llvm.org GIT mirror llvm / release_31 docs / SourceLevelDebugging.html
release_31

Tree @release_31 (Download .tar.gz)

SourceLevelDebugging.html @release_31raw · history · blame

   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
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
2587
2588
2589
2590
2591
2592
2593
2594
2595
2596
2597
2598
2599
2600
2601
2602
2603
2604
2605
2606
2607
2608
2609
2610
2611
2612
2613
2614
2615
2616
2617
2618
2619
2620
2621
2622
2623
2624
2625
2626
2627
2628
2629
2630
2631
2632
2633
2634
2635
2636
2637
2638
2639
2640
2641
2642
2643
2644
2645
2646
2647
2648
2649
2650
2651
2652
2653
2654
2655
2656
2657
2658
2659
2660
2661
2662
2663
2664
2665
2666
2667
2668
2669
2670
2671
2672
2673
2674
2675
2676
2677
2678
2679
2680
2681
2682
2683
2684
2685
2686
2687
2688
2689
2690
2691
2692
2693
2694
2695
2696
2697
2698
2699
2700
2701
2702
2703
2704
2705
2706
2707
2708
2709
2710
2711
2712
2713
2714
2715
2716
2717
2718
2719
2720
2721
2722
2723
2724
2725
2726
2727
2728
2729
2730
2731
2732
2733
2734
2735
2736
2737
2738
2739
2740
2741
2742
2743
2744
2745
2746
2747
2748
2749
2750
2751
2752
2753
2754
2755
2756
2757
2758
2759
2760
2761
2762
2763
2764
2765
2766
2767
2768
2769
2770
2771
2772
2773
2774
2775
2776
2777
2778
2779
2780
2781
2782
2783
2784
2785
2786
2787
2788
2789
2790
2791
2792
2793
2794
2795
2796
2797
2798
2799
2800
2801
2802
2803
2804
2805
2806
2807
2808
2809
2810
2811
2812
2813
2814
2815
2816
2817
2818
2819
2820
2821
2822
2823
2824
2825
2826
2827
2828
2829
2830
2831
2832
2833
2834
2835
2836
2837
2838
2839
2840
2841
2842
2843
2844
2845
2846
2847
2848
2849
2850
2851
2852
2853
2854
2855
2856
2857
2858
2859
2860
2861
2862
<!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>Source Level Debugging with LLVM</title>
  <link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>

<h1>Source Level Debugging with LLVM</h1>

<table class="layout" style="width:100%">
  <tr class="layout">
    <td class="left">
<ul>
  <li><a href="#introduction">Introduction</a>
  <ol>
    <li><a href="#phil">Philosophy behind LLVM debugging information</a></li>
    <li><a href="#consumers">Debug information consumers</a></li>
    <li><a href="#debugopt">Debugging optimized code</a></li>
  </ol></li>
  <li><a href="#format">Debugging information format</a>
  <ol>
    <li><a href="#debug_info_descriptors">Debug information descriptors</a>
    <ul>
      <li><a href="#format_compile_units">Compile unit descriptors</a></li>
      <li><a href="#format_files">File descriptors</a></li>
      <li><a href="#format_global_variables">Global variable descriptors</a></li>
      <li><a href="#format_subprograms">Subprogram descriptors</a></li>
      <li><a href="#format_blocks">Block descriptors</a></li>
      <li><a href="#format_basic_type">Basic type descriptors</a></li>
      <li><a href="#format_derived_type">Derived type descriptors</a></li>
      <li><a href="#format_composite_type">Composite type descriptors</a></li>
      <li><a href="#format_subrange">Subrange descriptors</a></li>
      <li><a href="#format_enumeration">Enumerator descriptors</a></li>
      <li><a href="#format_variables">Local variables</a></li>
    </ul></li>
    <li><a href="#format_common_intrinsics">Debugger intrinsic functions</a>
      <ul>
      <li><a href="#format_common_declare">llvm.dbg.declare</a></li>
      <li><a href="#format_common_value">llvm.dbg.value</a></li>
    </ul></li>
  </ol></li>
  <li><a href="#format_common_lifetime">Object lifetimes and scoping</a></li>
  <li><a href="#ccxx_frontend">C/C++ front-end specific debug information</a>
  <ol>
    <li><a href="#ccxx_compile_units">C/C++ source file information</a></li>
    <li><a href="#ccxx_global_variable">C/C++ global variable information</a></li>
    <li><a href="#ccxx_subprogram">C/C++ function information</a></li>
    <li><a href="#ccxx_basic_types">C/C++ basic types</a></li>
    <li><a href="#ccxx_derived_types">C/C++ derived types</a></li>
    <li><a href="#ccxx_composite_types">C/C++ struct/union types</a></li>
    <li><a href="#ccxx_enumeration_types">C/C++ enumeration types</a></li>
  </ol></li>
  <li><a href="#llvmdwarfextension">LLVM Dwarf Extensions</a>
    <ol>
      <li><a href="#objcproperty">Debugging Information Extension
	  for Objective C Properties</a>
        <ul>
	  <li><a href="#objcpropertyintroduction">Introduction</a></li>
	  <li><a href="#objcpropertyproposal">Proposal</a></li>
	  <li><a href="#objcpropertynewattributes">New DWARF Attributes</a></li>
	  <li><a href="#objcpropertynewconstants">New DWARF Constants</a></li>
        </ul>
      </li>
      <li><a href="#acceltable">Name Accelerator Tables</a>
        <ul>
          <li><a href="#acceltableintroduction">Introduction</a></li>
          <li><a href="#acceltablehashes">Hash Tables</a></li>
          <li><a href="#acceltabledetails">Details</a></li>
          <li><a href="#acceltablecontents">Contents</a></li>
          <li><a href="#acceltableextensions">Language Extensions and File Format Changes</a></li>
        </ul>
      </li>
    </ol>
  </li>
</ul>
</td>
<td class="right">
<img src="img/venusflytrap.jpg" alt="A leafy and green bug eater" width="247"
height="369">
</td>
</tr></table>

<div class="doc_author">
  <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a>
            and <a href="mailto:jlaskey@mac.com">Jim Laskey</a></p>
</div>


<!-- *********************************************************************** -->
<h2><a name="introduction">Introduction</a></h2>
<!-- *********************************************************************** -->

<div>

<p>This document is the central repository for all information pertaining to
   debug information in LLVM.  It describes the <a href="#format">actual format
   that the LLVM debug information</a> takes, which is useful for those
   interested in creating front-ends or dealing directly with the information.
   Further, this document provides specific examples of what debug information
   for C/C++ looks like.</p>

<!-- ======================================================================= -->
<h3>
  <a name="phil">Philosophy behind LLVM debugging information</a>
</h3>

<div>

<p>The idea of the LLVM debugging information is to capture how the important
   pieces of the source-language's Abstract Syntax Tree map onto LLVM code.
   Several design aspects have shaped the solution that appears here.  The
   important ones are:</p>

<ul>
  <li>Debugging information should have very little impact on the rest of the
      compiler.  No transformations, analyses, or code generators should need to
      be modified because of debugging information.</li>

  <li>LLVM optimizations should interact in <a href="#debugopt">well-defined and
      easily described ways</a> with the debugging information.</li>

  <li>Because LLVM is designed to support arbitrary programming languages,
      LLVM-to-LLVM tools should not need to know anything about the semantics of
      the source-level-language.</li>

  <li>Source-level languages are often <b>widely</b> different from one another.
      LLVM should not put any restrictions of the flavor of the source-language,
      and the debugging information should work with any language.</li>

  <li>With code generator support, it should be possible to use an LLVM compiler
      to compile a program to native machine code and standard debugging
      formats.  This allows compatibility with traditional machine-code level
      debuggers, like GDB or DBX.</li>
</ul>

<p>The approach used by the LLVM implementation is to use a small set
   of <a href="#format_common_intrinsics">intrinsic functions</a> to define a
   mapping between LLVM program objects and the source-level objects.  The
   description of the source-level program is maintained in LLVM metadata
   in an <a href="#ccxx_frontend">implementation-defined format</a>
   (the C/C++ front-end currently uses working draft 7 of
   the <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3
   standard</a>).</p>

<p>When a program is being debugged, a debugger interacts with the user and
   turns the stored debug information into source-language specific information.
   As such, a debugger must be aware of the source-language, and is thus tied to
   a specific language or family of languages.</p>

</div>

<!-- ======================================================================= -->
<h3>
  <a name="consumers">Debug information consumers</a>
</h3>

<div>

<p>The role of debug information is to provide meta information normally
   stripped away during the compilation process.  This meta information provides
   an LLVM user a relationship between generated code and the original program
   source code.</p>

<p>Currently, debug information is consumed by DwarfDebug to produce dwarf
   information used by the gdb debugger.  Other targets could use the same
   information to produce stabs or other debug forms.</p>

<p>It would also be reasonable to use debug information to feed profiling tools
   for analysis of generated code, or, tools for reconstructing the original
   source from generated code.</p>

<p>TODO - expound a bit more.</p>

</div>

<!-- ======================================================================= -->
<h3>
  <a name="debugopt">Debugging optimized code</a>
</h3>

<div>

<p>An extremely high priority of LLVM debugging information is to make it
   interact well with optimizations and analysis.  In particular, the LLVM debug
   information provides the following guarantees:</p>

<ul>
  <li>LLVM debug information <b>always provides information to accurately read
      the source-level state of the program</b>, regardless of which LLVM
      optimizations have been run, and without any modification to the
      optimizations themselves.  However, some optimizations may impact the
      ability to modify the current state of the program with a debugger, such
      as setting program variables, or calling functions that have been
      deleted.</li>

  <li>As desired, LLVM optimizations can be upgraded to be aware of the LLVM
      debugging information, allowing them to update the debugging information
      as they perform aggressive optimizations.  This means that, with effort,
      the LLVM optimizers could optimize debug code just as well as non-debug
      code.</li>

  <li>LLVM debug information does not prevent optimizations from
      happening (for example inlining, basic block reordering/merging/cleanup,
      tail duplication, etc).</li>

  <li>LLVM debug information is automatically optimized along with the rest of
      the program, using existing facilities.  For example, duplicate
      information is automatically merged by the linker, and unused information
      is automatically removed.</li>
</ul>

<p>Basically, the debug information allows you to compile a program with
   "<tt>-O0 -g</tt>" and get full debug information, allowing you to arbitrarily
   modify the program as it executes from a debugger.  Compiling a program with
   "<tt>-O3 -g</tt>" gives you full debug information that is always available
   and accurate for reading (e.g., you get accurate stack traces despite tail
   call elimination and inlining), but you might lose the ability to modify the
   program and call functions where were optimized out of the program, or
   inlined away completely.</p>

<p><a href="TestingGuide.html#quicktestsuite">LLVM test suite</a> provides a
   framework to test optimizer's handling of debugging information. It can be
   run like this:</p>

<div class="doc_code">
<pre>
% cd llvm/projects/test-suite/MultiSource/Benchmarks  # or some other level
% make TEST=dbgopt
</pre>
</div>

<p>This will test impact of debugging information on optimization passes. If
   debugging information influences optimization passes then it will be reported
   as a failure. See <a href="TestingGuide.html">TestingGuide</a> for more
   information on LLVM test infrastructure and how to run various tests.</p>

</div>

</div>

<!-- *********************************************************************** -->
<h2>
  <a name="format">Debugging information format</a>
</h2>
<!-- *********************************************************************** -->

<div>

<p>LLVM debugging information has been carefully designed to make it possible
   for the optimizer to optimize the program and debugging information without
   necessarily having to know anything about debugging information.  In
   particular, the use of metadata avoids duplicated debugging information from
   the beginning, and the global dead code elimination pass automatically
   deletes debugging information for a function if it decides to delete the
   function. </p>

<p>To do this, most of the debugging information (descriptors for types,
   variables, functions, source files, etc) is inserted by the language
   front-end in the form of LLVM metadata. </p>

<p>Debug information is designed to be agnostic about the target debugger and
   debugging information representation (e.g. DWARF/Stabs/etc).  It uses a
   generic pass to decode the information that represents variables, types,
   functions, namespaces, etc: this allows for arbitrary source-language
   semantics and type-systems to be used, as long as there is a module
   written for the target debugger to interpret the information. </p>

<p>To provide basic functionality, the LLVM debugger does have to make some
   assumptions about the source-level language being debugged, though it keeps
   these to a minimum.  The only common features that the LLVM debugger assumes
   exist are <a href="#format_files">source files</a>,
   and <a href="#format_global_variables">program objects</a>.  These abstract
   objects are used by a debugger to form stack traces, show information about
   local variables, etc.</p>

<p>This section of the documentation first describes the representation aspects
   common to any source-language.  The <a href="#ccxx_frontend">next section</a>
   describes the data layout conventions used by the C and C++ front-ends.</p>

<!-- ======================================================================= -->
<h3>
  <a name="debug_info_descriptors">Debug information descriptors</a>
</h3>

<div>

<p>In consideration of the complexity and volume of debug information, LLVM
   provides a specification for well formed debug descriptors. </p>

<p>Consumers of LLVM debug information expect the descriptors for program
   objects to start in a canonical format, but the descriptors can include
   additional information appended at the end that is source-language
   specific. All LLVM debugging information is versioned, allowing backwards
   compatibility in the case that the core structures need to change in some
   way.  Also, all debugging information objects start with a tag to indicate
   what type of object it is.  The source-language is allowed to define its own
   objects, by using unreserved tag numbers.  We recommend using with tags in
   the range 0x1000 through 0x2000 (there is a defined enum DW_TAG_user_base =
   0x1000.)</p>

<p>The fields of debug descriptors used internally by LLVM
   are restricted to only the simple data types <tt>i32</tt>, <tt>i1</tt>,
   <tt>float</tt>, <tt>double</tt>, <tt>mdstring</tt> and <tt>mdnode</tt>. </p>

<div class="doc_code">
<pre>
!1 = metadata !{
  i32,   ;; A tag
  ...
}
</pre>
</div>

<p><a name="LLVMDebugVersion">The first field of a descriptor is always an
   <tt>i32</tt> containing a tag value identifying the content of the
   descriptor.  The remaining fields are specific to the descriptor.  The values
   of tags are loosely bound to the tag values of DWARF information entries.
   However, that does not restrict the use of the information supplied to DWARF
   targets.  To facilitate versioning of debug information, the tag is augmented
   with the current debug version (LLVMDebugVersion = 8 &lt;&lt; 16 or
   0x80000 or 524288.)</a></p>

<p>The details of the various descriptors follow.</p>

<!-- ======================================================================= -->
<h4>
  <a name="format_compile_units">Compile unit descriptors</a>
</h4>

<div>

<div class="doc_code">
<pre>
!0 = metadata !{
  i32,       ;; Tag = 17 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
             ;; (DW_TAG_compile_unit)
  i32,       ;; Unused field.
  i32,       ;; DWARF language identifier (ex. DW_LANG_C89)
  metadata,  ;; Source file name
  metadata,  ;; Source file directory (includes trailing slash)
  metadata   ;; Producer (ex. "4.0.1 LLVM (LLVM research group)")
  i1,        ;; True if this is a main compile unit.
  i1,        ;; True if this is optimized.
  metadata,  ;; Flags
  i32        ;; Runtime version
  metadata   ;; List of enums types
  metadata   ;; List of retained types
  metadata   ;; List of subprograms
  metadata   ;; List of global variables
}
</pre>
</div>

<p>These descriptors contain a source language ID for the file (we use the DWARF
   3.0 ID numbers, such as <tt>DW_LANG_C89</tt>, <tt>DW_LANG_C_plus_plus</tt>,
   <tt>DW_LANG_Cobol74</tt>, etc), three strings describing the filename,
   working directory of the compiler, and an identifier string for the compiler
   that produced it.</p>

<p>Compile unit descriptors provide the root context for objects declared in a
   specific compilation unit. File descriptors are defined using this context.
   These descriptors are collected by a named metadata
   <tt>!llvm.dbg.cu</tt>. Compile unit descriptor keeps track of subprograms,
   global variables and type information.

</div>

<!-- ======================================================================= -->
<h4>
  <a name="format_files">File descriptors</a>
</h4>

<div>

<div class="doc_code">
<pre>
!0 = metadata !{
  i32,       ;; Tag = 41 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
             ;; (DW_TAG_file_type)
  metadata,  ;; Source file name
  metadata,  ;; Source file directory (includes trailing slash)
  metadata   ;; Unused
}
</pre>
</div>

<p>These descriptors contain information for a file. Global variables and top
   level functions would be defined using this context.k File descriptors also
   provide context for source line correspondence. </p>

<p>Each input file is encoded as a separate file descriptor in LLVM debugging
   information output. </p>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="format_global_variables">Global variable descriptors</a>
</h4>

<div>

<div class="doc_code">
<pre>
!1 = metadata !{
  i32,      ;; Tag = 52 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
            ;; (DW_TAG_variable)
  i32,      ;; Unused field.
  metadata, ;; Reference to context descriptor
  metadata, ;; Name
  metadata, ;; Display name (fully qualified C++ name)
  metadata, ;; MIPS linkage name (for C++)
  metadata, ;; Reference to file where defined
  i32,      ;; Line number where defined
  metadata, ;; Reference to type descriptor
  i1,       ;; True if the global is local to compile unit (static)
  i1,       ;; True if the global is defined in the compile unit (not extern)
  {}*       ;; Reference to the global variable
}
</pre>
</div>

<p>These descriptors provide debug information about globals variables.  The
provide details such as name, type and where the variable is defined. All
global variables are collected inside the named metadata
<tt>!llvm.dbg.cu</tt>.</p>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="format_subprograms">Subprogram descriptors</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32,      ;; Tag = 46 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
            ;; (DW_TAG_subprogram)
  i32,      ;; Unused field.
  metadata, ;; Reference to context descriptor
  metadata, ;; Name
  metadata, ;; Display name (fully qualified C++ name)
  metadata, ;; MIPS linkage name (for C++)
  metadata, ;; Reference to file where defined
  i32,      ;; Line number where defined
  metadata, ;; Reference to type descriptor
  i1,       ;; True if the global is local to compile unit (static)
  i1,       ;; True if the global is defined in the compile unit (not extern)
  i32,      ;; Line number where the scope of the subprogram begins
  i32,      ;; Virtuality, e.g. dwarf::DW_VIRTUALITY__virtual
  i32,      ;; Index into a virtual function
  metadata, ;; indicates which base type contains the vtable pointer for the
            ;; derived class
  i32,      ;; Flags - Artifical, Private, Protected, Explicit, Prototyped.
  i1,       ;; isOptimized
  Function *,;; Pointer to LLVM function
  metadata, ;; Lists function template parameters
  metadata  ;; Function declaration descriptor
  metadata  ;; List of function variables
}
</pre>
</div>

<p>These descriptors provide debug information about functions, methods and
   subprograms.  They provide details such as name, return types and the source
   location where the subprogram is defined.
</p>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="format_blocks">Block descriptors</a>
</h4>

<div>

<div class="doc_code">
<pre>
!3 = metadata !{
  i32,     ;; Tag = 11 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_lexical_block)
  metadata,;; Reference to context descriptor
  i32,     ;; Line number
  i32,     ;; Column number
  metadata,;; Reference to source file
  i32      ;; Unique ID to identify blocks from a template function
}
</pre>
</div>

<p>This descriptor provides debug information about nested blocks within a
   subprogram. The line number and column numbers are used to dinstinguish
   two lexical blocks at same depth. </p>

<div class="doc_code">
<pre>
!3 = metadata !{
  i32,     ;; Tag = 11 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_lexical_block)
  metadata ;; Reference to the scope we're annotating with a file change
  metadata,;; Reference to the file the scope is enclosed in.
}
</pre>
</div>

<p>This descriptor provides a wrapper around a lexical scope to handle file
   changes in the middle of a lexical block.</p>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="format_basic_type">Basic type descriptors</a>
</h4>

<div>

<div class="doc_code">
<pre>
!4 = metadata !{
  i32,      ;; Tag = 36 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
            ;; (DW_TAG_base_type)
  metadata, ;; Reference to context
  metadata, ;; Name (may be "" for anonymous types)
  metadata, ;; Reference to file where defined (may be NULL)
  i32,      ;; Line number where defined (may be 0)
  i64,      ;; Size in bits
  i64,      ;; Alignment in bits
  i64,      ;; Offset in bits
  i32,      ;; Flags
  i32       ;; DWARF type encoding
}
</pre>
</div>

<p>These descriptors define primitive types used in the code. Example int, bool
   and float.  The context provides the scope of the type, which is usually the
   top level.  Since basic types are not usually user defined the context
   and line number can be left as NULL and 0.  The size, alignment and offset
   are expressed in bits and can be 64 bit values.  The alignment is used to
   round the offset when embedded in a
   <a href="#format_composite_type">composite type</a> (example to keep float
   doubles on 64 bit boundaries.) The offset is the bit offset if embedded in
   a <a href="#format_composite_type">composite type</a>.</p>

<p>The type encoding provides the details of the type.  The values are typically
   one of the following:</p>

<div class="doc_code">
<pre>
DW_ATE_address       = 1
DW_ATE_boolean       = 2
DW_ATE_float         = 4
DW_ATE_signed        = 5
DW_ATE_signed_char   = 6
DW_ATE_unsigned      = 7
DW_ATE_unsigned_char = 8
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="format_derived_type">Derived type descriptors</a>
</h4>

<div>

<div class="doc_code">
<pre>
!5 = metadata !{
  i32,      ;; Tag (see below)
  metadata, ;; Reference to context
  metadata, ;; Name (may be "" for anonymous types)
  metadata, ;; Reference to file where defined (may be NULL)
  i32,      ;; Line number where defined (may be 0)
  i64,      ;; Size in bits
  i64,      ;; Alignment in bits
  i64,      ;; Offset in bits
  i32,      ;; Flags to encode attributes, e.g. private
  metadata, ;; Reference to type derived from
  metadata, ;; (optional) Name of the Objective C property associated with
            ;; Objective-C an ivar
  metadata, ;; (optional) Name of the Objective C property getter selector.
  metadata, ;; (optional) Name of the Objective C property setter selector.
  i32       ;; (optional) Objective C property attributes.
}
</pre>
</div>

<p>These descriptors are used to define types derived from other types.  The
value of the tag varies depending on the meaning.  The following are possible
tag values:</p>

<div class="doc_code">
<pre>
DW_TAG_formal_parameter = 5
DW_TAG_member           = 13
DW_TAG_pointer_type     = 15
DW_TAG_reference_type   = 16
DW_TAG_typedef          = 22
DW_TAG_const_type       = 38
DW_TAG_volatile_type    = 53
DW_TAG_restrict_type    = 55
</pre>
</div>

<p><tt>DW_TAG_member</tt> is used to define a member of
   a <a href="#format_composite_type">composite type</a>
   or <a href="#format_subprograms">subprogram</a>.  The type of the member is
   the <a href="#format_derived_type">derived
   type</a>. <tt>DW_TAG_formal_parameter</tt> is used to define a member which
   is a formal argument of a subprogram.</p>

<p><tt>DW_TAG_typedef</tt> is used to provide a name for the derived type.</p>

<p><tt>DW_TAG_pointer_type</tt>, <tt>DW_TAG_reference_type</tt>,
   <tt>DW_TAG_const_type</tt>, <tt>DW_TAG_volatile_type</tt> and
   <tt>DW_TAG_restrict_type</tt> are used to qualify
   the <a href="#format_derived_type">derived type</a>. </p>

<p><a href="#format_derived_type">Derived type</a> location can be determined
   from the context and line number.  The size, alignment and offset are
   expressed in bits and can be 64 bit values.  The alignment is used to round
   the offset when embedded in a <a href="#format_composite_type">composite
   type</a> (example to keep float doubles on 64 bit boundaries.) The offset is
   the bit offset if embedded in a <a href="#format_composite_type">composite
   type</a>.</p>

<p>Note that the <tt>void *</tt> type is expressed as a type derived from NULL.
</p>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="format_composite_type">Composite type descriptors</a>
</h4>

<div>

<div class="doc_code">
<pre>
!6 = metadata !{
  i32,      ;; Tag (see below)
  metadata, ;; Reference to context
  metadata, ;; Name (may be "" for anonymous types)
  metadata, ;; Reference to file where defined (may be NULL)
  i32,      ;; Line number where defined (may be 0)
  i64,      ;; Size in bits
  i64,      ;; Alignment in bits
  i64,      ;; Offset in bits
  i32,      ;; Flags
  metadata, ;; Reference to type derived from
  metadata, ;; Reference to array of member descriptors
  i32       ;; Runtime languages
}
</pre>
</div>

<p>These descriptors are used to define types that are composed of 0 or more
elements.  The value of the tag varies depending on the meaning.  The following
are possible tag values:</p>

<div class="doc_code">
<pre>
DW_TAG_array_type       = 1
DW_TAG_enumeration_type = 4
DW_TAG_structure_type   = 19
DW_TAG_union_type       = 23
DW_TAG_vector_type      = 259
DW_TAG_subroutine_type  = 21
DW_TAG_inheritance      = 28
</pre>
</div>

<p>The vector flag indicates that an array type is a native packed vector.</p>

<p>The members of array types (tag = <tt>DW_TAG_array_type</tt>) or vector types
   (tag = <tt>DW_TAG_vector_type</tt>) are <a href="#format_subrange">subrange
   descriptors</a>, each representing the range of subscripts at that level of
   indexing.</p>

<p>The members of enumeration types (tag = <tt>DW_TAG_enumeration_type</tt>) are
   <a href="#format_enumeration">enumerator descriptors</a>, each representing
   the definition of enumeration value for the set. All enumeration type
   descriptors are collected inside the named metadata
   <tt>!llvm.dbg.cu</tt>.</p>

<p>The members of structure (tag = <tt>DW_TAG_structure_type</tt>) or union (tag
   = <tt>DW_TAG_union_type</tt>) types are any one of
   the <a href="#format_basic_type">basic</a>,
   <a href="#format_derived_type">derived</a>
   or <a href="#format_composite_type">composite</a> type descriptors, each
   representing a field member of the structure or union.</p>

<p>For C++ classes (tag = <tt>DW_TAG_structure_type</tt>), member descriptors
   provide information about base classes, static members and member
   functions. If a member is a <a href="#format_derived_type">derived type
   descriptor</a> and has a tag of <tt>DW_TAG_inheritance</tt>, then the type
   represents a base class. If the member of is
   a <a href="#format_global_variables">global variable descriptor</a> then it
   represents a static member.  And, if the member is
   a <a href="#format_subprograms">subprogram descriptor</a> then it represents
   a member function.  For static members and member
   functions, <tt>getName()</tt> returns the members link or the C++ mangled
   name.  <tt>getDisplayName()</tt> the simplied version of the name.</p>

<p>The first member of subroutine (tag = <tt>DW_TAG_subroutine_type</tt>) type
   elements is the return type for the subroutine.  The remaining elements are
   the formal arguments to the subroutine.</p>

<p><a href="#format_composite_type">Composite type</a> location can be
   determined from the context and line number.  The size, alignment and
   offset are expressed in bits and can be 64 bit values.  The alignment is used
   to round the offset when embedded in
   a <a href="#format_composite_type">composite type</a> (as an example, to keep
   float doubles on 64 bit boundaries.) The offset is the bit offset if embedded
   in a <a href="#format_composite_type">composite type</a>.</p>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="format_subrange">Subrange descriptors</a>
</h4>

<div>

<div class="doc_code">
<pre>
!42 = metadata !{
  i32,    ;; Tag = 33 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a> (DW_TAG_subrange_type)
  i64,    ;; Low value
  i64     ;; High value
}
</pre>
</div>

<p>These descriptors are used to define ranges of array subscripts for an array
   <a href="#format_composite_type">composite type</a>.  The low value defines
   the lower bounds typically zero for C/C++.  The high value is the upper
   bounds.  Values are 64 bit.  High - low + 1 is the size of the array.  If low
   > high the array bounds are not included in generated debugging information.
</p>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="format_enumeration">Enumerator descriptors</a>
</h4>

<div>

<div class="doc_code">
<pre>
!6 = metadata !{
  i32,      ;; Tag = 40 + <a href="#LLVMDebugVersion">LLVMDebugVersion</a>
            ;; (DW_TAG_enumerator)
  metadata, ;; Name
  i64       ;; Value
}
</pre>
</div>

<p>These descriptors are used to define members of an
   enumeration <a href="#format_composite_type">composite type</a>, it
   associates the name to the value.</p>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="format_variables">Local variables</a>
</h4>

<div>

<div class="doc_code">
<pre>
!7 = metadata !{
  i32,      ;; Tag (see below)
  metadata, ;; Context
  metadata, ;; Name
  metadata, ;; Reference to file where defined
  i32,      ;; 24 bit - Line number where defined
            ;; 8 bit - Argument number. 1 indicates 1st argument.
  metadata, ;; Type descriptor
  i32,      ;; flags
  metadata  ;; (optional) Reference to inline location
}
</pre>
</div>

<p>These descriptors are used to define variables local to a sub program.  The
   value of the tag depends on the usage of the variable:</p>

<div class="doc_code">
<pre>
DW_TAG_auto_variable   = 256
DW_TAG_arg_variable    = 257
DW_TAG_return_variable = 258
</pre>
</div>

<p>An auto variable is any variable declared in the body of the function.  An
   argument variable is any variable that appears as a formal argument to the
   function.  A return variable is used to track the result of a function and
   has no source correspondent.</p>

<p>The context is either the subprogram or block where the variable is defined.
   Name the source variable name.  Context and line indicate where the
   variable was defined. Type descriptor defines the declared type of the
   variable.</p>

</div>

</div>

<!-- ======================================================================= -->
<h3>
  <a name="format_common_intrinsics">Debugger intrinsic functions</a>
</h3>

<div>

<p>LLVM uses several intrinsic functions (name prefixed with "llvm.dbg") to
   provide debug information at various points in generated code.</p>

<!-- ======================================================================= -->
<h4>
  <a name="format_common_declare">llvm.dbg.declare</a>
</h4>

<div>
<pre>
  void %<a href="#format_common_declare">llvm.dbg.declare</a>(metadata, metadata)
</pre>

<p>This intrinsic provides information about a local element (e.g., variable). The
   first argument is metadata holding the alloca for the variable. The
   second argument is metadata containing a description of the variable.</p>
</div>

<!-- ======================================================================= -->
<h4>
  <a name="format_common_value">llvm.dbg.value</a>
</h4>

<div>
<pre>
  void %<a href="#format_common_value">llvm.dbg.value</a>(metadata, i64, metadata)
</pre>

<p>This intrinsic provides information when a user source variable is set to a
   new value.  The first argument is the new value (wrapped as metadata).  The
   second argument is the offset in the user source variable where the new value
   is written.  The third argument is metadata containing a description of the
   user source variable.</p>
</div>

</div>

<!-- ======================================================================= -->
<h3>
  <a name="format_common_lifetime">Object lifetimes and scoping</a>
</h3>

<div>
<p>In many languages, the local variables in functions can have their lifetimes
   or scopes limited to a subset of a function.  In the C family of languages,
   for example, variables are only live (readable and writable) within the
   source block that they are defined in.  In functional languages, values are
   only readable after they have been defined.  Though this is a very obvious
   concept, it is non-trivial to model in LLVM, because it has no notion of
   scoping in this sense, and does not want to be tied to a language's scoping
   rules.</p>

<p>In order to handle this, the LLVM debug format uses the metadata attached to
   llvm instructions to encode line number and scoping information. Consider
   the following C fragment, for example:</p>

<div class="doc_code">
<pre>
1.  void foo() {
2.    int X = 21;
3.    int Y = 22;
4.    {
5.      int Z = 23;
6.      Z = X;
7.    }
8.    X = Y;
9.  }
</pre>
</div>

<p>Compiled to LLVM, this function would be represented like this:</p>

<div class="doc_code">
<pre>
define void @foo() nounwind ssp {
entry:
  %X = alloca i32, align 4                        ; &lt;i32*&gt; [#uses=4]
  %Y = alloca i32, align 4                        ; &lt;i32*&gt; [#uses=4]
  %Z = alloca i32, align 4                        ; &lt;i32*&gt; [#uses=3]
  %0 = bitcast i32* %X to {}*                     ; &lt;{}*&gt; [#uses=1]
  call void @llvm.dbg.declare(metadata !{i32 * %X}, metadata !0), !dbg !7
  store i32 21, i32* %X, !dbg !8
  %1 = bitcast i32* %Y to {}*                     ; &lt;{}*&gt; [#uses=1]
  call void @llvm.dbg.declare(metadata !{i32 * %Y}, metadata !9), !dbg !10
  store i32 22, i32* %Y, !dbg !11
  %2 = bitcast i32* %Z to {}*                     ; &lt;{}*&gt; [#uses=1]
  call void @llvm.dbg.declare(metadata !{i32 * %Z}, metadata !12), !dbg !14
  store i32 23, i32* %Z, !dbg !15
  %tmp = load i32* %X, !dbg !16                   ; &lt;i32&gt; [#uses=1]
  %tmp1 = load i32* %Y, !dbg !16                  ; &lt;i32&gt; [#uses=1]
  %add = add nsw i32 %tmp, %tmp1, !dbg !16        ; &lt;i32&gt; [#uses=1]
  store i32 %add, i32* %Z, !dbg !16
  %tmp2 = load i32* %Y, !dbg !17                  ; &lt;i32&gt; [#uses=1]
  store i32 %tmp2, i32* %X, !dbg !17
  ret void, !dbg !18
}

declare void @llvm.dbg.declare(metadata, metadata) nounwind readnone

!0 = metadata !{i32 459008, metadata !1, metadata !"X",
                metadata !3, i32 2, metadata !6}; [ DW_TAG_auto_variable ]
!1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]
!2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo", metadata !"foo",
               metadata !"foo", metadata !3, i32 1, metadata !4,
               i1 false, i1 true}; [DW_TAG_subprogram ]
!3 = metadata !{i32 458769, i32 0, i32 12, metadata !"foo.c",
                metadata !"/private/tmp", metadata !"clang 1.1", i1 true,
                i1 false, metadata !"", i32 0}; [DW_TAG_compile_unit ]
!4 = metadata !{i32 458773, metadata !3, metadata !"", null, i32 0, i64 0, i64 0,
                i64 0, i32 0, null, metadata !5, i32 0}; [DW_TAG_subroutine_type ]
!5 = metadata !{null}
!6 = metadata !{i32 458788, metadata !3, metadata !"int", metadata !3, i32 0,
                i64 32, i64 32, i64 0, i32 0, i32 5}; [DW_TAG_base_type ]
!7 = metadata !{i32 2, i32 7, metadata !1, null}
!8 = metadata !{i32 2, i32 3, metadata !1, null}
!9 = metadata !{i32 459008, metadata !1, metadata !"Y", metadata !3, i32 3,
                metadata !6}; [ DW_TAG_auto_variable ]
!10 = metadata !{i32 3, i32 7, metadata !1, null}
!11 = metadata !{i32 3, i32 3, metadata !1, null}
!12 = metadata !{i32 459008, metadata !13, metadata !"Z", metadata !3, i32 5,
                 metadata !6}; [ DW_TAG_auto_variable ]
!13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]
!14 = metadata !{i32 5, i32 9, metadata !13, null}
!15 = metadata !{i32 5, i32 5, metadata !13, null}
!16 = metadata !{i32 6, i32 5, metadata !13, null}
!17 = metadata !{i32 8, i32 3, metadata !1, null}
!18 = metadata !{i32 9, i32 1, metadata !2, null}
</pre>
</div>

<p>This example illustrates a few important details about LLVM debugging
   information. In particular, it shows how the <tt>llvm.dbg.declare</tt>
   intrinsic and location information, which are attached to an instruction,
   are applied together to allow a debugger to analyze the relationship between
   statements, variable definitions, and the code used to implement the
   function.</p>

<div class="doc_code">
<pre>
call void @llvm.dbg.declare(metadata, metadata !0), !dbg !7
</pre>
</div>

<p>The first intrinsic
   <tt>%<a href="#format_common_declare">llvm.dbg.declare</a></tt>
   encodes debugging information for the variable <tt>X</tt>. The metadata
   <tt>!dbg !7</tt> attached to the intrinsic provides scope information for the
   variable <tt>X</tt>.</p>

<div class="doc_code">
<pre>
!7 = metadata !{i32 2, i32 7, metadata !1, null}
!1 = metadata !{i32 458763, metadata !2}; [DW_TAG_lexical_block ]
!2 = metadata !{i32 458798, i32 0, metadata !3, metadata !"foo",
                metadata !"foo", metadata !"foo", metadata !3, i32 1,
                metadata !4, i1 false, i1 true}; [DW_TAG_subprogram ]
</pre>
</div>

<p>Here <tt>!7</tt> is metadata providing location information. It has four
   fields: line number, column number, scope, and original scope. The original
   scope represents inline location if this instruction is inlined inside a
   caller, and is null otherwise. In this example, scope is encoded by
   <tt>!1</tt>. <tt>!1</tt> represents a lexical block inside the scope
   <tt>!2</tt>, where <tt>!2</tt> is a
   <a href="#format_subprograms">subprogram descriptor</a>. This way the
   location information attached to the intrinsics indicates that the
   variable <tt>X</tt> is declared at line number 2 at a function level scope in
   function <tt>foo</tt>.</p>

<p>Now lets take another example.</p>

<div class="doc_code">
<pre>
call void @llvm.dbg.declare(metadata, metadata !12), !dbg !14
</pre>
</div>

<p>The second intrinsic
   <tt>%<a href="#format_common_declare">llvm.dbg.declare</a></tt>
   encodes debugging information for variable <tt>Z</tt>. The metadata
   <tt>!dbg !14</tt> attached to the intrinsic provides scope information for
   the variable <tt>Z</tt>.</p>

<div class="doc_code">
<pre>
!13 = metadata !{i32 458763, metadata !1}; [DW_TAG_lexical_block ]
!14 = metadata !{i32 5, i32 9, metadata !13, null}
</pre>
</div>

<p>Here <tt>!14</tt> indicates that <tt>Z</tt> is declared at line number 5 and
   column number 9 inside of lexical scope <tt>!13</tt>. The lexical scope
   itself resides inside of lexical scope <tt>!1</tt> described above.</p>

<p>The scope information attached with each instruction provides a
   straightforward way to find instructions covered by a scope.</p>

</div>

</div>

<!-- *********************************************************************** -->
<h2>
  <a name="ccxx_frontend">C/C++ front-end specific debug information</a>
</h2>
<!-- *********************************************************************** -->

<div>

<p>The C and C++ front-ends represent information about the program in a format
   that is effectively identical
   to <a href="http://www.eagercon.com/dwarf/dwarf3std.htm">DWARF 3.0</a> in
   terms of information content.  This allows code generators to trivially
   support native debuggers by generating standard dwarf information, and
   contains enough information for non-dwarf targets to translate it as
   needed.</p>

<p>This section describes the forms used to represent C and C++ programs. Other
   languages could pattern themselves after this (which itself is tuned to
   representing programs in the same way that DWARF 3 does), or they could
   choose to provide completely different forms if they don't fit into the DWARF
   model.  As support for debugging information gets added to the various LLVM
   source-language front-ends, the information used should be documented
   here.</p>

<p>The following sections provide examples of various C/C++ constructs and the
   debug information that would best describe those constructs.</p>

<!-- ======================================================================= -->
<h3>
  <a name="ccxx_compile_units">C/C++ source file information</a>
</h3>

<div>

<p>Given the source files <tt>MySource.cpp</tt> and <tt>MyHeader.h</tt> located
   in the directory <tt>/Users/mine/sources</tt>, the following code:</p>

<div class="doc_code">
<pre>
#include "MyHeader.h"

int main(int argc, char *argv[]) {
  return 0;
}
</pre>
</div>

<p>a C/C++ front-end would generate the following descriptors:</p>

<div class="doc_code">
<pre>
...
;;
;; Define the compile unit for the main source file "/Users/mine/sources/MySource.cpp".
;;
!2 = metadata !{
  i32 524305,    ;; Tag
  i32 0,         ;; Unused
  i32 4,         ;; Language Id
  metadata !"MySource.cpp",
  metadata !"/Users/mine/sources",
  metadata !"4.2.1 (Based on Apple Inc. build 5649) (LLVM build 00)",
  i1 true,       ;; Main Compile Unit
  i1 false,      ;; Optimized compile unit
  metadata !"",  ;; Compiler flags
  i32 0}         ;; Runtime version

;;
;; Define the file for the file "/Users/mine/sources/MySource.cpp".
;;
!1 = metadata !{
  i32 524329,    ;; Tag
  metadata !"MySource.cpp",
  metadata !"/Users/mine/sources",
  metadata !2    ;; Compile unit
}

;;
;; Define the file for the file "/Users/mine/sources/Myheader.h"
;;
!3 = metadata !{
  i32 524329,    ;; Tag
  metadata !"Myheader.h"
  metadata !"/Users/mine/sources",
  metadata !2    ;; Compile unit
}

...
</pre>
</div>

<p>llvm::Instruction provides easy access to metadata attached with an
instruction. One can extract line number information encoded in LLVM IR
using <tt>Instruction::getMetadata()</tt> and
<tt>DILocation::getLineNumber()</tt>.
<pre>
 if (MDNode *N = I->getMetadata("dbg")) {  // Here I is an LLVM instruction
   DILocation Loc(N);                      // DILocation is in DebugInfo.h
   unsigned Line = Loc.getLineNumber();
   StringRef File = Loc.getFilename();
   StringRef Dir = Loc.getDirectory();
 }
</pre>
</div>

<!-- ======================================================================= -->
<h3>
  <a name="ccxx_global_variable">C/C++ global variable information</a>
</h3>

<div>

<p>Given an integer global variable declared as follows:</p>

<div class="doc_code">
<pre>
int MyGlobal = 100;
</pre>
</div>

<p>a C/C++ front-end would generate the following descriptors:</p>

<div class="doc_code">
<pre>
;;
;; Define the global itself.
;;
%MyGlobal = global int 100
...
;;
;; List of debug info of globals
;;
!llvm.dbg.cu = !{!0}

;; Define the compile unit.
!0 = metadata !{
  i32 786449,                       ;; Tag
  i32 0,                            ;; Context
  i32 4,                            ;; Language
  metadata !"foo.cpp",              ;; File
  metadata !"/Volumes/Data/tmp",    ;; Directory
  metadata !"clang version 3.1 ",   ;; Producer
  i1 true,                          ;; Deprecated field
  i1 false,                         ;; "isOptimized"?
  metadata !"",                     ;; Flags
  i32 0,                            ;; Runtime Version
  metadata !1,                      ;; Enum Types
  metadata !1,                      ;; Retained Types
  metadata !1,                      ;; Subprograms
  metadata !3                       ;; Global Variables
} ; [ DW_TAG_compile_unit ]

;; The Array of Global Variables
!3 = metadata !{
  metadata !4
}

!4 = metadata !{
  metadata !5
}

;;
;; Define the global variable itself.
;;
!5 = metadata !{
  i32 786484,                        ;; Tag
  i32 0,                             ;; Unused
  null,                              ;; Unused
  metadata !"MyGlobal",              ;; Name
  metadata !"MyGlobal",              ;; Display Name
  metadata !"",                      ;; Linkage Name
  metadata !6,                       ;; File
  i32 1,                             ;; Line
  metadata !7,                       ;; Type
  i32 0,                             ;; IsLocalToUnit
  i32 1,                             ;; IsDefinition
  i32* @MyGlobal                     ;; LLVM-IR Value
} ; [ DW_TAG_variable ]

;;
;; Define the file
;;
!6 = metadata !{
  i32 786473,                        ;; Tag
  metadata !"foo.cpp",               ;; File
  metadata !"/Volumes/Data/tmp",     ;; Directory
  null                               ;; Unused
} ; [ DW_TAG_file_type ]

;;
;; Define the type
;;
!7 = metadata !{
  i32 786468,                         ;; Tag
  null,                               ;; Unused
  metadata !"int",                    ;; Name
  null,                               ;; Unused
  i32 0,                              ;; Line
  i64 32,                             ;; Size in Bits
  i64 32,                             ;; Align in Bits
  i64 0,                              ;; Offset
  i32 0,                              ;; Flags
  i32 5                               ;; Encoding
} ; [ DW_TAG_base_type ]

</pre>
</div>

</div>

<!-- ======================================================================= -->
<h3>
  <a name="ccxx_subprogram">C/C++ function information</a>
</h3>

<div>

<p>Given a function declared as follows:</p>

<div class="doc_code">
<pre>
int main(int argc, char *argv[]) {
  return 0;
}
</pre>
</div>

<p>a C/C++ front-end would generate the following descriptors:</p>

<div class="doc_code">
<pre>
;;
;; Define the anchor for subprograms.  Note that the second field of the
;; anchor is 46, which is the same as the tag for subprograms
;; (46 = DW_TAG_subprogram.)
;;
!6 = metadata !{
  i32 524334,        ;; Tag
  i32 0,             ;; Unused
  metadata !1,       ;; Context
  metadata !"main",  ;; Name
  metadata !"main",  ;; Display name
  metadata !"main",  ;; Linkage name
  metadata !1,       ;; File
  i32 1,             ;; Line number
  metadata !4,       ;; Type
  i1 false,          ;; Is local
  i1 true,           ;; Is definition
  i32 0,             ;; Virtuality attribute, e.g. pure virtual function
  i32 0,             ;; Index into virtual table for C++ methods
  i32 0,             ;; Type that holds virtual table.
  i32 0,             ;; Flags
  i1 false,          ;; True if this function is optimized
  Function *,        ;; Pointer to llvm::Function
  null               ;; Function template parameters
}
;;
;; Define the subprogram itself.
;;
define i32 @main(i32 %argc, i8** %argv) {
...
}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h3>
  <a name="ccxx_basic_types">C/C++ basic types</a>
</h3>

<div>

<p>The following are the basic type descriptors for C/C++ core types:</p>

<!-- ======================================================================= -->
<h4>
  <a name="ccxx_basic_type_bool">bool</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"bool",  ;; Name
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 8,             ;; Size in Bits
  i64 8,             ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 2              ;; Encoding
}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="ccxx_basic_char">char</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"char",  ;; Name
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 8,             ;; Size in Bits
  i64 8,             ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 6              ;; Encoding
}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="ccxx_basic_unsigned_char">unsigned char</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"unsigned char",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 8,             ;; Size in Bits
  i64 8,             ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 8              ;; Encoding
}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="ccxx_basic_short">short</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"short int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 16,            ;; Size in Bits
  i64 16,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 5              ;; Encoding
}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="ccxx_basic_unsigned_short">unsigned short</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"short unsigned int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 16,            ;; Size in Bits
  i64 16,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 7              ;; Encoding
}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="ccxx_basic_int">int</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"int",   ;; Name
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 32,            ;; Size in Bits
  i64 32,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 5              ;; Encoding
}
</pre></div>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="ccxx_basic_unsigned_int">unsigned int</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"unsigned int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 32,            ;; Size in Bits
  i64 32,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 7              ;; Encoding
}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="ccxx_basic_long_long">long long</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"long long int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 64,            ;; Size in Bits
  i64 64,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 5              ;; Encoding
}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="ccxx_basic_unsigned_long_long">unsigned long long</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"long long unsigned int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 64,            ;; Size in Bits
  i64 64,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 7              ;; Encoding
}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="ccxx_basic_float">float</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"float",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 32,            ;; Size in Bits
  i64 32,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 4              ;; Encoding
}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="ccxx_basic_double">double</a>
</h4>

<div>

<div class="doc_code">
<pre>
!2 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"double",;; Name
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 64,            ;; Size in Bits
  i64 64,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 4              ;; Encoding
}
</pre>
</div>

</div>

</div>

<!-- ======================================================================= -->
<h3>
  <a name="ccxx_derived_types">C/C++ derived types</a>
</h3>

<div>

<p>Given the following as an example of C/C++ derived type:</p>

<div class="doc_code">
<pre>
typedef const int *IntPtr;
</pre>
</div>

<p>a C/C++ front-end would generate the following descriptors:</p>

<div class="doc_code">
<pre>
;;
;; Define the typedef "IntPtr".
;;
!2 = metadata !{
  i32 524310,          ;; Tag
  metadata !1,         ;; Context
  metadata !"IntPtr",  ;; Name
  metadata !3,         ;; File
  i32 0,               ;; Line number
  i64 0,               ;; Size in bits
  i64 0,               ;; Align in bits
  i64 0,               ;; Offset in bits
  i32 0,               ;; Flags
  metadata !4          ;; Derived From type
}

;;
;; Define the pointer type.
;;
!4 = metadata !{
  i32 524303,          ;; Tag
  metadata !1,         ;; Context
  metadata !"",        ;; Name
  metadata !1,         ;; File
  i32 0,               ;; Line number
  i64 64,              ;; Size in bits
  i64 64,              ;; Align in bits
  i64 0,               ;; Offset in bits
  i32 0,               ;; Flags
  metadata !5          ;; Derived From type
}
;;
;; Define the const type.
;;
!5 = metadata !{
  i32 524326,          ;; Tag
  metadata !1,         ;; Context
  metadata !"",        ;; Name
  metadata !1,         ;; File
  i32 0,               ;; Line number
  i64 32,              ;; Size in bits
  i64 32,              ;; Align in bits
  i64 0,               ;; Offset in bits
  i32 0,               ;; Flags
  metadata !6          ;; Derived From type
}
;;
;; Define the int type.
;;
!6 = metadata !{
  i32 524324,          ;; Tag
  metadata !1,         ;; Context
  metadata !"int",     ;; Name
  metadata !1,         ;; File
  i32 0,               ;; Line number
  i64 32,              ;; Size in bits
  i64 32,              ;; Align in bits
  i64 0,               ;; Offset in bits
  i32 0,               ;; Flags
  5                    ;; Encoding
}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h3>
  <a name="ccxx_composite_types">C/C++ struct/union types</a>
</h3>

<div>

<p>Given the following as an example of C/C++ struct type:</p>

<div class="doc_code">
<pre>
struct Color {
  unsigned Red;
  unsigned Green;
  unsigned Blue;
};
</pre>
</div>

<p>a C/C++ front-end would generate the following descriptors:</p>

<div class="doc_code">
<pre>
;;
;; Define basic type for unsigned int.
;;
!5 = metadata !{
  i32 524324,        ;; Tag
  metadata !1,       ;; Context
  metadata !"unsigned int",
  metadata !1,       ;; File
  i32 0,             ;; Line number
  i64 32,            ;; Size in Bits
  i64 32,            ;; Align in Bits
  i64 0,             ;; Offset in Bits
  i32 0,             ;; Flags
  i32 7              ;; Encoding
}
;;
;; Define composite type for struct Color.
;;
!2 = metadata !{
  i32 524307,        ;; Tag
  metadata !1,       ;; Context
  metadata !"Color", ;; Name
  metadata !1,       ;; Compile unit
  i32 1,             ;; Line number
  i64 96,            ;; Size in bits
  i64 32,            ;; Align in bits
  i64 0,             ;; Offset in bits
  i32 0,             ;; Flags
  null,              ;; Derived From
  metadata !3,       ;; Elements
  i32 0              ;; Runtime Language
}

;;
;; Define the Red field.
;;
!4 = metadata !{
  i32 524301,        ;; Tag
  metadata !1,       ;; Context
  metadata !"Red",   ;; Name
  metadata !1,       ;; File
  i32 2,             ;; Line number
  i64 32,            ;; Size in bits
  i64 32,            ;; Align in bits
  i64 0,             ;; Offset in bits
  i32 0,             ;; Flags
  metadata !5        ;; Derived From type
}

;;
;; Define the Green field.
;;
!6 = metadata !{
  i32 524301,        ;; Tag
  metadata !1,       ;; Context
  metadata !"Green", ;; Name
  metadata !1,       ;; File
  i32 3,             ;; Line number
  i64 32,            ;; Size in bits
  i64 32,            ;; Align in bits
  i64 32,             ;; Offset in bits
  i32 0,             ;; Flags
  metadata !5        ;; Derived From type
}

;;
;; Define the Blue field.
;;
!7 = metadata !{
  i32 524301,        ;; Tag
  metadata !1,       ;; Context
  metadata !"Blue",  ;; Name
  metadata !1,       ;; File
  i32 4,             ;; Line number
  i64 32,            ;; Size in bits
  i64 32,            ;; Align in bits
  i64 64,             ;; Offset in bits
  i32 0,             ;; Flags
  metadata !5        ;; Derived From type
}

;;
;; Define the array of fields used by the composite type Color.
;;
!3 = metadata !{metadata !4, metadata !6, metadata !7}
</pre>
</div>

</div>

<!-- ======================================================================= -->
<h3>
  <a name="ccxx_enumeration_types">C/C++ enumeration types</a>
</h3>

<div>

<p>Given the following as an example of C/C++ enumeration type:</p>

<div class="doc_code">
<pre>
enum Trees {
  Spruce = 100,
  Oak = 200,
  Maple = 300
};
</pre>
</div>

<p>a C/C++ front-end would generate the following descriptors:</p>

<div class="doc_code">
<pre>
;;
;; Define composite type for enum Trees
;;
!2 = metadata !{
  i32 524292,        ;; Tag
  metadata !1,       ;; Context
  metadata !"Trees", ;; Name
  metadata !1,       ;; File
  i32 1,             ;; Line number
  i64 32,            ;; Size in bits
  i64 32,            ;; Align in bits
  i64 0,             ;; Offset in bits
  i32 0,             ;; Flags
  null,              ;; Derived From type
  metadata !3,       ;; Elements
  i32 0              ;; Runtime language
}

;;
;; Define the array of enumerators used by composite type Trees.
;;
!3 = metadata !{metadata !4, metadata !5, metadata !6}

;;
;; Define Spruce enumerator.
;;
!4 = metadata !{i32 524328, metadata !"Spruce", i64 100}

;;
;; Define Oak enumerator.
;;
!5 = metadata !{i32 524328, metadata !"Oak", i64 200}

;;
;; Define Maple enumerator.
;;
!6 = metadata !{i32 524328, metadata !"Maple", i64 300}

</pre>
</div>

</div>

</div>


<!-- *********************************************************************** -->
<h2>
  <a name="llvmdwarfextension">Debugging information format</a>
</h2>
<!-- *********************************************************************** -->
<div>
<!-- ======================================================================= -->
<h3>
  <a name="objcproperty">Debugging Information Extension for Objective C Properties</a>
</h3>
<div>
<!-- *********************************************************************** -->
<h4>
  <a name="objcpropertyintroduction">Introduction</a>
</h4>
<!-- *********************************************************************** -->

<div>
<p>Objective C provides a simpler way to declare and define accessor methods
using declared properties. The language provides features to declare a
property and to let compiler synthesize accessor methods.
</p>

<p>The debugger lets developer inspect Objective C interfaces and their
instance variables and class variables. However, the debugger does not know
anything about the properties defined in Objective C interfaces. The debugger
consumes information generated by compiler in DWARF format. The format does
not support encoding of Objective C properties. This proposal describes DWARF
extensions to encode Objective C properties, which the debugger can use to let
developers inspect Objective C properties.
</p>

</div>


<!-- *********************************************************************** -->
<h4>
  <a name="objcpropertyproposal">Proposal</a>
</h4>
<!-- *********************************************************************** -->

<div>
<p>Objective C properties exist separately from class members. A property
can be defined only by &quot;setter&quot; and &quot;getter&quot; selectors, and
be calculated anew on each access.  Or a property can just be a direct access
to some declared ivar.  Finally it can have an ivar &quot;automatically
synthesized&quot; for it by the compiler, in which case the property can be
referred to in user code directly using the standard C dereference syntax as
well as through the property &quot;dot&quot; syntax, but there is no entry in
the @interface declaration corresponding to this ivar.
</p>
<p>
To facilitate debugging, these properties we will add a new DWARF TAG into the
DW_TAG_structure_type definition for the class to hold the description of a
given property, and a set of DWARF attributes that provide said description.
The property tag will also contain the name and declared type of the property.
</p>
<p>
If there is a related ivar, there will also be a DWARF property attribute placed
in the DW_TAG_member DIE for that ivar referring back to the property TAG for
that property. And in the case where the compiler synthesizes the ivar directly,
the compiler is expected to generate a DW_TAG_member for that ivar (with the
DW_AT_artificial set to 1), whose name will be the name used to access this
ivar directly in code, and with the property attribute pointing back to the
property it is backing.
</p>
<p>
The following examples will serve as illustration for our discussion:
</p>

<div class="doc_code">
<pre>
@interface I1 {
  int n2;
}

@property int p1;
@property int p2;
@end

@implementation I1
@synthesize p1;
@synthesize p2 = n2;
@end
</pre>
</div>

<p>
This produces the following DWARF (this is a &quot;pseudo dwarfdump&quot; output):
</p>
<div class="doc_code">
<pre>
0x00000100:  TAG_structure_type [7] *
               AT_APPLE_runtime_class( 0x10 )
               AT_name( "I1" )
               AT_decl_file( "Objc_Property.m" )
               AT_decl_line( 3 )

0x00000110    TAG_APPLE_property
                AT_name ( "p1" )
                AT_type ( {0x00000150} ( int ) )

0x00000120:   TAG_APPLE_property
                AT_name ( "p2" )
                AT_type ( {0x00000150} ( int ) )

0x00000130:   TAG_member [8]
                AT_name( "_p1" )
                AT_APPLE_property ( {0x00000110} "p1" )
                AT_type( {0x00000150} ( int ) )
                AT_artificial ( 0x1 )

0x00000140:    TAG_member [8]
                 AT_name( "n2" )
                 AT_APPLE_property ( {0x00000120} "p2" )
                 AT_type( {0x00000150} ( int ) )

0x00000150:  AT_type( ( int ) )
</pre>
</div>

<p> Note, the current convention is that the name of the ivar for an
auto-synthesized property is the name of the property from which it derives with
an underscore prepended, as is shown in the example.
But we actually don't need to know this convention, since we are given the name
of the ivar directly.
</p>

<p>
Also, it is common practice in ObjC to have different property declarations in
the @interface and @implementation - e.g. to provide a read-only property in
the interface,and a read-write interface in the implementation.  In that case,
the compiler should emit whichever property declaration will be in force in the
current translation unit.
</p>

<p> Developers can decorate a property with attributes which are encoded using
DW_AT_APPLE_property_attribute.
</p>

<div class="doc_code">
<pre>
@property (readonly, nonatomic) int pr;
</pre>
</div>
<p>
Which produces a property tag:
<p>
<div class="doc_code">
<pre>
TAG_APPLE_property [8]
  AT_name( "pr" )
  AT_type ( {0x00000147} (int) )
  AT_APPLE_property_attribute (DW_APPLE_PROPERTY_readonly, DW_APPLE_PROPERTY_nonatomic)
</pre>
</div>

<p> The setter and getter method names are attached to the property using
DW_AT_APPLE_property_setter and DW_AT_APPLE_property_getter attributes.
</p>
<div class="doc_code">
<pre>
@interface I1
@property (setter=myOwnP3Setter:) int p3;
-(void)myOwnP3Setter:(int)a;
@end

@implementation I1
@synthesize p3;
-(void)myOwnP3Setter:(int)a{ }
@end
</pre>
</div>

<p>
The DWARF for this would be:
</p>
<div class="doc_code">
<pre>
0x000003bd: TAG_structure_type [7] *
              AT_APPLE_runtime_class( 0x10 )
              AT_name( "I1" )
              AT_decl_file( "Objc_Property.m" )
              AT_decl_line( 3 )

0x000003cd      TAG_APPLE_property
                  AT_name ( "p3" )
                  AT_APPLE_property_setter ( "myOwnP3Setter:" )
                  AT_type( {0x00000147} ( int ) )

0x000003f3:     TAG_member [8]
                  AT_name( "_p3" )
                  AT_type ( {0x00000147} ( int ) )
                  AT_APPLE_property ( {0x000003cd} )
                  AT_artificial ( 0x1 )
</pre>
</div>

</div>

<!-- *********************************************************************** -->
<h4>
  <a name="objcpropertynewtags">New DWARF Tags</a>
</h4>
<!-- *********************************************************************** -->

<div>
<table border="1" cellspacing="0">
  <col width="200">
  <col width="200">
  <tr>
    <th>TAG</th>
    <th>Value</th>
  </tr>
  <tr>
    <td>DW_TAG_APPLE_property</td>
    <td>0x4200</td>
  </tr>
</table>

</div>

<!-- *********************************************************************** -->
<h4>
  <a name="objcpropertynewattributes">New DWARF Attributes</a>
</h4>
<!-- *********************************************************************** -->

<div>
<table border="1" cellspacing="0">
  <col width="200">
  <col width="200">
  <col width="200">
  <tr>
    <th>Attribute</th>
    <th>Value</th>
    <th>Classes</th>
  </tr>
  <tr>
    <td>DW_AT_APPLE_property</td>
    <td>0x3fed</td>
    <td>Reference</td>
  </tr>
  <tr>
    <td>DW_AT_APPLE_property_getter</td>
    <td>0x3fe9</td>
    <td>String</td>
  </tr>
  <tr>
    <td>DW_AT_APPLE_property_setter</td>
    <td>0x3fea</td>
    <td>String</td>
  </tr>
  <tr>
    <td>DW_AT_APPLE_property_attribute</td>
    <td>0x3feb</td>
    <td>Constant</td>
  </tr>
</table>

</div>

<!-- *********************************************************************** -->
<h4>
  <a name="objcpropertynewconstants">New DWARF Constants</a>
</h4>
<!-- *********************************************************************** -->

<div>
<table border="1" cellspacing="0">
  <col width="200">
  <col width="200">
  <tr>
    <th>Name</th>
    <th>Value</th>
  </tr>
  <tr>
    <td>DW_AT_APPLE_PROPERTY_readonly</td>
    <td>0x1</td>
  </tr>
  <tr>
    <td>DW_AT_APPLE_PROPERTY_readwrite</td>
    <td>0x2</td>
  </tr>
  <tr>
    <td>DW_AT_APPLE_PROPERTY_assign</td>
    <td>0x4</td>
  </tr>
  <tr>
    <td>DW_AT_APPLE_PROPERTY_retain</td>
    <td>0x8</td>
  </tr>
  <tr>
    <td>DW_AT_APPLE_PROPERTY_copy</td>
    <td>0x10</td>
  </tr>
  <tr>
    <td>DW_AT_APPLE_PROPERTY_nonatomic</td>
    <td>0x20</td>
  </tr>
</table>

</div>
</div>

<!-- ======================================================================= -->
<h3>
  <a name="acceltable">Name Accelerator Tables</a>
</h3>
<!-- ======================================================================= -->
<div>
<!-- ======================================================================= -->
<h4>
  <a name="acceltableintroduction">Introduction</a>
</h4>
<!-- ======================================================================= -->
<div>
<p>The .debug_pubnames and .debug_pubtypes formats are not what a debugger
  needs. The "pub" in the section name indicates that the entries in the
  table are publicly visible names only. This means no static or hidden
  functions show up in the .debug_pubnames. No static variables or private class
  variables are in the .debug_pubtypes. Many compilers add different things to
  these tables, so we can't rely upon the contents between gcc, icc, or clang.</p>

<p>The typical query given by users tends not to match up with the contents of
  these tables. For example, the DWARF spec states that "In the case of the
  name of a function member or static data member of a C++ structure, class or
  union, the name presented in the .debug_pubnames section is not the simple
  name given by the DW_AT_name attribute of the referenced debugging information
  entry, but rather the fully qualified name of the data or function member."
  So the only names in these tables for complex C++ entries is a fully
  qualified name.  Debugger users tend not to enter their search strings as
  "a::b::c(int,const Foo&) const", but rather as "c", "b::c" , or "a::b::c".  So
  the name entered in the name table must be demangled in order to chop it up
  appropriately and additional names must be manually entered into the table
  to make it effective as a name lookup table for debuggers to use.</p>

<p>All debuggers currently ignore the .debug_pubnames table as a result of
  its inconsistent and useless public-only name content making it a waste of
  space in the object file. These tables, when they are written to disk, are
  not sorted in any way, leaving every debugger to do its own parsing
  and sorting. These tables also include an inlined copy of the string values
  in the table itself making the tables much larger than they need to be on
  disk, especially for large C++ programs.</p>

<p>Can't we just fix the sections by adding all of the names we need to this
  table? No, because that is not what the tables are defined to contain and we
  won't know the difference between the old bad tables and the new good tables.
  At best we could make our own renamed sections that contain all of the data
  we need.</p>

<p>These tables are also insufficient for what a debugger like LLDB needs.
  LLDB uses clang for its expression parsing where LLDB acts as a PCH. LLDB is
  then often asked to look for type "foo" or namespace "bar", or list items in
  namespace "baz". Namespaces are not included in the pubnames or pubtypes
  tables. Since clang asks a lot of questions when it is parsing an expression,
  we need to be very fast when looking up names, as it happens a lot. Having new
  accelerator tables that are optimized for very quick lookups will benefit
  this type of debugging experience greatly.</p>

<p>We would like to generate name lookup tables that can be mapped into
  memory from disk, and used as is, with little or no up-front parsing. We would
  also be able to control the exact content of these different tables so they
  contain exactly what we need. The Name Accelerator Tables were designed
  to fix these issues. In order to solve these issues we need to:</p>

<ul>
  <li>Have a format that can be mapped into memory from disk and used as is</li>
  <li>Lookups should be very fast</li>
  <li>Extensible table format so these tables can be made by many producers</li>
  <li>Contain all of the names needed for typical lookups out of the box</li>
  <li>Strict rules for the contents of tables</li>
</ul>

<p>Table size is important and the accelerator table format should allow the
  reuse of strings from common string tables so the strings for the names are
  not duplicated. We also want to make sure the table is ready to be used as-is
  by simply mapping the table into memory with minimal header parsing.</p>

<p>The name lookups need to be fast and optimized for the kinds of lookups
  that debuggers tend to do. Optimally we would like to touch as few parts of
  the mapped table as possible when doing a name lookup and be able to quickly
  find the name entry we are looking for, or discover there are no matches. In
  the case of debuggers we optimized for lookups that fail most of the time.</p>

<p>Each table that is defined should have strict rules on exactly what is in
  the accelerator tables and documented so clients can rely on the content.</p>

</div>

<!-- ======================================================================= -->
<h4>
  <a name="acceltablehashes">Hash Tables</a>
</h4>
<!-- ======================================================================= -->

<div>
<h5>Standard Hash Tables</h5>

<p>Typical hash tables have a header, buckets, and each bucket points to the
bucket contents:
</p>

<div class="doc_code">
<pre>
.------------.
|  HEADER    |
|------------|
|  BUCKETS   |
|------------|
|  DATA      |
`------------'
</pre>
</div>

<p>The BUCKETS are an array of offsets to DATA for each hash:</p>

<div class="doc_code">
<pre>
.------------.
| 0x00001000 | BUCKETS[0]
| 0x00002000 | BUCKETS[1]
| 0x00002200 | BUCKETS[2]
| 0x000034f0 | BUCKETS[3]
|            | ...
| 0xXXXXXXXX | BUCKETS[n_buckets]
'------------'
</pre>
</div>

<p>So for bucket[3] in the example above, we have an offset into the table
  0x000034f0 which points to a chain of entries for the bucket. Each bucket
  must contain a next pointer, full 32 bit hash value, the string itself,
  and the data for the current string value.</p>

<div class="doc_code">
<pre>
            .------------.
0x000034f0: | 0x00003500 | next pointer
            | 0x12345678 | 32 bit hash
            | "erase"    | string value
            | data[n]    | HashData for this bucket
            |------------|
0x00003500: | 0x00003550 | next pointer
            | 0x29273623 | 32 bit hash
            | "dump"     | string value
            | data[n]    | HashData for this bucket
            |------------|
0x00003550: | 0x00000000 | next pointer
            | 0x82638293 | 32 bit hash
            | "main"     | string value
            | data[n]    | HashData for this bucket
            `------------'
</pre>
</div>

<p>The problem with this layout for debuggers is that we need to optimize for
  the negative lookup case where the symbol we're searching for is not present.
  So if we were to lookup "printf" in the table above, we would make a 32 hash
  for "printf", it might match bucket[3]. We would need to go to the offset
  0x000034f0 and start looking to see if our 32 bit hash matches. To do so, we
  need to read the next pointer, then read the hash, compare it, and skip to
  the next bucket. Each time we are skipping many bytes in memory and touching
  new cache pages just to do the compare on the full 32 bit hash. All of these
  accesses then tell us that we didn't have a match.</p>

<h5>Name Hash Tables</h5>

<p>To solve the issues mentioned above we have structured the hash tables
  a bit differently: a header, buckets, an array of all unique 32 bit hash
  values, followed by an array of hash value data offsets, one for each hash
  value, then the data for all hash values:</p>

<div class="doc_code">
<pre>
.-------------.
|  HEADER     |
|-------------|
|  BUCKETS    |
|-------------|
|  HASHES     |
|-------------|
|  OFFSETS    |
|-------------|
|  DATA       |
`-------------'
</pre>
</div>

<p>The BUCKETS in the name tables are an index into the HASHES array. By
  making all of the full 32 bit hash values contiguous in memory, we allow
  ourselves to efficiently check for a match while touching as little
  memory as possible. Most often checking the 32 bit hash values is as far as
  the lookup goes. If it does match, it usually is a match with no collisions.
  So for a table with "n_buckets" buckets, and "n_hashes" unique 32 bit hash
  values, we can clarify the contents of the BUCKETS, HASHES and OFFSETS as:</p>

<div class="doc_code">
<pre>
.-------------------------.
|  HEADER.magic           | uint32_t
|  HEADER.version         | uint16_t
|  HEADER.hash_function   | uint16_t
|  HEADER.bucket_count    | uint32_t
|  HEADER.hashes_count    | uint32_t
|  HEADER.header_data_len | uint32_t
|  HEADER_DATA            | HeaderData
|-------------------------|
|  BUCKETS                | uint32_t[n_buckets] // 32 bit hash indexes
|-------------------------|
|  HASHES                 | uint32_t[n_buckets] // 32 bit hash values
|-------------------------|
|  OFFSETS                | uint32_t[n_buckets] // 32 bit offsets to hash value data
|-------------------------|
|  ALL HASH DATA          |
`-------------------------'
</pre>
</div>

<p>So taking the exact same data from the standard hash example above we end up
  with:</p>

<div class="doc_code">
<pre>
            .------------.
            | HEADER     |
            |------------|
            |          0 | BUCKETS[0]
            |          2 | BUCKETS[1]
            |          5 | BUCKETS[2]
            |          6 | BUCKETS[3]
            |            | ...
            |        ... | BUCKETS[n_buckets]
            |------------|
            | 0x........ | HASHES[0]
            | 0x........ | HASHES[1]
            | 0x........ | HASHES[2]
            | 0x........ | HASHES[3]
            | 0x........ | HASHES[4]
            | 0x........ | HASHES[5]
            | 0x12345678 | HASHES[6]    hash for BUCKETS[3]
            | 0x29273623 | HASHES[7]    hash for BUCKETS[3]
            | 0x82638293 | HASHES[8]    hash for BUCKETS[3]
            | 0x........ | HASHES[9]
            | 0x........ | HASHES[10]
            | 0x........ | HASHES[11]
            | 0x........ | HASHES[12]
            | 0x........ | HASHES[13]
            | 0x........ | HASHES[n_hashes]
            |------------|
            | 0x........ | OFFSETS[0]
            | 0x........ | OFFSETS[1]
            | 0x........ | OFFSETS[2]
            | 0x........ | OFFSETS[3]
            | 0x........ | OFFSETS[4]
            | 0x........ | OFFSETS[5]
            | 0x000034f0 | OFFSETS[6]   offset for BUCKETS[3]
            | 0x00003500 | OFFSETS[7]   offset for BUCKETS[3]
            | 0x00003550 | OFFSETS[8]   offset for BUCKETS[3]
            | 0x........ | OFFSETS[9]
            | 0x........ | OFFSETS[10]
            | 0x........ | OFFSETS[11]
            | 0x........ | OFFSETS[12]
            | 0x........ | OFFSETS[13]
            | 0x........ | OFFSETS[n_hashes]
            |------------|
            |            |
            |            |
            |            |
            |            |
            |            |
            |------------|
0x000034f0: | 0x00001203 | .debug_str ("erase")
            | 0x00000004 | A 32 bit array count - number of HashData with name "erase"
            | 0x........ | HashData[0]
            | 0x........ | HashData[1]
            | 0x........ | HashData[2]
            | 0x........ | HashData[3]
            | 0x00000000 | String offset into .debug_str (terminate data for hash)
            |------------|
0x00003500: | 0x00001203 | String offset into .debug_str ("collision")
            | 0x00000002 | A 32 bit array count - number of HashData with name "collision"
            | 0x........ | HashData[0]
            | 0x........ | HashData[1]
            | 0x00001203 | String offset into .debug_str ("dump")
            | 0x00000003 | A 32 bit array count - number of HashData with name "dump"
            | 0x........ | HashData[0]
            | 0x........ | HashData[1]
            | 0x........ | HashData[2]
            | 0x00000000 | String offset into .debug_str (terminate data for hash)
            |------------|
0x00003550: | 0x00001203 | String offset into .debug_str ("main")
            | 0x00000009 | A 32 bit array count - number of HashData with name "main"
            | 0x........ | HashData[0]
            | 0x........ | HashData[1]
            | 0x........ | HashData[2]
            | 0x........ | HashData[3]
            | 0x........ | HashData[4]
            | 0x........ | HashData[5]
            | 0x........ | HashData[6]
            | 0x........ | HashData[7]
            | 0x........ | HashData[8]
            | 0x00000000 | String offset into .debug_str (terminate data for hash)
            `------------'
</pre>
</div>

<p>So we still have all of the same data, we just organize it more efficiently
  for debugger lookup. If we repeat the same "printf" lookup from above, we
  would hash "printf" and find it matches BUCKETS[3] by taking the 32 bit hash
  value and modulo it by n_buckets. BUCKETS[3] contains "6" which is the index
  into the HASHES table. We would then compare any consecutive 32 bit hashes
  values in the HASHES array as long as the hashes would be in BUCKETS[3]. We
  do this by verifying that each subsequent hash value modulo n_buckets is still
  3. In the case of a failed lookup we would access the memory for BUCKETS[3], and
  then compare a few consecutive 32 bit hashes before we know that we have no match.
  We don't end up marching through multiple words of memory and we really keep the
  number of processor data cache lines being accessed as small as possible.</p>

<p>The string hash that is used for these lookup tables is the Daniel J.
  Bernstein hash which is also used in the ELF GNU_HASH sections. It is a very
  good hash for all kinds of names in programs with very few hash collisions.</p>

<p>Empty buckets are designated by using an invalid hash index of UINT32_MAX.</p>
</div>

<!-- ======================================================================= -->
<h4>
  <a name="acceltabledetails">Details</a>
</h4>
<!-- ======================================================================= -->
<div>
<p>These name hash tables are designed to be generic where specializations of
  the table get to define additional data that goes into the header
  ("HeaderData"), how the string value is stored ("KeyType") and the content
  of the data for each hash value.</p>

<h5>Header Layout</h5>
<p>The header has a fixed part, and the specialized part. The exact format of
  the header is:</p>
<div class="doc_code">
<pre>
struct Header
{
  uint32_t   magic;           // 'HASH' magic value to allow endian detection
  uint16_t   version;         // Version number
  uint16_t   hash_function;   // The hash function enumeration that was used
  uint32_t   bucket_count;    // The number of buckets in this hash table
  uint32_t   hashes_count;    // The total number of unique hash values and hash data offsets in this table
  uint32_t   header_data_len; // The bytes to skip to get to the hash indexes (buckets) for correct alignment
                              // Specifically the length of the following HeaderData field - this does not
                              // include the size of the preceding fields
  HeaderData header_data;     // Implementation specific header data
};
</pre>
</div>
<p>The header starts with a 32 bit "magic" value which must be 'HASH' encoded as
  an ASCII integer. This allows the detection of the start of the hash table and
  also allows the table's byte order to be determined so the table can be
  correctly extracted. The "magic" value is followed by a 16 bit version number
  which allows the table to be revised and modified in the future. The current
  version number is 1. "hash_function" is a uint16_t enumeration that specifies
  which hash function was used to produce this table. The current values for the
  hash function enumerations include:</p>
<div class="doc_code">
<pre>
enum HashFunctionType
{
  eHashFunctionDJB = 0u, // Daniel J Bernstein hash function
};
</pre>
</div>
<p>"bucket_count" is a 32 bit unsigned integer that represents how many buckets
  are in the BUCKETS array. "hashes_count" is the number of unique 32 bit hash
  values that are in the HASHES array, and is the same number of offsets are
  contained in the OFFSETS array. "header_data_len" specifies the size in
  bytes of the HeaderData that is filled in by specialized versions of this
  table.</p>

<h5>Fixed Lookup</h5>
<p>The header is followed by the buckets, hashes, offsets, and hash value
  data.
<div class="doc_code">
<pre>
struct FixedTable
{
  uint32_t buckets[Header.bucket_count];  // An array of hash indexes into the "hashes[]" array below
  uint32_t hashes [Header.hashes_count];  // Every unique 32 bit hash for the entire table is in this table
  uint32_t offsets[Header.hashes_count];  // An offset that corresponds to each item in the "hashes[]" array above
};
</pre>
</div>
<p>"buckets" is an array of 32 bit indexes into the "hashes" array. The
  "hashes" array contains all of the 32 bit hash values for all names in the
  hash table. Each hash in the "hashes" table has an offset in the "offsets"
  array that points to the data for the hash value.</p>

<p>This table setup makes it very easy to repurpose these tables to contain
  different data, while keeping the lookup mechanism the same for all tables.
  This layout also makes it possible to save the table to disk and map it in
  later and do very efficient name lookups with little or no parsing.</p>

<p>DWARF lookup tables can be implemented in a variety of ways and can store
  a lot of information for each name. We want to make the DWARF tables
  extensible and able to store the data efficiently so we have used some of the
  DWARF features that enable efficient data storage to define exactly what kind
  of data we store for each name.</p>

<p>The "HeaderData" contains a definition of the contents of each HashData
  chunk. We might want to store an offset to all of the debug information
  entries (DIEs) for each name. To keep things extensible, we create a list of
  items, or Atoms, that are contained in the data for each name. First comes the
  type of the data in each atom:</p>
<div class="doc_code">
<pre>
enum AtomType
{
  eAtomTypeNULL       = 0u,
  eAtomTypeDIEOffset  = 1u,   // DIE offset, check form for encoding
  eAtomTypeCUOffset   = 2u,   // DIE offset of the compiler unit header that contains the item in question
  eAtomTypeTag        = 3u,   // DW_TAG_xxx value, should be encoded as DW_FORM_data1 (if no tags exceed 255) or DW_FORM_data2
  eAtomTypeNameFlags  = 4u,   // Flags from enum NameFlags
  eAtomTypeTypeFlags  = 5u,   // Flags from enum TypeFlags
};
</pre>
</div>
<p>The enumeration values and their meanings are:</p>
<div class="doc_code">
<pre>
  eAtomTypeNULL       - a termination atom that specifies the end of the atom list
  eAtomTypeDIEOffset  - an offset into the .debug_info section for the DWARF DIE for this name
  eAtomTypeCUOffset   - an offset into the .debug_info section for the CU that contains the DIE
  eAtomTypeDIETag     - The DW_TAG_XXX enumeration value so you don't have to parse the DWARF to see what it is
  eAtomTypeNameFlags  - Flags for functions and global variables (isFunction, isInlined, isExternal...)
  eAtomTypeTypeFlags  - Flags for types (isCXXClass, isObjCClass, ...)
</pre>
</div>
<p>Then we allow each atom type to define the atom type and how the data for
  each atom type data is encoded:</p>
<div class="doc_code">
<pre>
struct Atom
{
  uint16_t type;  // AtomType enum value
  uint16_t form;  // DWARF DW_FORM_XXX defines
};
</pre>
</div>
<p>The "form" type above is from the DWARF specification and defines the
  exact encoding of the data for the Atom type. See the DWARF specification for
  the DW_FORM_ definitions.</p>
<div class="doc_code">
<pre>
struct HeaderData
{
  uint32_t die_offset_base;
  uint32_t atom_count;
  Atoms    atoms[atom_count0];
};
</pre>
</div>
<p>"HeaderData" defines the base DIE offset that should be added to any atoms
  that are encoded using the DW_FORM_ref1, DW_FORM_ref2, DW_FORM_ref4,
  DW_FORM_ref8 or DW_FORM_ref_udata. It also defines what is contained in
  each "HashData" object -- Atom.form tells us how large each field will be in
  the HashData and the Atom.type tells us how this data should be interpreted.</p>

<p>For the current implementations of the ".apple_names" (all functions + globals),
  the ".apple_types" (names of all types that are defined), and the
  ".apple_namespaces" (all namespaces), we currently set the Atom array to be:</p>
<div class="doc_code">
<pre>
HeaderData.atom_count = 1;
HeaderData.atoms[0].type = eAtomTypeDIEOffset;
HeaderData.atoms[0].form = DW_FORM_data4;
</pre>
</div>
<p>This defines the contents to be the DIE offset (eAtomTypeDIEOffset) that is
  encoded as a 32 bit value (DW_FORM_data4). This allows a single name to have
  multiple matching DIEs in a single file, which could come up with an inlined
  function for instance. Future tables could include more information about the
  DIE such as flags indicating if the DIE is a function, method, block,
  or inlined.</p>

<p>The KeyType for the DWARF table is a 32 bit string table offset into the
  ".debug_str" table. The ".debug_str" is the string table for the DWARF which
  may already contain copies of all of the strings. This helps make sure, with
  help from the compiler, that we reuse the strings between all of the DWARF
  sections and keeps the hash table size down. Another benefit to having the
  compiler generate all strings as DW_FORM_strp in the debug info, is that
  DWARF parsing can be made much faster.</p>

<p>After a lookup is made, we get an offset into the hash data. The hash data
  needs to be able to deal with 32 bit hash collisions, so the chunk of data
  at the offset in the hash data consists of a triple:</p>
<div class="doc_code">
<pre>
uint32_t str_offset
uint32_t hash_data_count
HashData[hash_data_count]
</pre>
</div>
<p>If "str_offset" is zero, then the bucket contents are done. 99.9% of the
  hash data chunks contain a single item (no 32 bit hash collision):</p>
<div class="doc_code">
<pre>
.------------.
| 0x00001023 | uint32_t KeyType (.debug_str[0x0001023] => "main")
| 0x00000004 | uint32_t HashData count
| 0x........ | uint32_t HashData[0] DIE offset
| 0x........ | uint32_t HashData[1] DIE offset
| 0x........ | uint32_t HashData[2] DIE offset
| 0x........ | uint32_t HashData[3] DIE offset
| 0x00000000 | uint32_t KeyType (end of hash chain)
`------------'
</pre>
</div>
<p>If there are collisions, you will have multiple valid string offsets:</p>
<div class="doc_code">
<pre>
.------------.
| 0x00001023 | uint32_t KeyType (.debug_str[0x0001023] => "main")
| 0x00000004 | uint32_t HashData count
| 0x........ | uint32_t HashData[0] DIE offset
| 0x........ | uint32_t HashData[1] DIE offset
| 0x........ | uint32_t HashData[2] DIE offset
| 0x........ | uint32_t HashData[3] DIE offset
| 0x00002023 | uint32_t KeyType (.debug_str[0x0002023] => "print")
| 0x00000002 | uint32_t HashData count
| 0x........ | uint32_t HashData[0] DIE offset
| 0x........ | uint32_t HashData[1] DIE offset
| 0x00000000 | uint32_t KeyType (end of hash chain)
`------------'
</pre>
</div>
<p>Current testing with real world C++ binaries has shown that there is around 1
  32 bit hash collision per 100,000 name entries.</p>
</div>
<!-- ======================================================================= -->
<h4>
  <a name="acceltablecontents">Contents</a>
</h4>
<!-- ======================================================================= -->
<div>
<p>As we said, we want to strictly define exactly what is included in the
  different tables. For DWARF, we have 3 tables: ".apple_names", ".apple_types",
  and ".apple_namespaces".</p>

<p>".apple_names" sections should contain an entry for each DWARF DIE whose
  DW_TAG is a DW_TAG_label, DW_TAG_inlined_subroutine, or DW_TAG_subprogram that
  has address attributes: DW_AT_low_pc, DW_AT_high_pc, DW_AT_ranges or
  DW_AT_entry_pc. It also contains DW_TAG_variable DIEs that have a DW_OP_addr
  in the location (global and static variables). All global and static variables
  should be included, including those scoped withing functions and classes. For
  example using the following code:</p>
<div class="doc_code">
<pre>
static int var = 0;

void f ()
{
  static int var = 0;
}
</pre>
</div>
<p>Both of the static "var" variables would be included in the table. All
  functions should emit both their full names and their basenames. For C or C++,
  the full name is the mangled name (if available) which is usually in the
  DW_AT_MIPS_linkage_name attribute, and the DW_AT_name contains the function
  basename. If global or static variables have a mangled name in a
  DW_AT_MIPS_linkage_name attribute, this should be emitted along with the
  simple name found in the DW_AT_name attribute.</p>

<p>".apple_types" sections should contain an entry for each DWARF DIE whose
  tag is one of:</p>
<ul>
  <li>DW_TAG_array_type</li>
  <li>DW_TAG_class_type</li>
  <li>DW_TAG_enumeration_type</li>
  <li>DW_TAG_pointer_type</li>
  <li>DW_TAG_reference_type</li>
  <li>DW_TAG_string_type</li>
  <li>DW_TAG_structure_type</li>
  <li>DW_TAG_subroutine_type</li>
  <li>DW_TAG_typedef</li>
  <li>DW_TAG_union_type</li>
  <li>DW_TAG_ptr_to_member_type</li>
  <li>DW_TAG_set_type</li>
  <li>DW_TAG_subrange_type</li>
  <li>DW_TAG_base_type</li>
  <li>DW_TAG_const_type</li>
  <li>DW_TAG_constant</li>
  <li>DW_TAG_file_type</li>
  <li>DW_TAG_namelist</li>
  <li>DW_TAG_packed_type</li>
  <li>DW_TAG_volatile_type</li>
  <li>DW_TAG_restrict_type</li>
  <li>DW_TAG_interface_type</li>
  <li>DW_TAG_unspecified_type</li>
  <li>DW_TAG_shared_type</li>
</ul>
<p>Only entries with a DW_AT_name attribute are included, and the entry must
  not be a forward declaration (DW_AT_declaration attribute with a non-zero value).
  For example, using the following code:</p>
<div class="doc_code">
<pre>
int main ()
{
  int *b = 0;
  return *b;
}
</pre>
</div>
<p>We get a few type DIEs:</p>
<div class="doc_code">
<pre>
0x00000067:     TAG_base_type [5]
                AT_encoding( DW_ATE_signed )
                AT_name( "int" )
                AT_byte_size( 0x04 )

0x0000006e:     TAG_pointer_type [6]
                AT_type( {0x00000067} ( int ) )
                AT_byte_size( 0x08 )
</pre>
</div>
<p>The DW_TAG_pointer_type is not included because it does not have a DW_AT_name.</p>

<p>".apple_namespaces" section should contain all DW_TAG_namespace DIEs. If
  we run into a namespace that has no name this is an anonymous namespace,
  and the name should be output as "(anonymous namespace)" (without the quotes).
  Why? This matches the output of the abi::cxa_demangle() that is in the standard
  C++ library that demangles mangled names.</p>
</div>

<!-- ======================================================================= -->
<h4>
  <a name="acceltableextensions">Language Extensions and File Format Changes</a>
</h4>
<!-- ======================================================================= -->
<div>
<h5>Objective-C Extensions</h5>
<p>".apple_objc" section should contain all DW_TAG_subprogram DIEs for an
  Objective-C class. The name used in the hash table is the name of the
  Objective-C class itself. If the Objective-C class has a category, then an
  entry is made for both the class name without the category, and for the class
  name with the category. So if we have a DIE at offset 0x1234 with a name
  of method "-[NSString(my_additions) stringWithSpecialString:]", we would add
  an entry for "NSString" that points to DIE 0x1234, and an entry for
  "NSString(my_additions)" that points to 0x1234. This allows us to quickly
  track down all Objective-C methods for an Objective-C class when doing
  expressions. It is needed because of the dynamic nature of Objective-C where
  anyone can add methods to a class. The DWARF for Objective-C methods is also
  emitted differently from C++ classes where the methods are not usually
  contained in the class definition, they are scattered about across one or more
  compile units. Categories can also be defined in different shared libraries.
  So we need to be able to quickly find all of the methods and class functions
  given the Objective-C class name, or quickly find all methods and class
  functions for a class + category name. This table does not contain any selector
  names, it just maps Objective-C class names (or class names + category) to all
  of the methods and class functions. The selectors are added as function
  basenames in the .debug_names section.</p>

<p>In the ".apple_names" section for Objective-C functions, the full name is the
  entire function name with the brackets ("-[NSString stringWithCString:]") and the
  basename is the selector only ("stringWithCString:").</p>

<h5>Mach-O Changes</h5>
<p>The sections names for the apple hash tables are for non mach-o files. For
  mach-o files, the sections should be contained in the "__DWARF" segment with
  names as follows:</p>
<ul>
  <li>".apple_names" -> "__apple_names"</li>
  <li>".apple_types" -> "__apple_types"</li>
  <li>".apple_namespaces" -> "__apple_namespac" (16 character limit)</li>
  <li> ".apple_objc" -> "__apple_objc"</li>
</ul>
</div>
</div>
</div>

<!-- *********************************************************************** -->

<hr>
<address>
  <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
  src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a>
  <a href="http://validator.w3.org/check/referer"><img
  src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>

  <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
  <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
  Last modified: $Date$
</address>

</body>
</html>