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
|
<!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>Accurate Garbage Collection with LLVM</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
<style type="text/css">
.rowhead { text-align: left; background: inherit; }
.indent { padding-left: 1em; }
.optl { color: #BFBFBF; }
</style>
</head>
<body>
<div class="doc_title">
Accurate Garbage Collection with LLVM
</div>
<ol>
<li><a href="#introduction">Introduction</a>
<ul>
<li><a href="#feature">GC features provided and algorithms
supported</a></li>
</ul>
</li>
<li><a href="#usage">Using the collectors</a>
<ul>
<li><a href="#shadow-stack">ShadowStack -
A highly portable collector</a></li>
<li><a href="#semispace">SemiSpace -
A simple copying collector runtime</a></li>
<li><a href="#ocaml">Ocaml -
An Objective Caml-compatible collector</a></li>
</ul>
</li>
<li><a href="#core">Core support</a>
<ul>
<li><a href="#gcattr">Specifying GC code generation:
<tt>gc "..."</tt></a></li>
<li><a href="#gcroot">Identifying GC roots on the stack:
<tt>llvm.gcroot</tt></a></li>
<li><a href="#barriers">Reading and writing references in the heap</a>
<ul>
<li><a href="#gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a></li>
<li><a href="#gcread">Read barrier: <tt>llvm.gcread</tt></a></li>
</ul>
</li>
</ul>
</li>
<li><a href="#runtime">Recommended runtime interface</a>
<ul>
<li><a href="#initialize">Garbage collector startup and
initialization</a></li>
<li><a href="#allocate">Allocating memory from the GC</a></li>
<li><a href="#explicit">Explicit invocation of the garbage
collector</a></li>
<li><a href="#traceroots">Tracing GC pointers from the program
stack</a></li>
<li><a href="#staticroots">Tracing GC pointers from static roots</a></li>
</ul>
</li>
<li><a href="#plugin">Implementing a collector plugin</a>
<ul>
<li><a href="#collector-algos">Overview of available features</a></li>
<li><a href="#stack-map">Computing stack maps</a></li>
<li><a href="#init-roots">Initializing roots to null:
<tt>InitRoots</tt></a></li>
<li><a href="#custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>,
<tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a></li>
<li><a href="#safe-points">Generating safe points:
<tt>NeededSafePoints</tt></a></li>
<li><a href="#assembly">Emitting assembly code:
<tt>GCMetadataPrinter</tt></a></li>
</ul>
</li>
<li><a href="#runtime-impl">Implementing a collector runtime</a>
<ul>
<li><a href="#gcdescriptors">Tracing GC pointers from heap
objects</a></li>
</ul>
</li>
<li><a href="#references">References</a></li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
Gordon Henriksen</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="introduction">Introduction</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Garbage collection is a widely used technique that frees the programmer from
having to know the lifetimes of heap objects, making software easier to produce
and maintain. Many programming languages rely on garbage collection for
automatic memory management. There are two primary forms of garbage collection:
conservative and accurate.</p>
<p>Conservative garbage collection often does not require any special support
from either the language or the compiler: it can handle non-type-safe
programming languages (such as C/C++) and does not require any special
information from the compiler. The
<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/">Boehm collector</a> is
an example of a state-of-the-art conservative collector.</p>
<p>Accurate garbage collection requires the ability to identify all pointers in
the program at run-time (which requires that the source-language be type-safe in
most cases). Identifying pointers at run-time requires compiler support to
locate all places that hold live pointer variables at run-time, including the
<a href="#gcroot">processor stack and registers</a>.</p>
<p>Conservative garbage collection is attractive because it does not require any
special compiler support, but it does have problems. In particular, because the
conservative garbage collector cannot <i>know</i> that a particular word in the
machine is a pointer, it cannot move live objects in the heap (preventing the
use of compacting and generational GC algorithms) and it can occasionally suffer
from memory leaks due to integer values that happen to point to objects in the
program. In addition, some aggressive compiler transformations can break
conservative garbage collectors (though these seem rare in practice).</p>
<p>Accurate garbage collectors do not suffer from any of these problems, but
they can suffer from degraded scalar optimization of the program. In particular,
because the runtime must be able to identify and update all pointers active in
the program, some optimizations are less effective. In practice, however, the
locality and performance benefits of using aggressive garbage allocation
techniques dominates any low-level losses.</p>
<p>This document describes the mechanisms and interfaces provided by LLVM to
support accurate garbage collection.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="feature">GC features provided and algorithms supported</a>
</div>
<div class="doc_text">
<p>LLVM's intermediate representation provides <a href="#intrinsics">garbage
collection intrinsics</a> that offer support for a broad class of
collector models. For instance, the intrinsics permit:</p>
<ul>
<li>semi-space collectors</li>
<li>mark-sweep collectors</li>
<li>generational collectors</li>
<li>reference counting</li>
<li>incremental collectors</li>
<li>concurrent collectors</li>
<li>cooperative collectors</li>
</ul>
<p>We hope that the primitive support built into the LLVM IR is sufficient to
support a broad class of garbage collected languages including Scheme, ML, Java,
C#, Perl, Python, Lua, Ruby, other scripting languages, and more.</p>
<p>However, LLVM does not itself implement a garbage collector. This is because
collectors are tightly coupled to object models, and LLVM is agnostic to object
models. Since LLVM is agnostic to object models, it would be inappropriate for
LLVM to dictate any particular collector. Instead, LLVM provides a framework for
garbage collector implementations in two manners:</p>
<ul>
<li><b>At compile time</b> with <a href="#plugin">collector plugins</a> for
the compiler. Collector plugins have ready access to important garbage
collector algorithms. Leveraging these tools, it is straightforward to
emit type-accurate stack maps for your runtime in as little as ~100 lines of
C++ code.</li>
<li><b>At runtime</b> with <a href="#runtime">suggested runtime
interfaces</a>, which allow front-end compilers to support a range of
collection runtimes.</li>
</ul>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="usage">Using the collectors</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>In general, using a collector implies:</p>
<ul>
<li>Emitting compatible code, including initialization in the main
program if necessary.</li>
<li>Loading a compiler plugin if the collector is not statically linked with
your compiler. For <tt>llc</tt>, use the <tt>-load</tt> option.</li>
<li>Selecting the collection algorithm by applying the <tt>gc "..."</tt>
attribute to your garbage collected functions, or equivalently with
the <tt>setGC</tt> method.</li>
<li>Linking your final executable with the garbage collector runtime.</li>
</ul>
<p>This table summarizes the available runtimes.</p>
<table>
<tr>
<th>Collector</th>
<th><tt>gc</tt> attribute</th>
<th>Linkage</th>
<th><tt>gcroot</tt></th>
<th><tt>gcread</tt></th>
<th><tt>gcwrite</tt></th>
</tr>
<tr valign="baseline">
<td><a href="#semispace">SemiSpace</a></td>
<td><tt>gc "shadow-stack"</tt></td>
<td>TODO FIXME</td>
<td>required</td>
<td>optional</td>
<td>optional</td>
</tr>
<tr valign="baseline">
<td><a href="#ocaml">Ocaml</a></td>
<td><tt>gc "ocaml"</tt></td>
<td><i>provided by ocamlopt</i></td>
<td>required</td>
<td>optional</td>
<td>optional</td>
</tr>
</table>
<p>The sections for <a href="#intrinsics">Collection intrinsics</a> and
<a href="#runtime">Recommended runtime interface</a> detail the interfaces that
collectors may require user programs to utilize.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="shadow-stack">ShadowStack - A highly portable collector</a>
</div>
<div class="doc_code"><tt>
Collector *llvm::createShadowStackCollector();
</tt></div>
<div class="doc_text">
<p>The ShadowStack backend is invoked with the <tt>gc "shadow-stack"</tt>
function attribute.
Unlike many collectors which rely on a cooperative code generator to generate
stack maps, this algorithm carefully maintains a linked list of stack root
descriptors [<a href="#henderson02">Henderson2002</a>]. This so-called "shadow
stack" mirrors the machine stack. Maintaining this data structure is slower
than using stack maps, but has a significant portability advantage because it
requires no special support from the target code generator.</p>
<p>The ShadowStack collector does not use read or write barriers, so the user
program may use <tt>load</tt> and <tt>store</tt> instead of <tt>llvm.gcread</tt>
and <tt>llvm.gcwrite</tt>.</p>
<p>ShadowStack is a code generator plugin only. It must be paired with a
compatible runtime.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="semispace">SemiSpace - A simple copying collector runtime</a>
</div>
<div class="doc_text">
<p>The SemiSpace runtime implements the <a href="runtime">suggested
runtime interface</a> and is compatible with the ShadowStack backend.</p>
<p>SemiSpace is a very simple copying collector. When it starts up, it
allocates two blocks of memory for the heap. It uses a simple bump-pointer
allocator to allocate memory from the first block until it runs out of space.
When it runs out of space, it traces through all of the roots of the program,
copying blocks to the other half of the memory space.</p>
<p>This runtime is highly experimental and has not been used in a real project.
Enhancements would be welcomed.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="ocaml">Ocaml - An Objective Caml-compatible collector</a>
</div>
<div class="doc_code"><tt>
Collector *llvm::createOcamlCollector();
</tt></div>
<div class="doc_text">
<p>The ocaml backend is invoked with the <tt>gc "ocaml"</tt> function attribute.
It supports the
<a href="http://caml.inria.fr/">Objective Caml</a> language runtime by emitting
a type-accurate stack map in the form of an ocaml 3.10.0-compatible frametable.
The linkage requirements are satisfied automatically by the <tt>ocamlopt</tt>
compiler when linking an executable.</p>
<p>The ocaml collector does not use read or write barriers, so the user program
may use <tt>load</tt> and <tt>store</tt> instead of <tt>llvm.gcread</tt> and
<tt>llvm.gcwrite</tt>.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="core">Core support</a><a name="intrinsics"></a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>This section describes the garbage collection facilities provided by the
<a href="LangRef.html">LLVM intermediate representation</a>.</p>
<p>These facilities are limited to those strictly necessary for compilation.
They are not intended to be a complete interface to any garbage collector.
Notably, heap allocation is not among the supplied primitives. A user program
will also need to interface with the runtime, using either the
<a href="#runtime">suggested runtime interface</a> or another interface
specified by the runtime.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="gcattr">Specifying GC code generation: <tt>gc "..."</tt></a>
</div>
<div class="doc_code"><tt>
define <i>ty</i> @<i>name</i>(...) <u>gc "<i>collector</i>"</u> { ...
</tt></div>
<div class="doc_text">
<p>The <tt>gc</tt> function attribute is used to specify the desired collector
algorithm to the compiler. It is equivalent to specifying the collector name
programmatically using the <tt>setGC</tt> method of <tt>Function</tt>.</p>
<p>Specifying the collector on a per-function basis allows LLVM to link together
programs that use different garbage collection algorithms.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="gcroot">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
</div>
<div class="doc_code"><tt>
void @llvm.gcroot(i8** %ptrloc, i8* %metadata)
</tt></div>
<div class="doc_text">
<p>The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM of a pointer
variable on the stack. The first argument <b>must</b> be a value referring to an alloca instruction
or a bitcast of an alloca. The second contains a pointer to metadata that
should be associated with the pointer, and <b>must</b> be a constant or global
value address. If your target collector uses tags, use a null pointer for
metadata.</p>
<p>Consider the following fragment of Java code:</p>
<pre>
{
Object X; // A null-initialized reference to an object
...
}
</pre>
<p>This block (which may be located in the middle of a function or in a loop
nest), could be compiled to this LLVM code:</p>
<pre>
Entry:
;; In the entry block for the function, allocate the
;; stack space for X, which is an LLVM pointer.
%X = alloca %Object*
;; Tell LLVM that the stack space is a stack root.
;; Java has type-tags on objects, so we pass null as metadata.
%tmp = bitcast %Object** %X to i8**
call void @llvm.gcroot(i8** %X, i8* null)
...
;; "CodeBlock" is the block corresponding to the start
;; of the scope above.
CodeBlock:
;; Java null-initializes pointers.
store %Object* null, %Object** %X
...
;; As the pointer goes out of scope, store a null value into
;; it, to indicate that the value is no longer live.
store %Object* null, %Object** %X
...
</pre>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="barriers">Reading and writing references in the heap</a>
</div>
<div class="doc_text">
<p>Some collectors need to be informed when the mutator (the program that needs
garbage collection) either reads a pointer from or writes a pointer to a field
of a heap object. The code fragments inserted at these points are called
<em>read barriers</em> and <em>write barriers</em>, respectively. The amount of
code that needs to be executed is usually quite small and not on the critical
path of any computation, so the overall performance impact of the barrier is
tolerable.</p>
<p>Barriers often require access to the <em>object pointer</em> rather than the
<em>derived pointer</em> (which is a pointer to the field within the
object). Accordingly, these intrinsics take both pointers as separate arguments
for completeness. In this snippet, <tt>%object</tt> is the object pointer, and
<tt>%derived</tt> is the derived pointer:</p>
<blockquote><pre>
;; An array type.
%class.Array = type { %class.Object, i32, [0 x %class.Object*] }
...
;; Load the object pointer from a gcroot.
%object = load %class.Array** %object_addr
;; Compute the derived pointer.
%derived = getelementptr %object, i32 0, i32 2, i32 %n</pre></blockquote>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a>
</div>
<div class="doc_code"><tt>
void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived)
</tt></div>
<div class="doc_text">
<p>For write barriers, LLVM provides the <tt>llvm.gcwrite</tt> intrinsic
function. It has exactly the same semantics as a non-volatile <tt>store</tt> to
the derived pointer (the third argument).</p>
<p>Many important algorithms require write barriers, including generational
and concurrent collectors. Additionally, write barriers could be used to
implement reference counting.</p>
<p>The use of this intrinsic is optional if the target collector does use
write barriers. If so, the collector will replace it with the corresponding
<tt>store</tt>.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsubsection">
<a name="gcread">Read barrier: <tt>llvm.gcread</tt></a>
</div>
<div class="doc_code"><tt>
i8* @llvm.gcread(i8* %object, i8** %derived)<br>
</tt></div>
<div class="doc_text">
<p>For read barriers, LLVM provides the <tt>llvm.gcread</tt> intrinsic function.
It has exactly the same semantics as a non-volatile <tt>load</tt> from the
derived pointer (the second argument).</p>
<p>Read barriers are needed by fewer algorithms than write barriers, and may
have a greater performance impact since pointer reads are more frequent than
writes.</p>
<p>As with <tt>llvm.gcwrite</tt>, a target collector might not require the use
of this intrinsic.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="runtime">Recommended runtime interface</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>LLVM specifies the following recommended runtime interface to the garbage
collection at runtime. A program should use these interfaces to accomplish the
tasks not supported by the intrinsics.</p>
<p>Unlike the intrinsics, which are integral to LLVM's code generator, there is
nothing unique about these interfaces; a front-end compiler and runtime are free
to agree to a different specification.</p>
<p class="doc_warning">Note: This interface is a work in progress.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="initialize">Garbage collector startup and initialization</a>
</div>
<div class="doc_text">
<div class="doc_code"><tt>
void llvm_gc_initialize(unsigned InitialHeapSize);
</tt></div>
<p>
The <tt>llvm_gc_initialize</tt> function should be called once before any other
garbage collection functions are called. This gives the garbage collector the
chance to initialize itself and allocate the heap. The initial heap size to
allocate should be specified as an argument.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="allocate">Allocating memory from the GC</a>
</div>
<div class="doc_text">
<div class="doc_code"><tt>
void *llvm_gc_allocate(unsigned Size);
</tt></div>
<p>The <tt>llvm_gc_allocate</tt> function is a global function defined by the
garbage collector implementation to allocate memory. It returns a
zeroed-out block of memory of the specified size, sufficiently aligned to store
any object.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="explicit">Explicit invocation of the garbage collector</a>
</div>
<div class="doc_text">
<div class="doc_code"><tt>
void llvm_gc_collect();
</tt></div>
<p>
The <tt>llvm_gc_collect</tt> function is exported by the garbage collector
implementations to provide a full collection, even when the heap is not
exhausted. This can be used by end-user code as a hint, and may be ignored by
the garbage collector.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="traceroots">Tracing GC pointers from the program stack</a>
</div>
<div class="doc_text">
<div class="doc_code"><tt>
void llvm_cg_walk_gcroots(void (*FP)(void **Root, void *Meta));
</tt></div>
<p>
The <tt>llvm_cg_walk_gcroots</tt> function is a function provided by the code
generator that iterates through all of the GC roots on the stack, calling the
specified function pointer with each record. For each GC root, the address of
the pointer and the meta-data (from the <a
href="#gcroot"><tt>llvm.gcroot</tt></a> intrinsic) are provided.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="staticroots">Tracing GC pointers from static roots</a>
</div>
<div class="doc_text">
TODO
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="plugin">Implementing a collector plugin</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>User code specifies which GC code generation to use with the <tt>gc</tt>
function attribute or, equivalently, with the <tt>setGC</tt> method of
<tt>Function</tt>.</p>
<p>To implement a GC plugin, it is necessary to subclass
<tt>llvm::GCStrategy</tt>, which can be accomplished in a few lines of
boilerplate code. LLVM's infrastructure provides access to several important
algorithms. For an uncontroversial collector, all that remains may be to emit
the assembly code for the collector's unique stack map data structure, which
might be accomplished in as few as 100 LOC.</p>
<p>This is not the appropriate place to implement a garbage collected heap or a
garbage collector itself. That code should exist in the language's runtime
library. The compiler plugin is responsible for generating code which is
compatible with that runtime library.</p>
<p>To subclass <tt>llvm::GCStrategy</tt> and register it with the compiler:</p>
<blockquote><pre>// lib/MyGC/MyGC.cpp - Example LLVM GC plugin
#include "llvm/CodeGen/GCStrategy.h"
#include "llvm/CodeGen/GCMetadata.h"
#include "llvm/Support/Compiler.h"
using namespace llvm;
namespace {
class VISIBILITY_HIDDEN MyGC : public GCStrategy {
public:
MyGC() {}
};
GCRegistry::Add<MyGC>
X("mygc", "My bespoke garbage collector.");
}</pre></blockquote>
<p>Using the LLVM makefiles (like the <a
href="http://llvm.org/viewvc/llvm-project/llvm/trunk/projects/sample/">sample
project</a>), this can be built into a plugin using a simple makefile:</p>
<blockquote><pre
># lib/MyGC/Makefile
LEVEL := ../..
LIBRARYNAME = <var>MyGC</var>
LOADABLE_MODULE = 1
include $(LEVEL)/Makefile.common</pre></blockquote>
<p>Once the plugin is compiled, code using it may be compiled using <tt>llc
-load=<var>MyGC.so</var></tt> (though <var>MyGC.so</var> may have some other
platform-specific extension):</p>
<blockquote><pre
>$ cat sample.ll
define void @f() gc "mygc" {
entry:
ret void
}
$ llvm-as < sample.ll | llc -load=MyGC.so</pre></blockquote>
<p>It is also possible to statically link the collector plugin into tools, such
as a language-specific compiler front-end.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="collector-algos">Overview of available features</a>
</div>
<div class="doc_text">
<p>The boilerplate collector above does nothing. More specifically:</p>
<ul>
<li><tt>llvm.gcread</tt> calls are replaced with the corresponding
<tt>load</tt> instruction.</li>
<li><tt>llvm.gcwrite</tt> calls are replaced with the corresponding
<tt>store</tt> instruction.</li>
<li>No stack map is emitted, and no safe points are added.</li>
</ul>
<p><tt>Collector</tt> provides a range of features through which a plugin
collector may do useful work. This matrix summarizes the supported (and planned)
features and correlates them with the collection techniques which typically
require them.</p>
<table>
<tr>
<th>Algorithm</th>
<th>Done</th>
<th>shadow stack</th>
<th>refcount</th>
<th>mark-sweep</th>
<th>copying</th>
<th>incremental</th>
<th>threaded</th>
<th>concurrent</th>
</tr>
<tr>
<th class="rowhead"><a href="#stack-map">stack map</a></th>
<td>✔</td>
<td></td>
<td></td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
</tr>
<tr>
<th class="rowhead"><a href="#init-roots">initialize roots</a></th>
<td>✔</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
</tr>
<tr class="doc_warning">
<th class="rowhead">derived pointers</th>
<td>NO</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>✘*</td>
<td>✘*</td>
</tr>
<tr>
<th class="rowhead"><em><a href="#custom">custom lowering</a></em></th>
<td>✔</td>
<th></th>
<th></th>
<th></th>
<th></th>
<th></th>
<th></th>
<th></th>
</tr>
<tr>
<th class="rowhead indent">gcroot</th>
<td>✔</td>
<td>✘</td>
<td>✘</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
</tr>
<tr>
<th class="rowhead indent">gcwrite</th>
<td>✔</td>
<td></td>
<td>✘</td>
<td></td>
<td></td>
<td>✘</td>
<td></td>
<td>✘</td>
</tr>
<tr>
<th class="rowhead indent">gcread</th>
<td>✔</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>✘</td>
</tr>
<tr>
<th class="rowhead"><em><a href="#safe-points">safe points</a></em></th>
<td></td>
<th></th>
<th></th>
<th></th>
<th></th>
<th></th>
<th></th>
<th></th>
</tr>
<tr>
<th class="rowhead indent">in calls</th>
<td>✔</td>
<td></td>
<td></td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
</tr>
<tr>
<th class="rowhead indent">before calls</th>
<td>✔</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>✘</td>
<td>✘</td>
</tr>
<tr class="doc_warning">
<th class="rowhead indent">for loops</th>
<td>NO</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>✘</td>
<td>✘</td>
</tr>
<tr>
<th class="rowhead indent">before escape</th>
<td>✔</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>✘</td>
<td>✘</td>
</tr>
<tr class="doc_warning">
<th class="rowhead">emit code at safe points</th>
<td>NO</td>
<td></td>
<td></td>
<td></td>
<td></td>
<td></td>
<td>✘</td>
<td>✘</td>
</tr>
<tr>
<th class="rowhead"><em>output</em></th>
<td></td>
<th></th>
<th></th>
<th></th>
<th></th>
<th></th>
<th></th>
<th></th>
</tr>
<tr>
<th class="rowhead indent"><a href="#assembly">assembly</a></th>
<td>✔</td>
<td></td>
<td></td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
<td>✘</td>
</tr>
<tr class="doc_warning">
<th class="rowhead indent">JIT</th>
<td>NO</td>
<td></td>
<td></td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
</tr>
<tr class="doc_warning">
<th class="rowhead indent">obj</th>
<td>NO</td>
<td></td>
<td></td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
</tr>
<tr class="doc_warning">
<th class="rowhead">live analysis</th>
<td>NO</td>
<td></td>
<td></td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
</tr>
<tr class="doc_warning">
<th class="rowhead">register map</th>
<td>NO</td>
<td></td>
<td></td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
<td class="optl">✘</td>
</tr>
<tr>
<td colspan="10">
<div><span class="doc_warning">*</span> Derived pointers only pose a
hazard to copying collectors.</div>
<div><span class="optl">✘</span> in gray denotes a feature which
could be utilized if available.</div>
</td>
</tr>
</table>
<p>To be clear, the collection techniques above are defined as:</p>
<dl>
<dt>Shadow Stack</dt>
<dd>The mutator carefully maintains a linked list of stack root
descriptors.</dd>
<dt>Reference Counting</dt>
<dd>The mutator maintains a reference count for each object and frees an
object when its count falls to zero.</dd>
<dt>Mark-Sweep</dt>
<dd>When the heap is exhausted, the collector marks reachable objects starting
from the roots, then deallocates unreachable objects in a sweep
phase.</dd>
<dt>Copying</dt>
<dd>As reachability analysis proceeds, the collector copies objects from one
heap area to another, compacting them in the process. Copying collectors
enable highly efficient "bump pointer" allocation and can improve locality
of reference.</dd>
<dt>Incremental</dt>
<dd>(Including generational collectors.) Incremental collectors generally have
all the properties of a copying collector (regardless of whether the
mature heap is compacting), but bring the added complexity of requiring
write barriers.</dd>
<dt>Threaded</dt>
<dd>Denotes a multithreaded mutator; the collector must still stop the mutator
("stop the world") before beginning reachability analysis. Stopping a
multithreaded mutator is a complicated problem. It generally requires
highly platform specific code in the runtime, and the production of
carefully designed machine code at safe points.</dd>
<dt>Concurrent</dt>
<dd>In this technique, the mutator and the collector run concurrently, with
the goal of eliminating pause times. In a <em>cooperative</em> collector,
the mutator further aids with collection should a pause occur, allowing
collection to take advantage of multiprocessor hosts. The "stop the world"
problem of threaded collectors is generally still present to a limited
extent. Sophisticated marking algorithms are necessary. Read barriers may
be necessary.</dd>
</dl>
<p>As the matrix indicates, LLVM's garbage collection infrastructure is already
suitable for a wide variety of collectors, but does not currently extend to
multithreaded programs. This will be added in the future as there is
interest.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="stack-map">Computing stack maps</a>
</div>
<div class="doc_text">
<blockquote><pre
>for (iterator I = begin(), E = end(); I != E; ++I) {
GCFunctionInfo *FI = *I;
unsigned FrameSize = FI->getFrameSize();
size_t RootCount = FI->roots_size();
for (GCFunctionInfo::roots_iterator RI = FI->roots_begin(),
RE = FI->roots_end();
RI != RE; ++RI) {
int RootNum = RI->Num;
int RootStackOffset = RI->StackOffset;
Constant *RootMetadata = RI->Metadata;
}
}</pre></blockquote>
<p>LLVM automatically computes a stack map. All a <tt>GCStrategy</tt> needs to do
is access it using <tt>GCFunctionMetadata::roots_begin()</tt> and
-<tt>end()</tt>. If the <tt>llvm.gcroot</tt> intrinsic is eliminated before code
generation by a custom lowering pass, LLVM's stack map will be empty.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="init-roots">Initializing roots to null: <tt>InitRoots</tt></a>
</div>
<div class="doc_text">
<blockquote><pre
>MyGC::MyGC() {
InitRoots = true;
}</pre></blockquote>
<p>When set, LLVM will automatically initialize each root to <tt>null</tt> upon
entry to the function. This prevents the GC's sweep phase from visiting
uninitialized pointers, which will almost certainly cause it to crash. This
initialization occurs before custom lowering, so the two may be used
together.</p>
<p>Since LLVM does not yet compute liveness information, there is no means of
distinguishing an uninitialized stack root from an initialized one. Therefore,
this feature should be used by all GC plugins. It is enabled by default.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>,
<tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a>
</div>
<div class="doc_text">
<p>For GCs which use barriers or unusual treatment of stack roots, these
flags allow the collector to perform arbitrary transformations of the LLVM
IR:</p>
<blockquote><pre
>class MyGC : public GCStrategy {
public:
MyGC() {
CustomRoots = true;
CustomReadBarriers = true;
CustomWriteBarriers = true;
}
virtual bool initializeCustomLowering(Module &M);
virtual bool performCustomLowering(Function &F);
};</pre></blockquote>
<p>If any of these flags are set, then LLVM suppresses its default lowering for
the corresponding intrinsics and instead calls
<tt>performCustomLowering</tt>.</p>
<p>LLVM's default action for each intrinsic is as follows:</p>
<ul>
<li><tt>llvm.gcroot</tt>: Pass through to the code generator to generate a
stack map.</li>
<li><tt>llvm.gcread</tt>: Substitute a <tt>load</tt> instruction.</li>
<li><tt>llvm.gcwrite</tt>: Substitute a <tt>store</tt> instruction.</li>
</ul>
<p>If <tt>CustomReadBarriers</tt> or <tt>CustomWriteBarriers</tt> are specified,
then <tt>performCustomLowering</tt> <strong>must</strong> eliminate the
corresponding barriers.</p>
<p><tt>performCustomLowering</tt> must comply with the same restrictions as <a
href="WritingAnLLVMPass.html#runOnFunction"><tt
>FunctionPass::runOnFunction</tt></a>.
Likewise, <tt>initializeCustomLowering</tt> has the same semantics as <a
href="WritingAnLLVMPass.html#doInitialization_mod"><tt
>Pass::doInitialization(Module&)</tt></a>.</p>
<p>The following can be used as a template:</p>
<blockquote><pre
>#include "llvm/Module.h"
#include "llvm/IntrinsicInst.h"
bool MyGC::initializeCustomLowering(Module &M) {
return false;
}
bool MyGC::performCustomLowering(Function &F) {
bool MadeChange = false;
for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; )
if (IntrinsicInst *CI = dyn_cast<IntrinsicInst>(II++))
if (Function *F = CI->getCalledFunction())
switch (F->getIntrinsicID()) {
case Intrinsic::gcwrite:
// Handle llvm.gcwrite.
CI->eraseFromParent();
MadeChange = true;
break;
case Intrinsic::gcread:
// Handle llvm.gcread.
CI->eraseFromParent();
MadeChange = true;
break;
case Intrinsic::gcroot:
// Handle llvm.gcroot.
CI->eraseFromParent();
MadeChange = true;
break;
}
return MadeChange;
}</pre></blockquote>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="safe-points">Generating safe points: <tt>NeededSafePoints</tt></a>
</div>
<div class="doc_text">
<p>LLVM can compute four kinds of safe points:</p>
<blockquote><pre
>namespace GC {
/// PointKind - The type of a collector-safe point.
///
enum PointKind {
Loop, //< Instr is a loop (backwards branch).
Return, //< Instr is a return instruction.
PreCall, //< Instr is a call instruction.
PostCall //< Instr is the return address of a call.
};
}</pre></blockquote>
<p>A collector can request any combination of the four by setting the
<tt>NeededSafePoints</tt> mask:</p>
<blockquote><pre
>MyGC::MyGC() {
NeededSafePoints = 1 << GC::Loop
| 1 << GC::Return
| 1 << GC::PreCall
| 1 << GC::PostCall;
}</pre></blockquote>
<p>It can then use the following routines to access safe points.</p>
<blockquote><pre
>for (iterator I = begin(), E = end(); I != E; ++I) {
GCFunctionInfo *MD = *I;
size_t PointCount = MD->size();
for (GCFunctionInfo::iterator PI = MD->begin(),
PE = MD->end(); PI != PE; ++PI) {
GC::PointKind PointKind = PI->Kind;
unsigned PointNum = PI->Num;
}
}
</pre></blockquote>
<p>Almost every collector requires <tt>PostCall</tt> safe points, since these
correspond to the moments when the function is suspended during a call to a
subroutine.</p>
<p>Threaded programs generally require <tt>Loop</tt> safe points to guarantee
that the application will reach a safe point within a bounded amount of time,
even if it is executing a long-running loop which contains no function
calls.</p>
<p>Threaded collectors may also require <tt>Return</tt> and <tt>PreCall</tt>
safe points to implement "stop the world" techniques using self-modifying code,
where it is important that the program not exit the function without reaching a
safe point (because only the topmost function has been patched).</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="assembly">Emitting assembly code: <tt>GCMetadataPrinter</tt></a>
</div>
<div class="doc_text">
<p>LLVM allows a GC to print arbitrary assembly code before and after the rest
of a module's assembly code. At the end of the module, the GC can print stack
maps built by the code generator. (At the beginning, this information is not
yet computed.)</p>
<p>Since AsmWriter and CodeGen are separate components of LLVM, a separate
abstract base class and registry is provided for printing assembly code, the
<tt>GCMetadaPrinter</tt> and <tt>GCMetadaPrinterRegistry</tt>. The AsmWriter
will look for such a subclass if the <tt>GCStrategy</tt> sets
<tt>UsesMetadata</tt>:</p>
<blockquote><pre
>MyGC::MyGC() {
UsesMetadata = true;
}</pre></blockquote>
<p>Note that LLVM does not currently have analogous APIs to support code
generation in the JIT, nor using the object writers.</p>
<blockquote><pre
>// lib/MyGC/MyGCPrinter.cpp - Example LLVM GC printer
#include "llvm/CodeGen/GCMetadataPrinter.h"
#include "llvm/Support/Compiler.h"
using namespace llvm;
namespace {
class VISIBILITY_HIDDEN MyGCPrinter : public GCMetadataPrinter {
public:
virtual void beginAssembly(std::ostream &OS, AsmPrinter &AP,
const TargetAsmInfo &TAI);
virtual void finishAssembly(std::ostream &OS, AsmPrinter &AP,
const TargetAsmInfo &TAI);
};
GCMetadataPrinterRegistry::Add<MyGCPrinter>
X("mygc", "My bespoke garbage collector.");
}</pre></blockquote>
<p>The collector should use <tt>AsmPrinter</tt> and <tt>TargetAsmInfo</tt> to
print portable assembly code to the <tt>std::ostream</tt>. The collector itself
contains the stack map for the entire module, and may access the
<tt>GCFunctionInfo</tt> using its own <tt>begin()</tt> and <tt>end()</tt>
methods. Here's a realistic example:</p>
<blockquote><pre
>#include "llvm/CodeGen/AsmPrinter.h"
#include "llvm/Function.h"
#include "llvm/Target/TargetMachine.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Target/TargetAsmInfo.h"
void MyGCPrinter::beginAssembly(std::ostream &OS, AsmPrinter &AP,
const TargetAsmInfo &TAI) {
// Nothing to do.
}
void MyGCPrinter::finishAssembly(std::ostream &OS, AsmPrinter &AP,
const TargetAsmInfo &TAI) {
// Set up for emitting addresses.
const char *AddressDirective;
int AddressAlignLog;
if (AP.TM.getTargetData()->getPointerSize() == sizeof(int32_t)) {
AddressDirective = TAI.getData32bitsDirective();
AddressAlignLog = 2;
} else {
AddressDirective = TAI.getData64bitsDirective();
AddressAlignLog = 3;
}
// Put this in the data section.
AP.SwitchToDataSection(TAI.getDataSection());
// For each function...
for (iterator FI = begin(), FE = end(); FI != FE; ++FI) {
GCFunctionInfo &MD = **FI;
// Emit this data structure:
//
// struct {
// int32_t PointCount;
// struct {
// void *SafePointAddress;
// int32_t LiveCount;
// int32_t LiveOffsets[LiveCount];
// } Points[PointCount];
// } __gcmap_<FUNCTIONNAME>;
// Align to address width.
AP.EmitAlignment(AddressAlignLog);
// Emit the symbol by which the stack map can be found.
std::string Symbol;
Symbol += TAI.getGlobalPrefix();
Symbol += "__gcmap_";
Symbol += MD.getFunction().getName();
if (const char *GlobalDirective = TAI.getGlobalDirective())
OS << GlobalDirective << Symbol << "\n";
OS << TAI.getGlobalPrefix() << Symbol << ":\n";
// Emit PointCount.
AP.EmitInt32(MD.size());
AP.EOL("safe point count");
// And each safe point...
for (GCFunctionInfo::iterator PI = MD.begin(),
PE = MD.end(); PI != PE; ++PI) {
// Align to address width.
AP.EmitAlignment(AddressAlignLog);
// Emit the address of the safe point.
OS << AddressDirective
<< TAI.getPrivateGlobalPrefix() << "label" << PI->Num;
AP.EOL("safe point address");
// Emit the stack frame size.
AP.EmitInt32(MD.getFrameSize());
AP.EOL("stack frame size");
// Emit the number of live roots in the function.
AP.EmitInt32(MD.live_size(PI));
AP.EOL("live root count");
// And for each live root...
for (GCFunctionInfo::live_iterator LI = MD.live_begin(PI),
LE = MD.live_end(PI);
LI != LE; ++LI) {
// Print its offset within the stack frame.
AP.EmitInt32(LI->StackOffset);
AP.EOL("stack offset");
}
}
}
}
</pre></blockquote>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="runtime-impl">Implementing a collector runtime</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>Implementing a garbage collector for LLVM is fairly straightforward. The
LLVM garbage collectors are provided in a form that makes them easy to link into
the language-specific runtime that a language front-end would use. They require
functionality from the language-specific runtime to get information about <a
href="#gcdescriptors">where pointers are located in heap objects</a>.</p>
<p>The implementation must include the
<a href="#allocate"><tt>llvm_gc_allocate</tt></a> and
<a href="#explicit"><tt>llvm_gc_collect</tt></a> functions. To do this, it will
probably have to <a href="#traceroots">trace through the roots
from the stack</a> and understand the <a href="#gcdescriptors">GC descriptors
for heap objects</a>. Luckily, there are some <a href="#usage">example
implementations</a> available.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="gcdescriptors">Tracing GC pointers from heap objects</a>
</div>
<div class="doc_text">
<p>
The three most common ways to keep track of where pointers live in heap objects
are (listed in order of space overhead required):</p>
<ol>
<li>In languages with polymorphic objects, pointers from an object header are
usually used to identify the GC pointers in the heap object. This is common for
object-oriented languages like Self, Smalltalk, Java, or C#.</li>
<li>If heap objects are not polymorphic, often the "shape" of the heap can be
determined from the roots of the heap or from some other meta-data [<a
href="#appel89">Appel89</a>, <a href="#goldberg91">Goldberg91</a>, <a
href="#tolmach94">Tolmach94</a>]. In this case, the garbage collector can
propagate the information around from meta data stored with the roots. This
often eliminates the need to have a header on objects in the heap. This is
common in the ML family.</li>
<li>If all heap objects have pointers in the same locations, or pointers can be
distinguished just by looking at them (e.g., the low order bit is clear), no
book-keeping is needed at all. This is common for Lisp-like languages.</li>
</ol>
<p>The LLVM garbage collectors are capable of supporting all of these styles of
language, including ones that mix various implementations. To do this, it
allows the source-language to associate meta-data with the <a
href="#gcroot">stack roots</a>, and the heap tracing routines can propagate the
information. In addition, LLVM allows the front-end to extract GC information
in any form from a specific object pointer (this supports situations #1 and #3).
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="references">References</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p><a name="appel89">[Appel89]</a> Runtime Tags Aren't Necessary. Andrew
W. Appel. Lisp and Symbolic Computation 19(7):703-705, July 1989.</p>
<p><a name="goldberg91">[Goldberg91]</a> Tag-free garbage collection for
strongly typed programming languages. Benjamin Goldberg. ACM SIGPLAN
PLDI'91.</p>
<p><a name="tolmach94">[Tolmach94]</a> Tag-free garbage collection using
explicit type parameters. Andrew Tolmach. Proceedings of the 1994 ACM
conference on LISP and functional programming.</p>
<p><a name="henderson02">[Henderson2002]</a> <a
href="http://citeseer.ist.psu.edu/henderson02accurate.html">
Accurate Garbage Collection in an Uncooperative Environment</a>.
Fergus Henderson. International Symposium on Memory Management 2002.</p>
</div>
<!-- *********************************************************************** -->
<hr>
<address>
<a href="http://jigsaw.w3.org/css-validator/check/referer"><img
src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
<a href="http://validator.w3.org/check/referer"><img
src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a>
<a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
<a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
Last modified: $Date$
</address>
</body>
</html>
|
