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
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
|
<!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:sabre@nondot.org">Chris Lattner</a>,
<a href="mailto:isanbard@gmail.com">Bill Wendling</a>,
<a href="mailto:pronesto@gmail.com">Fernando Magno Quintao
Pereira</a> and
<a href="mailto:jlaskey@mac.com">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>
<div class="doc_text">
<p>The SelectionDAG optimization phase is run multiple times for code
generation, immediately after the DAG is built and once after each
legalization. The first run of the pass allows the initial code to be
cleaned up (e.g. performing optimizations that depend on knowing that the
operators have restricted type inputs). Subsequent runs of the pass clean up
the messy code generated by the Legalize passes, which allows Legalize to be
very simple (it can focus on making code legal instead of focusing on
generating <em>good</em> and legal code).</p>
<p>One important class of optimizations performed is optimizing inserted sign
and zero extension instructions. We currently use ad-hoc techniques, but
could move to more rigorous techniques in the future. Here are some good
papers on the subject:</p>
<p>"<a href="http://www.eecs.harvard.edu/~nr/pubs/widen-abstract.html">Widening
integer arithmetic</a>"<br>
Kevin Redwine and Norman Ramsey<br>
International Conference on Compiler Construction (CC) 2004</p>
<p>"<a href="http://portal.acm.org/citation.cfm?doid=512529.512552">Effective
sign extension elimination</a>"<br>
Motohiro Kawahito, Hideaki Komatsu, and Toshio Nakatani<br>
Proceedings of the ACM SIGPLAN 2002 Conference on Programming Language Design
and Implementation.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="selectiondag_select">SelectionDAG Select Phase</a>
</div>
<div class="doc_text">
<p>The Select phase is the bulk of the target-specific code for instruction
selection. This phase takes a legal SelectionDAG as input, pattern matches
the instructions supported by the target to this DAG, and produces a new DAG
of target code. For example, consider the following LLVM fragment:</p>
<div class="doc_code">
<pre>
%t1 = add float %W, %X
%t2 = mul float %t1, %Y
%t3 = add float %t2, %Z
</pre>
</div>
<p>This LLVM code corresponds to a SelectionDAG that looks basically like
this:</p>
<div class="doc_code">
<pre>
(fadd:f32 (fmul:f32 (fadd:f32 W, X), Y), Z)
</pre>
</div>
<p>If a target supports floating point multiply-and-add (FMA) operations, one of
the adds can be merged with the multiply. On the PowerPC, for example, the
output of the instruction selector might look like this DAG:</p>
<div class="doc_code">
<pre>
(FMADDS (FADDS W, X), Y, Z)
</pre>
</div>
<p>The <tt>FMADDS</tt> instruction is a ternary instruction that multiplies its
first two operands and adds the third (as single-precision floating-point
numbers). The <tt>FADDS</tt> instruction is a simple binary single-precision
add instruction. To perform this pattern match, the PowerPC backend includes
the following instruction definitions:</p>
<div class="doc_code">
<pre>
def FMADDS : AForm_1<59, 29,
(ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRC, F4RC:$FRB),
"fmadds $FRT, $FRA, $FRC, $FRB",
[<b>(set F4RC:$FRT, (fadd (fmul F4RC:$FRA, F4RC:$FRC),
F4RC:$FRB))</b>]>;
def FADDS : AForm_2<59, 21,
(ops F4RC:$FRT, F4RC:$FRA, F4RC:$FRB),
"fadds $FRT, $FRA, $FRB",
[<b>(set F4RC:$FRT, (fadd F4RC:$FRA, F4RC:$FRB))</b>]>;
</pre>
</div>
<p>The portion of the instruction definition in bold indicates the pattern used
to match the instruction. The DAG operators
(like <tt>fmul</tt>/<tt>fadd</tt>) are defined in
the <tt>lib/Target/TargetSelectionDAG.td</tt> file. "<tt>F4RC</tt>" is the
register class of the input and result values.</p>
<p>The TableGen DAG instruction selector generator reads the instruction
patterns in the <tt>.td</tt> file and automatically builds parts of the
pattern matching code for your target. It has the following strengths:</p>
<ul>
<li>At compiler-compiler time, it analyzes your instruction patterns and tells
you if your patterns make sense or not.</li>
<li>It can handle arbitrary constraints on operands for the pattern match. In
particular, it is straight-forward to say things like "match any immediate
that is a 13-bit sign-extended value". For examples, see the
<tt>immSExt16</tt> and related <tt>tblgen</tt> classes in the PowerPC
backend.</li>
<li>It knows several important identities for the patterns defined. For
example, it knows that addition is commutative, so it allows the
<tt>FMADDS</tt> pattern above to match "<tt>(fadd X, (fmul Y, Z))</tt>" as
well as "<tt>(fadd (fmul X, Y), Z)</tt>", without the target author having
to specially handle this case.</li>
<li>It has a full-featured type-inferencing system. In particular, you should
rarely have to explicitly tell the system what type parts of your patterns
are. In the <tt>FMADDS</tt> case above, we didn't have to tell
<tt>tblgen</tt> that all of the nodes in the pattern are of type 'f32'.
It was able to infer and propagate this knowledge from the fact that
<tt>F4RC</tt> has type 'f32'.</li>
<li>Targets can define their own (and rely on built-in) "pattern fragments".
Pattern fragments are chunks of reusable patterns that get inlined into
your patterns during compiler-compiler time. For example, the integer
"<tt>(not x)</tt>" operation is actually defined as a pattern fragment
that expands as "<tt>(xor x, -1)</tt>", since the SelectionDAG does not
have a native '<tt>not</tt>' operation. Targets can define their own
short-hand fragments as they see fit. See the definition of
'<tt>not</tt>' and '<tt>ineg</tt>' for examples.</li>
<li>In addition to instructions, targets can specify arbitrary patterns that
map to one or more instructions using the 'Pat' class. For example, the
PowerPC has no way to load an arbitrary integer immediate into a register
in one instruction. To tell tblgen how to do this, it defines:
<br>
<br>
<div class="doc_code">
<pre>
// Arbitrary immediate support. Implement in terms of LIS/ORI.
def : Pat<(i32 imm:$imm),
(ORI (LIS (HI16 imm:$imm)), (LO16 imm:$imm))>;
</pre>
</div>
<br>
If none of the single-instruction patterns for loading an immediate into a
register match, this will be used. This rule says "match an arbitrary i32
immediate, turning it into an <tt>ORI</tt> ('or a 16-bit immediate') and
an <tt>LIS</tt> ('load 16-bit immediate, where the immediate is shifted to
the left 16 bits') instruction". To make this work, the
<tt>LO16</tt>/<tt>HI16</tt> node transformations are used to manipulate
the input immediate (in this case, take the high or low 16-bits of the
immediate).</li>
<li>While the system does automate a lot, it still allows you to write custom
C++ code to match special cases if there is something that is hard to
express.</li>
</ul>
<p>While it has many strengths, the system currently has some limitations,
primarily because it is a work in progress and is not yet finished:</p>
<ul>
<li>Overall, there is no way to define or match SelectionDAG nodes that define
multiple values (e.g. <tt>SMUL_LOHI</tt>, <tt>LOAD</tt>, <tt>CALL</tt>,
etc). This is the biggest reason that you currently still <em>have
to</em> write custom C++ code for your instruction selector.</li>
<li>There is no great way to support matching complex addressing modes yet.
In the future, we will extend pattern fragments to allow them to define
multiple values (e.g. the four operands of the <a href="#x86_memory">X86
addressing mode</a>, which are currently matched with custom C++ code).
In addition, we'll extend fragments so that a fragment can match multiple
different patterns.</li>
<li>We don't automatically infer flags like isStore/isLoad yet.</li>
<li>We don't automatically generate the set of supported registers and
operations for the <a href="#selectiondag_legalize">Legalizer</a>
yet.</li>
<li>We don't have a way of tying in custom legalized nodes yet.</li>
</ul>
<p>Despite these limitations, the instruction selector generator is still quite
useful for most of the binary and logical operations in typical instruction
sets. If you run into any problems or can't figure out how to do something,
please let Chris know!</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="selectiondag_sched">SelectionDAG Scheduling and Formation Phase</a>
</div>
<div class="doc_text">
<p>The scheduling phase takes the DAG of target instructions from the selection
phase and assigns an order. The scheduler can pick an order depending on
various constraints of the machines (i.e. order for minimal register pressure
or try to cover instruction latencies). Once an order is established, the
DAG is converted to a list
of <tt><a href="#machineinstr">MachineInstr</a></tt>s and the SelectionDAG is
destroyed.</p>
<p>Note that this phase is logically separate from the instruction selection
phase, but is tied to it closely in the code because it operates on
SelectionDAGs.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="selectiondag_future">Future directions for the SelectionDAG</a>
</div>
<div class="doc_text">
<ol>
<li>Optional function-at-a-time selection.</li>
<li>Auto-generate entire selector from <tt>.td</tt> file.</li>
</ol>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ssamco">SSA-based Machine Code Optimizations</a>
</div>
<div class="doc_text"><p>To Be Written</p></div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="liveintervals">Live Intervals</a>
</div>
<div class="doc_text">
<p>Live Intervals are the ranges (intervals) where a variable is <i>live</i>.
They are used by some <a href="#regalloc">register allocator</a> passes to
determine if two or more virtual registers which require the same physical
register are live at the same point in the program (i.e., they conflict).
When this situation occurs, one virtual register must be <i>spilled</i>.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="livevariable_analysis">Live Variable Analysis</a>
</div>
<div class="doc_text">
<p>The first step in determining the live intervals of variables is to calculate
the set of registers that are immediately dead after the instruction (i.e.,
the instruction calculates the value, but it is never used) and the set of
registers that are used by the instruction, but are never used after the
instruction (i.e., they are killed). Live variable information is computed
for each <i>virtual</i> register and <i>register allocatable</i> physical
register in the function. This is done in a very efficient manner because it
uses SSA to sparsely compute lifetime information for virtual registers
(which are in SSA form) and only has to track physical registers within a
block. Before register allocation, LLVM can assume that physical registers
are only live within a single basic block. This allows it to do a single,
local analysis to resolve physical register lifetimes within each basic
block. If a physical register is not register allocatable (e.g., a stack
pointer or condition codes), it is not tracked.</p>
<p>Physical registers may be live in to or out of a function. Live in values are
typically arguments in registers. Live out values are typically return values
in registers. Live in values are marked as such, and are given a dummy
"defining" instruction during live intervals analysis. If the last basic
block of a function is a <tt>return</tt>, then it's marked as using all live
out values in the function.</p>
<p><tt>PHI</tt> nodes need to be handled specially, because the calculation of
the live variable information from a depth first traversal of the CFG of the
function won't guarantee that a virtual register used by the <tt>PHI</tt>
node is defined before it's used. When a <tt>PHI</tt> node is encountered,
only the definition is handled, because the uses will be handled in other
basic blocks.</p>
<p>For each <tt>PHI</tt> node of the current basic block, we simulate an
assignment at the end of the current basic block and traverse the successor
basic blocks. If a successor basic block has a <tt>PHI</tt> node and one of
the <tt>PHI</tt> node's operands is coming from the current basic block, then
the variable is marked as <i>alive</i> within the current basic block and all
of its predecessor basic blocks, until the basic block with the defining
instruction is encountered.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="liveintervals_analysis">Live Intervals Analysis</a>
</div>
<div class="doc_text">
<p>We now have the information available to perform the live intervals analysis
and build the live intervals themselves. We start off by numbering the basic
blocks and machine instructions. We then handle the "live-in" values. These
are in physical registers, so the physical register is assumed to be killed
by the end of the basic block. Live intervals for virtual registers are
computed for some ordering of the machine instructions <tt>[1, N]</tt>. A
live interval is an interval <tt>[i, j)</tt>, where <tt>1 <= i <= j
< N</tt>, for which a variable is live.</p>
<p><i><b>More to come...</b></i></p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="regalloc">Register Allocation</a>
</div>
<div class="doc_text">
<p>The <i>Register Allocation problem</i> consists in mapping a program
<i>P<sub>v</sub></i>, that can use an unbounded number of virtual registers,
to a program <i>P<sub>p</sub></i> that contains a finite (possibly small)
number of physical registers. Each target architecture has a different number
of physical registers. If the number of physical registers is not enough to
accommodate all the virtual registers, some of them will have to be mapped
into memory. These virtuals are called <i>spilled virtuals</i>.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="regAlloc_represent">How registers are represented in LLVM</a>
</div>
<div class="doc_text">
<p>In LLVM, physical registers are denoted by integer numbers that normally
range from 1 to 1023. To see how this numbering is defined for a particular
architecture, you can read the <tt>GenRegisterNames.inc</tt> file for that
architecture. For instance, by
inspecting <tt>lib/Target/X86/X86GenRegisterNames.inc</tt> we see that the
32-bit register <tt>EAX</tt> is denoted by 15, and the MMX register
<tt>MM0</tt> is mapped to 48.</p>
<p>Some architectures contain registers that share the same physical location. A
notable example is the X86 platform. For instance, in the X86 architecture,
the registers <tt>EAX</tt>, <tt>AX</tt> and <tt>AL</tt> share the first eight
bits. These physical registers are marked as <i>aliased</i> in LLVM. Given a
particular architecture, you can check which registers are aliased by
inspecting its <tt>RegisterInfo.td</tt> file. Moreover, the method
<tt>TargetRegisterInfo::getAliasSet(p_reg)</tt> returns an array containing
all the physical registers aliased to the register <tt>p_reg</tt>.</p>
<p>Physical registers, in LLVM, are grouped in <i>Register Classes</i>.
Elements in the same register class are functionally equivalent, and can be
interchangeably used. Each virtual register can only be mapped to physical
registers of a particular class. For instance, in the X86 architecture, some
virtuals can only be allocated to 8 bit registers. A register class is
described by <tt>TargetRegisterClass</tt> objects. To discover if a virtual
register is compatible with a given physical, this code can be used:</p>
<div class="doc_code">
<pre>
bool RegMapping_Fer::compatible_class(MachineFunction &mf,
unsigned v_reg,
unsigned p_reg) {
assert(TargetRegisterInfo::isPhysicalRegister(p_reg) &&
"Target register must be physical");
const TargetRegisterClass *trc = mf.getRegInfo().getRegClass(v_reg);
return trc->contains(p_reg);
}
</pre>
</div>
<p>Sometimes, mostly for debugging purposes, it is useful to change the number
of physical registers available in the target architecture. This must be done
statically, inside the <tt>TargetRegsterInfo.td</tt> file. Just <tt>grep</tt>
for <tt>RegisterClass</tt>, the last parameter of which is a list of
registers. Just commenting some out is one simple way to avoid them being
used. A more polite way is to explicitly exclude some registers from
the <i>allocation order</i>. See the definition of the <tt>GR</tt> register
class in <tt>lib/Target/IA64/IA64RegisterInfo.td</tt> for an example of this
(e.g., <tt>numReservedRegs</tt> registers are hidden.)</p>
<p>Virtual registers are also denoted by integer numbers. Contrary to physical
registers, different virtual registers never share the same number. The
smallest virtual register is normally assigned the number 1024. This may
change, so, in order to know which is the first virtual register, you should
access <tt>TargetRegisterInfo::FirstVirtualRegister</tt>. Any register whose
number is greater than or equal
to <tt>TargetRegisterInfo::FirstVirtualRegister</tt> is considered a virtual
register. Whereas physical registers are statically defined in
a <tt>TargetRegisterInfo.td</tt> file and cannot be created by the
application developer, that is not the case with virtual registers. In order
to create new virtual registers, use the
method <tt>MachineRegisterInfo::createVirtualRegister()</tt>. This method
will return a virtual register with the highest code.</p>
<p>Before register allocation, the operands of an instruction are mostly virtual
registers, although physical registers may also be used. In order to check if
a given machine operand is a register, use the boolean
function <tt>MachineOperand::isRegister()</tt>. To obtain the integer code of
a register, use <tt>MachineOperand::getReg()</tt>. An instruction may define
or use a register. For instance, <tt>ADD reg:1026 := reg:1025 reg:1024</tt>
defines the registers 1024, and uses registers 1025 and 1026. Given a
register operand, the method <tt>MachineOperand::isUse()</tt> informs if that
register is being used by the instruction. The
method <tt>MachineOperand::isDef()</tt> informs if that registers is being
defined.</p>
<p>We will call physical registers present in the LLVM bitcode before register
allocation <i>pre-colored registers</i>. Pre-colored registers are used in
many different situations, for instance, to pass parameters of functions
calls, and to store results of particular instructions. There are two types
of pre-colored registers: the ones <i>implicitly</i> defined, and
those <i>explicitly</i> defined. Explicitly defined registers are normal
operands, and can be accessed
with <tt>MachineInstr::getOperand(int)::getReg()</tt>. In order to check
which registers are implicitly defined by an instruction, use
the <tt>TargetInstrInfo::get(opcode)::ImplicitDefs</tt>,
where <tt>opcode</tt> is the opcode of the target instruction. One important
difference between explicit and implicit physical registers is that the
latter are defined statically for each instruction, whereas the former may
vary depending on the program being compiled. For example, an instruction
that represents a function call will always implicitly define or use the same
set of physical registers. To read the registers implicitly used by an
instruction,
use <tt>TargetInstrInfo::get(opcode)::ImplicitUses</tt>. Pre-colored
registers impose constraints on any register allocation algorithm. The
register allocator must make sure that none of them is been overwritten by
the values of virtual registers while still alive.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="regAlloc_howTo">Mapping virtual registers to physical registers</a>
</div>
<div class="doc_text">
<p>There are two ways to map virtual registers to physical registers (or to
memory slots). The first way, that we will call <i>direct mapping</i>, is
based on the use of methods of the classes <tt>TargetRegisterInfo</tt>,
and <tt>MachineOperand</tt>. The second way, that we will call <i>indirect
mapping</i>, relies on the <tt>VirtRegMap</tt> class in order to insert loads
and stores sending and getting values to and from memory.</p>
<p>The direct mapping provides more flexibility to the developer of the register
allocator; however, it is more error prone, and demands more implementation
work. Basically, the programmer will have to specify where load and store
instructions should be inserted in the target function being compiled in
order to get and store values in memory. To assign a physical register to a
virtual register present in a given operand,
use <tt>MachineOperand::setReg(p_reg)</tt>. To insert a store instruction,
use <tt>TargetRegisterInfo::storeRegToStackSlot(...)</tt>, and to insert a
load instruction, use <tt>TargetRegisterInfo::loadRegFromStackSlot</tt>.</p>
<p>The indirect mapping shields the application developer from the complexities
of inserting load and store instructions. In order to map a virtual register
to a physical one, use <tt>VirtRegMap::assignVirt2Phys(vreg, preg)</tt>. In
order to map a certain virtual register to memory,
use <tt>VirtRegMap::assignVirt2StackSlot(vreg)</tt>. This method will return
the stack slot where <tt>vreg</tt>'s value will be located. If it is
necessary to map another virtual register to the same stack slot,
use <tt>VirtRegMap::assignVirt2StackSlot(vreg, stack_location)</tt>. One
important point to consider when using the indirect mapping, is that even if
a virtual register is mapped to memory, it still needs to be mapped to a
physical register. This physical register is the location where the virtual
register is supposed to be found before being stored or after being
reloaded.</p>
<p>If the indirect strategy is used, after all the virtual registers have been
mapped to physical registers or stack slots, it is necessary to use a spiller
object to place load and store instructions in the code. Every virtual that
has been mapped to a stack slot will be stored to memory after been defined
and will be loaded before being used. The implementation of the spiller tries
to recycle load/store instructions, avoiding unnecessary instructions. For an
example of how to invoke the spiller,
see <tt>RegAllocLinearScan::runOnMachineFunction</tt>
in <tt>lib/CodeGen/RegAllocLinearScan.cpp</tt>.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="regAlloc_twoAddr">Handling two address instructions</a>
</div>
<div class="doc_text">
<p>With very rare exceptions (e.g., function calls), the LLVM machine code
instructions are three address instructions. That is, each instruction is
expected to define at most one register, and to use at most two registers.
However, some architectures use two address instructions. In this case, the
defined register is also one of the used register. For instance, an
instruction such as <tt>ADD %EAX, %EBX</tt>, in X86 is actually equivalent
to <tt>%EAX = %EAX + %EBX</tt>.</p>
<p>In order to produce correct code, LLVM must convert three address
instructions that represent two address instructions into true two address
instructions. LLVM provides the pass <tt>TwoAddressInstructionPass</tt> for
this specific purpose. It must be run before register allocation takes
place. After its execution, the resulting code may no longer be in SSA
form. This happens, for instance, in situations where an instruction such
as <tt>%a = ADD %b %c</tt> is converted to two instructions such as:</p>
<div class="doc_code">
<pre>
%a = MOVE %b
%a = ADD %a %c
</pre>
</div>
<p>Notice that, internally, the second instruction is represented as
<tt>ADD %a[def/use] %c</tt>. I.e., the register operand <tt>%a</tt> is both
used and defined by the instruction.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="regAlloc_ssaDecon">The SSA deconstruction phase</a>
</div>
<div class="doc_text">
<p>An important transformation that happens during register allocation is called
the <i>SSA Deconstruction Phase</i>. The SSA form simplifies many analyses
that are performed on the control flow graph of programs. However,
traditional instruction sets do not implement PHI instructions. Thus, in
order to generate executable code, compilers must replace PHI instructions
with other instructions that preserve their semantics.</p>
<p>There are many ways in which PHI instructions can safely be removed from the
target code. The most traditional PHI deconstruction algorithm replaces PHI
instructions with copy instructions. That is the strategy adopted by
LLVM. The SSA deconstruction algorithm is implemented
in <tt>lib/CodeGen/PHIElimination.cpp</tt>. In order to invoke this pass, the
identifier <tt>PHIEliminationID</tt> must be marked as required in the code
of the register allocator.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="regAlloc_fold">Instruction folding</a>
</div>
<div class="doc_text">
<p><i>Instruction folding</i> is an optimization performed during register
allocation that removes unnecessary copy instructions. For instance, a
sequence of instructions such as:</p>
<div class="doc_code">
<pre>
%EBX = LOAD %mem_address
%EAX = COPY %EBX
</pre>
</div>
<p>can be safely substituted by the single instruction:</p>
<div class="doc_code">
<pre>
%EAX = LOAD %mem_address
</pre>
</div>
<p>Instructions can be folded with
the <tt>TargetRegisterInfo::foldMemoryOperand(...)</tt> method. Care must be
taken when folding instructions; a folded instruction can be quite different
from the original
instruction. See <tt>LiveIntervals::addIntervalsForSpills</tt>
in <tt>lib/CodeGen/LiveIntervalAnalysis.cpp</tt> for an example of its
use.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="regAlloc_builtIn">Built in register allocators</a>
</div>
<div class="doc_text">
<p>The LLVM infrastructure provides the application developer with three
different register allocators:</p>
<ul>
<li><i>Simple</i> — This is a very simple implementation that does not
keep values in registers across instructions. This register allocator
immediately spills every value right after it is computed, and reloads all
used operands from memory to temporary registers before each
instruction.</li>
<li><i>Local</i> — This register allocator is an improvement on the
<i>Simple</i> implementation. It allocates registers on a basic block
level, attempting to keep values in registers and reusing registers as
appropriate.</li>
<li><i>Linear Scan</i> — <i>The default allocator</i>. This is the
well-know linear scan register allocator. Whereas the
<i>Simple</i> and <i>Local</i> algorithms use a direct mapping
implementation technique, the <i>Linear Scan</i> implementation
uses a spiller in order to place load and stores.</li>
</ul>
<p>The type of register allocator used in <tt>llc</tt> can be chosen with the
command line option <tt>-regalloc=...</tt>:</p>
<div class="doc_code">
<pre>
$ llc -f -regalloc=simple file.bc -o sp.s;
$ llc -f -regalloc=local file.bc -o lc.s;
$ llc -f -regalloc=linearscan file.bc -o ln.s;
</pre>
</div>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="proepicode">Prolog/Epilog Code Insertion</a>
</div>
<div class="doc_text"><p>To Be Written</p></div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="latemco">Late Machine Code Optimizations</a>
</div>
<div class="doc_text"><p>To Be Written</p></div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="codeemit">Code Emission</a>
</div>
<div class="doc_text"><p>To Be Written</p></div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="codeemit_asm">Generating Assembly Code</a>
</div>
<div class="doc_text"><p>To Be Written</p></div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="codeemit_bin">Generating Binary Machine Code</a>
</div>
<div class="doc_text">
<p>For the JIT or <tt>.o</tt> file writer</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="targetimpls">Target-specific Implementation Notes</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section of the document explains features or design decisions that are
specific to the code generator for a particular target.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="tailcallopt">Tail call optimization</a>
</div>
<div class="doc_text">
<p>Tail call optimization, callee reusing the stack of the caller, is currently
supported on x86/x86-64 and PowerPC. It is performed if:</p>
<ul>
<li>Caller and callee have the calling convention <tt>fastcc</tt>.</li>
<li>The call is a tail call - in tail position (ret immediately follows call
and ret uses value of call or is void).</li>
<li>Option <tt>-tailcallopt</tt> is enabled.</li>
<li>Platform specific constraints are met.</li>
</ul>
<p>x86/x86-64 constraints:</p>
<ul>
<li>No variable argument lists are used.</li>
<li>On x86-64 when generating GOT/PIC code only module-local calls (visibility
= hidden or protected) are supported.</li>
</ul>
<p>PowerPC constraints:</p>
<ul>
<li>No variable argument lists are used.</li>
<li>No byval parameters are used.</li>
<li>On ppc32/64 GOT/PIC only module-local calls (visibility = hidden or protected) are supported.</li>
</ul>
<p>Example:</p>
<p>Call as <tt>llc -tailcallopt test.ll</tt>.</p>
<div class="doc_code">
<pre>
declare fastcc i32 @tailcallee(i32 inreg %a1, i32 inreg %a2, i32 %a3, i32 %a4)
define fastcc i32 @tailcaller(i32 %in1, i32 %in2) {
%l1 = add i32 %in1, %in2
%tmp = tail call fastcc i32 @tailcallee(i32 %in1 inreg, i32 %in2 inreg, i32 %in1, i32 %l1)
ret i32 %tmp
}
</pre>
</div>
<p>Implications of <tt>-tailcallopt</tt>:</p>
<p>To support tail call optimization in situations where the callee has more
arguments than the caller a 'callee pops arguments' convention is used. This
currently causes each <tt>fastcc</tt> call that is not tail call optimized
(because one or more of above constraints are not met) to be followed by a
readjustment of the stack. So performance might be worse in such cases.</p>
<p>On x86 and x86-64 one register is reserved for indirect tail calls (e.g via a
function pointer). So there is one less register for integer argument
passing. For x86 this means 2 registers (if <tt>inreg</tt> parameter
attribute is used) and for x86-64 this means 5 register are used.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="x86">The X86 backend</a>
</div>
<div class="doc_text">
<p>The X86 code generator lives in the <tt>lib/Target/X86</tt> directory. This
code generator is capable of targeting a variety of x86-32 and x86-64
processors, and includes support for ISA extensions such as MMX and SSE.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="x86_tt">X86 Target Triples supported</a>
</div>
<div class="doc_text">
<p>The following are the known target triples that are supported by the X86
backend. This is not an exhaustive list, and it would be useful to add those
that people test.</p>
<ul>
<li><b>i686-pc-linux-gnu</b> — Linux</li>
<li><b>i386-unknown-freebsd5.3</b> — FreeBSD 5.3</li>
<li><b>i686-pc-cygwin</b> — Cygwin on Win32</li>
<li><b>i686-pc-mingw32</b> — MingW on Win32</li>
<li><b>i386-pc-mingw32msvc</b> — MingW crosscompiler on Linux</li>
<li><b>i686-apple-darwin*</b> — Apple Darwin on X86</li>
<li><b>x86_64-unknown-linux-gnu</b> — Linux</li>
</ul>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="x86_cc">X86 Calling Conventions supported</a>
</div>
<div class="doc_text">
<p>The following target-specific calling conventions are known to backend:</p>
<ul>
<li><b>x86_StdCall</b> — stdcall calling convention seen on Microsoft
Windows platform (CC ID = 64).</li>
<li><b>x86_FastCall</b> — fastcall calling convention seen on Microsoft
Windows platform (CC ID = 65).</li>
</ul>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="x86_memory">Representing X86 addressing modes in MachineInstrs</a>
</div>
<div class="doc_text">
<p>The x86 has a very flexible way of accessing memory. It is capable of
forming memory addresses of the following expression directly in integer
instructions (which use ModR/M addressing):</p>
<div class="doc_code">
<pre>
Base + [1,2,4,8] * IndexReg + Disp32
</pre>
</div>
<p>In order to represent this, LLVM tracks no less than 4 operands for each
memory operand of this form. This means that the "load" form of
'<tt>mov</tt>' has the following <tt>MachineOperand</tt>s in this order:</p>
<div class="doc_code">
<pre>
Index: 0 | 1 2 3 4
Meaning: DestReg, | BaseReg, Scale, IndexReg, Displacement
OperandTy: VirtReg, | VirtReg, UnsImm, VirtReg, SignExtImm
</pre>
</div>
<p>Stores, and all other instructions, treat the four memory operands in the
same way and in the same order.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="x86_memory">X86 address spaces supported</a>
</div>
<div class="doc_text">
<p>x86 has an experimental feature which provides
the ability to perform loads and stores to different address spaces
via the x86 segment registers. A segment override prefix byte on an
instruction causes the instruction's memory access to go to the specified
segment. LLVM address space 0 is the default address space, which includes
the stack, and any unqualified memory accesses in a program. Address spaces
1-255 are currently reserved for user-defined code. The GS-segment is
represented by address space 256, while the FS-segment is represented by
address space 257. Other x86 segments have yet to be allocated address space
numbers.</p>
<p>While these address spaces may seem similar to TLS via the
<tt>thread_local</tt> keyword, and often use the same underlying hardware,
there are some fundamental differences.</p>
<p>The <tt>thread_local</tt> keyword applies to global variables and
specifies that they are to be allocated in thread-local memory. There are
no type qualifiers involved, and these variables can be pointed to with
normal pointers and accessed with normal loads and stores.
The <tt>thread_local</tt> keyword is target-independent at the LLVM IR
level (though LLVM doesn't yet have implementations of it for some
configurations).<p>
<p>Special address spaces, in contrast, apply to static types. Every
load and store has a particular address space in its address operand type,
and this is what determines which address space is accessed.
LLVM ignores these special address space qualifiers on global variables,
and does not provide a way to directly allocate storage in them.
At the LLVM IR level, the behavior of these special address spaces depends
in part on the underlying OS or runtime environment, and they are specific
to x86 (and LLVM doesn't yet handle them correctly in some cases).</p>
<p>Some operating systems and runtime environments use (or may in the future
use) the FS/GS-segment registers for various low-level purposes, so care
should be taken when considering them.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="x86_names">Instruction naming</a>
</div>
<div class="doc_text">
<p>An instruction name consists of the base name, a default operand size, and a
a character per operand with an optional special size. For example:</p>
<div class="doc_code">
<pre>
ADD8rr -> add, 8-bit register, 8-bit register
IMUL16rmi -> imul, 16-bit register, 16-bit memory, 16-bit immediate
IMUL16rmi8 -> imul, 16-bit register, 16-bit memory, 8-bit immediate
MOVSX32rm16 -> movsx, 32-bit register, 16-bit memory
</pre>
</div>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ppc">The PowerPC backend</a>
</div>
<div class="doc_text">
<p>The PowerPC code generator lives in the lib/Target/PowerPC directory. The
code generation is retargetable to several variations or <i>subtargets</i> of
the PowerPC ISA; including ppc32, ppc64 and altivec.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="ppc_abi">LLVM PowerPC ABI</a>
</div>
<div class="doc_text">
<p>LLVM follows the AIX PowerPC ABI, with two deviations. LLVM uses a PC
relative (PIC) or static addressing for accessing global values, so no TOC
(r2) is used. Second, r31 is used as a frame pointer to allow dynamic growth
of a stack frame. LLVM takes advantage of having no TOC to provide space to
save the frame pointer in the PowerPC linkage area of the caller frame.
Other details of PowerPC ABI can be found at <a href=
"http://developer.apple.com/documentation/DeveloperTools/Conceptual/LowLevelABI/Articles/32bitPowerPC.html"
>PowerPC ABI.</a> Note: This link describes the 32 bit ABI. The 64 bit ABI
is similar except space for GPRs are 8 bytes wide (not 4) and r13 is reserved
for system use.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="ppc_frame">Frame Layout</a>
</div>
<div class="doc_text">
<p>The size of a PowerPC frame is usually fixed for the duration of a
function's invocation. Since the frame is fixed size, all references
into the frame can be accessed via fixed offsets from the stack pointer. The
exception to this is when dynamic alloca or variable sized arrays are
present, then a base pointer (r31) is used as a proxy for the stack pointer
and stack pointer is free to grow or shrink. A base pointer is also used if
llvm-gcc is not passed the -fomit-frame-pointer flag. The stack pointer is
always aligned to 16 bytes, so that space allocated for altivec vectors will
be properly aligned.</p>
<p>An invocation frame is laid out as follows (low memory at top);</p>
<table class="layout">
<tr>
<td>Linkage<br><br></td>
</tr>
<tr>
<td>Parameter area<br><br></td>
</tr>
<tr>
<td>Dynamic area<br><br></td>
</tr>
<tr>
<td>Locals area<br><br></td>
</tr>
<tr>
<td>Saved registers area<br><br></td>
</tr>
<tr style="border-style: none hidden none hidden;">
<td><br></td>
</tr>
<tr>
<td>Previous Frame<br><br></td>
</tr>
</table>
<p>The <i>linkage</i> area is used by a callee to save special registers prior
to allocating its own frame. Only three entries are relevant to LLVM. The
first entry is the previous stack pointer (sp), aka link. This allows
probing tools like gdb or exception handlers to quickly scan the frames in
the stack. A function epilog can also use the link to pop the frame from the
stack. The third entry in the linkage area is used to save the return
address from the lr register. Finally, as mentioned above, the last entry is
used to save the previous frame pointer (r31.) The entries in the linkage
area are the size of a GPR, thus the linkage area is 24 bytes long in 32 bit
mode and 48 bytes in 64 bit mode.</p>
<p>32 bit linkage area</p>
<table class="layout">
<tr>
<td>0</td>
<td>Saved SP (r1)</td>
</tr>
<tr>
<td>4</td>
<td>Saved CR</td>
</tr>
<tr>
<td>8</td>
<td>Saved LR</td>
</tr>
<tr>
<td>12</td>
<td>Reserved</td>
</tr>
<tr>
<td>16</td>
<td>Reserved</td>
</tr>
<tr>
<td>20</td>
<td>Saved FP (r31)</td>
</tr>
</table>
<p>64 bit linkage area</p>
<table class="layout">
<tr>
<td>0</td>
<td>Saved SP (r1)</td>
</tr>
<tr>
<td>8</td>
<td>Saved CR</td>
</tr>
<tr>
<td>16</td>
<td>Saved LR</td>
</tr>
<tr>
<td>24</td>
<td>Reserved</td>
</tr>
<tr>
<td>32</td>
<td>Reserved</td>
</tr>
<tr>
<td>40</td>
<td>Saved FP (r31)</td>
</tr>
</table>
<p>The <i>parameter area</i> is used to store arguments being passed to a callee
function. Following the PowerPC ABI, the first few arguments are actually
passed in registers, with the space in the parameter area unused. However,
if there are not enough registers or the callee is a thunk or vararg
function, these register arguments can be spilled into the parameter area.
Thus, the parameter area must be large enough to store all the parameters for
the largest call sequence made by the caller. The size must also be
minimally large enough to spill registers r3-r10. This allows callees blind
to the call signature, such as thunks and vararg functions, enough space to
cache the argument registers. Therefore, the parameter area is minimally 32
bytes (64 bytes in 64 bit mode.) Also note that since the parameter area is
a fixed offset from the top of the frame, that a callee can access its spilt
arguments using fixed offsets from the stack pointer (or base pointer.)</p>
<p>Combining the information about the linkage, parameter areas and alignment. A
stack frame is minimally 64 bytes in 32 bit mode and 128 bytes in 64 bit
mode.</p>
<p>The <i>dynamic area</i> starts out as size zero. If a function uses dynamic
alloca then space is added to the stack, the linkage and parameter areas are
shifted to top of stack, and the new space is available immediately below the
linkage and parameter areas. The cost of shifting the linkage and parameter
areas is minor since only the link value needs to be copied. The link value
can be easily fetched by adding the original frame size to the base pointer.
Note that allocations in the dynamic space need to observe 16 byte
alignment.</p>
<p>The <i>locals area</i> is where the llvm compiler reserves space for local
variables.</p>
<p>The <i>saved registers area</i> is where the llvm compiler spills callee
saved registers on entry to the callee.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="ppc_prolog">Prolog/Epilog</a>
</div>
<div class="doc_text">
<p>The llvm prolog and epilog are the same as described in the PowerPC ABI, with
the following exceptions. Callee saved registers are spilled after the frame
is created. This allows the llvm epilog/prolog support to be common with
other targets. The base pointer callee saved register r31 is saved in the
TOC slot of linkage area. This simplifies allocation of space for the base
pointer and makes it convenient to locate programatically and during
debugging.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">
<a name="ppc_dynamic">Dynamic Allocation</a>
</div>
<div class="doc_text">
<p><i>TODO - More to come.</i></p>
</div>
<!-- *********************************************************************** -->
<hr>
<address>
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
<a href="http://validator.w3.org/check/referer"><img
src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
</address>
</body>
</html>
|
