFunction</a>,
465 <a href="#machinebasicblock">MachineBasicBlock</a>, and <a
466 href="#machineinstr"><tt>MachineInstr</tt></a> instances
467 (defined in include/llvm/CodeGen). This representation is completely target
468 agnostic, representing instructions in their most abstract form: an opcode and a
469 series of operands. This representation is designed to support both SSA
470 representation for machine code, as well as a register allocated, non-SSA form.
475 <!-- ======================================================================= -->
476 <div class="doc_subsection">
477 <a name="machineinstr">The <tt>MachineInstr</tt> class</a>
480 <div class="doc_text">
482 <p>Target machine instructions are represented as instances of the
483 <tt>MachineInstr</tt> class. This class is an extremely abstract way of
484 representing machine instructions. In particular, it only keeps track of
485 an opcode number and a set of operands.</p>
487 <p>The opcode number is a simple unsigned number that only has meaning to a
488 specific backend. All of the instructions for a target should be defined in
489 the <tt>*InstrInfo.td</tt> file for the target. The opcode enum values
490 are auto-generated from this description. The <tt>MachineInstr</tt> class does
491 not have any information about how to interpret the instruction (i.e., what the
492 semantics of the instruction are): for that you must refer to the
493 <tt><a href="#targetinstrinfo">TargetInstrInfo</a></tt> class.</p>
495 <p>The operands of a machine instruction can be of several different types:
496 they can be a register reference, constant integer, basic block reference, etc.
497 In addition, a machine operand should be marked as a def or a use of the value
498 (though only registers are allowed to be defs).</p>
500 <p>By convention, the LLVM code generator orders instruction operands so that
501 all register definitions come before the register uses, even on architectures
502 that are normally printed in other orders. For example, the SPARC add
503 instruction: "<tt>add %i1, %i2, %i3</tt>" adds the "%i1", and "%i2" registers
504 and stores the result into the "%i3" register. In the LLVM code generator,
505 the operands should be stored as "<tt>%i3, %i1, %i2</tt>": with the destination
508 <p>Keeping destination (definition) operands at the beginning of the operand
509 list has several advantages. In particular, the debugging printer will print
510 the instruction like this:</p>
516 <p>If the first operand is a def, and it is also easier to <a
517 href="#buildmi">create instructions</a> whose only def is the first
522 <!-- _______________________________________________________________________ -->
523 <div class="doc_subsubsection">
524 <a name="buildmi">Using the <tt>MachineInstrBuilder.h</tt> functions</a>
527 <div class="doc_text">
529 <p>Machine instructions are created by using the <tt>BuildMI</tt> functions,
530 located in the <tt>include/llvm/CodeGen/MachineInstrBuilder.h</tt> file. The
531 <tt>BuildMI</tt> functions make it easy to build arbitrary machine
532 instructions. Usage of the <tt>BuildMI</tt> functions look like this:
536 // Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42')
537 // instruction. The '1' specifies how many operands will be added.
538 MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42);
540 // Create the same instr, but insert it at the end of a basic block.
541 MachineBasicBlock &MBB = ...
542 BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42);
544 // Create the same instr, but insert it before a specified iterator point.
545 MachineBasicBlock::iterator MBBI = ...
546 BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42);
548 // Create a 'cmp Reg, 0' instruction, no destination reg.
549 MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0);
550 // Create an 'sahf' instruction which takes no operands and stores nothing.
551 MI = BuildMI(X86::SAHF, 0);
553 // Create a self looping branch instruction.
554 BuildMI(MBB, X86::JNE, 1).addMBB(&MBB);
558 The key thing to remember with the <tt>BuildMI</tt> functions is that you have
559 to specify the number of operands that the machine instruction will take. This
560 allows for efficient memory allocation. You also need to specify if operands
561 default to be uses of values, not definitions. If you need to add a definition
562 operand (other than the optional destination register), you must explicitly
568 <!-- _______________________________________________________________________ -->
569 <div class="doc_subsubsection">
570 <a name="fixedregs">Fixed (preassigned) registers</a>
573 <div class="doc_text">
575 <p>One important issue that the code generator needs to be aware of is the
576 presence of fixed registers. In particular, there are often places in the
577 instruction stream where the register allocator <em>must</em> arrange for a
578 particular value to be in a particular register. This can occur due to
579 limitations of the instruction set (e.g., the X86 can only do a 32-bit divide
580 with the <tt>EAX</tt>/<tt>EDX</tt> registers), or external factors like calling
581 conventions. In any case, the instruction selector should emit code that
582 copies a virtual register into or out of a physical register when needed.</p>
584 <p>For example, consider this simple LLVM example:</p>
587 int %test(int %X, int %Y) {
593 <p>The X86 instruction selector produces this machine code for the div
595 "<tt>llc X.bc -march=x86 -print-machineinstrs</tt>" to get this):</p>
599 %EAX = mov %reg1024 ;; Copy X (in reg1024) into EAX
600 %reg1027 = sar %reg1024, 31
601 %EDX = mov %reg1027 ;; Sign extend X into EDX
602 idiv %reg1025 ;; Divide by Y (in reg1025)
603 %reg1026 = mov %EAX ;; Read the result (Z) out of EAX
606 %EAX = mov %reg1026 ;; 32-bit return value goes in EAX
610 <p>By the end of code generation, the register allocator has coalesced
611 the registers and deleted the resultant identity moves, producing the
615 ;; X is in EAX, Y is in ECX
622 <p>This approach is extremely general (if it can handle the X86 architecture,
623 it can handle anything!) and allows all of the target specific
624 knowledge about the instruction stream to be isolated in the instruction
625 selector. Note that physical registers should have a short lifetime for good
626 code generation, and all physical registers are assumed dead on entry and
627 exit of basic blocks (before register allocation). Thus if you need a value
628 to be live across basic block boundaries, it <em>must</em> live in a virtual
633 <!-- _______________________________________________________________________ -->
634 <div class="doc_subsubsection">
635 <a name="ssa">Machine code SSA form</a>
638 <div class="doc_text">
640 <p><tt>MachineInstr</tt>'s are initially selected in SSA-form, and
641 are maintained in SSA-form until register allocation happens. For the most
642 part, this is trivially simple since LLVM is already in SSA form: LLVM PHI nodes
643 become machine code PHI nodes, and virtual registers are only allowed to have a
644 single definition.</p>
646 <p>After register allocation, machine code is no longer in SSA-form, as there
647 are no virtual registers left in the code.</p>
651 <!-- ======================================================================= -->
652 <div class="doc_subsection">
653 <a name="machinebasicblock">The <tt>MachineBasicBlock</tt> class</a>
656 <div class="doc_text">
658 <p>The <tt>MachineBasicBlock</tt> class contains a list of machine instructions
659 (<a href="#machineinstr">MachineInstr</a> instances). It roughly corresponds to
660 the LLVM code input to the instruction selector, but there can be a one-to-many
661 mapping (i.e. one LLVM basic block can map to multiple machine basic blocks).
662 The MachineBasicBlock class has a "<tt>getBasicBlock</tt>" method, which returns
663 the LLVM basic block that it comes from.
668 <!-- ======================================================================= -->
669 <div class="doc_subsection">
670 <a name="machinefunction">The <tt>MachineFunction</tt> class</a>
673 <div class="doc_text">
675 <p>The <tt>MachineFunction</tt> class contains a list of machine basic blocks
676 (<a href="#machinebasicblock">MachineBasicBlock</a> instances). It corresponds
677 one-to-one with the LLVM function input to the instruction selector. In
678 addition to a list of basic blocks, the <tt>MachineFunction</tt> contains a
679 the MachineConstantPool, MachineFrameInfo, MachineFunctionInfo,
680 SSARegMap, and a set of live in and live out registers for the function. See
681 <tt>MachineFunction.h</tt> for more information.
688 <!-- *********************************************************************** -->
689 <div class="doc_section">
690 <a name="codegenalgs">Target-independent code generation algorithms</a>
692 <!-- *********************************************************************** -->
694 <div class="doc_text">
696 <p>This section documents the phases described in the <a
697 href="#high-level-design">high-level design of the code generator</a>. It
698 explains how they work and some of the rationale behind their design.</p>
702 <!-- ======================================================================= -->
703 <div class="doc_subsection">
704 <a name="instselect">Instruction Selection</a>
707 <div class="doc_text">
709 Instruction Selection is the process of translating LLVM code presented to the
710 code generator into target-specific machine instructions. There are several
711 well-known ways to do this in the literature. In LLVM there are two main forms:
712 the SelectionDAG based instruction selector framework and an old-style 'simple'
713 instruction selector (which effectively peephole selects each LLVM instruction
714 into a series of machine instructions). We recommend that all targets use the
715 SelectionDAG infrastructure.
718 <p>Portions of the DAG instruction selector are generated from the target
719 description files (<tt>*.td</tt>) files. Eventually, we aim for the entire
720 instruction selector to be generated from these <tt>.td</tt> files.</p>
723 <!-- _______________________________________________________________________ -->
724 <div class="doc_subsubsection">
725 <a name="selectiondag_intro">Introduction to SelectionDAGs</a>
728 <div class="doc_text">
731 The SelectionDAG provides an abstraction for code representation in a way that
732 is amenable to instruction selection using automatic techniques
733 (e.g. dynamic-programming based optimal pattern matching selectors), It is also
734 well suited to other phases of code generation; in particular, instruction scheduling. Additionally, the SelectionDAG provides a host representation where a
735 large variety of very-low-level (but target-independent)
736 <a href="#selectiondag_optimize">optimizations</a> may be
737 performed: ones which require extensive information about the instructions
738 efficiently supported by the target.
742 The SelectionDAG is a Directed-Acyclic-Graph whose nodes are instances of the
743 <tt>SDNode</tt> class. The primary payload of the <tt>SDNode</tt> is its
744 operation code (Opcode) that indicates what operation the node performs.
745 The various operation node types are described at the top of the
746 <tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt> file. Depending on the
747 operation, nodes may contain additional information (e.g. the condition code
748 for a SETCC node) contained in a derived class.</p>
750 <p>Although most operations define a single value, each node in the graph may
751 define multiple values. For example, a combined div/rem operation will define
752 both the dividend and the remainder. Many other situations require multiple
753 values as well. Each node also has some number of operands, which are edges
754 to the node defining the used value. Because nodes may define multiple values,
755 edges are represented by instances of the <tt>SDOperand</tt> class, which is
756 a <SDNode, unsigned> pair, indicating the node and result
757 value being used, respectively. Each value produced by an SDNode has an
758 associated MVT::ValueType, indicating what type the value is.
762 SelectionDAGs contain two different kinds of values: those that represent data
763 flow and those that represent control flow dependencies. Data values are simple
764 edges with an integer or floating point value type. Control edges are
765 represented as "chain" edges which are of type MVT::Other. These edges provide
766 an ordering between nodes that have side effects (such as
767 loads/stores/calls/return/etc). All nodes that have side effects should take a
768 token chain as input and produce a new one as output. By convention, token
769 chain inputs are always operand #0, and chain results are always the last
770 value produced by an operation.</p>
773 A SelectionDAG has designated "Entry" and "Root" nodes. The Entry node is
774 always a marker node with an Opcode of ISD::EntryToken. The Root node is the
775 final side-effecting node in the token chain. For example, in a single basic
776 block function, this would be the return node.
780 One important concept for SelectionDAGs is the notion of a "legal" vs. "illegal"
781 DAG. A legal DAG for a target is one that only uses supported operations and
782 supported types. On PowerPC, for example, a DAG with any values of i1, i8, i16,
783 or i64 type would be illegal. The <a href="#selectiondag_legalize">legalize</a>
784 phase is responsible for turning an illegal DAG into a legal DAG.
788 <!-- _______________________________________________________________________ -->
789 <div class="doc_subsubsection">
790 <a name="selectiondag_process">SelectionDAG Instruction Selection Process</a>
793 <div class="doc_text">
796 SelectionDAG-based instruction selection consists of the following steps:
800 <li><a href="#selectiondag_build">Build initial DAG</a> - This stage performs
801 a simple translation from the input LLVM code to an illegal SelectionDAG.
803 <li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> - This stage
804 performs simple optimizations on the SelectionDAG to simplify it and
805 recognize meta instructions (like rotates and div/rem pairs) for
806 targets that support these meta operations. This makes the resultant code
807 more efficient and the 'select instructions from DAG' phase (below) simpler.
809 <li><a href="#selectiondag_legalize">Legalize SelectionDAG</a> - This stage
810 converts the illegal SelectionDAG to a legal SelectionDAG, by eliminating
811 unsupported operations and data types.</li>
812 <li><a href="#selectiondag_optimize">Optimize SelectionDAG (#2)</a> - This
813 second run of the SelectionDAG optimized the newly legalized DAG, to
814 eliminate inefficiencies introduced by legalization.</li>
815 <li><a href="#selectiondag_select">Select instructions from DAG</a> - Finally,
816 the target instruction selector matches the DAG operations to target
817 instructions. This process translates the target-independent input DAG into
818 another DAG of target instructions.</li>
819 <li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation</a>
820 - The last phase assigns a linear order to the instructions in the
821 target-instruction DAG and emits them into the MachineFunction being
822 compiled. This step uses traditional prepass scheduling techniques.</li>
825 <p>After all of these steps are complete, the SelectionDAG is destroyed and the
826 rest of the code generation passes are run.</p>
830 <!-- _______________________________________________________________________ -->
831 <div class="doc_subsubsection">
832 <a name="selectiondag_build">Initial SelectionDAG Construction</a>
835 <div class="doc_text">
838 The initial SelectionDAG is naively peephole expanded from the LLVM input by
839 the <tt>SelectionDAGLowering</tt> class in the SelectionDAGISel.cpp file. The
840 intent of this pass is to expose as much low-level, target-specific details
841 to the SelectionDAG as possible. This pass is mostly hard-coded (e.g. an LLVM
842 add turns into an SDNode add while a geteelementptr is expanded into the obvious
843 arithmetic). This pass requires target-specific hooks to lower calls and
844 returns, varargs, etc. For these features, the TargetLowering interface is
850 <!-- _______________________________________________________________________ -->
851 <div class="doc_subsubsection">
852 <a name="selectiondag_legalize">SelectionDAG Legalize Phase</a>
855 <div class="doc_text">
857 <p>The Legalize phase is in charge of converting a DAG to only use the types and
858 operations that are natively supported by the target. This involves two major
862 <li><p>Convert values of unsupported types to values of supported types.</p>
863 <p>There are two main ways of doing this: promoting a small type to a larger
864 type (e.g. f32 -> f64, or i16 -> i32), and breaking up large
866 to smaller ones (e.g. implementing i64 with i32 operations where
867 possible). Type conversions can insert sign and zero extensions as
868 needed to make sure that the final code has the same behavior as the
872 <li><p>Eliminate operations that are not supported by the target in a supported
874 <p>Targets often have wierd constraints, such as not supporting every
875 operation on every supported datatype (e.g. X86 does not support byte
876 conditional moves). Legalize takes care of either open-coding another
877 sequence of operations to emulate the operation (this is known as
878 expansion), promoting to a larger type that supports the operation
879 (promotion), or using a target-specific hook to implement the
885 Instead of using a Legalize pass, we could require that every target-specific
886 <a href="#selectiondag_optimize">selector</a> supports and expands every
887 operator and type even if they are not supported and may require many
888 instructions to implement (in fact, this is the approach taken by the
889 "simple" selectors). However, using a Legalize pass allows all of the
890 cannonicalization patterns to be shared across targets which makes it very
891 easy to optimize the cannonicalized code because it is still in the form of
897 <!-- _______________________________________________________________________ -->
898 <div class="doc_subsubsection">
899 <a name="selectiondag_optimize">SelectionDAG Optimization Phase: the DAG
903 <div class="doc_text">
906 The SelectionDAG optimization phase is run twice for code generation: once
907 immediately after the DAG is built and once after legalization. The first run
908 of the pass allows the initial code to be cleaned up (e.g. performing
909 optimizations that depend on knowing that the operators have restricted type
910 inputs). The second run of the pass cleans up the messy code generated by the
911 Legalize pass, allowing Legalize to be very simple since it can ignore many
916 One important class of optimizations performed is optimizing inserted sign and
917 zero extension instructions. We currently use ad-hoc techniques, but could move
918 to more rigorous techniques in the future. Here are some good
919 papers on the subject:</p>
922 "<a href="http://www.eecs.harvard.edu/~nr/pubs/widen-abstract.html">Widening
923 integer arithmetic</a>"<br>
924 Kevin Redwine and Norman Ramsey<br>
925 International Conference on Compiler Construction (CC) 2004
930 "<a href="http://portal.acm.org/citation.cfm?doid=512529.512552">Effective
931 sign extension elimination</a>"<br>
932 Motohiro Kawahito, Hideaki Komatsu, and Toshio Nakatani<br>
933 Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language Design
939 <!-- _______________________________________________________________________ -->
940 <div class="doc_subsubsection">
941 <a name="selectiondag_select">SelectionDAG Select Phase</a>
944 <div class="doc_text">
946 <p>The Select phase is the bulk of the target-specific code for instruction
947 selection. This phase takes a legal SelectionDAG as input, and does simple
948 pattern matching on the DAG to generate code. In time, the Select phase will
949 be automatically generated from the target's InstrInfo.td file, which is why we
950 want to make the Select phase as simple and mechanical as possible.</p>
954 <!-- _______________________________________________________________________ -->
955 <div class="doc_subsubsection">
956 <a name="selectiondag_sched">SelectionDAG Scheduling and Formation Phase</a>
959 <div class="doc_text">
961 <p>The scheduling phase takes the DAG of target instructions from the selection
962 phase and assigns an order. The scheduler can pick an order depending on
963 various constraints of the machines (i.e. order for minimal register pressure or
964 try to cover instruction latencies). Once an order is established, the DAG is
965 converted to a list of <a href="#machineinstr">MachineInstr</a>s and the
966 Selection DAG is destroyed.
971 <!-- _______________________________________________________________________ -->
972 <div class="doc_subsubsection">
973 <a name="selectiondag_future">Future directions for the SelectionDAG</a>
976 <div class="doc_text">
979 <li>Optional function-at-a-time selection.</li>
980 <li>Auto-generate entire selector from .td file.</li>
986 <!-- ======================================================================= -->
987 <div class="doc_subsection">
988 <a name="ssamco">SSA-based Machine Code Optimizations</a>
990 <div class="doc_text"><p>To Be Written</p></div>
991 <!-- ======================================================================= -->
992 <div class="doc_subsection">
993 <a name="regalloc">Register Allocation</a>
995 <div class="doc_text"><p>To Be Written</p></div>
996 <!-- ======================================================================= -->
997 <div class="doc_subsection">
998 <a name="proepicode">Prolog/Epilog Code Insertion</a>
1000 <div class="doc_text"><p>To Be Written</p></div>
1001 <!-- ======================================================================= -->
1002 <div class="doc_subsection">
1003 <a name="latemco">Late Machine Code Optimizations</a>
1005 <div class="doc_text"><p>To Be Written</p></div>
1006 <!-- ======================================================================= -->
1007 <div class="doc_subsection">
1008 <a name="codeemit">Code Emission</a>
1012 <!-- _______________________________________________________________________ -->
1013 <div class="doc_subsubsection">
1014 <a name="codeemit_asm">Generating Assembly Code</a>
1017 <div class="doc_text">
1022 <!-- _______________________________________________________________________ -->
1023 <div class="doc_subsubsection">
1024 <a name="codeemit_bin">Generating Binary Machine Code</a>
1027 <div class="doc_text">
1028 <p>For the JIT or .o file writer</p>
1032 <!-- *********************************************************************** -->
1033 <div class="doc_section">
1034 <a name="targetimpls">Target-specific Implementation Notes</a>
1036 <!-- *********************************************************************** -->
1038 <div class="doc_text">
1040 <p>This section of the document explains features or design decisions that
1041 are specific to the code generator for a particular target.</p>
1046 <!-- ======================================================================= -->
1047 <div class="doc_subsection">
1048 <a name="x86">The X86 backend</a>
1051 <div class="doc_text">
1054 The X86 code generator lives in the <tt>lib/Target/X86</tt> directory. This
1055 code generator currently targets a generic P6-like processor. As such, it
1056 produces a few P6-and-above instructions (like conditional moves), but it does
1057 not make use of newer features like MMX or SSE. In the future, the X86 backend
1058 will have sub-target support added for specific processor families and
1059 implementations.</p>
1063 <!-- _______________________________________________________________________ -->
1064 <div class="doc_subsubsection">
1065 <a name="x86_tt">X86 Target Triples Supported</a>
1068 <div class="doc_text">
1070 The following are the known target triples that are supported by the X86
1071 backend. This is not an exhaustive list, but it would be useful to add those
1076 <li><b>i686-pc-linux-gnu</b> - Linux</li>
1077 <li><b>i386-unknown-freebsd5.3</b> - FreeBSD 5.3</li>
1078 <li><b>i686-pc-cygwin</b> - Cygwin on Win32</li>
1079 <li><b>i686-pc-mingw32</b> - MingW on Win32</li>
1080 <li><b>i686-apple-darwin*</b> - Apple Darwin on X86</li>
1085 <!-- _______________________________________________________________________ -->
1086 <div class="doc_subsubsection">
1087 <a name="x86_memory">Representing X86 addressing modes in MachineInstrs</a>
1090 <div class="doc_text">
1092 <p>The x86 has a very flexible way of accessing memory. It is capable of
1093 forming memory addresses of the following expression directly in integer
1094 instructions (which use ModR/M addressing):</p>
1097 Base+[1,2,4,8]*IndexReg+Disp32
1100 <p>In order to represent this, LLVM tracks no less than 4 operands for each
1101 memory operand of this form. This means that the "load" form of 'mov' has the
1102 following <tt>MachineOperand</tt>s in this order:</p>
1106 Meaning: DestReg, | BaseReg, Scale, IndexReg, Displacement
1107 OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm
1110 <p>Stores, and all other instructions, treat the four memory operands in the
1111 same way, in the same order.</p>
1115 <!-- _______________________________________________________________________ -->
1116 <div class="doc_subsubsection">
1117 <a name="x86_names">Instruction naming</a>
1120 <div class="doc_text">
1123 An instruction name consists of the base name, a default operand size, and a
1124 a character per operand with an optional special size. For example:</p>
1127 <tt>ADD8rr</tt> -> add, 8-bit register, 8-bit register<br>
1128 <tt>IMUL16rmi</tt> -> imul, 16-bit register, 16-bit memory, 16-bit immediate<br>
1129 <tt>IMUL16rmi8</tt> -> imul, 16-bit register, 16-bit memory, 8-bit immediate<br>
1130 <tt>MOVSX32rm16</tt> -> movsx, 32-bit register, 16-bit memory
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