forked from llvm-mirror/llvm
-
Notifications
You must be signed in to change notification settings - Fork 0
/
CodeGenerator.html
2125 lines (1711 loc) · 89.4 KB
/
CodeGenerator.html
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8">
<title>The LLVM Target-Independent Code Generator</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<div class="doc_title">
The LLVM Target-Independent Code Generator
</div>
<ol>
<li><a href="#introduction">Introduction</a>
<ul>
<li><a href="#required">Required components in the code generator</a></li>
<li><a href="#high-level-design">The high-level design of the code
generator</a></li>
<li><a href="#tablegen">Using TableGen for target description</a></li>
</ul>
</li>
<li><a href="#targetdesc">Target description classes</a>
<ul>
<li><a href="#targetmachine">The <tt>TargetMachine</tt> class</a></li>
<li><a href="#targetdata">The <tt>TargetData</tt> class</a></li>
<li><a href="#targetlowering">The <tt>TargetLowering</tt> class</a></li>
<li><a href="#targetregisterinfo">The <tt>TargetRegisterInfo</tt> class</a></li>
<li><a href="#targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a></li>
<li><a href="#targetframeinfo">The <tt>TargetFrameInfo</tt> class</a></li>
<li><a href="#targetsubtarget">The <tt>TargetSubtarget</tt> class</a></li>
<li><a href="#targetjitinfo">The <tt>TargetJITInfo</tt> class</a></li>
</ul>
</li>
<li><a href="#codegendesc">Machine code description classes</a>
<ul>
<li><a href="#machineinstr">The <tt>MachineInstr</tt> class</a></li>
<li><a href="#machinebasicblock">The <tt>MachineBasicBlock</tt>
class</a></li>
<li><a href="#machinefunction">The <tt>MachineFunction</tt> class</a></li>
</ul>
</li>
<li><a href="#codegenalgs">Target-independent code generation algorithms</a>
<ul>
<li><a href="#instselect">Instruction Selection</a>
<ul>
<li><a href="#selectiondag_intro">Introduction to SelectionDAGs</a></li>
<li><a href="#selectiondag_process">SelectionDAG Code Generation
Process</a></li>
<li><a href="#selectiondag_build">Initial SelectionDAG
Construction</a></li>
<li><a href="#selectiondag_legalize_types">SelectionDAG LegalizeTypes Phase</a></li>
<li><a href="#selectiondag_legalize">SelectionDAG Legalize Phase</a></li>
<li><a href="#selectiondag_optimize">SelectionDAG Optimization
Phase: the DAG Combiner</a></li>
<li><a href="#selectiondag_select">SelectionDAG Select Phase</a></li>
<li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation
Phase</a></li>
<li><a href="#selectiondag_future">Future directions for the
SelectionDAG</a></li>
</ul></li>
<li><a href="#liveintervals">Live Intervals</a>
<ul>
<li><a href="#livevariable_analysis">Live Variable Analysis</a></li>
<li><a href="#liveintervals_analysis">Live Intervals Analysis</a></li>
</ul></li>
<li><a href="#regalloc">Register Allocation</a>
<ul>
<li><a href="#regAlloc_represent">How registers are represented in
LLVM</a></li>
<li><a href="#regAlloc_howTo">Mapping virtual registers to physical
registers</a></li>
<li><a href="#regAlloc_twoAddr">Handling two address instructions</a></li>
<li><a href="#regAlloc_ssaDecon">The SSA deconstruction phase</a></li>
<li><a href="#regAlloc_fold">Instruction folding</a></li>
<li><a href="#regAlloc_builtIn">Built in register allocators</a></li>
</ul></li>
<li><a href="#codeemit">Code Emission</a>
<ul>
<li><a href="#codeemit_asm">Generating Assembly Code</a></li>
<li><a href="#codeemit_bin">Generating Binary Machine Code</a></li>
</ul></li>
</ul>
</li>
<li><a href="#targetimpls">Target-specific Implementation Notes</a>
<ul>
<li><a href="#tailcallopt">Tail call optimization</a></li>
<li><a href="#x86">The X86 backend</a></li>
<li><a href="#ppc">The PowerPC backend</a>
<ul>
<li><a href="#ppc_abi">LLVM PowerPC ABI</a></li>
<li><a href="#ppc_frame">Frame Layout</a></li>
<li><a href="#ppc_prolog">Prolog/Epilog</a></li>
<li><a href="#ppc_dynamic">Dynamic Allocation</a></li>
</ul></li>
</ul></li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:[email protected]">Chris Lattner</a>,
<a href="mailto:[email protected]">Bill Wendling</a>,
<a href="mailto:[email protected]">Fernando Magno Quintao
Pereira</a> and
<a href="mailto:[email protected]">Jim Laskey</a></p>
</div>
<div class="doc_warning">
<p>Warning: This is a work in progress.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="introduction">Introduction</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>The LLVM target-independent code generator is a framework that provides a
suite of reusable components for translating the LLVM internal representation
to the machine code for a specified target—either in assembly form
(suitable for a static compiler) or in binary machine code format (usable for
a JIT compiler). The LLVM target-independent code generator consists of five
main components:</p>
<ol>
<li><a href="#targetdesc">Abstract target description</a> interfaces which
capture important properties about various aspects of the machine,
independently of how they will be used. These interfaces are defined in
<tt>include/llvm/Target/</tt>.</li>
<li>Classes used to represent the <a href="#codegendesc">machine code</a>
being generated for a target. These classes are intended to be abstract
enough to represent the machine code for <i>any</i> target machine. These
classes are defined in <tt>include/llvm/CodeGen/</tt>.</li>
<li><a href="#codegenalgs">Target-independent algorithms</a> used to implement
various phases of native code generation (register allocation, scheduling,
stack frame representation, etc). This code lives
in <tt>lib/CodeGen/</tt>.</li>
<li><a href="#targetimpls">Implementations of the abstract target description
interfaces</a> for particular targets. These machine descriptions make
use of the components provided by LLVM, and can optionally provide custom
target-specific passes, to build complete code generators for a specific
target. Target descriptions live in <tt>lib/Target/</tt>.</li>
<li><a href="#jit">The target-independent JIT components</a>. The LLVM JIT is
completely target independent (it uses the <tt>TargetJITInfo</tt>
structure to interface for target-specific issues. The code for the
target-independent JIT lives in <tt>lib/ExecutionEngine/JIT</tt>.</li>
</ol>
<p>Depending on which part of the code generator you are interested in working
on, different pieces of this will be useful to you. In any case, you should
be familiar with the <a href="#targetdesc">target description</a>
and <a href="#codegendesc">machine code representation</a> classes. If you
want to add a backend for a new target, you will need
to <a href="#targetimpls">implement the target description</a> classes for
your new target and understand the <a href="LangRef.html">LLVM code
representation</a>. If you are interested in implementing a
new <a href="#codegenalgs">code generation algorithm</a>, it should only
depend on the target-description and machine code representation classes,
ensuring that it is portable.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="required">Required components in the code generator</a>
</div>
<div class="doc_text">
<p>The two pieces of the LLVM code generator are the high-level interface to the
code generator and the set of reusable components that can be used to build
target-specific backends. The two most important interfaces
(<a href="#targetmachine"><tt>TargetMachine</tt></a>
and <a href="#targetdata"><tt>TargetData</tt></a>) are the only ones that are
required to be defined for a backend to fit into the LLVM system, but the
others must be defined if the reusable code generator components are going to
be used.</p>
<p>This design has two important implications. The first is that LLVM can
support completely non-traditional code generation targets. For example, the
C backend does not require register allocation, instruction selection, or any
of the other standard components provided by the system. As such, it only
implements these two interfaces, and does its own thing. Another example of
a code generator like this is a (purely hypothetical) backend that converts
LLVM to the GCC RTL form and uses GCC to emit machine code for a target.</p>
<p>This design also implies that it is possible to design and implement
radically different code generators in the LLVM system that do not make use
of any of the built-in components. Doing so is not recommended at all, but
could be required for radically different targets that do not fit into the
LLVM machine description model: FPGAs for example.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="high-level-design">The high-level design of the code generator</a>
</div>
<div class="doc_text">
<p>The LLVM target-independent code generator is designed to support efficient
and quality code generation for standard register-based microprocessors.
Code generation in this model is divided into the following stages:</p>
<ol>
<li><b><a href="#instselect">Instruction Selection</a></b> — This phase
determines an efficient way to express the input LLVM code in the target
instruction set. This stage produces the initial code for the program in
the target instruction set, then makes use of virtual registers in SSA
form and physical registers that represent any required register
assignments due to target constraints or calling conventions. This step
turns the LLVM code into a DAG of target instructions.</li>
<li><b><a href="#selectiondag_sched">Scheduling and Formation</a></b> —
This phase takes the DAG of target instructions produced by the
instruction selection phase, determines an ordering of the instructions,
then emits the instructions
as <tt><a href="#machineinstr">MachineInstr</a></tt>s with that ordering.
Note that we describe this in the <a href="#instselect">instruction
selection section</a> because it operates on
a <a href="#selectiondag_intro">SelectionDAG</a>.</li>
<li><b><a href="#ssamco">SSA-based Machine Code Optimizations</a></b> —
This optional stage consists of a series of machine-code optimizations
that operate on the SSA-form produced by the instruction selector.
Optimizations like modulo-scheduling or peephole optimization work
here.</li>
<li><b><a href="#regalloc">Register Allocation</a></b> — The target code
is transformed from an infinite virtual register file in SSA form to the
concrete register file used by the target. This phase introduces spill
code and eliminates all virtual register references from the program.</li>
<li><b><a href="#proepicode">Prolog/Epilog Code Insertion</a></b> — Once
the machine code has been generated for the function and the amount of
stack space required is known (used for LLVM alloca's and spill slots),
the prolog and epilog code for the function can be inserted and "abstract
stack location references" can be eliminated. This stage is responsible
for implementing optimizations like frame-pointer elimination and stack
packing.</li>
<li><b><a href="#latemco">Late Machine Code Optimizations</a></b> —
Optimizations that operate on "final" machine code can go here, such as
spill code scheduling and peephole optimizations.</li>
<li><b><a href="#codeemit">Code Emission</a></b> — The final stage
actually puts out the code for the current function, either in the target
assembler format or in machine code.</li>
</ol>
<p>The code generator is based on the assumption that the instruction selector
will use an optimal pattern matching selector to create high-quality
sequences of native instructions. Alternative code generator designs based
on pattern expansion and aggressive iterative peephole optimization are much
slower. This design permits efficient compilation (important for JIT
environments) and aggressive optimization (used when generating code offline)
by allowing components of varying levels of sophistication to be used for any
step of compilation.</p>
<p>In addition to these stages, target implementations can insert arbitrary
target-specific passes into the flow. For example, the X86 target uses a
special pass to handle the 80x87 floating point stack architecture. Other
targets with unusual requirements can be supported with custom passes as
needed.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="tablegen">Using TableGen for target description</a>
</div>
<div class="doc_text">
<p>The target description classes require a detailed description of the target
architecture. These target descriptions often have a large amount of common
information (e.g., an <tt>add</tt> instruction is almost identical to a
<tt>sub</tt> instruction). In order to allow the maximum amount of
commonality to be factored out, the LLVM code generator uses
the <a href="TableGenFundamentals.html">TableGen</a> tool to describe big
chunks of the target machine, which allows the use of domain-specific and
target-specific abstractions to reduce the amount of repetition.</p>
<p>As LLVM continues to be developed and refined, we plan to move more and more
of the target description to the <tt>.td</tt> form. Doing so gives us a
number of advantages. The most important is that it makes it easier to port
LLVM because it reduces the amount of C++ code that has to be written, and
the surface area of the code generator that needs to be understood before
someone can get something working. Second, it makes it easier to change
things. In particular, if tables and other things are all emitted
by <tt>tblgen</tt>, we only need a change in one place (<tt>tblgen</tt>) to
update all of the targets to a new interface.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="targetdesc">Target description classes</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>The LLVM target description classes (located in the
<tt>include/llvm/Target</tt> directory) provide an abstract description of
the target machine independent of any particular client. These classes are
designed to capture the <i>abstract</i> properties of the target (such as the
instructions and registers it has), and do not incorporate any particular
pieces of code generation algorithms.</p>
<p>All of the target description classes (except the
<tt><a href="#targetdata">TargetData</a></tt> class) are designed to be
subclassed by the concrete target implementation, and have virtual methods
implemented. To get to these implementations, the
<tt><a href="#targetmachine">TargetMachine</a></tt> class provides accessors
that should be implemented by the target.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="targetmachine">The <tt>TargetMachine</tt> class</a>
</div>
<div class="doc_text">
<p>The <tt>TargetMachine</tt> class provides virtual methods that are used to
access the target-specific implementations of the various target description
classes via the <tt>get*Info</tt> methods (<tt>getInstrInfo</tt>,
<tt>getRegisterInfo</tt>, <tt>getFrameInfo</tt>, etc.). This class is
designed to be specialized by a concrete target implementation
(e.g., <tt>X86TargetMachine</tt>) which implements the various virtual
methods. The only required target description class is
the <a href="#targetdata"><tt>TargetData</tt></a> class, but if the code
generator components are to be used, the other interfaces should be
implemented as well.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="targetdata">The <tt>TargetData</tt> class</a>
</div>
<div class="doc_text">
<p>The <tt>TargetData</tt> class is the only required target description class,
and it is the only class that is not extensible (you cannot derived a new
class from it). <tt>TargetData</tt> specifies information about how the
target lays out memory for structures, the alignment requirements for various
data types, the size of pointers in the target, and whether the target is
little-endian or big-endian.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="targetlowering">The <tt>TargetLowering</tt> class</a>
</div>
<div class="doc_text">
<p>The <tt>TargetLowering</tt> class is used by SelectionDAG based instruction
selectors primarily to describe how LLVM code should be lowered to
SelectionDAG operations. Among other things, this class indicates:</p>
<ul>
<li>an initial register class to use for various <tt>ValueType</tt>s,</li>
<li>which operations are natively supported by the target machine,</li>
<li>the return type of <tt>setcc</tt> operations,</li>
<li>the type to use for shift amounts, and</li>
<li>various high-level characteristics, like whether it is profitable to turn
division by a constant into a multiplication sequence</li>
</ul>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="targetregisterinfo">The <tt>TargetRegisterInfo</tt> class</a>
</div>
<div class="doc_text">
<p>The <tt>TargetRegisterInfo</tt> class is used to describe the register file
of the target and any interactions between the registers.</p>
<p>Registers in the code generator are represented in the code generator by
unsigned integers. Physical registers (those that actually exist in the
target description) are unique small numbers, and virtual registers are
generally large. Note that register #0 is reserved as a flag value.</p>
<p>Each register in the processor description has an associated
<tt>TargetRegisterDesc</tt> entry, which provides a textual name for the
register (used for assembly output and debugging dumps) and a set of aliases
(used to indicate whether one register overlaps with another).</p>
<p>In addition to the per-register description, the <tt>TargetRegisterInfo</tt>
class exposes a set of processor specific register classes (instances of the
<tt>TargetRegisterClass</tt> class). Each register class contains sets of
registers that have the same properties (for example, they are all 32-bit
integer registers). Each SSA virtual register created by the instruction
selector has an associated register class. When the register allocator runs,
it replaces virtual registers with a physical register in the set.</p>
<p>The target-specific implementations of these classes is auto-generated from
a <a href="TableGenFundamentals.html">TableGen</a> description of the
register file.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a>
</div>
<div class="doc_text">
<p>The <tt>TargetInstrInfo</tt> class is used to describe the machine
instructions supported by the target. It is essentially an array of
<tt>TargetInstrDescriptor</tt> objects, each of which describes one
instruction the target supports. Descriptors define things like the mnemonic
for the opcode, the number of operands, the list of implicit register uses
and defs, whether the instruction has certain target-independent properties
(accesses memory, is commutable, etc), and holds any target-specific
flags.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="targetframeinfo">The <tt>TargetFrameInfo</tt> class</a>
</div>
<div class="doc_text">
<p>The <tt>TargetFrameInfo</tt> class is used to provide information about the
stack frame layout of the target. It holds the direction of stack growth, the
known stack alignment on entry to each function, and the offset to the local
area. The offset to the local area is the offset from the stack pointer on
function entry to the first location where function data (local variables,
spill locations) can be stored.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="targetsubtarget">The <tt>TargetSubtarget</tt> class</a>
</div>
<div class="doc_text">
<p>The <tt>TargetSubtarget</tt> class is used to provide information about the
specific chip set being targeted. A sub-target informs code generation of
which instructions are supported, instruction latencies and instruction
execution itinerary; i.e., which processing units are used, in what order,
and for how long.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="targetjitinfo">The <tt>TargetJITInfo</tt> class</a>
</div>
<div class="doc_text">
<p>The <tt>TargetJITInfo</tt> class exposes an abstract interface used by the
Just-In-Time code generator to perform target-specific activities, such as
emitting stubs. If a <tt>TargetMachine</tt> supports JIT code generation, it
should provide one of these objects through the <tt>getJITInfo</tt>
method.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="codegendesc">Machine code description classes</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>At the high-level, LLVM code is translated to a machine specific
representation formed out of
<a href="#machinefunction"><tt>MachineFunction</tt></a>,
<a href="#machinebasicblock"><tt>MachineBasicBlock</tt></a>,
and <a href="#machineinstr"><tt>MachineInstr</tt></a> instances (defined
in <tt>include/llvm/CodeGen</tt>). This representation is completely target
agnostic, representing instructions in their most abstract form: an opcode
and a series of operands. This representation is designed to support both an
SSA representation for machine code, as well as a register allocated, non-SSA
form.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="machineinstr">The <tt>MachineInstr</tt> class</a>
</div>
<div class="doc_text">
<p>Target machine instructions are represented as instances of the
<tt>MachineInstr</tt> class. This class is an extremely abstract way of
representing machine instructions. In particular, it only keeps track of an
opcode number and a set of operands.</p>
<p>The opcode number is a simple unsigned integer that only has meaning to a
specific backend. All of the instructions for a target should be defined in
the <tt>*InstrInfo.td</tt> file for the target. The opcode enum values are
auto-generated from this description. The <tt>MachineInstr</tt> class does
not have any information about how to interpret the instruction (i.e., what
the semantics of the instruction are); for that you must refer to the
<tt><a href="#targetinstrinfo">TargetInstrInfo</a></tt> class.</p>
<p>The operands of a machine instruction can be of several different types: a
register reference, a constant integer, a basic block reference, etc. In
addition, a machine operand should be marked as a def or a use of the value
(though only registers are allowed to be defs).</p>
<p>By convention, the LLVM code generator orders instruction operands so that
all register definitions come before the register uses, even on architectures
that are normally printed in other orders. For example, the SPARC add
instruction: "<tt>add %i1, %i2, %i3</tt>" adds the "%i1", and "%i2" registers
and stores the result into the "%i3" register. In the LLVM code generator,
the operands should be stored as "<tt>%i3, %i1, %i2</tt>": with the
destination first.</p>
<p>Keeping destination (definition) operands at the beginning of the operand
list has several advantages. In particular, the debugging printer will print
the instruction like this:</p>
<div class="doc_code">
<pre>
%r3 = add %i1, %i2
</pre>
</div>
<p>Also if the first operand is a def, it is easier to <a href="#buildmi">create
instructions</a> whose only def is the first operand.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="buildmi">Using the <tt>MachineInstrBuilder.h</tt> functions</a>
</div>
<div class="doc_text">
<p>Machine instructions are created by using the <tt>BuildMI</tt> functions,
located in the <tt>include/llvm/CodeGen/MachineInstrBuilder.h</tt> file. The
<tt>BuildMI</tt> functions make it easy to build arbitrary machine
instructions. Usage of the <tt>BuildMI</tt> functions look like this:</p>
<div class="doc_code">
<pre>
// Create a 'DestReg = mov 42' (rendered in X86 assembly as 'mov DestReg, 42')
// instruction. The '1' specifies how many operands will be added.
MachineInstr *MI = BuildMI(X86::MOV32ri, 1, DestReg).addImm(42);
// Create the same instr, but insert it at the end of a basic block.
MachineBasicBlock &MBB = ...
BuildMI(MBB, X86::MOV32ri, 1, DestReg).addImm(42);
// Create the same instr, but insert it before a specified iterator point.
MachineBasicBlock::iterator MBBI = ...
BuildMI(MBB, MBBI, X86::MOV32ri, 1, DestReg).addImm(42);
// Create a 'cmp Reg, 0' instruction, no destination reg.
MI = BuildMI(X86::CMP32ri, 2).addReg(Reg).addImm(0);
// Create an 'sahf' instruction which takes no operands and stores nothing.
MI = BuildMI(X86::SAHF, 0);
// Create a self looping branch instruction.
BuildMI(MBB, X86::JNE, 1).addMBB(&MBB);
</pre>
</div>
<p>The key thing to remember with the <tt>BuildMI</tt> functions is that you
have to specify the number of operands that the machine instruction will
take. This allows for efficient memory allocation. You also need to specify
if operands default to be uses of values, not definitions. If you need to
add a definition operand (other than the optional destination register), you
must explicitly mark it as such:</p>
<div class="doc_code">
<pre>
MI.addReg(Reg, RegState::Define);
</pre>
</div>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="fixedregs">Fixed (preassigned) registers</a>
</div>
<div class="doc_text">
<p>One important issue that the code generator needs to be aware of is the
presence of fixed registers. In particular, there are often places in the
instruction stream where the register allocator <em>must</em> arrange for a
particular value to be in a particular register. This can occur due to
limitations of the instruction set (e.g., the X86 can only do a 32-bit divide
with the <tt>EAX</tt>/<tt>EDX</tt> registers), or external factors like
calling conventions. In any case, the instruction selector should emit code
that copies a virtual register into or out of a physical register when
needed.</p>
<p>For example, consider this simple LLVM example:</p>
<div class="doc_code">
<pre>
define i32 @test(i32 %X, i32 %Y) {
%Z = udiv i32 %X, %Y
ret i32 %Z
}
</pre>
</div>
<p>The X86 instruction selector produces this machine code for the <tt>div</tt>
and <tt>ret</tt> (use "<tt>llc X.bc -march=x86 -print-machineinstrs</tt>" to
get this):</p>
<div class="doc_code">
<pre>
;; Start of div
%EAX = mov %reg1024 ;; Copy X (in reg1024) into EAX
%reg1027 = sar %reg1024, 31
%EDX = mov %reg1027 ;; Sign extend X into EDX
idiv %reg1025 ;; Divide by Y (in reg1025)
%reg1026 = mov %EAX ;; Read the result (Z) out of EAX
;; Start of ret
%EAX = mov %reg1026 ;; 32-bit return value goes in EAX
ret
</pre>
</div>
<p>By the end of code generation, the register allocator has coalesced the
registers and deleted the resultant identity moves producing the following
code:</p>
<div class="doc_code">
<pre>
;; X is in EAX, Y is in ECX
mov %EAX, %EDX
sar %EDX, 31
idiv %ECX
ret
</pre>
</div>
<p>This approach is extremely general (if it can handle the X86 architecture, it
can handle anything!) and allows all of the target specific knowledge about
the instruction stream to be isolated in the instruction selector. Note that
physical registers should have a short lifetime for good code generation, and
all physical registers are assumed dead on entry to and exit from basic
blocks (before register allocation). Thus, if you need a value to be live
across basic block boundaries, it <em>must</em> live in a virtual
register.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="ssa">Machine code in SSA form</a>
</div>
<div class="doc_text">
<p><tt>MachineInstr</tt>'s are initially selected in SSA-form, and are
maintained in SSA-form until register allocation happens. For the most part,
this is trivially simple since LLVM is already in SSA form; LLVM PHI nodes
become machine code PHI nodes, and virtual registers are only allowed to have
a single definition.</p>
<p>After register allocation, machine code is no longer in SSA-form because
there are no virtual registers left in the code.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="machinebasicblock">The <tt>MachineBasicBlock</tt> class</a>
</div>
<div class="doc_text">
<p>The <tt>MachineBasicBlock</tt> class contains a list of machine instructions
(<tt><a href="#machineinstr">MachineInstr</a></tt> instances). It roughly
corresponds to the LLVM code input to the instruction selector, but there can
be a one-to-many mapping (i.e. one LLVM basic block can map to multiple
machine basic blocks). The <tt>MachineBasicBlock</tt> class has a
"<tt>getBasicBlock</tt>" method, which returns the LLVM basic block that it
comes from.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="machinefunction">The <tt>MachineFunction</tt> class</a>
</div>
<div class="doc_text">
<p>The <tt>MachineFunction</tt> class contains a list of machine basic blocks
(<tt><a href="#machinebasicblock">MachineBasicBlock</a></tt> instances). It
corresponds one-to-one with the LLVM function input to the instruction
selector. In addition to a list of basic blocks,
the <tt>MachineFunction</tt> contains a a <tt>MachineConstantPool</tt>,
a <tt>MachineFrameInfo</tt>, a <tt>MachineFunctionInfo</tt>, and a
<tt>MachineRegisterInfo</tt>. See
<tt>include/llvm/CodeGen/MachineFunction.h</tt> for more information.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="codegenalgs">Target-independent code generation algorithms</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section documents the phases described in the
<a href="#high-level-design">high-level design of the code generator</a>.
It explains how they work and some of the rationale behind their design.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="instselect">Instruction Selection</a>
</div>
<div class="doc_text">
<p>Instruction Selection is the process of translating LLVM code presented to
the code generator into target-specific machine instructions. There are
several well-known ways to do this in the literature. LLVM uses a
SelectionDAG based instruction selector.</p>
<p>Portions of the DAG instruction selector are generated from the target
description (<tt>*.td</tt>) files. Our goal is for the entire instruction
selector to be generated from these <tt>.td</tt> files, though currently
there are still things that require custom C++ code.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="selectiondag_intro">Introduction to SelectionDAGs</a>
</div>
<div class="doc_text">
<p>The SelectionDAG provides an abstraction for code representation in a way
that is amenable to instruction selection using automatic techniques
(e.g. dynamic-programming based optimal pattern matching selectors). It is
also well-suited to other phases of code generation; in particular,
instruction scheduling (SelectionDAG's are very close to scheduling DAGs
post-selection). Additionally, the SelectionDAG provides a host
representation where a large variety of very-low-level (but
target-independent) <a href="#selectiondag_optimize">optimizations</a> may be
performed; ones which require extensive information about the instructions
efficiently supported by the target.</p>
<p>The SelectionDAG is a Directed-Acyclic-Graph whose nodes are instances of the
<tt>SDNode</tt> class. The primary payload of the <tt>SDNode</tt> is its
operation code (Opcode) that indicates what operation the node performs and
the operands to the operation. The various operation node types are
described at the top of the <tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt>
file.</p>
<p>Although most operations define a single value, each node in the graph may
define multiple values. For example, a combined div/rem operation will
define both the dividend and the remainder. Many other situations require
multiple values as well. Each node also has some number of operands, which
are edges to the node defining the used value. Because nodes may define
multiple values, edges are represented by instances of the <tt>SDValue</tt>
class, which is a <tt><SDNode, unsigned></tt> pair, indicating the node
and result value being used, respectively. Each value produced by
an <tt>SDNode</tt> has an associated <tt>MVT</tt> (Machine Value Type)
indicating what the type of the value is.</p>
<p>SelectionDAGs contain two different kinds of values: those that represent
data flow and those that represent control flow dependencies. Data values
are simple edges with an integer or floating point value type. Control edges
are represented as "chain" edges which are of type <tt>MVT::Other</tt>.
These edges provide an ordering between nodes that have side effects (such as
loads, stores, calls, returns, etc). All nodes that have side effects should
take a token chain as input and produce a new one as output. By convention,
token chain inputs are always operand #0, and chain results are always the
last value produced by an operation.</p>
<p>A SelectionDAG has designated "Entry" and "Root" nodes. The Entry node is
always a marker node with an Opcode of <tt>ISD::EntryToken</tt>. The Root
node is the final side-effecting node in the token chain. For example, in a
single basic block function it would be the return node.</p>
<p>One important concept for SelectionDAGs is the notion of a "legal" vs.
"illegal" DAG. A legal DAG for a target is one that only uses supported
operations and supported types. On a 32-bit PowerPC, for example, a DAG with
a value of type i1, i8, i16, or i64 would be illegal, as would a DAG that
uses a SREM or UREM operation. The
<a href="#selectinodag_legalize_types">legalize types</a> and
<a href="#selectiondag_legalize">legalize operations</a> phases are
responsible for turning an illegal DAG into a legal DAG.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="selectiondag_process">SelectionDAG Instruction Selection Process</a>
</div>
<div class="doc_text">
<p>SelectionDAG-based instruction selection consists of the following steps:</p>
<ol>
<li><a href="#selectiondag_build">Build initial DAG</a> — This stage
performs a simple translation from the input LLVM code to an illegal
SelectionDAG.</li>
<li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> — This
stage performs simple optimizations on the SelectionDAG to simplify it,
and recognize meta instructions (like rotates
and <tt>div</tt>/<tt>rem</tt> pairs) for targets that support these meta
operations. This makes the resultant code more efficient and
the <a href="#selectiondag_select">select instructions from DAG</a> phase
(below) simpler.</li>
<li><a href="#selectiondag_legalize_types">Legalize SelectionDAG Types</a>
— This stage transforms SelectionDAG nodes to eliminate any types
that are unsupported on the target.</li>
<li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> — The
SelectionDAG optimizer is run to clean up redundancies exposed by type
legalization.</li>
<li><a href="#selectiondag_legalize">Legalize SelectionDAG Types</a> —
This stage transforms SelectionDAG nodes to eliminate any types that are
unsupported on the target.</li>
<li><a href="#selectiondag_optimize">Optimize SelectionDAG</a> — The
SelectionDAG optimizer is run to eliminate inefficiencies introduced by
operation legalization.</li>
<li><a href="#selectiondag_select">Select instructions from DAG</a> —
Finally, the target instruction selector matches the DAG operations to
target instructions. This process translates the target-independent input
DAG into another DAG of target instructions.</li>
<li><a href="#selectiondag_sched">SelectionDAG Scheduling and Formation</a>
— The last phase assigns a linear order to the instructions in the
target-instruction DAG and emits them into the MachineFunction being
compiled. This step uses traditional prepass scheduling techniques.</li>
</ol>
<p>After all of these steps are complete, the SelectionDAG is destroyed and the
rest of the code generation passes are run.</p>
<p>One great way to visualize what is going on here is to take advantage of a
few LLC command line options. The following options pop up a window
displaying the SelectionDAG at specific times (if you only get errors printed
to the console while using this, you probably
<a href="ProgrammersManual.html#ViewGraph">need to configure your system</a>
to add support for it).</p>
<ul>
<li><tt>-view-dag-combine1-dags</tt> displays the DAG after being built,
before the first optimization pass.</li>
<li><tt>-view-legalize-dags</tt> displays the DAG before Legalization.</li>
<li><tt>-view-dag-combine2-dags</tt> displays the DAG before the second
optimization pass.</li>
<li><tt>-view-isel-dags</tt> displays the DAG before the Select phase.</li>
<li><tt>-view-sched-dags</tt> displays the DAG before Scheduling.</li>
</ul>
<p>The <tt>-view-sunit-dags</tt> displays the Scheduler's dependency graph.
This graph is based on the final SelectionDAG, with nodes that must be
scheduled together bundled into a single scheduling-unit node, and with
immediate operands and other nodes that aren't relevant for scheduling
omitted.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="selectiondag_build">Initial SelectionDAG Construction</a>
</div>
<div class="doc_text">
<p>The initial SelectionDAG is naïvely peephole expanded from the LLVM
input by the <tt>SelectionDAGLowering</tt> class in the
<tt>lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp</tt> file. The intent of
this pass is to expose as much low-level, target-specific details to the
SelectionDAG as possible. This pass is mostly hard-coded (e.g. an
LLVM <tt>add</tt> turns into an <tt>SDNode add</tt> while a
<tt>getelementptr</tt> is expanded into the obvious arithmetic). This pass
requires target-specific hooks to lower calls, returns, varargs, etc. For
these features, the <tt><a href="#targetlowering">TargetLowering</a></tt>
interface is used.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="selectiondag_legalize_types">SelectionDAG LegalizeTypes Phase</a>
</div>
<div class="doc_text">
<p>The Legalize phase is in charge of converting a DAG to only use the types
that are natively supported by the target.</p>
<p>There are two main ways of converting values of unsupported scalar types to
values of supported types: converting small types to larger types
("promoting"), and breaking up large integer types into smaller ones
("expanding"). For example, a target might require that all f32 values are
promoted to f64 and that all i1/i8/i16 values are promoted to i32. The same
target might require that all i64 values be expanded into pairs of i32
values. These changes can insert sign and zero extensions as needed to make
sure that the final code has the same behavior as the input.</p>
<p>There are two main ways of converting values of unsupported vector types to
value of supported types: splitting vector types, multiple times if
necessary, until a legal type is found, and extending vector types by adding
elements to the end to round them out to legal types ("widening"). If a
vector gets split all the way down to single-element parts with no supported
vector type being found, the elements are converted to scalars
("scalarizing").</p>
<p>A target implementation tells the legalizer which types are supported (and
which register class to use for them) by calling the
<tt>addRegisterClass</tt> method in its TargetLowering constructor.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="selectiondag_legalize">SelectionDAG Legalize Phase</a>
</div>
<div class="doc_text">
<p>The Legalize phase is in charge of converting a DAG to only use the
operations that are natively supported by the target.</p>
<p>Targets often have weird constraints, such as not supporting every operation
on every supported datatype (e.g. X86 does not support byte conditional moves
and PowerPC does not support sign-extending loads from a 16-bit memory
location). Legalize takes care of this by open-coding another sequence of
operations to emulate the operation ("expansion"), by promoting one type to a
larger type that supports the operation ("promotion"), or by using a
target-specific hook to implement the legalization ("custom").</p>
<p>A target implementation tells the legalizer which operations are not
supported (and which of the above three actions to take) by calling the
<tt>setOperationAction</tt> method in its <tt>TargetLowering</tt>
constructor.</p>
<p>Prior to the existence of the Legalize passes, we required that every target
<a href="#selectiondag_optimize">selector</a> supported and handled every
operator and type even if they are not natively supported. The introduction
of the Legalize phases allows all of the canonicalization patterns to be
shared across targets, and makes it very easy to optimize the canonicalized
code because it is still in the form of a DAG.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="selectiondag_optimize">SelectionDAG Optimization Phase: the DAG
Combiner</a>
</div>