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
|
<!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>Writing an LLVM Pass</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<h1>
Writing an LLVM Pass
</h1>
<ol>
<li><a href="#introduction">Introduction - What is a pass?</a></li>
<li><a href="#quickstart">Quick Start - Writing hello world</a>
<ul>
<li><a href="#makefile">Setting up the build environment</a></li>
<li><a href="#basiccode">Basic code required</a></li>
<li><a href="#running">Running a pass with <tt>opt</tt></a></li>
</ul></li>
<li><a href="#passtype">Pass classes and requirements</a>
<ul>
<li><a href="#ImmutablePass">The <tt>ImmutablePass</tt> class</a></li>
<li><a href="#ModulePass">The <tt>ModulePass</tt> class</a>
<ul>
<li><a href="#runOnModule">The <tt>runOnModule</tt> method</a></li>
</ul></li>
<li><a href="#CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
<ul>
<li><a href="#doInitialization_scc">The <tt>doInitialization(CallGraph
&)</tt> method</a></li>
<li><a href="#runOnSCC">The <tt>runOnSCC</tt> method</a></li>
<li><a href="#doFinalization_scc">The <tt>doFinalization(CallGraph
&)</tt> method</a></li>
</ul></li>
<li><a href="#FunctionPass">The <tt>FunctionPass</tt> class</a>
<ul>
<li><a href="#doInitialization_mod">The <tt>doInitialization(Module
&)</tt> method</a></li>
<li><a href="#runOnFunction">The <tt>runOnFunction</tt> method</a></li>
<li><a href="#doFinalization_mod">The <tt>doFinalization(Module
&)</tt> method</a></li>
</ul></li>
<li><a href="#LoopPass">The <tt>LoopPass</tt> class</a>
<ul>
<li><a href="#doInitialization_loop">The <tt>doInitialization(Loop *,
LPPassManager &)</tt> method</a></li>
<li><a href="#runOnLoop">The <tt>runOnLoop</tt> method</a></li>
<li><a href="#doFinalization_loop">The <tt>doFinalization()
</tt> method</a></li>
</ul></li>
<li><a href="#RegionPass">The <tt>RegionPass</tt> class</a>
<ul>
<li><a href="#doInitialization_region">The <tt>doInitialization(Region *,
RGPassManager &)</tt> method</a></li>
<li><a href="#runOnRegion">The <tt>runOnRegion</tt> method</a></li>
<li><a href="#doFinalization_region">The <tt>doFinalization()
</tt> method</a></li>
</ul></li>
<li><a href="#BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
<ul>
<li><a href="#doInitialization_fn">The <tt>doInitialization(Function
&)</tt> method</a></li>
<li><a href="#runOnBasicBlock">The <tt>runOnBasicBlock</tt>
method</a></li>
<li><a href="#doFinalization_fn">The <tt>doFinalization(Function
&)</tt> method</a></li>
</ul></li>
<li><a href="#MachineFunctionPass">The <tt>MachineFunctionPass</tt>
class</a>
<ul>
<li><a href="#runOnMachineFunction">The
<tt>runOnMachineFunction(MachineFunction &)</tt> method</a></li>
</ul></li>
</ul>
<li><a href="#registration">Pass Registration</a>
<ul>
<li><a href="#print">The <tt>print</tt> method</a></li>
</ul></li>
<li><a href="#interaction">Specifying interactions between passes</a>
<ul>
<li><a href="#getAnalysisUsage">The <tt>getAnalysisUsage</tt>
method</a></li>
<li><a href="#AU::addRequired">The <tt>AnalysisUsage::addRequired<></tt> and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods</a></li>
<li><a href="#AU::addPreserved">The <tt>AnalysisUsage::addPreserved<></tt> method</a></li>
<li><a href="#AU::examples">Example implementations of <tt>getAnalysisUsage</tt></a></li>
<li><a href="#getAnalysis">The <tt>getAnalysis<></tt> and
<tt>getAnalysisIfAvailable<></tt> methods</a></li>
</ul></li>
<li><a href="#analysisgroup">Implementing Analysis Groups</a>
<ul>
<li><a href="#agconcepts">Analysis Group Concepts</a></li>
<li><a href="#registerag">Using <tt>RegisterAnalysisGroup</tt></a></li>
</ul></li>
<li><a href="#passStatistics">Pass Statistics</a>
<li><a href="#passmanager">What PassManager does</a>
<ul>
<li><a href="#releaseMemory">The <tt>releaseMemory</tt> method</a></li>
</ul></li>
<li><a href="#registering">Registering dynamically loaded passes</a>
<ul>
<li><a href="#registering_existing">Using existing registries</a></li>
<li><a href="#registering_new">Creating new registries</a></li>
</ul></li>
<li><a href="#debughints">Using GDB with dynamically loaded passes</a>
<ul>
<li><a href="#breakpoint">Setting a breakpoint in your pass</a></li>
<li><a href="#debugmisc">Miscellaneous Problems</a></li>
</ul></li>
<li><a href="#future">Future extensions planned</a>
<ul>
<li><a href="#SMP">Multithreaded LLVM</a></li>
</ul></li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
<a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="introduction">Introduction - What is a pass?</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>The LLVM Pass Framework is an important part of the LLVM system, because LLVM
passes are where most of the interesting parts of the compiler exist. Passes
perform the transformations and optimizations that make up the compiler, they
build the analysis results that are used by these transformations, and they are,
above all, a structuring technique for compiler code.</p>
<p>All LLVM passes are subclasses of the <tt><a
href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>
class, which implement functionality by overriding virtual methods inherited
from <tt>Pass</tt>. Depending on how your pass works, you should inherit from
the <tt><a href="#ModulePass">ModulePass</a></tt>, <tt><a
href="#CallGraphSCCPass">CallGraphSCCPass</a></tt>, <tt><a
href="#FunctionPass">FunctionPass</a></tt>, or <tt><a
href="#LoopPass">LoopPass</a></tt>, or <tt><a
href="#RegionPass">RegionPass</a></tt>, or <tt><a
href="#BasicBlockPass">BasicBlockPass</a></tt> classes, which gives the system
more information about what your pass does, and how it can be combined with
other passes. One of the main features of the LLVM Pass Framework is that it
schedules passes to run in an efficient way based on the constraints that your
pass meets (which are indicated by which class they derive from).</p>
<p>We start by showing you how to construct a pass, everything from setting up
the code, to compiling, loading, and executing it. After the basics are down,
more advanced features are discussed.</p>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="quickstart">Quick Start - Writing hello world</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>Here we describe how to write the "hello world" of passes. The "Hello" pass
is designed to simply print out the name of non-external functions that exist in
the program being compiled. It does not modify the program at all, it just
inspects it. The source code and files for this pass are available in the LLVM
source tree in the <tt>lib/Transforms/Hello</tt> directory.</p>
<!-- ======================================================================= -->
<h3>
<a name="makefile">Setting up the build environment</a>
</h3>
<div>
<p>First, configure and build LLVM. This needs to be done directly inside the
LLVM source tree rather than in a separate objects directory.
Next, you need to create a new directory somewhere in the LLVM source
base. For this example, we'll assume that you made
<tt>lib/Transforms/Hello</tt>. Finally, you must set up a build script
(Makefile) that will compile the source code for the new pass. To do this,
copy the following into <tt>Makefile</tt>:</p>
<hr>
<div class="doc_code"><pre>
# Makefile for hello pass
# Path to top level of LLVM hierarchy
LEVEL = ../../..
# Name of the library to build
LIBRARYNAME = Hello
# Make the shared library become a loadable module so the tools can
# dlopen/dlsym on the resulting library.
LOADABLE_MODULE = 1
# Include the makefile implementation stuff
include $(LEVEL)/Makefile.common
</pre></div>
<p>This makefile specifies that all of the <tt>.cpp</tt> files in the current
directory are to be compiled and linked together into a shared object
<tt>$(LEVEL)/Debug+Asserts/lib/Hello.so</tt> that can be dynamically loaded by
the <tt>opt</tt> or <tt>bugpoint</tt> tools via their <tt>-load</tt> options.
If your operating system uses a suffix other than .so (such as windows or
Mac OS/X), the appropriate extension will be used.</p>
<p>If you are used CMake to build LLVM, see
<a href="CMake.html#passdev">Developing an LLVM pass with CMake</a>.</p>
<p>Now that we have the build scripts set up, we just need to write the code for
the pass itself.</p>
</div>
<!-- ======================================================================= -->
<h3>
<a name="basiccode">Basic code required</a>
</h3>
<div>
<p>Now that we have a way to compile our new pass, we just have to write it.
Start out with:</p>
<div class="doc_code">
<pre>
<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
<b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>"
</pre>
</div>
<p>Which are needed because we are writing a <tt><a
href="http://llvm.org/doxygen/classllvm_1_1Pass.html">Pass</a></tt>,
we are operating on <tt><a
href="http://llvm.org/doxygen/classllvm_1_1Function.html">Function</a></tt>'s,
and we will be doing some printing.</p>
<p>Next we have:</p>
<div class="doc_code">
<pre>
<b>using namespace llvm;</b>
</pre>
</div>
<p>... which is required because the functions from the include files
live in the llvm namespace.</p>
<p>Next we have:</p>
<div class="doc_code">
<pre>
<b>namespace</b> {
</pre>
</div>
<p>... which starts out an anonymous namespace. Anonymous namespaces are to C++
what the "<tt>static</tt>" keyword is to C (at global scope). It makes the
things declared inside of the anonymous namespace visible only to the current
file. If you're not familiar with them, consult a decent C++ book for more
information.</p>
<p>Next, we declare our pass itself:</p>
<div class="doc_code">
<pre>
<b>struct</b> Hello : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
</pre>
</div>
<p>This declares a "<tt>Hello</tt>" class that is a subclass of <tt><a
href="http://llvm.org/doxygen/classllvm_1_1FunctionPass.html">FunctionPass</a></tt>.
The different builtin pass subclasses are described in detail <a
href="#passtype">later</a>, but for now, know that <a
href="#FunctionPass"><tt>FunctionPass</tt></a>'s operate on a function at a
time.</p>
<div class="doc_code">
<pre>
static char ID;
Hello() : FunctionPass(ID) {}
</pre>
</div>
<p>This declares pass identifier used by LLVM to identify pass. This allows LLVM
to avoid using expensive C++ runtime information.</p>
<div class="doc_code">
<pre>
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
errs() << "<i>Hello: </i>";
errs().write_escaped(F.getName()) << "\n";
<b>return false</b>;
}
}; <i>// end of struct Hello</i>
} <i>// end of anonymous namespace</i>
</pre>
</div>
<p>We declare a "<a href="#runOnFunction"><tt>runOnFunction</tt></a>" method,
which overloads an abstract virtual method inherited from <a
href="#FunctionPass"><tt>FunctionPass</tt></a>. This is where we are supposed
to do our thing, so we just print out our message with the name of each
function.</p>
<div class="doc_code">
<pre>
char Hello::ID = 0;
</pre>
</div>
<p>We initialize pass ID here. LLVM uses ID's address to identify a pass, so
initialization value is not important.</p>
<div class="doc_code">
<pre>
static RegisterPass<Hello> X("<i>hello</i>", "<i>Hello World Pass</i>",
false /* Only looks at CFG */,
false /* Analysis Pass */);
</pre>
</div>
<p>Lastly, we <a href="#registration">register our class</a> <tt>Hello</tt>,
giving it a command line argument "<tt>hello</tt>", and a name "<tt>Hello World
Pass</tt>". The last two arguments describe its behavior: if a pass walks CFG
without modifying it then the third argument is set to <tt>true</tt>; if a pass
is an analysis pass, for example dominator tree pass, then <tt>true</tt> is
supplied as the fourth argument.</p>
<p>As a whole, the <tt>.cpp</tt> file looks like:</p>
<div class="doc_code">
<pre>
<b>#include</b> "<a href="http://llvm.org/doxygen/Pass_8h-source.html">llvm/Pass.h</a>"
<b>#include</b> "<a href="http://llvm.org/doxygen/Function_8h-source.html">llvm/Function.h</a>"
<b>#include</b> "<a href="http://llvm.org/doxygen/raw__ostream_8h.html">llvm/Support/raw_ostream.h</a>"
<b>using namespace llvm;</b>
<b>namespace</b> {
<b>struct Hello</b> : <b>public</b> <a href="#FunctionPass">FunctionPass</a> {
static char ID;
Hello() : FunctionPass(ID) {}
<b>virtual bool</b> <a href="#runOnFunction">runOnFunction</a>(Function &F) {
errs() << "<i>Hello: </i>";
errs().write_escaped(F.getName()) << '\n';
<b>return false</b>;
}
};
}
char Hello::ID = 0;
static RegisterPass<Hello> X("hello", "Hello World Pass", false, false);
</pre>
</div>
<p>Now that it's all together, compile the file with a simple "<tt>gmake</tt>"
command in the local directory and you should get a new file
"<tt>Debug+Asserts/lib/Hello.so</tt>" under the top level directory of the LLVM
source tree (not in the local directory). Note that everything in this file is
contained in an anonymous namespace — this reflects the fact that passes
are self contained units that do not need external interfaces (although they can
have them) to be useful.</p>
</div>
<!-- ======================================================================= -->
<h3>
<a name="running">Running a pass with <tt>opt</tt></a>
</h3>
<div>
<p>Now that you have a brand new shiny shared object file, we can use the
<tt>opt</tt> command to run an LLVM program through your pass. Because you
registered your pass with <tt>RegisterPass</tt>, you will be able to
use the <tt>opt</tt> tool to access it, once loaded.</p>
<p>To test it, follow the example at the end of the <a
href="GettingStarted.html">Getting Started Guide</a> to compile "Hello World" to
LLVM. We can now run the bitcode file (<tt>hello.bc</tt>) for the program
through our transformation like this (or course, any bitcode file will
work):</p>
<div class="doc_code"><pre>
$ opt -load ../../../Debug+Asserts/lib/Hello.so -hello < hello.bc > /dev/null
Hello: __main
Hello: puts
Hello: main
</pre></div>
<p>The '<tt>-load</tt>' option specifies that '<tt>opt</tt>' should load your
pass as a shared object, which makes '<tt>-hello</tt>' a valid command line
argument (which is one reason you need to <a href="#registration">register your
pass</a>). Because the hello pass does not modify the program in any
interesting way, we just throw away the result of <tt>opt</tt> (sending it to
<tt>/dev/null</tt>).</p>
<p>To see what happened to the other string you registered, try running
<tt>opt</tt> with the <tt>-help</tt> option:</p>
<div class="doc_code"><pre>
$ opt -load ../../../Debug+Asserts/lib/Hello.so -help
OVERVIEW: llvm .bc -> .bc modular optimizer
USAGE: opt [options] <input bitcode>
OPTIONS:
Optimizations available:
...
-funcresolve - Resolve Functions
-gcse - Global Common Subexpression Elimination
-globaldce - Dead Global Elimination
<b>-hello - Hello World Pass</b>
-indvars - Canonicalize Induction Variables
-inline - Function Integration/Inlining
-instcombine - Combine redundant instructions
...
</pre></div>
<p>The pass name get added as the information string for your pass, giving some
documentation to users of <tt>opt</tt>. Now that you have a working pass, you
would go ahead and make it do the cool transformations you want. Once you get
it all working and tested, it may become useful to find out how fast your pass
is. The <a href="#passManager"><tt>PassManager</tt></a> provides a nice command
line option (<tt>--time-passes</tt>) that allows you to get information about
the execution time of your pass along with the other passes you queue up. For
example:</p>
<div class="doc_code"><pre>
$ opt -load ../../../Debug+Asserts/lib/Hello.so -hello -time-passes < hello.bc > /dev/null
Hello: __main
Hello: puts
Hello: main
===============================================================================
... Pass execution timing report ...
===============================================================================
Total Execution Time: 0.02 seconds (0.0479059 wall clock)
---User Time--- --System Time-- --User+System-- ---Wall Time--- --- Pass Name ---
0.0100 (100.0%) 0.0000 ( 0.0%) 0.0100 ( 50.0%) 0.0402 ( 84.0%) Bitcode Writer
0.0000 ( 0.0%) 0.0100 (100.0%) 0.0100 ( 50.0%) 0.0031 ( 6.4%) Dominator Set Construction
0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0013 ( 2.7%) Module Verifier
<b> 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0000 ( 0.0%) 0.0033 ( 6.9%) Hello World Pass</b>
0.0100 (100.0%) 0.0100 (100.0%) 0.0200 (100.0%) 0.0479 (100.0%) TOTAL
</pre></div>
<p>As you can see, our implementation above is pretty fast :). The additional
passes listed are automatically inserted by the '<tt>opt</tt>' tool to verify
that the LLVM emitted by your pass is still valid and well formed LLVM, which
hasn't been broken somehow.</p>
<p>Now that you have seen the basics of the mechanics behind passes, we can talk
about some more details of how they work and how to use them.</p>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="passtype">Pass classes and requirements</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>One of the first things that you should do when designing a new pass is to
decide what class you should subclass for your pass. The <a
href="#basiccode">Hello World</a> example uses the <tt><a
href="#FunctionPass">FunctionPass</a></tt> class for its implementation, but we
did not discuss why or when this should occur. Here we talk about the classes
available, from the most general to the most specific.</p>
<p>When choosing a superclass for your Pass, you should choose the <b>most
specific</b> class possible, while still being able to meet the requirements
listed. This gives the LLVM Pass Infrastructure information necessary to
optimize how passes are run, so that the resultant compiler isn't unnecessarily
slow.</p>
<!-- ======================================================================= -->
<h3>
<a name="ImmutablePass">The <tt>ImmutablePass</tt> class</a>
</h3>
<div>
<p>The most plain and boring type of pass is the "<tt><a
href="http://llvm.org/doxygen/classllvm_1_1ImmutablePass.html">ImmutablePass</a></tt>"
class. This pass type is used for passes that do not have to be run, do not
change state, and never need to be updated. This is not a normal type of
transformation or analysis, but can provide information about the current
compiler configuration.</p>
<p>Although this pass class is very infrequently used, it is important for
providing information about the current target machine being compiled for, and
other static information that can affect the various transformations.</p>
<p><tt>ImmutablePass</tt>es never invalidate other transformations, are never
invalidated, and are never "run".</p>
</div>
<!-- ======================================================================= -->
<h3>
<a name="ModulePass">The <tt>ModulePass</tt> class</a>
</h3>
<div>
<p>The "<tt><a
href="http://llvm.org/doxygen/classllvm_1_1ModulePass.html">ModulePass</a></tt>"
class is the most general of all superclasses that you can use. Deriving from
<tt>ModulePass</tt> indicates that your pass uses the entire program as a unit,
referring to function bodies in no predictable order, or adding and removing
functions. Because nothing is known about the behavior of <tt>ModulePass</tt>
subclasses, no optimization can be done for their execution.</p>
<p>A module pass can use function level passes (e.g. dominators) using
the getAnalysis interface
<tt>getAnalysis<DominatorTree>(llvm::Function *)</tt> to provide the
function to retrieve analysis result for, if the function pass does not require
any module or immutable passes. Note that this can only be done for functions for which the
analysis ran, e.g. in the case of dominators you should only ask for the
DominatorTree for function definitions, not declarations.</p>
<p>To write a correct <tt>ModulePass</tt> subclass, derive from
<tt>ModulePass</tt> and overload the <tt>runOnModule</tt> method with the
following signature:</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="runOnModule">The <tt>runOnModule</tt> method</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnModule(Module &M) = 0;
</pre></div>
<p>The <tt>runOnModule</tt> method performs the interesting work of the pass.
It should return true if the module was modified by the transformation and
false otherwise.</p>
</div>
</div>
<!-- ======================================================================= -->
<h3>
<a name="CallGraphSCCPass">The <tt>CallGraphSCCPass</tt> class</a>
</h3>
<div>
<p>The "<tt><a
href="http://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html">CallGraphSCCPass</a></tt>"
is used by passes that need to traverse the program bottom-up on the call graph
(callees before callers). Deriving from CallGraphSCCPass provides some
mechanics for building and traversing the CallGraph, but also allows the system
to optimize execution of CallGraphSCCPass's. If your pass meets the
requirements outlined below, and doesn't meet the requirements of a <tt><a
href="#FunctionPass">FunctionPass</a></tt> or <tt><a
href="#BasicBlockPass">BasicBlockPass</a></tt>, you should derive from
<tt>CallGraphSCCPass</tt>.</p>
<p><b>TODO</b>: explain briefly what SCC, Tarjan's algo, and B-U mean.</p>
<p>To be explicit, <tt>CallGraphSCCPass</tt> subclasses are:</p>
<ol>
<li>... <em>not allowed</em> to inspect or modify any <tt>Function</tt>s other
than those in the current SCC and the direct callers and direct callees of the
SCC.</li>
<li>... <em>required</em> to preserve the current CallGraph object, updating it
to reflect any changes made to the program.</li>
<li>... <em>not allowed</em> to add or remove SCC's from the current Module,
though they may change the contents of an SCC.</li>
<li>... <em>allowed</em> to add or remove global variables from the current
Module.</li>
<li>... <em>allowed</em> to maintain state across invocations of
<a href="#runOnSCC"><tt>runOnSCC</tt></a> (including global data).</li>
</ol>
<p>Implementing a <tt>CallGraphSCCPass</tt> is slightly tricky in some cases
because it has to handle SCCs with more than one node in it. All of the virtual
methods described below should return true if they modified the program, or
false if they didn't.</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="doInitialization_scc">
The <tt>doInitialization(CallGraph &)</tt> method
</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(CallGraph &CG);
</pre></div>
<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
<tt>CallGraphSCCPass</tt>'s are not allowed to do. They can add and remove
functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
is designed to do simple initialization type of stuff that does not depend on
the SCCs being processed. The <tt>doInitialization</tt> method call is not
scheduled to overlap with any other pass executions (thus it should be very
fast).</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="runOnSCC">The <tt>runOnSCC</tt> method</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnSCC(CallGraphSCC &SCC) = 0;
</pre></div>
<p>The <tt>runOnSCC</tt> method performs the interesting work of the pass, and
should return true if the module was modified by the transformation, false
otherwise.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="doFinalization_scc">
The <tt>doFinalization(CallGraph &)</tt> method
</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization(CallGraph &CG);
</pre></div>
<p>The <tt>doFinalization</tt> method is an infrequently used method that is
called when the pass framework has finished calling <a
href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
program being compiled.</p>
</div>
</div>
<!-- ======================================================================= -->
<h3>
<a name="FunctionPass">The <tt>FunctionPass</tt> class</a>
</h3>
<div>
<p>In contrast to <tt>ModulePass</tt> subclasses, <tt><a
href="http://llvm.org/doxygen/classllvm_1_1Pass.html">FunctionPass</a></tt>
subclasses do have a predictable, local behavior that can be expected by the
system. All <tt>FunctionPass</tt> execute on each function in the program
independent of all of the other functions in the program.
<tt>FunctionPass</tt>'s do not require that they are executed in a particular
order, and <tt>FunctionPass</tt>'s do not modify external functions.</p>
<p>To be explicit, <tt>FunctionPass</tt> subclasses are not allowed to:</p>
<ol>
<li>Modify a Function other than the one currently being processed.</li>
<li>Add or remove Function's from the current Module.</li>
<li>Add or remove global variables from the current Module.</li>
<li>Maintain state across invocations of
<a href="#runOnFunction"><tt>runOnFunction</tt></a> (including global data)</li>
</ol>
<p>Implementing a <tt>FunctionPass</tt> is usually straightforward (See the <a
href="#basiccode">Hello World</a> pass for example). <tt>FunctionPass</tt>'s
may overload three virtual methods to do their work. All of these methods
should return true if they modified the program, or false if they didn't.</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="doInitialization_mod">
The <tt>doInitialization(Module &)</tt> method
</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(Module &M);
</pre></div>
<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
<tt>FunctionPass</tt>'s are not allowed to do. They can add and remove
functions, get pointers to functions, etc. The <tt>doInitialization</tt> method
is designed to do simple initialization type of stuff that does not depend on
the functions being processed. The <tt>doInitialization</tt> method call is not
scheduled to overlap with any other pass executions (thus it should be very
fast).</p>
<p>A good example of how this method should be used is the <a
href="http://llvm.org/doxygen/LowerAllocations_8cpp-source.html">LowerAllocations</a>
pass. This pass converts <tt>malloc</tt> and <tt>free</tt> instructions into
platform dependent <tt>malloc()</tt> and <tt>free()</tt> function calls. It
uses the <tt>doInitialization</tt> method to get a reference to the malloc and
free functions that it needs, adding prototypes to the module if necessary.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="runOnFunction">The <tt>runOnFunction</tt> method</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnFunction(Function &F) = 0;
</pre></div><p>
<p>The <tt>runOnFunction</tt> method must be implemented by your subclass to do
the transformation or analysis work of your pass. As usual, a true value should
be returned if the function is modified.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="doFinalization_mod">
The <tt>doFinalization(Module &)</tt> method
</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization(Module &M);
</pre></div>
<p>The <tt>doFinalization</tt> method is an infrequently used method that is
called when the pass framework has finished calling <a
href="#runOnFunction"><tt>runOnFunction</tt></a> for every function in the
program being compiled.</p>
</div>
</div>
<!-- ======================================================================= -->
<h3>
<a name="LoopPass">The <tt>LoopPass</tt> class </a>
</h3>
<div>
<p> All <tt>LoopPass</tt> execute on each loop in the function independent of
all of the other loops in the function. <tt>LoopPass</tt> processes loops in
loop nest order such that outer most loop is processed last. </p>
<p> <tt>LoopPass</tt> subclasses are allowed to update loop nest using
<tt>LPPassManager</tt> interface. Implementing a loop pass is usually
straightforward. <tt>LoopPass</tt>'s may overload three virtual methods to
do their work. All these methods should return true if they modified the
program, or false if they didn't. </p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="doInitialization_loop">
The <tt>doInitialization(Loop *,LPPassManager &)</tt> method
</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(Loop *, LPPassManager &LPM);
</pre></div>
<p>The <tt>doInitialization</tt> method is designed to do simple initialization
type of stuff that does not depend on the functions being processed. The
<tt>doInitialization</tt> method call is not scheduled to overlap with any
other pass executions (thus it should be very fast). LPPassManager
interface should be used to access Function or Module level analysis
information.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="runOnLoop">The <tt>runOnLoop</tt> method</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnLoop(Loop *, LPPassManager &LPM) = 0;
</pre></div><p>
<p>The <tt>runOnLoop</tt> method must be implemented by your subclass to do
the transformation or analysis work of your pass. As usual, a true value should
be returned if the function is modified. <tt>LPPassManager</tt> interface
should be used to update loop nest.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="doFinalization_loop">The <tt>doFinalization()</tt> method</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization();
</pre></div>
<p>The <tt>doFinalization</tt> method is an infrequently used method that is
called when the pass framework has finished calling <a
href="#runOnLoop"><tt>runOnLoop</tt></a> for every loop in the
program being compiled. </p>
</div>
</div>
<!-- ======================================================================= -->
<h3>
<a name="RegionPass">The <tt>RegionPass</tt> class </a>
</h3>
<div>
<p> <tt>RegionPass</tt> is similar to <a href="#LoopPass"><tt>LoopPass</tt></a>,
but executes on each single entry single exit region in the function.
<tt>RegionPass</tt> processes regions in nested order such that the outer most
region is processed last. </p>
<p> <tt>RegionPass</tt> subclasses are allowed to update the region tree by using
the <tt>RGPassManager</tt> interface. You may overload three virtual methods of
<tt>RegionPass</tt> to implement your own region pass. All these
methods should return true if they modified the program, or false if they didn not.
</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="doInitialization_region">
The <tt>doInitialization(Region *, RGPassManager &)</tt> method
</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(Region *, RGPassManager &RGM);
</pre></div>
<p>The <tt>doInitialization</tt> method is designed to do simple initialization
type of stuff that does not depend on the functions being processed. The
<tt>doInitialization</tt> method call is not scheduled to overlap with any
other pass executions (thus it should be very fast). RPPassManager
interface should be used to access Function or Module level analysis
information.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="runOnRegion">The <tt>runOnRegion</tt> method</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnRegion(Region *, RGPassManager &RGM) = 0;
</pre></div><p>
<p>The <tt>runOnRegion</tt> method must be implemented by your subclass to do
the transformation or analysis work of your pass. As usual, a true value should
be returned if the region is modified. <tt>RGPassManager</tt> interface
should be used to update region tree.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="doFinalization_region">The <tt>doFinalization()</tt> method</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization();
</pre></div>
<p>The <tt>doFinalization</tt> method is an infrequently used method that is
called when the pass framework has finished calling <a
href="#runOnRegion"><tt>runOnRegion</tt></a> for every region in the
program being compiled. </p>
</div>
</div>
<!-- ======================================================================= -->
<h3>
<a name="BasicBlockPass">The <tt>BasicBlockPass</tt> class</a>
</h3>
<div>
<p><tt>BasicBlockPass</tt>'s are just like <a
href="#FunctionPass"><tt>FunctionPass</tt></a>'s, except that they must limit
their scope of inspection and modification to a single basic block at a time.
As such, they are <b>not</b> allowed to do any of the following:</p>
<ol>
<li>Modify or inspect any basic blocks outside of the current one</li>
<li>Maintain state across invocations of
<a href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a></li>
<li>Modify the control flow graph (by altering terminator instructions)</li>
<li>Any of the things forbidden for
<a href="#FunctionPass"><tt>FunctionPass</tt></a>es.</li>
</ol>
<p><tt>BasicBlockPass</tt>es are useful for traditional local and "peephole"
optimizations. They may override the same <a
href="#doInitialization_mod"><tt>doInitialization(Module &)</tt></a> and <a
href="#doFinalization_mod"><tt>doFinalization(Module &)</tt></a> methods that <a
href="#FunctionPass"><tt>FunctionPass</tt></a>'s have, but also have the following virtual methods that may also be implemented:</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="doInitialization_fn">
The <tt>doInitialization(Function &)</tt> method
</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doInitialization(Function &F);
</pre></div>
<p>The <tt>doIninitialize</tt> method is allowed to do most of the things that
<tt>BasicBlockPass</tt>'s are not allowed to do, but that
<tt>FunctionPass</tt>'s can. The <tt>doInitialization</tt> method is designed
to do simple initialization that does not depend on the
BasicBlocks being processed. The <tt>doInitialization</tt> method call is not
scheduled to overlap with any other pass executions (thus it should be very
fast).</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="runOnBasicBlock">The <tt>runOnBasicBlock</tt> method</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnBasicBlock(BasicBlock &BB) = 0;
</pre></div>
<p>Override this function to do the work of the <tt>BasicBlockPass</tt>. This
function is not allowed to inspect or modify basic blocks other than the
parameter, and are not allowed to modify the CFG. A true value must be returned
if the basic block is modified.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="doFinalization_fn">
The <tt>doFinalization(Function &)</tt> method
</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> doFinalization(Function &F);
</pre></div>
<p>The <tt>doFinalization</tt> method is an infrequently used method that is
called when the pass framework has finished calling <a
href="#runOnBasicBlock"><tt>runOnBasicBlock</tt></a> for every BasicBlock in the
program being compiled. This can be used to perform per-function
finalization.</p>
</div>
</div>
<!-- ======================================================================= -->
<h3>
<a name="MachineFunctionPass">The <tt>MachineFunctionPass</tt> class</a>
</h3>
<div>
<p>A <tt>MachineFunctionPass</tt> is a part of the LLVM code generator that
executes on the machine-dependent representation of each LLVM function in the
program.</p>
<p>Code generator passes are registered and initialized specially by
<tt>TargetMachine::addPassesToEmitFile</tt> and similar routines, so they
cannot generally be run from the <tt>opt</tt> or <tt>bugpoint</tt>
commands.</p>
<p>A <tt>MachineFunctionPass</tt> is also a <tt>FunctionPass</tt>, so all
the restrictions that apply to a <tt>FunctionPass</tt> also apply to it.
<tt>MachineFunctionPass</tt>es also have additional restrictions. In particular,
<tt>MachineFunctionPass</tt>es are not allowed to do any of the following:</p>
<ol>
<li>Modify or create any LLVM IR Instructions, BasicBlocks, Arguments,
Functions, GlobalVariables, GlobalAliases, or Modules.</li>
<li>Modify a MachineFunction other than the one currently being processed.</li>
<li>Maintain state across invocations of <a
href="#runOnMachineFunction"><tt>runOnMachineFunction</tt></a> (including global
data)</li>
</ol>
<!-- _______________________________________________________________________ -->
<h4>
<a name="runOnMachineFunction">
The <tt>runOnMachineFunction(MachineFunction &MF)</tt> method
</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual bool</b> runOnMachineFunction(MachineFunction &MF) = 0;
</pre></div>
<p><tt>runOnMachineFunction</tt> can be considered the main entry point of a
<tt>MachineFunctionPass</tt>; that is, you should override this method to do the
work of your <tt>MachineFunctionPass</tt>.</p>
<p>The <tt>runOnMachineFunction</tt> method is called on every
<tt>MachineFunction</tt> in a <tt>Module</tt>, so that the
<tt>MachineFunctionPass</tt> may perform optimizations on the machine-dependent
representation of the function. If you want to get at the LLVM <tt>Function</tt>
for the <tt>MachineFunction</tt> you're working on, use
<tt>MachineFunction</tt>'s <tt>getFunction()</tt> accessor method -- but
remember, you may not modify the LLVM <tt>Function</tt> or its contents from a
<tt>MachineFunctionPass</tt>.</p>
</div>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="registration">Pass registration</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>In the <a href="#basiccode">Hello World</a> example pass we illustrated how
pass registration works, and discussed some of the reasons that it is used and
what it does. Here we discuss how and why passes are registered.</p>
<p>As we saw above, passes are registered with the <b><tt>RegisterPass</tt></b>
template. The template parameter is the name of the pass that is to be used on
the command line to specify that the pass should be added to a program (for
example, with <tt>opt</tt> or <tt>bugpoint</tt>). The first argument is the
name of the pass, which is to be used for the <tt>-help</tt> output of
programs, as
well as for debug output generated by the <tt>--debug-pass</tt> option.</p>
<p>If you want your pass to be easily dumpable, you should
implement the virtual <tt>print</tt> method:</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="print">The <tt>print</tt> method</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual void</b> print(std::ostream &O, <b>const</b> Module *M) <b>const</b>;
</pre></div>
<p>The <tt>print</tt> method must be implemented by "analyses" in order to print
a human readable version of the analysis results. This is useful for debugging
an analysis itself, as well as for other people to figure out how an analysis
works. Use the <tt>opt -analyze</tt> argument to invoke this method.</p>
<p>The <tt>llvm::OStream</tt> parameter specifies the stream to write the results on,
and the <tt>Module</tt> parameter gives a pointer to the top level module of the
program that has been analyzed. Note however that this pointer may be null in
certain circumstances (such as calling the <tt>Pass::dump()</tt> from a
debugger), so it should only be used to enhance debug output, it should not be
depended on.</p>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="interaction">Specifying interactions between passes</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>One of the main responsibilities of the <tt>PassManager</tt> is to make sure
that passes interact with each other correctly. Because <tt>PassManager</tt>
tries to <a href="#passmanager">optimize the execution of passes</a> it must
know how the passes interact with each other and what dependencies exist between
the various passes. To track this, each pass can declare the set of passes that
are required to be executed before the current pass, and the passes which are
invalidated by the current pass.</p>
<p>Typically this functionality is used to require that analysis results are
computed before your pass is run. Running arbitrary transformation passes can
invalidate the computed analysis results, which is what the invalidation set
specifies. If a pass does not implement the <tt><a
href="#getAnalysisUsage">getAnalysisUsage</a></tt> method, it defaults to not
having any prerequisite passes, and invalidating <b>all</b> other passes.</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="getAnalysisUsage">The <tt>getAnalysisUsage</tt> method</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual void</b> getAnalysisUsage(AnalysisUsage &Info) <b>const</b>;
</pre></div>
<p>By implementing the <tt>getAnalysisUsage</tt> method, the required and
invalidated sets may be specified for your transformation. The implementation
should fill in the <tt><a
href="http://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html">AnalysisUsage</a></tt>
object with information about which passes are required and not invalidated. To
do this, a pass may call any of the following methods on the AnalysisUsage
object:</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="AU::addRequired">
The <tt>AnalysisUsage::addRequired<></tt>
and <tt>AnalysisUsage::addRequiredTransitive<></tt> methods
</a>
</h4>
<div>
<p>
If your pass requires a previous pass to be executed (an analysis for example),
it can use one of these methods to arrange for it to be run before your pass.
LLVM has many different types of analyses and passes that can be required,
spanning the range from <tt>DominatorSet</tt> to <tt>BreakCriticalEdges</tt>.
Requiring <tt>BreakCriticalEdges</tt>, for example, guarantees that there will
be no critical edges in the CFG when your pass has been run.
</p>
<p>
Some analyses chain to other analyses to do their job. For example, an <a
href="AliasAnalysis.html">AliasAnalysis</a> implementation is required to <a
href="AliasAnalysis.html#chaining">chain</a> to other alias analysis passes. In
cases where analyses chain, the <tt>addRequiredTransitive</tt> method should be
used instead of the <tt>addRequired</tt> method. This informs the PassManager
that the transitively required pass should be alive as long as the requiring
pass is.
</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="AU::addPreserved">
The <tt>AnalysisUsage::addPreserved<></tt> method
</a>
</h4>
<div>
<p>
One of the jobs of the PassManager is to optimize how and when analyses are run.
In particular, it attempts to avoid recomputing data unless it needs to. For
this reason, passes are allowed to declare that they preserve (i.e., they don't
invalidate) an existing analysis if it's available. For example, a simple
constant folding pass would not modify the CFG, so it can't possibly affect the
results of dominator analysis. By default, all passes are assumed to invalidate
all others.
</p>
<p>
The <tt>AnalysisUsage</tt> class provides several methods which are useful in
certain circumstances that are related to <tt>addPreserved</tt>. In particular,
the <tt>setPreservesAll</tt> method can be called to indicate that the pass does
not modify the LLVM program at all (which is true for analyses), and the
<tt>setPreservesCFG</tt> method can be used by transformations that change
instructions in the program but do not modify the CFG or terminator instructions
(note that this property is implicitly set for <a
href="#BasicBlockPass">BasicBlockPass</a>'s).
</p>
<p>
<tt>addPreserved</tt> is particularly useful for transformations like
<tt>BreakCriticalEdges</tt>. This pass knows how to update a small set of loop
and dominator related analyses if they exist, so it can preserve them, despite
the fact that it hacks on the CFG.
</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="AU::examples">
Example implementations of <tt>getAnalysisUsage</tt>
</a>
</h4>
<div>
<div class="doc_code"><pre>
<i>// This example modifies the program, but does not modify the CFG</i>
<b>void</b> <a href="http://llvm.org/doxygen/structLICM.html">LICM</a>::getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
AU.setPreservesCFG();
AU.addRequired<<a href="http://llvm.org/doxygen/classllvm_1_1LoopInfo.html">LoopInfo</a>>();
}
</pre></div>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="getAnalysis">
The <tt>getAnalysis<></tt> and
<tt>getAnalysisIfAvailable<></tt> methods
</a>
</h4>
<div>
<p>The <tt>Pass::getAnalysis<></tt> method is automatically inherited by
your class, providing you with access to the passes that you declared that you
required with the <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a>
method. It takes a single template argument that specifies which pass class you
want, and returns a reference to that pass. For example:</p>
<div class="doc_code"><pre>
bool LICM::runOnFunction(Function &F) {
LoopInfo &LI = getAnalysis<LoopInfo>();
...
}
</pre></div>
<p>This method call returns a reference to the pass desired. You may get a
runtime assertion failure if you attempt to get an analysis that you did not
declare as required in your <a
href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> implementation. This
method can be called by your <tt>run*</tt> method implementation, or by any
other local method invoked by your <tt>run*</tt> method.
A module level pass can use function level analysis info using this interface.
For example:</p>
<div class="doc_code"><pre>
bool ModuleLevelPass::runOnModule(Module &M) {
...
DominatorTree &DT = getAnalysis<DominatorTree>(Func);
...
}
</pre></div>
<p>In above example, runOnFunction for DominatorTree is called by pass manager
before returning a reference to the desired pass.</p>
<p>
If your pass is capable of updating analyses if they exist (e.g.,
<tt>BreakCriticalEdges</tt>, as described above), you can use the
<tt>getAnalysisIfAvailable</tt> method, which returns a pointer to the analysis
if it is active. For example:</p>
<div class="doc_code"><pre>
...
if (DominatorSet *DS = getAnalysisIfAvailable<DominatorSet>()) {
<i>// A DominatorSet is active. This code will update it.</i>
}
...
</pre></div>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="analysisgroup">Implementing Analysis Groups</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>Now that we understand the basics of how passes are defined, how they are
used, and how they are required from other passes, it's time to get a little bit
fancier. All of the pass relationships that we have seen so far are very
simple: one pass depends on one other specific pass to be run before it can run.
For many applications, this is great, for others, more flexibility is
required.</p>
<p>In particular, some analyses are defined such that there is a single simple
interface to the analysis results, but multiple ways of calculating them.
Consider alias analysis for example. The most trivial alias analysis returns
"may alias" for any alias query. The most sophisticated analysis a
flow-sensitive, context-sensitive interprocedural analysis that can take a
significant amount of time to execute (and obviously, there is a lot of room
between these two extremes for other implementations). To cleanly support
situations like this, the LLVM Pass Infrastructure supports the notion of
Analysis Groups.</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="agconcepts">Analysis Group Concepts</a>
</h4>
<div>
<p>An Analysis Group is a single simple interface that may be implemented by
multiple different passes. Analysis Groups can be given human readable names
just like passes, but unlike passes, they need not derive from the <tt>Pass</tt>
class. An analysis group may have one or more implementations, one of which is
the "default" implementation.</p>
<p>Analysis groups are used by client passes just like other passes are: the
<tt>AnalysisUsage::addRequired()</tt> and <tt>Pass::getAnalysis()</tt> methods.
In order to resolve this requirement, the <a href="#passmanager">PassManager</a>
scans the available passes to see if any implementations of the analysis group
are available. If none is available, the default implementation is created for
the pass to use. All standard rules for <A href="#interaction">interaction
between passes</a> still apply.</p>
<p>Although <a href="#registration">Pass Registration</a> is optional for normal
passes, all analysis group implementations must be registered, and must use the
<A href="#registerag"><tt>INITIALIZE_AG_PASS</tt></a> template to join the
implementation pool. Also, a default implementation of the interface
<b>must</b> be registered with <A
href="#registerag"><tt>RegisterAnalysisGroup</tt></a>.</p>
<p>As a concrete example of an Analysis Group in action, consider the <a
href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>
analysis group. The default implementation of the alias analysis interface (the
<tt><a
href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">basicaa</a></tt>
pass) just does a few simple checks that don't require significant analysis to
compute (such as: two different globals can never alias each other, etc).
Passes that use the <tt><a
href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
interface (for example the <tt><a
href="http://llvm.org/doxygen/structGCSE.html">gcse</a></tt> pass), do
not care which implementation of alias analysis is actually provided, they just
use the designated interface.</p>
<p>From the user's perspective, commands work just like normal. Issuing the
command '<tt>opt -gcse ...</tt>' will cause the <tt>basicaa</tt> class to be
instantiated and added to the pass sequence. Issuing the command '<tt>opt
-somefancyaa -gcse ...</tt>' will cause the <tt>gcse</tt> pass to use the
<tt>somefancyaa</tt> alias analysis (which doesn't actually exist, it's just a
hypothetical example) instead.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="registerag">Using <tt>RegisterAnalysisGroup</tt></a>
</h4>
<div>
<p>The <tt>RegisterAnalysisGroup</tt> template is used to register the analysis
group itself, while the <tt>INITIALIZE_AG_PASS</tt> is used to add pass
implementations to the analysis group. First,
an analysis group should be registered, with a human readable name
provided for it.
Unlike registration of passes, there is no command line argument to be specified
for the Analysis Group Interface itself, because it is "abstract":</p>
<div class="doc_code"><pre>
<b>static</b> RegisterAnalysisGroup<<a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>> A("<i>Alias Analysis</i>");
</pre></div>
<p>Once the analysis is registered, passes can declare that they are valid
implementations of the interface by using the following code:</p>
<div class="doc_code"><pre>
<b>namespace</b> {
//<i> Declare that we implement the AliasAnalysis interface</i>
INITIALIZE_AG_PASS(FancyAA, <a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, "<i>somefancyaa</i>",
"<i>A more complex alias analysis implementation</i>",
false, // <i>Is CFG Only?</i>
true, // <i>Is Analysis?</i>
false, // <i>Is default Analysis Group implementation?</i>
);
}
</pre></div>
<p>This just shows a class <tt>FancyAA</tt> that
uses the <tt>INITIALIZE_AG_PASS</tt> macro both to register and
to "join" the <tt><a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a></tt>
analysis group. Every implementation of an analysis group should join using
this macro.</p>
<div class="doc_code"><pre>
<b>namespace</b> {
//<i> Declare that we implement the AliasAnalysis interface</i>
INITIALIZE_AG_PASS(BasicAA, <a href="http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html">AliasAnalysis</a>, "<i>basicaa</i>",
"<i>Basic Alias Analysis (default AA impl)</i>",
false, // <i>Is CFG Only?</i>
true, // <i>Is Analysis?</i>
true, // <i>Is default Analysis Group implementation?</i>
);
}
</pre></div>
<p>Here we show how the default implementation is specified (using the final
argument to the <tt>INITIALIZE_AG_PASS</tt> template). There must be exactly
one default implementation available at all times for an Analysis Group to be
used. Only default implementation can derive from <tt>ImmutablePass</tt>.
Here we declare that the
<tt><a href="http://llvm.org/doxygen/structBasicAliasAnalysis.html">BasicAliasAnalysis</a></tt>
pass is the default implementation for the interface.</p>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="passStatistics">Pass Statistics</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>The <a
href="http://llvm.org/doxygen/Statistic_8h-source.html"><tt>Statistic</tt></a>
class is designed to be an easy way to expose various success
metrics from passes. These statistics are printed at the end of a
run, when the -stats command line option is enabled on the command
line. See the <a href="http://llvm.org/docs/ProgrammersManual.html#Statistic">Statistics section</a> in the Programmer's Manual for details.
</div>
<!-- *********************************************************************** -->
<h2>
<a name="passmanager">What PassManager does</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>The <a
href="http://llvm.org/doxygen/PassManager_8h-source.html"><tt>PassManager</tt></a>
<a
href="http://llvm.org/doxygen/classllvm_1_1PassManager.html">class</a>
takes a list of passes, ensures their <a href="#interaction">prerequisites</a>
are set up correctly, and then schedules passes to run efficiently. All of the
LLVM tools that run passes use the <tt>PassManager</tt> for execution of these
passes.</p>
<p>The <tt>PassManager</tt> does two main things to try to reduce the execution
time of a series of passes:</p>
<ol>
<li><b>Share analysis results</b> - The PassManager attempts to avoid
recomputing analysis results as much as possible. This means keeping track of
which analyses are available already, which analyses get invalidated, and which
analyses are needed to be run for a pass. An important part of work is that the
<tt>PassManager</tt> tracks the exact lifetime of all analysis results, allowing
it to <a href="#releaseMemory">free memory</a> allocated to holding analysis
results as soon as they are no longer needed.</li>
<li><b>Pipeline the execution of passes on the program</b> - The
<tt>PassManager</tt> attempts to get better cache and memory usage behavior out
of a series of passes by pipelining the passes together. This means that, given
a series of consecutive <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s, it
will execute all of the <a href="#FunctionPass"><tt>FunctionPass</tt></a>'s on
the first function, then all of the <a
href="#FunctionPass"><tt>FunctionPass</tt></a>es on the second function,
etc... until the entire program has been run through the passes.
<p>This improves the cache behavior of the compiler, because it is only touching
the LLVM program representation for a single function at a time, instead of
traversing the entire program. It reduces the memory consumption of compiler,
because, for example, only one <a
href="http://llvm.org/doxygen/classllvm_1_1DominatorSet.html"><tt>DominatorSet</tt></a>
needs to be calculated at a time. This also makes it possible to implement
some <a
href="#SMP">interesting enhancements</a> in the future.</p></li>
</ol>
<p>The effectiveness of the <tt>PassManager</tt> is influenced directly by how
much information it has about the behaviors of the passes it is scheduling. For
example, the "preserved" set is intentionally conservative in the face of an
unimplemented <a href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method.
Not implementing when it should be implemented will have the effect of not
allowing any analysis results to live across the execution of your pass.</p>
<p>The <tt>PassManager</tt> class exposes a <tt>--debug-pass</tt> command line
options that is useful for debugging pass execution, seeing how things work, and
diagnosing when you should be preserving more analyses than you currently are
(To get information about all of the variants of the <tt>--debug-pass</tt>
option, just type '<tt>opt -help-hidden</tt>').</p>
<p>By using the <tt>--debug-pass=Structure</tt> option, for example, we can see
how our <a href="#basiccode">Hello World</a> pass interacts with other passes.
Lets try it out with the <tt>gcse</tt> and <tt>licm</tt> passes:</p>
<div class="doc_code"><pre>
$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -licm --debug-pass=Structure < hello.bc > /dev/null
Module Pass Manager
Function Pass Manager
Dominator Set Construction
Immediate Dominators Construction
Global Common Subexpression Elimination
-- Immediate Dominators Construction
-- Global Common Subexpression Elimination
Natural Loop Construction
Loop Invariant Code Motion
-- Natural Loop Construction
-- Loop Invariant Code Motion
Module Verifier
-- Dominator Set Construction
-- Module Verifier
Bitcode Writer
--Bitcode Writer
</pre></div>
<p>This output shows us when passes are constructed and when the analysis
results are known to be dead (prefixed with '<tt>--</tt>'). Here we see that
GCSE uses dominator and immediate dominator information to do its job. The LICM
pass uses natural loop information, which uses dominator sets, but not immediate
dominators. Because immediate dominators are no longer useful after the GCSE
pass, it is immediately destroyed. The dominator sets are then reused to
compute natural loop information, which is then used by the LICM pass.</p>
<p>After the LICM pass, the module verifier runs (which is automatically added
by the '<tt>opt</tt>' tool), which uses the dominator set to check that the
resultant LLVM code is well formed. After it finishes, the dominator set
information is destroyed, after being computed once, and shared by three
passes.</p>
<p>Lets see how this changes when we run the <a href="#basiccode">Hello
World</a> pass in between the two passes:</p>
<div class="doc_code"><pre>
$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
Module Pass Manager
Function Pass Manager
Dominator Set Construction
Immediate Dominators Construction
Global Common Subexpression Elimination
<b>-- Dominator Set Construction</b>
-- Immediate Dominators Construction
-- Global Common Subexpression Elimination
<b> Hello World Pass
-- Hello World Pass
Dominator Set Construction</b>
Natural Loop Construction
Loop Invariant Code Motion
-- Natural Loop Construction
-- Loop Invariant Code Motion
Module Verifier
-- Dominator Set Construction
-- Module Verifier
Bitcode Writer
--Bitcode Writer
Hello: __main
Hello: puts
Hello: main
</pre></div>
<p>Here we see that the <a href="#basiccode">Hello World</a> pass has killed the
Dominator Set pass, even though it doesn't modify the code at all! To fix this,
we need to add the following <a
href="#getAnalysisUsage"><tt>getAnalysisUsage</tt></a> method to our pass:</p>
<div class="doc_code"><pre>
<i>// We don't modify the program, so we preserve all analyses</i>
<b>virtual void</b> getAnalysisUsage(AnalysisUsage &AU) <b>const</b> {
AU.setPreservesAll();
}
</pre></div>
<p>Now when we run our pass, we get this output:</p>
<div class="doc_code"><pre>
$ opt -load ../../../Debug+Asserts/lib/Hello.so -gcse -hello -licm --debug-pass=Structure < hello.bc > /dev/null
Pass Arguments: -gcse -hello -licm
Module Pass Manager
Function Pass Manager
Dominator Set Construction
Immediate Dominators Construction
Global Common Subexpression Elimination
-- Immediate Dominators Construction
-- Global Common Subexpression Elimination
Hello World Pass
-- Hello World Pass
Natural Loop Construction
Loop Invariant Code Motion
-- Loop Invariant Code Motion
-- Natural Loop Construction
Module Verifier
-- Dominator Set Construction
-- Module Verifier
Bitcode Writer
--Bitcode Writer
Hello: __main
Hello: puts
Hello: main
</pre></div>
<p>Which shows that we don't accidentally invalidate dominator information
anymore, and therefore do not have to compute it twice.</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="releaseMemory">The <tt>releaseMemory</tt> method</a>
</h4>
<div>
<div class="doc_code"><pre>
<b>virtual void</b> releaseMemory();
</pre></div>
<p>The <tt>PassManager</tt> automatically determines when to compute analysis
results, and how long to keep them around for. Because the lifetime of the pass
object itself is effectively the entire duration of the compilation process, we
need some way to free analysis results when they are no longer useful. The
<tt>releaseMemory</tt> virtual method is the way to do this.</p>
<p>If you are writing an analysis or any other pass that retains a significant
amount of state (for use by another pass which "requires" your pass and uses the
<a href="#getAnalysis">getAnalysis</a> method) you should implement
<tt>releaseMemory</tt> to, well, release the memory allocated to maintain this
internal state. This method is called after the <tt>run*</tt> method for the
class, before the next call of <tt>run*</tt> in your pass.</p>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="registering">Registering dynamically loaded passes</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p><i>Size matters</i> when constructing production quality tools using llvm,
both for the purposes of distribution, and for regulating the resident code size
when running on the target system. Therefore, it becomes desirable to
selectively use some passes, while omitting others and maintain the flexibility
to change configurations later on. You want to be able to do all this, and,
provide feedback to the user. This is where pass registration comes into
play.</p>
<p>The fundamental mechanisms for pass registration are the
<tt>MachinePassRegistry</tt> class and subclasses of
<tt>MachinePassRegistryNode</tt>.</p>
<p>An instance of <tt>MachinePassRegistry</tt> is used to maintain a list of
<tt>MachinePassRegistryNode</tt> objects. This instance maintains the list and
communicates additions and deletions to the command line interface.</p>
<p>An instance of <tt>MachinePassRegistryNode</tt> subclass is used to maintain
information provided about a particular pass. This information includes the
command line name, the command help string and the address of the function used
to create an instance of the pass. A global static constructor of one of these
instances <i>registers</i> with a corresponding <tt>MachinePassRegistry</tt>,
the static destructor <i>unregisters</i>. Thus a pass that is statically linked
in the tool will be registered at start up. A dynamically loaded pass will
register on load and unregister at unload.</p>
<!-- _______________________________________________________________________ -->
<h3>
<a name="registering_existing">Using existing registries</a>
</h3>
<div>
<p>There are predefined registries to track instruction scheduling
(<tt>RegisterScheduler</tt>) and register allocation (<tt>RegisterRegAlloc</tt>)
machine passes. Here we will describe how to <i>register</i> a register
allocator machine pass.</p>
<p>Implement your register allocator machine pass. In your register allocator
.cpp file add the following include;</p>
<div class="doc_code"><pre>
#include "llvm/CodeGen/RegAllocRegistry.h"
</pre></div>
<p>Also in your register allocator .cpp file, define a creator function in the
form; </p>
<div class="doc_code"><pre>
FunctionPass *createMyRegisterAllocator() {
return new MyRegisterAllocator();
}
</pre></div>
<p>Note that the signature of this function should match the type of
<tt>RegisterRegAlloc::FunctionPassCtor</tt>. In the same file add the
"installing" declaration, in the form;</p>
<div class="doc_code"><pre>
static RegisterRegAlloc myRegAlloc("myregalloc",
" my register allocator help string",
createMyRegisterAllocator);
</pre></div>
<p>Note the two spaces prior to the help string produces a tidy result on the
-help query.</p>
<div class="doc_code"><pre>
$ llc -help
...
-regalloc - Register allocator to use (default=linearscan)
=linearscan - linear scan register allocator
=local - local register allocator
=simple - simple register allocator
=myregalloc - my register allocator help string
...
</pre></div>
<p>And that's it. The user is now free to use <tt>-regalloc=myregalloc</tt> as
an option. Registering instruction schedulers is similar except use the
<tt>RegisterScheduler</tt> class. Note that the
<tt>RegisterScheduler::FunctionPassCtor</tt> is significantly different from
<tt>RegisterRegAlloc::FunctionPassCtor</tt>.</p>
<p>To force the load/linking of your register allocator into the llc/lli tools,
add your creator function's global declaration to "Passes.h" and add a "pseudo"
call line to <tt>llvm/Codegen/LinkAllCodegenComponents.h</tt>.</p>
</div>
<!-- _______________________________________________________________________ -->
<h3>
<a name="registering_new">Creating new registries</a>
</h3>
<div>
<p>The easiest way to get started is to clone one of the existing registries; we
recommend <tt>llvm/CodeGen/RegAllocRegistry.h</tt>. The key things to modify
are the class name and the <tt>FunctionPassCtor</tt> type.</p>
<p>Then you need to declare the registry. Example: if your pass registry is
<tt>RegisterMyPasses</tt> then define;</p>
<div class="doc_code"><pre>
MachinePassRegistry RegisterMyPasses::Registry;
</pre></div>
<p>And finally, declare the command line option for your passes. Example:</p>
<div class="doc_code"><pre>
cl::opt<RegisterMyPasses::FunctionPassCtor, false,
RegisterPassParser<RegisterMyPasses> >
MyPassOpt("mypass",
cl::init(&createDefaultMyPass),
cl::desc("my pass option help"));
</pre></div>
<p>Here the command option is "mypass", with createDefaultMyPass as the default
creator.</p>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="debughints">Using GDB with dynamically loaded passes</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>Unfortunately, using GDB with dynamically loaded passes is not as easy as it
should be. First of all, you can't set a breakpoint in a shared object that has
not been loaded yet, and second of all there are problems with inlined functions
in shared objects. Here are some suggestions to debugging your pass with
GDB.</p>
<p>For sake of discussion, I'm going to assume that you are debugging a
transformation invoked by <tt>opt</tt>, although nothing described here depends
on that.</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="breakpoint">Setting a breakpoint in your pass</a>
</h4>
<div>
<p>First thing you do is start <tt>gdb</tt> on the <tt>opt</tt> process:</p>
<div class="doc_code"><pre>
$ <b>gdb opt</b>
GNU gdb 5.0
Copyright 2000 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB. Type "show warranty" for details.
This GDB was configured as "sparc-sun-solaris2.6"...
(gdb)
</pre></div>
<p>Note that <tt>opt</tt> has a lot of debugging information in it, so it takes
time to load. Be patient. Since we cannot set a breakpoint in our pass yet
(the shared object isn't loaded until runtime), we must execute the process, and
have it stop before it invokes our pass, but after it has loaded the shared
object. The most foolproof way of doing this is to set a breakpoint in
<tt>PassManager::run</tt> and then run the process with the arguments you
want:</p>
<div class="doc_code"><pre>
(gdb) <b>break llvm::PassManager::run</b>
Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
(gdb) <b>run test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption]</b>
Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption]
Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
70 bool PassManager::run(Module &M) { return PM->run(M); }
(gdb)
</pre></div>
<p>Once the <tt>opt</tt> stops in the <tt>PassManager::run</tt> method you are
now free to set breakpoints in your pass so that you can trace through execution
or do other standard debugging stuff.</p>
</div>
<!-- _______________________________________________________________________ -->
<h4>
<a name="debugmisc">Miscellaneous Problems</a>
</h4>
<div>
<p>Once you have the basics down, there are a couple of problems that GDB has,
some with solutions, some without.</p>
<ul>
<li>Inline functions have bogus stack information. In general, GDB does a
pretty good job getting stack traces and stepping through inline functions.
When a pass is dynamically loaded however, it somehow completely loses this
capability. The only solution I know of is to de-inline a function (move it
from the body of a class to a .cpp file).</li>
<li>Restarting the program breaks breakpoints. After following the information
above, you have succeeded in getting some breakpoints planted in your pass. Nex
thing you know, you restart the program (i.e., you type '<tt>run</tt>' again),
and you start getting errors about breakpoints being unsettable. The only way I
have found to "fix" this problem is to <tt>delete</tt> the breakpoints that are
already set in your pass, run the program, and re-set the breakpoints once
execution stops in <tt>PassManager::run</tt>.</li>
</ul>
<p>Hopefully these tips will help with common case debugging situations. If
you'd like to contribute some tips of your own, just contact <a
href="mailto:sabre@nondot.org">Chris</a>.</p>
</div>
</div>
<!-- *********************************************************************** -->
<h2>
<a name="future">Future extensions planned</a>
</h2>
<!-- *********************************************************************** -->
<div>
<p>Although the LLVM Pass Infrastructure is very capable as it stands, and does
some nifty stuff, there are things we'd like to add in the future. Here is
where we are going:</p>
<!-- _______________________________________________________________________ -->
<h4>
<a name="SMP">Multithreaded LLVM</a>
</h4>
<div>
<p>Multiple CPU machines are becoming more common and compilation can never be
fast enough: obviously we should allow for a multithreaded compiler. Because of
the semantics defined for passes above (specifically they cannot maintain state
across invocations of their <tt>run*</tt> methods), a nice clean way to
implement a multithreaded compiler would be for the <tt>PassManager</tt> class
to create multiple instances of each pass object, and allow the separate
instances to be hacking on different parts of the program at the same time.</p>
<p>This implementation would prevent each of the passes from having to implement
multithreaded constructs, requiring only the LLVM core to have locking in a few
places (for global resources). Although this is a simple extension, we simply
haven't had time (or multiprocessor machines, thus a reason) to implement this.
Despite that, we have kept the LLVM passes SMP ready, and you should too.</p>
</div>
</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>
|
