summaryrefslogtreecommitdiff
path: root/docs/BytecodeFormat.html
blob: 51177492a9ecd7885a565876c1f9edc00636d41a (plain) 
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
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154

<!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>LLVM Bytecode File Format</title>
  <link rel="stylesheet" href="llvm.css" type="text/css">
  <style type="text/css">
    TR, TD { border: 2px solid gray; padding-left: 4pt; padding-right: 4pt; 
             padding-top: 2pt; padding-bottom: 2pt; }
    TH { border: 2px solid gray; font-weight: bold; font-size: 105%; }
    TABLE { text-align: center; border: 2px solid black; 
      border-collapse: collapse; margin-top: 1em; margin-left: 1em; 
      margin-right: 1em; margin-bottom: 1em; }
    .td_left { border: 2px solid gray; text-align: left; }
  </style>
</head>
<body>
<div class="doc_title"> LLVM Bytecode File Format </div>
<ol>
  <li><a href="#abstract">Abstract</a></li>
  <li><a href="#concepts">Concepts</a>
    <ol>
      <li><a href="#blocks">Blocks</a></li>
      <li><a href="#lists">Lists</a></li>
      <li><a href="#fields">Fields</a></li>
      <li><a href="#align">Alignment</a></li>
      <li><a href="#vbr">Variable Bit-Rate Encoding</a></li>
      <li><a href="#encoding">Encoding Primitives</a></li>
      <li><a href="#slots">Slots</a></li>
    </ol>
  </li>
  <li><a href="#general">General Structure</a> </li>
  <li><a href="#blockdefs">Block Definitions</a>
    <ol>
      <li><a href="#signature">Signature Block</a></li>
      <li><a href="#module">Module Block</a></li>
      <li><a href="#globaltypes">Global Type Pool</a></li>
      <li><a href="#globalinfo">Module Info Block</a></li>
      <li><a href="#constantpool">Global Constant Pool</a></li>
      <li><a href="#functiondefs">Function Definition</a></li>
      <li><a href="#compactiontable">Compaction Table</a></li>
      <li><a href="#instructionlist">Instructions List</a></li>
      <li><a href="#instructions">Instructions</a></li>
      <li><a href="#symtab">Symbol Table</a></li>
    </ol>
  </li>
  <li><a href="#versiondiffs">Version Differences</a>
    <ol>
      <li><a href="#vers13">Version 1.3 Differences From 1.4</a></li>
      <li><a href="#vers12">Version 1.2 Differences From 1.3</a></li>
      <li><a href="#vers11">Version 1.1 Differences From 1.2</a></li>
      <li><a href="#vers10">Version 1.0 Differences From 1.1</a></li>
    </ol>
  </li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="abstract">Abstract </a></div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This document describes the LLVM bytecode file format. It specifies
the binary encoding rules of the bytecode file format so that
equivalent systems can encode bytecode files correctly. The LLVM
bytecode representation is used to store the intermediate
representation on disk in compacted form.</p>
<p>The LLVM bytecode format may change in the future, but LLVM will
always be backwards compatible with older formats. This document will
only describe the most current version of the bytecode format. See <a
 href="#versiondiffs">Version Differences</a> for the details on how
the current version is different from previous versions.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="concepts">Concepts</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section describes the general concepts of the bytecode file
format without getting into specific layout details. It is recommended
that you read this section thoroughly before interpreting the detailed
descriptions.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="blocks">Blocks</a> </div>
<div class="doc_text">
<p>LLVM bytecode files consist simply of a sequence of blocks of bytes
using a binary encoding Each block begins with an header of two
unsigned integers. The first value identifies the type of block and the
second value provides the size of the block in bytes. The block
identifier is used because it is possible for entire blocks to be
omitted from the file if they are empty. The block identifier helps the
reader determine which kind of block is next in the file. Note that
blocks can be nested within other blocks.</p>
<p> All blocks are variable length, and the block header specifies the
size of the block. All blocks begin on a byte index that is aligned to
an even 32-bit boundary. That is, the first block is 32-bit aligned
because it starts at offset 0. Each block is padded with zero fill
bytes to ensure that the next block also starts on a 32-bit boundary.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="lists">Lists</a> </div>
<div class="doc_text">
<p>LLVM Bytecode blocks often contain lists of things of a similar
type. For example, a function contains a list of instructions and a
function type contains a list of argument types. There are two basic
types of lists: length lists (<a href="#llist">llist</a>), and null
terminated lists (<a href="#zlist">zlist</a>), as described below in
the <a href="#encoding">Encoding Primitives</a>.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="fields">Fields</a> </div>
<div class="doc_text">
<p>Fields are units of information that LLVM knows how to write atomically. Most 
fields have a uniform length or some kind of length indication built into their 
encoding. For example, a constant string (array of bytes) is written simply as 
the length followed by the characters. Although this is similar to a list, 
constant strings are treated atomically and are thus fields.</p>
<p>Fields use a condensed bit format specific to the type of information
they must contain. As few bits as possible are written for each field. The
sections that follow will provide the details on how these fields are
written and how the bits are to be interpreted.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="align">Alignment</a> </div>
<div class="doc_text">
  <p>To support cross-platform differences, the bytecode file is aligned on 
  certain boundaries. This means that a small amount of padding (at most 3 
  bytes) will be added to ensure that the next entry is aligned to a 32-bit 
  boundary.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="vbr">Variable Bit-Rate Encoding</a>
</div>
<div class="doc_text">
<p>Most of the values written to LLVM bytecode files are small integers. To 
minimize the number of bytes written for these quantities, an encoding scheme 
similar to UTF-8 is used to write integer data. The scheme is known as
variable bit rate (vbr) encoding. In this encoding, the high bit of
each byte is used to indicate if more bytes follow. If (byte &amp;
0x80) is non-zero in any given byte, it means there is another byte
immediately following that also contributes to the value. For the final
byte (byte &amp; 0x80) is false (the high bit is not set). In each byte
only the low seven bits contribute to the value. Consequently 32-bit
quantities can take from one to <em>five</em> bytes to encode. In
general, smaller quantities will encode in fewer bytes, as follows:</p>
<table>
  <tbody>
    <tr>
      <th>Byte #</th>
      <th>Significant Bits</th>
      <th>Maximum Value</th>
    </tr>
    <tr>
      <td>1</td>
      <td>0-6</td>
      <td>127</td>
    </tr>
    <tr>
      <td>2</td>
      <td>7-13</td>
      <td>16,383</td>
    </tr>
    <tr>
      <td>3</td>
      <td>14-20</td>
      <td>2,097,151</td>
    </tr>
    <tr>
      <td>4</td>
      <td>21-27</td>
      <td>268,435,455</td>
    </tr>
    <tr>
      <td>5</td>
      <td>28-34</td>
      <td>34,359,738,367</td>
    </tr>
    <tr>
      <td>6</td>
      <td>35-41</td>
      <td>4,398,046,511,103</td>
    </tr>
    <tr>
      <td>7</td>
      <td>42-48</td>
      <td>562,949,953,421,311</td>
    </tr>
    <tr>
      <td>8</td>
      <td>49-55</td>
      <td>72,057,594,037,927,935</td>
    </tr>
    <tr>
      <td>9</td>
      <td>56-62</td>
      <td>9,223,372,036,854,775,807</td>
    </tr>
    <tr>
      <td>10</td>
      <td>63-69</td>
      <td>1,180,591,620,717,411,303,423</td>
    </tr>
  </tbody>
</table>
<p>Note that in practice, the tenth byte could only encode bit 63 since
the maximum quantity to use this encoding is a 64-bit integer.</p>
<p><em>Signed</em> VBR values are encoded with the standard vbr
encoding, but with the sign bit as the low order bit instead of the
high order bit. This allows small negative quantities to be encoded
efficiently. For example, -3
is encoded as "((3 &lt;&lt; 1) | 1)" and 3 is encoded as "(3 &lt;&lt;
1) | 0)", emitted with the standard vbr encoding above.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="encoding">Encoding Primitives</a> </div>
<div class="doc_text">
<p>Each field in the bytecode format is encoded into the file using a
small set of primitive formats. The table below defines the encoding
rules for the various primitives used and gives them each a type name.
The type names used in the descriptions of blocks and fields in the <a
 href="#details">Detailed Layout</a>next section. Any type name with
the suffix <em>_vbr</em> indicates a quantity that is encoded using
variable bit rate encoding as described above.</p>
<table class="doc_table">
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Rule</b></th>
    </tr>
    <tr>
      <td><a name="unsigned"><b>unsigned</b></a></td>
      <td class="td_left">A 32-bit unsigned integer that always occupies four 
      consecutive bytes. The unsigned integer is encoded using LSB first 
      ordering. That is bits 2<sup>0</sup> through 2<sup>7</sup> are in the 
      byte with the lowest file offset (little endian).</td>
    </tr>
    <tr>
      <td style="vertical-align: top;"><a name="uint24_vbr">
        <b>uint24_vbr</b></a></td>
      <td style="vertical-align: top; text-align: left;">A 24-bit unsigned 
      integer that occupies from one to four bytes using variable bit rate 
      encoding.</td>
    </tr>
    <tr>
      <td><a name="uint32_vbr"><b>uint32_vbr</b></a></td>
      <td class="td_left">A 32-bit unsigned integer that occupies from one to 
        five bytes using variable bit rate encoding.</td>
    </tr>
    <tr>
      <td><a name="uint64_vbr"><b>uint64_vbr</b></a></td>
      <td class="td_left">A 64-bit unsigned integer that occupies from one to ten 
        bytes using variable bit rate encoding.</td>
    </tr>
    <tr>
      <td><a name="int64_vbr"><b>int64_vbr</b></a></td>
      <td class="td_left">A 64-bit signed integer that occupies from one to ten 
        bytes using the signed variable bit rate encoding.</td>
    </tr>
    <tr>
      <td><a name="char"><b>char</b></a></td>
      <td class="td_left">A single unsigned character encoded into one byte</td>
    </tr>
    <tr>
      <td><a name="bit"><b>bit(n-m)</b></a></td>
      <td class="td_left">A set of bit within some larger integer field. The values 
        of <code>n</code> and <code>m</code> specify the inclusive range of bits 
        that define the subfield. The value for <code>m</code> may be omitted if 
        its the same as <code>n</code>.</td>
    </tr>
    <tr>
      <td style="vertical-align: top;"><b><a name="float"><b>float</b></a></b></td>
      <td style="vertical-align: top; text-align: left;">A floating point value encoded 
        as a 32-bit IEEE value written in little-endian form.<br>
      </td>
    </tr>
    <tr>
      <td style="vertical-align: top;"><b><b><a name="double"><b>double</b></a></b></b></td>
      <td style="vertical-align: top; text-align: left;">A floating point value encoded 
        as a64-bit IEEE value written in little-endian form</td>
    </tr>
    <tr>
      <td><a name="string"><b>string</b></a></td>
      <td class="td_left">A uint32_vbr indicating the type of the
constant string which also includes its length, immediately followed by
the characters of the string. There is no terminating null byte in the
string.</td>
    </tr>
    <tr>
      <td><a name="data"><b>data</b></a></td>
      <td class="td_left">An arbitrarily long segment of data to which
no interpretation is implied. This is used for constant initializers.<br>
      </td>
    </tr>
    <tr>
      <td><a name="llist"><b>llist(x)</b></a></td>
      <td class="td_left">A length list of x. This means the list is
encoded as an <a href="#uint32_vbr">uint32_vbr</a> providing the
length of the list, followed by a sequence of that many "x" items. This
implies that the reader should iterate the number of times provided by
the length.</td>
    </tr>
    <tr>
      <td><a name="zlist"><b>zlist(x)</b></a></td>
      <td class="td_left">A zero-terminated list of x. This means the
list is encoded as a sequence of an indeterminate number of "x" items,
followed by an <a href="#uint32_vbr">uint32_vbr</a> terminating value.
This implies that none of the "x" items can have a zero value (or else
the list terminates).</td>
    </tr>
    <tr>
      <td><a name="block"><b>block</b></a></td>
      <td class="td_left">A block of data that is logically related. A
block is an unsigned 32-bit integer that encodes the type of the block
in the low 5 bits and the size of the block in the high 27 bits. The
length does not include the block header or any alignment bytes at the
end of the block. Blocks may compose other blocks. </td>
    </tr>
  </tbody>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="notation">Field Notation</a> </div>
<div class="doc_text">
<p>In the detailed block and field descriptions that follow, a regex
like notation is used to describe optional and repeated fields. A very
limited subset of regex is used to describe these, as given in the
following table: </p>
<table class="doc_table">
  <tbody>
    <tr>
      <th><b>Character</b></th>
      <th class="td_left"><b>Meaning</b></th>
    </tr>
    <tr>
      <td><b><code>?</code></b></td>
      <td class="td_left">The question mark indicates 0 or 1
occurrences of the thing preceding it.</td>
    </tr>
    <tr>
      <td><b><code>*</code></b></td>
      <td class="td_left">The asterisk indicates 0 or more occurrences
of the thing preceding it.</td>
    </tr>
    <tr>
      <td><b><code>+</code></b></td>
      <td class="td_left">The plus sign indicates 1 or more occurrences
of the thing preceding it.</td>
    </tr>
    <tr>
      <td><b><code>()</code></b></td>
      <td class="td_left">Parentheses are used for grouping.</td>
    </tr>
    <tr>
      <td><b><code>,</code></b></td>
      <td class="td_left">The comma separates sequential fields.</td>
    </tr>
  </tbody>
</table>
<p>So, for example, consider the following specifications:</p>
<div class="doc_code">
<ol>
  <li><code>string?</code></li>
  <li><code>(uint32_vbr,uin32_vbr)+</code></li>
  <li><code>(unsigned?,uint32_vbr)*</code></li>
  <li><code>(llist(unsigned))?</code></li>
</ol>
</div>
<p>with the following interpretations:</p>
<ol>
  <li>An optional string. Matches either nothing or a single string</li>
  <li>One or more pairs of uint32_vbr.</li>
  <li>Zero or more occurrences of either an unsigned followed by a
uint32_vbr or just a uint32_vbr.</li>
  <li>An optional length list of unsigned values.</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="slots">Slots</a> </div>
<div class="doc_text">
<p>The bytecode format uses the notion of a "slot" to reference Types
and Values. Since the bytecode file is a <em>direct</em> representation of
LLVM's intermediate representation, there is a need to represent pointers in
the file.  Slots are used for this purpose. For example, if one has the following
assembly:
</p>
<div class="doc_code"><code> %MyType = type { int, sbyte }<br>
%MyVar = external global %MyType
</code></div>
<p>there are two definitions. The definition of <tt>%MyVar</tt> uses <tt>%MyType</tt>.
In the C++ IR this linkage between <tt>%MyVar</tt> and <tt>%MyType</tt>
is explicit through the use of C++ pointers. In bytecode, however, there's no
ability to store memory addresses. Instead, we compute and write out
slot numbers for every Type and Value written to the file.</p>
<p>A slot number is simply an unsigned 32-bit integer encoded in the variable
bit rate scheme (see <a href="#encoding">encoding</a>). This ensures that
low slot numbers are encoded in one byte. Through various bits of magic LLVM
attempts to always keep the slot numbers low. The first attempt is to associate
slot numbers with their "type plane". That is, Values of the same type
are written to the bytecode file in a list (sequentially). Their order in 
that list determines their slot number. This means that slot #1 doesn't mean
anything unless you also specify for which type you want slot #1. Types are
always written to the file first (in the <a href="#globaltypes">Global Type 
Pool</a>) and in such a way that both forward and backward references of the 
types can often be resolved with a single pass through the type pool. </p>
<p>Slot numbers are also kept small by rearranging their order. Because
of the structure of LLVM, certain values are much more likely to be used
frequently in the body of a function. For this reason, a compaction table is
provided in the body of a function if its use would make the function body 
smaller.  Suppose you have a function body that uses just the types "int*" and
"{double}" but uses them thousands of time. Its worthwhile to ensure that the 
slot number for these types are low so they can be encoded in a single byte 
(via vbr). This is exactly what the compaction table does.</p>
<p>In summary then, a slot number can be though of as just a vbr encoded index 
into a list of Type* or Value*. To keep slot numbers low, Value* are indexed by
two slot numbers: the "type plane index" (type slot) and the "value index"
(value slot).</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="general">General Structure</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section provides the general structure of the LLVM bytecode
file format. The bytecode file format requires blocks to be in a
certain order and nested in a particular way so that an LLVM module can
be constructed efficiently from the contents of the file. This ordering
defines a general structure for bytecode files as shown below. The
table below shows the order in which all block types may appear. Please
note that some of the blocks are optional and some may be repeated. The
structure is fairly loose because optional blocks, if empty, are
completely omitted from the file.</p>
<table>
  <tbody>
    <tr>
      <th>ID</th>
      <th>Parent</th>
      <th>Optional?</th>
      <th>Repeated?</th>
      <th>Level</th>
      <th>Block Type</th>
      <th>Description</th>
    </tr>
    <tr>
      <td>N/A</td>
      <td>File</td>
      <td>No</td>
      <td>No</td>
      <td>0</td>
      <td class="td_left"><a href="#signature">Signature</a></td>
      <td class="td_left">This contains the file signature (magic
number) that identifies the file as LLVM bytecode.</td>
    </tr>
    <tr>
      <td>0x01</td>
      <td>File</td>
      <td>No</td>
      <td>No</td>
      <td>0</td>
      <td class="td_left"><a href="#module">Module</a></td>
      <td class="td_left">This is the top level block in a bytecode
file. It contains all the other blocks. </td>
    </tr>
    <tr>
      <td>0x06</td>
      <td>Module</td>
      <td>No</td>
      <td>No</td>
      <td>1</td>
      <td class="td_left">&nbsp;&nbsp;&nbsp;<a href="#globaltypes">Global&nbsp;Type&nbsp;Pool</a></td>
      <td class="td_left">This block contains all the global (module)
level types.</td>
    </tr>
    <tr>
      <td>0x05</td>
      <td>Module</td>
      <td>No</td>
      <td>No</td>
      <td>1</td>
      <td class="td_left">&nbsp;&nbsp;&nbsp;<a href="#globalinfo">Module&nbsp;Globals&nbsp;Info</a></td>
      <td class="td_left">This block contains the type, constness, and
linkage for each of the global variables in the module. It also
contains the type of the functions and the constant initializers.</td>
    </tr>
    <tr>
      <td>0x03</td>
      <td>Module</td>
      <td>Yes</td>
      <td>No</td>
      <td>1</td>
      <td class="td_left">&nbsp;&nbsp;&nbsp;<a href="#constantpool">Module&nbsp;Constant&nbsp;Pool</a></td>
      <td class="td_left">This block contains all the global constants
except function arguments, global values and constant strings.</td>
    </tr>
    <tr>
      <td>0x02</td>
      <td>Module</td>
      <td>Yes</td>
      <td>Yes</td>
      <td>1</td>
      <td class="td_left">&nbsp;&nbsp;&nbsp;<a href="#functiondefs">Function&nbsp;Definitions</a>*</td>
      <td class="td_left">One function block is written for each
function in the module. The function block contains the instructions,
compaction table, type constant pool, and symbol table for the function.</td>
    </tr>
    <tr>
      <td>0x03</td>
      <td>Function</td>
      <td>Yes</td>
      <td>No</td>
      <td>2</td>
      <td class="td_left">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a
 href="#constantpool">Function&nbsp;Constant&nbsp;Pool</a></td>
      <td class="td_left">Any constants (including types) used solely
within the function are emitted here in the function constant pool. </td>
    </tr>
    <tr>
      <td>0x08</td>
      <td>Function</td>
      <td>Yes</td>
      <td>No</td>
      <td>2</td>
      <td class="td_left">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a
 href="#compactiontable">Compaction&nbsp;Table</a></td>
      <td class="td_left">This table reduces bytecode size by providing
a funtion-local mapping of type and value slot numbers to their global
slot numbers</td>
    </tr>
    <tr>
      <td>0x07</td>
      <td>Function</td>
      <td>No</td>
      <td>No</td>
      <td>2</td>
      <td class="td_left">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a
 href="#instructionlist">Instruction&nbsp;List</a></td>
      <td class="td_left">This block contains all the instructions of
the function. The basic blocks are inferred by terminating
instructions. </td>
    </tr>
    <tr>
      <td>0x04</td>
      <td>Function</td>
      <td>Yes</td>
      <td>No</td>
      <td>2</td>
      <td class="td_left">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<a
 href="#symtab">Function&nbsp;Symbol&nbsp;Table</a></td>
      <td class="td_left">This symbol table provides the names for the
function specific values used (basic block labels mostly).</td>
    </tr>
    <tr>
      <td>0x04</td>
      <td>Module</td>
      <td>Yes</td>
      <td>No</td>
      <td>1</td>
      <td class="td_left">&nbsp;&nbsp;&nbsp;<a href="#symtab">Module&nbsp;Symbol&nbsp;Table</a></td>
      <td class="td_left">This symbol table provides the names for the
various entries in the file that are not function specific (global
vars, and functions mostly).</td>
    </tr>
  </tbody>
</table>
<p>Use the links in the table for details about the contents of each of
the block types.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section"> <a name="blockdefs">Block Definitions</a> </div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section provides the detailed layout of the individual block
types in the LLVM bytecode file format. </p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="signature">Signature Block</a> </div>
<div class="doc_text">
<p>The signature occurs in every LLVM bytecode file and is always first.
It simply provides a few bytes of data to identify the file as being an LLVM
bytecode file. This block is always four bytes in length and differs from the
other blocks because there is no identifier and no block length at the start
of the block. Essentially, this block is just the "magic number" for the file.
</p>
<p>There are two types of signatures for LLVM bytecode: uncompressed and
compressed as shown in the table below. </p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Uncompressed</b></th>
      <th class="td_left"><b>Compressed</b></th>
    </tr>
    <tr>
      <td><a href="#char">char</a></td>
      <td class="td_left">Constant "l" (0x6C)</td>
      <td class="td_left">Constant "l" (0x6C)</td>
    </tr>
    <tr>
      <td><a href="#char">char</a></td>
      <td class="td_left">Constant "l" (0x6C)</td>
      <td class="td_left">Constant "l" (0x6C)</td>
    </tr>
    <tr>
      <td><a href="#char">char</a></td>
      <td class="td_left">Constant "v" (0x76)</td>
      <td class="td_left">Constant "v" (0x76)</td>
    </tr>
    <tr>
      <td><a href="#char">char</a></td>
      <td class="td_left">Constant "m" (0x6D)</td>
      <td class="td_left">Constant "c" (0x63)</td>
    </tr>
    <tr>
      <td><a href="#char">char</a></td>
      <td class="td_left">N/A</td>
      <td class="td_left">'0'=null,'1'=gzip,'2'=bzip2</td>
    </tr>
  </tbody>
</table>
<p>In other words, the uncompressed signature is just the characters 'llvm'
while the compressed signature is the characters 'llvc' followed by an ascii
digit ('0', '1', or '2') that indicates the kind of compression used. A value of
'0' indicates that null compression was used. This can happen when compression
was requested on a platform that wasn't configured for gzip or bzip2. A value of
'1' means that the rest of the file is compressed using the gzip algorithm and
should be uncompressed before interpretation. A value of '2' means that the rest
of the file is compressed using the bzip2 algorithm and should be uncompressed
before interpretation. In all cases, the data resulting from uncompression
should be interpreted as if it occurred immediately after the 'llvm'
signature (i.e. the uncompressed data begins with the 
<a href="#module">Module Block</a></p>
<p><b>NOTE:</b> As of LLVM 1.4, all bytecode files produced by the LLVM tools
are compressed by default. To disable compression, pass the 
<tt>--disable-compression</tt> option to the tool, if it supports it.
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="module">Module Block</a> </div>
<div class="doc_text">
<p>The module block contains a small pre-amble and all the other blocks in
the file. The table below shows the structure of the module block. Note that it
only provides the module identifier, size of the module block, and the format
information. Everything else is contained in other blocks, described in other
sections.</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#unsigned">unsigned</a><br></td>
      <td class="td_left"><a href="#mod_header">Module Block Identifier
          (0x01)</a></td>
    </tr>
    <tr>
      <td><a href="#unsigned">unsigned</a></td>
      <td class="td_left"><a href="#mod_header">Module Block Size</a></td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left"><a href="#format">Format Information</a></td>
    </tr>
    <tr>
      <td><a href="#block">block</a></td>
      <td class="td_left"><a href="#globaltypes">Global Type Pool</a></td>
    </tr>
    <tr>
      <td><a href="#block">block</a></td>
      <td class="td_left"><a href="#globalinfo">Module Globals Info</a></td>
    </tr>
    <tr>
      <td><a href="#block">block</a></td>
      <td class="td_left"><a href="#constantpool">Module Constant Pool</a></td>
    </tr>
    <tr>
      <td><a href="#block">block</a>*</td>
      <td class="td_left"><a href="#functiondefs">Function Definitions</a></td>
    </tr>
    <tr>
      <td><a href="#block">block</a></td>
      <td class="td_left"><a href="#symtab">Module Symbol Table</a></td>
    </tr>
  </tbody>
</table>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="mod_header">Module Block Header</a></div>
<div class="doc_text">
  <p>The block header for the module block uses a longer format than the other
  blocks in a bytecode file. Specifically, instead of encoding the type and size
  of the block into a 32-bit integer with 5-bits for type and 27-bits for size,
  the module block header uses two 32-bit unsigned values, one for type, and one
  for size. While the 2<sup>27</sup> byte limit on block size is sufficient for the blocks
  contained in the module, it isn't sufficient for the module block itself
  because we want to ensure that bytecode files as large as 2<sup>32</sup> bytes
  are possible. For this reason, the module block (and only the module block)
  uses a long format header.</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="format">Format Information</a></div>
<div class="doc_text">
<p>The format information field is encoded into a <a href="#uint32_vbr">uint32_vbr</a>
as shown in the following table.</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#bit">bit(0)</a></td>
      <td class="td_left">Target is big endian?</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(1)</a></td>
      <td class="td_left">On target pointers are 64-bit?</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(2)</a></td>
      <td class="td_left">Target has no endianess?</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(3)</a></td>
      <td class="td_left">Target has no pointer size?</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(4-31)</a></td>
      <td class="td_left">Bytecode format version</td>
    </tr>
  </tbody>
</table>
<p>
Of particular note, the bytecode format number is simply a 28-bit
monotonically increasing integer that identifies the version of the bytecode
format (which is not directly related to the LLVM release number). The
bytecode versions defined so far are (note that this document only
describes the latest version, 1.3):</p>
<ul>
  <li>#0: LLVM 1.0 &amp; 1.1</li>
  <li>#1: LLVM 1.2</li>
  <li>#2: LLVM 1.2.5 (not released)</li>
  <li>#3: LLVM 1.3</li>
  <li>#4: LLVM 1.3.x (not released)</li>
  <li>#5: LLVM 1.4 and newer</li>
  </li>
</ul>
<p>Note that we plan to eventually expand the target description
capabilities
of bytecode files to <a href="http://llvm.org/PR263">target
triples</a>.
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="globaltypes">Global Type Pool</a> </div>
<div class="doc_text">
<p>The global type pool consists of type definitions. Their order of appearance
in the file determines their type slot number (0 based). Slot numbers are
used to replace pointers in the intermediate representation. Each slot number 
uniquely identifies one entry in a type plane (a collection of values of the
same type).  Since all values have types and are associated with the order in 
which the type pool is written, the global type pool <em>must</em> be written 
as the first block of a module. If it is not, attempts to read the file will
fail because both forward and backward type resolution will not be possible.</p>
<p>The type pool is simply a list of type definitions, as shown in the
table below.</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#unsigned">block</a></td>
      <td class="td_left">Type Pool Identifier (0x06) + Size<br>
      </td>
    </tr>
    <tr>
      <td><a href="#llist">llist</a>(<a href="#type">type</a>)</td>
      <td class="td_left">A length list of type definitions.</td>
    </tr>
  </tbody>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="type">Type Definitions</a></div>
<div class="doc_text">
<p>Types in the type pool are defined using a different format for each kind
of type, as given in the following sections.</p>
<h3>Primitive Types</h3>
<p>The primitive types encompass the basic integer and floating point
types. They are encoded simply as their TypeID.</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Type ID for the primitive types (values 1 to
11) <sup>1</sup></td>
    </tr>
  </tbody>
</table>
Notes:
<ol>
  <li>The values for the Type IDs for the primitive types are provided
by the definition of the <code>llvm::Type::TypeID</code> enumeration
in <code>include/llvm/Type.h</code>. The enumeration gives the
following mapping:
    <ol>
      <li>bool</li>
      <li>ubyte</li>
      <li>sbyte</li>
      <li>ushort</li>
      <li>short</li>
      <li>uint</li>
      <li>int</li>
      <li>ulong</li>
      <li>long</li>
      <li>float</li>
      <li>double</li>
    </ol>
  </li>
</ol>
<h3>Function Types</h3>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Type ID for function types (13)</td>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Type slot number of function's return type.</td>
    </tr>
    <tr>
      <td><a href="#llist">llist</a>(<a href="#uint24_vbr">uint24_vbr</a>)</td>
      <td class="td_left">Type slot number of each argument's type.</td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a>?</td>
      <td class="td_left">Value 0 if this is a varargs function,
missing otherwise.</td>
    </tr>
  </tbody>
</table>
<h3>Structure Types</h3>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Type ID for structure types (14)</td>
    </tr>
    <tr>
      <td><a href="#zlist">zlist</a>(<a href="#uint24_vbr">uint24_vbr</a>)</td>
      <td class="td_left">Slot number of each of the element's fields.</td>
    </tr>
  </tbody>
</table>
<h3>Array Types</h3>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Type ID for Array Types (15)</td>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Type slot number of array's element type.</td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">The number of elements in the array.</td>
    </tr>
  </tbody>
</table>
<h3>Pointer Types</h3>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Type ID For Pointer Types (16)</td>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Type slot number of pointer's element type.</td>
    </tr>
  </tbody>
</table>
<h3>Opaque Types</h3>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Type ID For Opaque Types (17)</td>
    </tr>
  </tbody>
</table>
<h3>Packed Types</h3>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Type ID for Packed Types (18)</td>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Slot number of packed vector's element type.</td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">The number of elements in the packed vector.</td>
    </tr>
  </tbody>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="globalinfo">Module Global Info</a>
</div>
<div class="doc_text">
<p>The module global info block contains the definitions of all global
variables including their initializers and the <em>declaration</em> of
all functions. The format is shown in the table below:</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#block">block</a></td>
      <td class="td_left">Module global info identifier (0x05) + size</td>
    </tr>
    <tr>
      <td><a href="#zlist">zlist</a>(<a href="#globalvar">globalvar</a>)</td>
      <td class="td_left">A zero terminated list of global var
definitions occurring in the module.</td>
    </tr>
    <tr>
      <td><a href="#zlist">zlist</a>(<a href="#funcfield">funcfield</a>)</td>
      <td class="td_left">A zero terminated list of function definitions
occurring in the module.</td>
    </tr>
    <tr>
      <td><a href="#llist">llist</a>(<a href="#string">string</a>)</td>
      <td class="td_left">A length list
of strings that specify the names of the libraries that this module
depends upon.</td>
    </tr>
    <tr>
      <td><a href="#string">string</a></td>
      <td class="td_left">The target
triple for the module (blank means no target triple specified, i.e. a
platform independent module).</td>
    </tr>
    <tr>
      <td><a href="#llist">llist</a>(<a href="#string">string</a>)</td>
      <td class="td_left">A length list
of strings that defines a table of section strings for globals.  A global's
SectionID is an index into this table.</td>
    </tr>
    <tr>
      <td><a href="#string">string</a></td>
      <td class="td_left">The inline asm block for this module.</td>
    </tr>
  </tbody>
</table>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="globalvar">Global Variable Field</a>
</div>

<div class="doc_text">

<p>Global variables are written using an <a href="#uint32_vbr">uint32_vbr</a>
that encodes information about the global variable, an optional extension vbr,
and a an optional initializers for the global var.</p>

<p>The table below provides the bit layout of the first <a
 href="#uint32_vbr">uint32_vbr</a> that describes the global variable.</p>
 
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#bit">bit(0)</a></td>
      <td class="td_left">Is constant?</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(1)</a></td>
      <td class="td_left">Has initializer? Note that this bit
determines whether the constant initializer field (described below)
follows. </td>
    </tr>
    <tr>
      <td><a href="#bit">bit(2-4)</a></td>
      <td class="td_left">Linkage type: 0=External, 1=Weak,
2=Appending, 3=Internal, 4=LinkOnce</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(5-31)</a></td>
      <td class="td_left">Type slot number of type for the global variable.</td>
    </tr>
  </tbody>
</table>

<p>When the Linkage type is set to 3 (internal) and the initializer field is set
to 0 (an invalid combination), an extension word follows the first <a
href="#uint32_vbr">uint32_vbr</a> which encodes the real linkage and init flag,
and can includes more information:</p>

<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#bit">bit(0)</a></td>
      <td class="td_left">Has initializer?  Indicates the real value of the "Has
        initializer" field for the global. </td>
    </tr>
    <tr>
      <td><a href="#bit">bit(2-4)</a></td>
      <td class="td_left">Linkage type: Indicates the real value of the "linkage
        type" field for the global.</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(4-8)</a></td>
      <td class="td_left">The log-base-2 of the alignment for the global.</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(9)</a></td>
      <td class="td_left">If this bit is set, a SectionID follows this vbr.</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(10-31)</a></td>
      <td class="td_left">Currently unassigned.</td>
    </tr>
  </tbody>
</table>

<p>If the SectionID bit is set above, the following field is included:</p>

<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a>
      </td>
      <td class="td_left">An optional section ID number, specifying the string
        to use for the section of the global.  This an index (+1) of an entry
        into the SectionID llist in the <a href="#globalinfo">Module Global
        Info</a> block.  If this value is 0 or not present, the global has an
        empty section string.</td>
    </tr>
  </tbody>
</table>

<p>If the "Has initializer" field is set, the following field is included:</p>

<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a>
      </td>
      <td class="td_left">An optional value slot number for the global 
          variable's constant initializer.</td>
    </tr>
  </tbody>
</table>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="funcfield">Function Field</a>
</div>
<div class="doc_text">
<p>Functions are written using an <a href="#uint32_vbr">uint32_vbr</a>
that encodes information about the function and a set of flags.  If needed,
an extension word may follow this first field.</p>

<p>The table below provides the bit layout of the <a
href="#uint32_vbr">uint32_vbr</a> that describes the function.</p>

<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#bit">bit(0-3)</a></td>
      <td class="td_left">
      Encodes the calling convention number of the function. The
      CC number of the function is the value of this field minus one.
      </td>
    </tr>
    <tr>
      <td><a href="#bit">bit(4)</a></td>
      <td class="td_left">If this bit is set to 1, the indicated function is
      external, and there is no <a href="#functiondefs">Function Definiton
      Block</a> in the bytecode file for the function.</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(5-30)</a></td>
      <td class="td_left">Type slot number of type for the function.</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(31)</a></td>
      <td class="td_left">Indicates whether an extension word follows.</td>
    </tr>
  </tbody>
</table>

<p>If bit(31) is set, an additional <a href="#uint32_vbr">uint32_vbr</a> word
follows with the following fields:</p>

<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#bit">bit(0-4)</a></td>
      <td class="td_left">The log-base-2 of the alignment for the function.</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(5-9)</a></td>
      <td class="td_left">The top nibble of the calling convention.</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(10)</a></td>
      <td class="td_left">If this bit is set, a SectionID follows this vbr.</td>
    </tr>
    <tr>
      <td><a href="#bit">bit(11-31)</a></td>
      <td class="td_left">Currently unassigned.</td>
    </tr>
  </tbody>
</table>

<p>If the SectionID bit is set above, the following field is included:</p>

<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a>
      </td>
      <td class="td_left">An optional section ID number, specifying the string
        to use for the section of the function.  This an index (+1) of an entry
        into the SectionID llist in the <a href="#globalinfo">Module Global
        Info</a> block.  If this value is 0 or not present, the function has an
        empty section string.</td>
    </tr>
  </tbody>
</table>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="constantpool">Constant Pool</a> </div>
<div class="doc_text">
<p>A constant pool defines as set of constant values. There are
actually two types of constant pool blocks: one for modules and one for
functions. For modules, the block begins with the constant strings
encountered anywhere in the module. For functions, the block begins
with types only encountered in the function. In both cases the header
is identical. The tables that follow, show the header, module constant
pool preamble, function constant pool preamble, and the part common to
both function and module constant pools.</p>
<p><b>Common Block Header</b></p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#block">block</a></td>
      <td class="td_left">Constant pool identifier (0x03) + size<br>
      </td>
    </tr>
  </tbody>
</table>
<p><b>Module Constant Pool Preamble (constant strings)</b></p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">The number of constant strings that follow.</td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">Zero. This identifies the following "plane"
as containing the constant strings. This is needed to identify it
uniquely from other constant planes that follow. </td>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a>+</td>
      <td class="td_left">Type slot number of the constant string's type.
Note that the constant string's type implicitly defines the length of
the string. </td>
    </tr>
  </tbody>
</table>
<p><b>Function Constant Pool Preamble (function types)</b></p>
<p>The structure of the types for functions is identical to the <a
 href="#globaltypes">Global Type Pool</a>. Please refer to that section
for the details. </p>
<p><b>Common Part (other constants)</b></p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">Number of entries in this type plane.</td>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Type slot number of this plane.</td>
    </tr>
    <tr>
      <td><a href="#constant">constant</a>+</td>
      <td class="td_left">The definition of a constant (see below).</td>
    </tr>
  </tbody>
</table>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="constant">Simple Constant Pool
Entries</a></div>

<div class="doc_text">

<p>Constant pool entries come in many shapes and flavors. The sections that
follow define the format for each of them. All constants start with a <a
 href="#uint32_vbr">uint32_vbr</a> encoded integer that provides the
number of operands for the constant. For primitive, structure, and
array constants, this will always be zero to indicate that the form of the 
constant is solely determined by its type. In this case, we have the following
field definitions, based on type:</p>

<ul>
  <li><b>Bool</b>. This is written as an <a href="#uint32_vbr">uint32_vbr</a>
of value 1U or 0U.</li>
  <li><b>Signed Integers (sbyte,short,int,long)</b>. These are written
as an <a href="#int64_vbr">int64_vbr</a> with the corresponding value.</li>
  <li><b>Unsigned Integers (ubyte,ushort,uint,ulong)</b>. These are
written as an <a href="#uint64_vbr">uint64_vbr</a> with the
corresponding value. </li>
  <li><b>Floating Point</b>. Both the float and double types are
written literally in binary format.</li>
  <li><b>Arrays</b>. Arrays are written simply as a list of <a
 href="#uint32_vbr">uint32_vbr</a> encoded value slot numbers to the constant
element values.</li>
  <li><b>Structures</b>. Structures are written simply as a list of <a
 href="#uint32_vbr">uint32_vbr</a> encoded value slot numbers to the constant
field values of the structure.</li>
</ul>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Undef Entries</a></div>

<div class="doc_text">
<p>When the number of operands to the constant is one, we have an 'undef' value
of the specified type.</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Inline Assembler Entries</a></div>

<div class="doc_text">
<p>Inline Assembler entries are stored in the constant pool, though they are not
   officially LLVM constants.  These entries are marked with a value of
   "4294967295" (all ones) for the number of operands.  They are encoded as
   follows:</p>
   
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#string">string</a></td>
      <td class="td_left">The asm string.</td>
    </tr>
    <tr>
      <td><a href="#string">string</a></td>
      <td class="td_left">The constraints string.</td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">Flags</sup></td>
    </tr>
  </tbody>
</table>

<p>Currently, the only defined flag, the low bit, indicates whether or not the
   inline assembler has side effects.</p>

</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Constant Expression Entries</a></div>

<div class="doc_text">

<p>Otherwise, we have a constant expression.  The format of the constant
expression is specified in the table below, and the number is equal to the
number of operands+1.</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">Op code of the instruction for the constant
expression.</td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">The value slot number of the constant value for an
operand.<sup>1</sup></td>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">The type slot number for the type of the constant
value for an operand.<sup>1</sup></td>
    </tr>
  </tbody>
</table>
Notes:
<ol>
  <li>Both these fields are repeatable but only in pairs.</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="functiondefs">Function Definition</a></div>
<div class="doc_text">
<p>Function definitions contain the linkage, constant pool or
compaction table, instruction list, and symbol table for a function.
The following table shows the structure of a function definition.</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#block">block</a><br>
      </td>
      <td class="td_left">Function definition block identifier (0x02) +
size<br>
      </td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">The linkage type of the function: 0=External,
1=Weak, 2=Appending, 3=Internal, 4=LinkOnce<sup>1</sup></td>
    </tr>
    <tr>
      <td><a href="#block">block</a></td>
      <td class="td_left">The <a href="#constantpool">constant pool</a>
block for this function.<sup>2</sup></td>
    </tr>
    <tr>
      <td><a href="#block">block</a></td>
      <td class="td_left">The <a href="#compactiontable">compaction
table</a> block for the function.<sup>2</sup></td>
    </tr>
    <tr>
      <td><a href="#block">block</a></td>
      <td class="td_left">The <a href="#instructionlist">instruction
list</a> for the function.</td>
    </tr>
    <tr>
      <td><a href="#block">block</a></td>
      <td class="td_left">The function's <a href="#symtab">symbol
table</a> containing only those symbols pertinent to the function
(mostly block labels).</td>
    </tr>
  </tbody>
</table>
Notes:
<ol>
  <li>Note that if the linkage type is "External" then none of the
other fields will be present as the function is defined elsewhere.</li>
  <li>Note that only one of the constant pool or compaction table will
be written. Compaction tables are only written if they will actually
save bytecode space. If not, then a regular constant pool is written.</li>
</ol>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="compactiontable">Compaction Table</a>
</div>
<div class="doc_text">
<p>Compaction tables are part of a function definition. They are merely
a device for reducing the size of bytecode files. The size of a
bytecode file is dependent on the <em>values</em> of the slot numbers
used because larger values use more bytes in the variable bit rate
encoding scheme. Furthermore, the compressed instruction format
reserves only six bits for the type of the instruction. In large
modules, declaring hundreds or thousands of types, the values of the
slot numbers can be quite large. However, functions may use only a
small fraction of the global types. In such cases a compaction table is
created that maps the global type and value slot numbers to smaller
values used by a function. Functions will contain either a
function-specific constant pool <em>or</em> a compaction table but not
both. Compaction tables have the format shown in the table below.</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">The number of types that follow</td>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a>+</td>
      <td class="td_left">The type slot number in the global types of
the type that will be referenced in the function with the index of this
entry in the compaction table.</td>
    </tr>
    <tr>
      <td><a href="#type_len">type_len</a></td>
      <td class="td_left">An encoding of the type and number of values
that follow. This field's encoding varies depending on the size of the
type plane. See <a href="#type_len">Type and Length</a> for further
details.</td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a>+</td>
      <td class="td_left">The value slot number in the global values
that will be referenced in the function with the index of this entry in
the compaction table.</td>
    </tr>
  </tbody>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="type_len">Type and Length</a></div>
<div class="doc_text">
<p>The type and length of a compaction table type plane is encoded
differently depending on the length of the plane. For planes of length
1 or 2, the length is encoded into bits 0 and 1 of a <a
 href="#uint32_vbr">uint32_vbr</a> and the type is encoded into bits
2-31. Because type numbers are often small, this often saves an extra
byte per plane. If the length of the plane is greater than 2 then the
encoding uses a <a href="#uint32_vbr">uint32_vbr</a> for each of the
length and type, in that order.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="instructionlist">Instruction List</a></div>
<div class="doc_text">
<p>The instructions in a function are written as a simple list. Basic
blocks are inferred by the terminating instruction types. The format of
the block is given in the following table.</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#block">block</a><br>
      </td>
      <td class="td_left">Instruction list identifier (0x07) + size<br>
      </td>
    </tr>
    <tr>
      <td><a href="#instruction">instruction</a>+</td>
      <td class="td_left">An instruction. Instructions have a variety
of formats. See <a href="#instruction">Instructions</a> for details.</td>
    </tr>
  </tbody>
</table>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="instructions">Instructions</a></div>

<div class="doc_text">
<p>Instructions are written out one at a time as distinct units.  Each
instruction
record contains at least an <a href="#opcodes">opcode</a> and a type field, 
and may contain a <a href="#instoperands">list of operands</a> (whose
interpretation depends on the opcode). Based on the number of operands, the
<a href="#instencode">instruction is encoded</a> in a
dense format that tries to encoded each instruction into 32-bits if 
possible. </p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="opcodes">Instruction Opcodes</a></div>
<div class="doc_text">
  <p>Instructions encode an opcode that identifies the kind of instruction.
  Opcodes are an enumerated integer value. The specific values used depend on
  the version of LLVM you're using. The opcode values are defined in the
  <a href="http://llvm.org/cvsweb/cvsweb.cgi/llvm/include/llvm/Instruction.def">
  <tt>include/llvm/Instruction.def</tt></a> file. You should check there for the
  most recent definitions. The table below provides the opcodes defined as of
  the writing of this document. The table associates each opcode mnemonic with
  its enumeration value and the bytecode and LLVM version numbers in which the
  opcode was introduced.</p>
  <table>
    <tbody>
      <tr>
        <th>Opcode</th>
        <th>Number</th>
        <th>Bytecode Version</th>
        <th>LLVM Version</th>
      </tr>
      <tr><td colspan="4"><b>Terminator Instructions</b></td></tr>
      <tr><td>Ret</td><td>1</td><td>1</td><td>1.0</td></tr>
      <tr><td>Br</td><td>2</td><td>1</td><td>1.0</td></tr>
      <tr><td>Switch</td><td>3</td><td>1</td><td>1.0</td></tr>
      <tr><td>Invoke</td><td>4</td><td>1</td><td>1.0</td></tr>
      <tr><td>Unwind</td><td>5</td><td>1</td><td>1.0</td></tr>
      <tr><td>Unreachable</td><td>6</td><td>1</td><td>1.4</td></tr>
      <tr><td colspan="4"><b>Binary Operators</b></td></tr>
      <tr><td>Add</td><td>7</td><td>1</td><td>1.0</td></tr>
      <tr><td>Sub</td><td>8</td><td>1</td><td>1.0</td></tr>
      <tr><td>Mul</td><td>9</td><td>1</td><td>1.0</td></tr>
      <tr><td>Div</td><td>10</td><td>1</td><td>1.0</td></tr>
      <tr><td>Rem</td><td>11</td><td>1</td><td>1.0</td></tr>
      <tr><td colspan="4"><b>Logical Operators</b></td></tr>
      <tr><td>And</td><td>12</td><td>1</td><td>1.0</td></tr>
      <tr><td>Or</td><td>13</td><td>1</td><td>1.0</td></tr>
      <tr><td>Xor</td><td>14</td><td>1</td><td>1.0</td></tr>
      <tr><td colspan="4"><b>Binary Comparison Operators</b></td></tr>
      <tr><td>SetEQ</td><td>15</td><td>1</td><td>1.0</td></tr>
      <tr><td>SetNE</td><td>16</td><td>1</td><td>1.0</td></tr>
      <tr><td>SetLE</td><td>17</td><td>1</td><td>1.0</td></tr>
      <tr><td>SetGE</td><td>18</td><td>1</td><td>1.0</td></tr>
      <tr><td>SetLT</td><td>19</td><td>1</td><td>1.0</td></tr>
      <tr><td>SetGT</td><td>20</td><td>1</td><td>1.0</td></tr>
      <tr><td colspan="4"><b>Memory Operators</b></td></tr>
      <tr><td>Malloc</td><td>21</td><td>1</td><td>1.0</td></tr>
      <tr><td>Free</td><td>22</td><td>1</td><td>1.0</td></tr>
      <tr><td>Alloca</td><td>23</td><td>1</td><td>1.0</td></tr>
      <tr><td>Load</td><td>24</td><td>1</td><td>1.0</td></tr>
      <tr><td>Store</td><td>25</td><td>1</td><td>1.0</td></tr>
      <tr><td>GetElementPtr</td><td>26</td><td>1</td><td>1.0</td></tr>
      <tr><td colspan="4"><b>Other Operators</b></td></tr>
      <tr><td>PHI</td><td>27</td><td>1</td><td>1.0</td></tr>
      <tr><td>Cast</td><td>28</td><td>1</td><td>1.0</td></tr>
      <tr><td>Call</td><td>29</td><td>1</td><td>1.0</td></tr>
      <tr><td>Shl</td><td>30</td><td>1</td><td>1.0</td></tr>
      <tr><td>Shr</td><td>31</td><td>1</td><td>1.0</td></tr>
      <tr><td>VANext</td><td>32</td><td>1</td><td>1.0</td></tr>
      <tr><td>VAArg</td><td>33</td><td>1</td><td>1.0</td></tr>
      <tr><td>Select</td><td>34</td><td>2</td><td>1.2</td></tr>
      <tr><td colspan="4">
          <b>Pseudo Instructions<a href="#pi_note">*</a></b>
      </td></tr>
      <tr><td>Invoke+CC </td><td>56</td><td>5</td><td>1.5</td></tr>
      <tr><td>Invoke+FastCC</td><td>57</td><td>5</td><td>1.5</td></tr>
      <tr><td>Call+CC</td><td>58</td><td>5</td><td>1.5</td></tr>
      <tr><td>Call+FastCC+TailCall</td><td>59</td><td>5</td><td>1.5</td></tr>
      <tr><td>Call+FastCC</td><td>60</td><td>5</td><td>1.5</td></tr>
      <tr><td>Call+CCC+TailCall</td><td>61</td><td>5</td><td>1.5</td></tr>
      <tr><td>Load+Volatile</td><td>62</td><td>3</td><td>1.3</td></tr>
      <tr><td>Store+Volatile</td><td>63</td><td>3</td><td>1.3</td></tr>
    </tbody>
  </table>

<p><b><a name="pi_note">* Note: </a></b>
These aren't really opcodes from an LLVM language perspective. They encode
information into other opcodes without reserving space for that information. 
For example, opcode=63 is a Volatile Store. The opcode for this
instruction is 25 (Store) but we encode it as 63 to indicate that is a Volatile
Store. The same is done for the calling conventions and tail calls.
In each of these entries in range 56-63, the opcode is documented as the base
opcode (Invoke, Call, Store) plus some set of modifiers, as follows:</p>
<dl>
  <dt>CC</dt>
  <dd>This means an arbitrary calling convention is specified
  in a VBR that follows the opcode. This is used when the instruction cannot
  be encoded with one of the more compact forms.
  </dd>
  <dt>FastCC</dt>
  <dd>This indicates that the Call or Invoke is using the FastCC calling 
  convention.</dd>
  <dt>CCC</dt>
  <dd>This indicates that the Call or Invoke is using the native "C" calling 
  convention.</dd>
  <dt>TailCall</dt>
  <dd>This indicates that the Call has the 'tail' modifier.</dd>
</dl>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="instoperands">Instruction
Operands</a></div>

<div class="doc_text">
<p>
Based on the instruction opcode and type, the bytecode format implicitly (to 
save space) specifies the interpretation of the operand list.  For most
instructions, the type of each operand is implicit from the type of the 
instruction itself (e.g. the type of operands of a binary operator must match
the type of the instruction).  As such, the bytecode format generally only 
encodes the value number of the operand, not the type.</p>

<p>In some cases, however, this is not sufficient.  This section enumerates
those cases:</p>

<ul>
<li>getelementptr: the slot numbers for sequential type indexes are shifted up
two bits.  This allows the low order bits will encode the type of index used,
as follows: 0=uint, 1=int, 2=ulong, 3=long.</li>
<li>cast: the result type number is encoded as the second operand.</li>
<li>alloca/malloc: If the allocation has an explicit alignment, the log2 of the
    alignment is encoded as the second operand.</li>
<li>call: If the tail marker and calling convention cannot be <a 
    href="#pi_note">encoded into the opcode</a> of the call, it is passed as an
    additional operand.  The low bit of the operand is a flag indicating whether
    the call is a tail call.  The rest of the bits contain the calling 
    convention number (shifted left by one bit).</li>
</ul>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="instencode">Instruction 
Encoding</a></div>

<div class="doc_text">
<p>For brevity, instructions are written in one of four formats,
depending on the number of operands to the instruction. Each
instruction begins with a <a href="#uint32_vbr">uint32_vbr</a> that
encodes the type of the instruction as well as other things. The tables
that follow describe the format of this first part of each instruction.</p>
<p><b>Instruction Format 0</b></p>
<p>This format is used for a few instructions that can't easily be
shortened because they have large numbers of operands (e.g. PHI Node or
getelementptr). Each of the opcode, type, and operand fields is found in
successive fields.</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">Specifies the opcode of the instruction. Note
that for compatibility with the other instruction formats, the opcode
is shifted left by 2 bits. Bits 0 and 1 must have value zero for this
format.</td>
    </tr>
    <tr>
      <td><a href="#uint24_vbr">uint24_vbr</a></td>
      <td class="td_left">Provides the type slot number of the result type of
        the instruction.</td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">The number of operands that follow.</td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a>+</td>
      <td class="td_left">The slot number of the value(s) for the operand(s).
      </td>
    </tr>
  </tbody>
</table>

<p><b>Instruction Format 1</b></p>
<p>This format encodes the opcode, type and a single operand into a
single <a href="#uint32_vbr">uint32_vbr</a> as follows:</p>
<table>
  <tbody>
    <tr>
      <th><b>Bits</b></th>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td>0-1</td>
      <td>constant "1"</td>
      <td class="td_left">These two bits must be the value 1 which identifies 
        this as an instruction of format 1.</td>
    </tr>
    <tr>
      <td>2-7</td>
      <td><a href="#instructions">opcode</a></td>
      <td class="td_left">Specifies the opcode of the instruction. Note that 
        the maximum opcode value is 63.</td>
    </tr>
    <tr>
      <td>8-19</td>
      <td><a href="#unsigned">unsigned</a></td>
      <td class="td_left">Specifies the slot number of the type for this 
        instruction. Maximum slot number is 2<sup>12</sup>-1=4095.</td>
    </tr>
    <tr>
      <td>20-31</td>
      <td><a href="#unsigned">unsigned</a></td>
      <td class="td_left">Specifies the slot number of the value for the 
        first operand. Maximum slot number is 2<sup>12</sup>-1=4095. Note that 
        the value 2<sup>12</sup>-1 denotes zero operands.</td>
    </tr>
  </tbody>
</table>
<p><b>Instruction Format 2</b></p>
<p>This format encodes the opcode, type and two operands into a single <a
 href="#uint32_vbr">uint32_vbr</a> as follows:</p>
<table>
  <tbody>
    <tr>
      <th><b>Bits</b></th>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td>0-1</td>
      <td>constant "2"</td>
      <td class="td_left">These two bits must be the value 2 which identifies 
        this as an instruction of format 2.</td>
    </tr>
    <tr>
      <td>2-7</td>
      <td><a href="#instructions">opcode</a></td>
      <td class="td_left">Specifies the opcode of the instruction. Note that 
        the maximum opcode value is 63.</td>
    </tr>
    <tr>
      <td>8-15</td>
      <td><a href="#unsigned">unsigned</a></td>
      <td class="td_left">Specifies the slot number of the type for this 
        instruction. Maximum slot number is 2<sup>8</sup>-1=255.</td>
    </tr>
    <tr>
      <td>16-23</td>
      <td><a href="#unsigned">unsigned</a></td>
      <td class="td_left">Specifies the slot number of the value for the first 
        operand. Maximum slot number is 2<sup>8</sup>-1=255.</td>
    </tr>
    <tr>
      <td>24-31</td>
      <td><a href="#unsigned">unsigned</a></td>
      <td class="td_left">Specifies the slot number of the value for the second 
        operand. Maximum slot number is 2<sup>8</sup>-1=255.</td>
    </tr>
  </tbody>
</table>
<p><b>Instruction Format 3</b></p>
<p>This format encodes the opcode, type and three operands into a
single <a href="#uint32_vbr">uint32_vbr</a> as follows:</p>
<table>
  <tbody>
    <tr>
      <th><b>Bits</b></th>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td>0-1</td>
      <td>constant "3"</td>
      <td class="td_left">These two bits must be the value 3 which identifies 
        this as an instruction of format 3.</td>
    </tr>
    <tr>
      <td>2-7</td>
      <td><a href="#instructions">opcode</a></td>
      <td class="td_left">Specifies the opcode of the instruction. Note that 
        the maximum opcode value is 63.</td>
    </tr>
    <tr>
      <td>8-13</td>
      <td><a href="#unsigned">unsigned</a></td>
      <td class="td_left">Specifies the slot number of the type for this 
        instruction. Maximum slot number is 2<sup>6</sup>-1=63.</td>
    </tr>
    <tr>
      <td>14-19</td>
      <td><a href="#unsigned">unsigned</a></td>
      <td class="td_left">Specifies the slot number of the value for the first 
        operand. Maximum slot number is 2<sup>6</sup>-1=63.</td>
    </tr>
    <tr>
      <td>20-25</td>
      <td><a href="#unsigned">unsigned</a></td>
      <td class="td_left">Specifies the slot number of the value for the second
        operand. Maximum slot number is 2<sup>6</sup>-1=63.</td>
    </tr>
    <tr>
      <td>26-31</td>
      <td><a href="#unsigned">unsigned</a></td>
      <td class="td_left">Specifies the slot number of the value for the third
        operand. Maximum slot number is 2<sup>6</sup>-1=63.</td>
    </tr>
  </tbody>
</table>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="symtab">Symbol Table</a> </div>
<div class="doc_text">
<p>A symbol table can be put out in conjunction with a module or a function. A
symbol table has a list of name/type associations followed by a list of
name/value associations. The name/value associations are organized into "type
planes" so that all values of a common type are listed together.  Each type 
plane starts with the number of entries in the plane and the type slot number
for all the values in that plane (so the type can be looked up in the global 
type pool). For each entry in a type plane, the slot number of the value and 
the name associated with that value are written. The format is given in the 
table below. </p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#block">block</a><br>
      </td>
      <td class="td_left">Symbol Table Identifier (0x04)</td>
    </tr>
    <tr>
      <td><a href="#llist">llist</a>(<a href="#symtab_entry">type_entry</a>)</td>
      <td class="td_left">A length list of symbol table entries for
        <tt>Type</tt>s
      </td>
    </tr>
    <tr>
      <td><a href="#zlist">llist</a>(<a href="#symtab_plane">symtab_plane</a>)</td>
      <td class="td_left">A length list of "type planes" of symbol table
        entries for <tt>Value</tt>s</td>
    </tr>
  </tbody>
</table>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="type_entry">Symbol Table Type
Entry</a>
</div>
<div class="doc_text">
<p>A symbol table type entry associates a name with a type. The name is provided
simply as an array of chars. The type is provided as a type slot number (index)
into the global type pool. The format is given in the following table:</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint24_vbr</a></td>
      <td class="td_left">Type slot number of the type being given a
        name relative to the global type pool.
      </td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">Length of the character array that follows.</td>
    </tr>
    <tr>
      <td><a href="#char">char</a>+</td>
      <td class="td_left">The characters of the name.</td>
    </tr>
  </tbody>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"> <a name="symtab_plane">Symbol Table
Plane</a>
</div>
<div class="doc_text">
<p>A symbol table plane provides the symbol table entries for all
values of a common type. The encoding is given in the following table:</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">Number of entries in this plane.</td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">Type slot number of type for all values in this plane..</td>
    </tr>
    <tr>
      <td><a href="#value_entry">value_entry</a>+</td>
      <td class="td_left">The symbol table entries for to associate values with
        names.</td>
    </tr>
  </tbody>
</table>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection"><a name="value_entry">Symbol Table Value
Entry</a>
</div>
<div class="doc_text">
<p>A symbol table value entry provides the assocation between a value and the
name given to the value. The value is referenced by its slot number. The
format is given in the following table:</p>
<table>
  <tbody>
    <tr>
      <th><b>Type</b></th>
      <th class="td_left"><b>Field Description</b></th>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint24_vbr</a></td>
      <td class="td_left">Value slot number of the value being given a name.
      </td>
    </tr>
    <tr>
      <td><a href="#uint32_vbr">uint32_vbr</a></td>
      <td class="td_left">Length of the character array that follows.</td>
    </tr>
    <tr>
      <td><a href="#char">char</a>+</td>
      <td class="td_left">The characters of the name.</td>
    </tr>
  </tbody>
</table>
</div>

<!-- *********************************************************************** -->
<div class="doc_section"> <a name="versiondiffs">Version Differences</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section describes the differences in the Bytecode Format across
LLVM
versions. The versions are listed in reverse order because it assumes
the current version is as documented in the previous sections. Each
section here
describes the differences between that version and the one that <i>follows</i>.
</p>
</div>

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="vers13">Version 1.3 Differences From 
    1.4</a></div>
<!-- _______________________________________________________________________ -->

<div class="doc_subsubsection">Unreachable Instruction</div>
<div class="doc_text">
  <p>The LLVM <a href="LangRef.html#i_unreachable">Unreachable</a> instruction
  was added in version 1.4 of LLVM.  This caused all instruction numbers after
  it to shift down by one.</p>
</div>

<div class="doc_subsubsection">Function Flags</div>
<div class="doc_text">
  <p>LLVM bytecode versions prior to 1.4 did not include the 5 bit offset 
     in <a href="#funcfield">the function list</a> in the <a
     href="#globalinfo">Module Global Info</a> block.</p>
</div>

<div class="doc_subsubsection">Function Flags</div>
<div class="doc_text">
  <p>LLVM bytecode versions prior to 1.4 did not include the 'undef' constant
     value, which affects the encoding of <a href="#constant">Constant
     Fields</a>.</p>
</div>

<!--
<div class="doc_subsubsection">Aligned Data</div>
<div class="doc_text">
  <p>In version 1.3, certain data items were aligned to 32-bit boundaries. In
  version 1.4, alignment of data was done away with completely. The need for
  alignment has gone away and the only thing it adds is bytecode file size
  overhead. In most cases this overhead was small. However, in functions with
  large numbers of format 0 instructions (GEPs and PHIs with lots of parameters)
  or regular instructions with large valued operands (e.g. because there's just
  a lot of instructions in the function) the overhead can be extreme. In one
  test case, the overhead was 44,000 bytes (34% of the total file size).
  Consequently in release 1.4, the decision was made to eliminate alignment
  altogether.</p>
  <p>In version 1.3 format, the following bytecode constructs were aligned (i.e.
  they were followed by one to three bytes of padding):</p>
  <ul>
    <li>All blocks.</li>
    <li>Instructions using the long format (format 0).</li>
    <li>All call instructions that called a var args function.</li>
    <li>The target triple (a string field at the end of the module block).</li>
    <li>The version field (immediately following the signature).</li>
  </ul>
  <p>None of these constructs are aligned in version 1.4</p>
</div>
-->

<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="vers12">Version 1.2 Differences
From 1.3</a></div>
<!-- _______________________________________________________________________ -->

<div class="doc_subsubsection">Type Derives From Value</div>
<div class="doc_text">
<p>In version 1.2, the Type class in the LLVM IR derives from the Value
class. This is not the case in version 1.3. Consequently, in version
1.2 the notion of a "Type Type" was used to write out values that were
Types. The types always occuped plane 12 (corresponding to the
TypeTyID) of any type planed set of values. In 1.3 this representation
is not convenient because the TypeTyID (12) is not present and its
value is now used for LabelTyID. Consequently, the data structures
written that involve types do so by writing all the types first and
then each of the value planes according to those types. In version 1.2,
the types would have been written intermingled with the values.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Restricted getelementptr Types</div>
<div class="doc_text">
<p>In version 1.2, the getelementptr instruction required a ubyte type
index for accessing a structure field and a long type index for
accessing an array element. Consequently, it was only possible to
access structures of 255 or fewer elements. Starting in version 1.3,
this restriction was lifted. Structures must now be indexed with uint
constants. Arrays may now be indexed with int, uint, long, or ulong
typed values. The consequence of this was that the bytecode format had
to change in order to accommodate the larger range of structure indices.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Short Block Headers</div>
<div class="doc_text">
<p>In version 1.2, block headers were always 8 bytes being comprised of
both an unsigned integer type and an unsigned integer size. For very
small modules, these block headers turn out to be a large fraction of
the total bytecode file size. In an attempt to make these small files
smaller, the type and size information was encoded into a single
unsigned integer (4 bytes) comprised of 5 bits for the block type
(maximum 31 block types) and 27 bits for the block size (max
~134MBytes). These limits seemed sufficient for any blocks or sizes
forseen in the future. Note that the module block, which encloses all
the other blocks is still written as 8 bytes since bytecode files
larger than 134MBytes might be possible.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Dependent Libraries and Target Triples</div>
<div class="doc_text">
<p>In version 1.2, the bytecode format does not store module's target
triple or dependent. These fields have been added to the end of the <a
 href="#globalinfo">module global info block</a>. The purpose of these
fields is to allow a front end compiler to specifiy that the generated
module is specific to a particular target triple (operating
system/manufacturer/processor) which makes it non-portable; and to
allow front end compilers to specify the list of libraries that the
module depends on for successful linking.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Types Restricted to 24-bits</div>
<div class="doc_text">
<p>In version 1.2, type slot identifiers were written as 32-bit VBR
quantities. In 1.3 this has been reduced to 24-bits in order to ensure
that it is not possible to overflow the type field of a global variable
definition. 24-bits for type slot numbers is deemed sufficient for any
practical use of LLVM.</p>
</div>
<!-- _______________________________________________________________________ -->
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="vers11">Version 1.1 Differences
From 1.2 </a></div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Explicit Primitive Zeros</div>
<div class="doc_text">
<p>In version 1.1, the zero value for primitives was explicitly encoded
into the bytecode format. Since these zero values are constant values
in the LLVM IR and never change, there is no reason to explicitly
encode them. This explicit encoding was removed in version 1.2.</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsubsection">Inconsistent Module Global Info</div>
<div class="doc_text">
<p>In version 1.1, the Module Global Info block was not aligned causing
the next block to be read in on an unaligned boundary. This problem was
corrected in version 1.2.<br>
<br>
</p>
</div>
<!-- _______________________________________________________________________ -->
<div class="doc_subsection"><a name="vers10">Version 1.0 Differences
From 1.1</a></div>
<div class="doc_text">
<p>None. Version 1.0 and 1.1 bytecode formats are identical.</p>
</div>
<!-- *********************************************************************** -->
<hr>
<address> <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
 src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
<a href="http://validator.w3.org/check/referer"><img
 src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
<a href="mailto:rspencer@x10sys.com">Reid Spencer</a> and <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>
generated by cgit v1.2.3 (git 2.25.1) at 2024-05-04 21:18:36 +0000