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5 <title>Accurate Garbage Collection with LLVM</title>
6 <link rel="stylesheet" href="llvm.css" type="text/css">
7</head>
8<body>
9
10<div class="doc_title">
11 Accurate Garbage Collection with LLVM
12</div>
13
14<ol>
15 <li><a href="#introduction">Introduction</a>
16 <ul>
17 <li><a href="#feature">GC features provided and algorithms supported</a></li>
18 </ul>
19 </li>
20
21 <li><a href="#interfaces">Interfaces for user programs</a>
22 <ul>
23 <li><a href="#roots">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a></li>
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]24 <li><a href="#allocate">Allocating memory from the GC</a></li>
25 <li><a href="#barriers">Reading and writing references to the heap</a></li>
26 <li><a href="#explicit">Explicit invocation of the garbage collector</a></li>
27 </ul>
28 </li>
29
30 <li><a href="#gcimpl">Implementing a garbage collector</a>
31 <ul>
32 <li><a href="#llvm_gc_readwrite">Implementing <tt>llvm_gc_read</tt> and <tt>llvm_gc_write</tt></a></li>
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]33 <li><a href="#callbacks">Callback functions used to implement the garbage collector</a></li>
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]34 </ul>
35 </li>
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]36 <li><a href="#gcimpls">GC implementations available</a>
37 <ul>
38 <li><a href="#semispace">SemiSpace - A simple copying garbage collector</a></li>
Chris Lattner0b02dbc2004-07-09 05:03:54 +0000[diff] [blame]39 </ul>
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]40 </li>
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]41
42<!--
43 <li><a href="#codegen">Implementing GC support in a code generator</a></li>
44-->
45</ol>
46
47<div class="doc_author">
48 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
49</div>
50
51<!-- *********************************************************************** -->
52<div class="doc_section">
53 <a name="introduction">Introduction</a>
54</div>
55<!-- *********************************************************************** -->
56
57<div class="doc_text">
58
59<p>Garbage collection is a widely used technique that frees the programmer from
60having to know the life-times of heap objects, making software easier to produce
61and maintain. Many programming languages rely on garbage collection for
62automatic memory management. There are two primary forms of garbage collection:
63conservative and accurate.</p>
64
65<p>Conservative garbage collection often does not require any special support
66from either the language or the compiler: it can handle non-type-safe
67programming languages (such as C/C++) and does not require any special
Jeff Cohen65fc36b2007-04-18 17:26:14 +0000[diff] [blame]68information from the compiler. The
69<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/">Boehm collector</a> is
70an example of a state-of-the-art conservative collector.</p>
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]71
72<p>Accurate garbage collection requires the ability to identify all pointers in
73the program at run-time (which requires that the source-language be type-safe in
74most cases). Identifying pointers at run-time requires compiler support to
75locate all places that hold live pointer variables at run-time, including the
76<a href="#roots">processor stack and registers</a>.</p>
77
78<p>
79Conservative garbage collection is attractive because it does not require any
80special compiler support, but it does have problems. In particular, because the
81conservative garbage collector cannot <i>know</i> that a particular word in the
82machine is a pointer, it cannot move live objects in the heap (preventing the
83use of compacting and generational GC algorithms) and it can occasionally suffer
84from memory leaks due to integer values that happen to point to objects in the
85program. In addition, some aggressive compiler transformations can break
86conservative garbage collectors (though these seem rare in practice).
87</p>
88
89<p>
90Accurate garbage collectors do not suffer from any of these problems, but they
91can suffer from degraded scalar optimization of the program. In particular,
92because the runtime must be able to identify and update all pointers active in
93the program, some optimizations are less effective. In practice, however, the
94locality and performance benefits of using aggressive garbage allocation
95techniques dominates any low-level losses.
96</p>
97
98<p>
99This document describes the mechanisms and interfaces provided by LLVM to
100support accurate garbage collection.
101</p>
102
103</div>
104
105<!-- ======================================================================= -->
106<div class="doc_subsection">
107 <a name="feature">GC features provided and algorithms supported</a>
108</div>
109
110<div class="doc_text">
111
112<p>
113LLVM provides support for a broad class of garbage collection algorithms,
114including compacting semi-space collectors, mark-sweep collectors, generational
115collectors, and even reference counting implementations. It includes support
116for <a href="#barriers">read and write barriers</a>, and associating <a
117href="#roots">meta-data with stack objects</a> (used for tagless garbage
118collection). All LLVM code generators support garbage collection, including the
119C backend.
120</p>
121
122<p>
123We hope that the primitive support built into LLVM is sufficient to support a
124broad class of garbage collected languages, including Scheme, ML, scripting
125languages, Java, C#, etc. That said, the implemented garbage collectors may
126need to be extended to support language-specific features such as finalization,
127weak references, or other features. As these needs are identified and
128implemented, they should be added to this specification.
129</p>
130
131<p>
132LLVM does not currently support garbage collection of multi-threaded programs or
133GC-safe points other than function calls, but these will be added in the future
134as there is interest.
135</p>
136
137</div>
138
139<!-- *********************************************************************** -->
140<div class="doc_section">
141 <a name="interfaces">Interfaces for user programs</a>
142</div>
143<!-- *********************************************************************** -->
144
145<div class="doc_text">
146
147<p>This section describes the interfaces provided by LLVM and by the garbage
148collector run-time that should be used by user programs. As such, this is the
149interface that front-end authors should generate code for.
150</p>
151
152</div>
153
154<!-- ======================================================================= -->
155<div class="doc_subsection">
156 <a name="roots">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
157</div>
158
159<div class="doc_text">
160
161<div class="doc_code"><tt>
162 void %llvm.gcroot(&lt;ty&gt;** %ptrloc, &lt;ty2&gt;* %metadata)
163</tt></div>
164
165<p>
166The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM of a pointer variable
167on the stack. The first argument contains the address of the variable on the
168stack, and the second contains a pointer to metadata that should be associated
169with the pointer (which <b>must</b> be a constant or global value address). At
170runtime, the <tt>llvm.gcroot</tt> intrinsic stores a null pointer into the
171specified location to initialize the pointer.</p>
172
173<p>
174Consider the following fragment of Java code:
175</p>
176
177<pre>
178 {
179 Object X; // A null-initialized reference to an object
180 ...
181 }
182</pre>
183
184<p>
185This block (which may be located in the middle of a function or in a loop nest),
186could be compiled to this LLVM code:
187</p>
188
189<pre>
190Entry:
191 ;; In the entry block for the function, allocate the
192 ;; stack space for X, which is an LLVM pointer.
193 %X = alloca %Object*
194 ...
195
196 ;; "CodeBlock" is the block corresponding to the start
Reid Spencer03d186a2004-05-25 08:45:31 +0000[diff] [blame]197 ;; of the scope above.
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]198CodeBlock:
199 ;; Initialize the object, telling LLVM that it is now live.
200 ;; Java has type-tags on objects, so it doesn't need any
201 ;; metadata.
202 call void %llvm.gcroot(%Object** %X, sbyte* null)
203 ...
204
205 ;; As the pointer goes out of scope, store a null value into
206 ;; it, to indicate that the value is no longer live.
207 store %Object* null, %Object** %X
208 ...
209</pre>
210
211</div>
212
213<!-- ======================================================================= -->
214<div class="doc_subsection">
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]215 <a name="allocate">Allocating memory from the GC</a>
216</div>
217
218<div class="doc_text">
219
220<div class="doc_code"><tt>
221 sbyte *%llvm_gc_allocate(unsigned %Size)
222</tt></div>
223
224<p>The <tt>llvm_gc_allocate</tt> function is a global function defined by the
Chris Lattneraab3aff2004-06-09 03:59:05 +0000[diff] [blame]225garbage collector implementation to allocate memory. It returns a
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]226zeroed-out block of memory of the appropriate size.</p>
227
228</div>
229
230<!-- ======================================================================= -->
231<div class="doc_subsection">
232 <a name="barriers">Reading and writing references to the heap</a>
233</div>
234
235<div class="doc_text">
236
237<div class="doc_code"><tt>
Chris Lattner728f03f2004-07-22 05:49:38 +0000[diff] [blame]238 sbyte *%llvm.gcread(sbyte *, sbyte **)<br>
239 void %llvm.gcwrite(sbyte*, sbyte*, sbyte**)
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]240</tt></div>
241
242<p>Several of the more interesting garbage collectors (e.g., generational
243collectors) need to be informed when the mutator (the program that needs garbage
244collection) reads or writes object references into the heap. In the case of a
245generational collector, it needs to keep track of which "old" generation objects
246have references stored into them. The amount of code that typically needs to be
Chris Lattneraab3aff2004-06-09 03:59:05 +0000[diff] [blame]247executed is usually quite small (and not on the critical path of any
248computation), so the overall performance impact of the inserted code is
249tolerable.</p>
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]250
251<p>To support garbage collectors that use read or write barriers, LLVM provides
252the <tt>llvm.gcread</tt> and <tt>llvm.gcwrite</tt> intrinsics. The first
253intrinsic has exactly the same semantics as a non-volatile LLVM load and the
Chris Lattner728f03f2004-07-22 05:49:38 +0000[diff] [blame]254second has the same semantics as a non-volatile LLVM store, with the
255additions that they also take a pointer to the start of the memory
256object as an argument. At code generation
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]257time, these intrinsics are replaced with calls into the garbage collector
258(<tt><a href="#llvm_gc_readwrite">llvm_gc_read</a></tt> and <tt><a
259href="#llvm_gc_readwrite">llvm_gc_write</a></tt> respectively), which are then
260inlined into the code.
261</p>
262
263<p>
264If you are writing a front-end for a garbage collected language, every load or
265store of a reference from or to the heap should use these intrinsics instead of
266normal LLVM loads/stores.</p>
267
268</div>
269
270<!-- ======================================================================= -->
271<div class="doc_subsection">
272 <a name="initialize">Garbage collector startup and initialization</a>
273</div>
274
275<div class="doc_text">
276
277<div class="doc_code"><tt>
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]278 void %llvm_gc_initialize(unsigned %InitialHeapSize)
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]279</tt></div>
280
281<p>
282The <tt>llvm_gc_initialize</tt> function should be called once before any other
283garbage collection functions are called. This gives the garbage collector the
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]284chance to initialize itself and allocate the heap spaces. The initial heap size
285to allocate should be specified as an argument.
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]286</p>
287
288</div>
289
290<!-- ======================================================================= -->
291<div class="doc_subsection">
292 <a name="explicit">Explicit invocation of the garbage collector</a>
293</div>
294
295<div class="doc_text">
296
297<div class="doc_code"><tt>
298 void %llvm_gc_collect()
299</tt></div>
300
301<p>
302The <tt>llvm_gc_collect</tt> function is exported by the garbage collector
303implementations to provide a full collection, even when the heap is not
304exhausted. This can be used by end-user code as a hint, and may be ignored by
305the garbage collector.
306</p>
307
308</div>
309
310
311<!-- *********************************************************************** -->
312<div class="doc_section">
313 <a name="gcimpl">Implementing a garbage collector</a>
314</div>
315<!-- *********************************************************************** -->
316
317<div class="doc_text">
318
319<p>
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]320Implementing a garbage collector for LLVM is fairly straight-forward. The LLVM
321garbage collectors are provided in a form that makes them easy to link into the
322language-specific runtime that a language front-end would use. They require
323functionality from the language-specific runtime to get information about <a
324href="#gcdescriptors">where pointers are located in heap objects</a>.
325</p>
326
327<p>The
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]328implementation must include the <a
329href="#allocate"><tt>llvm_gc_allocate</tt></a> and <a
330href="#explicit"><tt>llvm_gc_collect</tt></a> functions, and it must implement
331the <a href="#llvm_gc_readwrite">read/write barrier</a> functions as well. To
332do this, it will probably have to <a href="#traceroots">trace through the roots
333from the stack</a> and understand the <a href="#gcdescriptors">GC descriptors
334for heap objects</a>. Luckily, there are some <a href="#gcimpls">example
335implementations</a> available.
336</p>
337</div>
338
339
340<!-- ======================================================================= -->
341<div class="doc_subsection">
342 <a name="llvm_gc_readwrite">Implementing <tt>llvm_gc_read</tt> and <tt>llvm_gc_write</tt></a>
343</div>
344
345<div class="doc_text">
346 <div class="doc_code"><tt>
Chris Lattner728f03f2004-07-22 05:49:38 +0000[diff] [blame]347 void *llvm_gc_read(void*, void **)<br>
348 void llvm_gc_write(void*, void *, void**)
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]349 </tt></div>
350
351<p>
352These functions <i>must</i> be implemented in every garbage collector, even if
353they do not need read/write barriers. In this case, just load or store the
354pointer, then return.
355</p>
356
357<p>
358If an actual read or write barrier is needed, it should be straight-forward to
Chris Lattner728f03f2004-07-22 05:49:38 +0000[diff] [blame]359implement it.
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]360</p>
361
362</div>
363
364<!-- ======================================================================= -->
365<div class="doc_subsection">
Chris Lattner0b02dbc2004-07-09 05:03:54 +0000[diff] [blame]366 <a name="callbacks">Callback functions used to implement the garbage collector</a>
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]367</div>
368
Chris Lattner0b02dbc2004-07-09 05:03:54 +0000[diff] [blame]369<div class="doc_text">
370<p>
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]371Garbage collector implementations make use of call-back functions that are
372implemented by other parts of the LLVM system.
Chris Lattner0b02dbc2004-07-09 05:03:54 +0000[diff] [blame]373</p>
374</div>
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]375
376<!--_________________________________________________________________________-->
377<div class="doc_subsubsection">
378 <a name="traceroots">Tracing GC pointers from the program stack</a>
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]379</div>
380
381<div class="doc_text">
382 <div class="doc_code"><tt>
383 void llvm_cg_walk_gcroots(void (*FP)(void **Root, void *Meta));
384 </tt></div>
385
386<p>
387The <tt>llvm_cg_walk_gcroots</tt> function is a function provided by the code
388generator that iterates through all of the GC roots on the stack, calling the
389specified function pointer with each record. For each GC root, the address of
390the pointer and the meta-data (from the <a
Duncan Sands8036ca42007-03-30 12:22:09 +0000[diff] [blame]391href="#roots"><tt>llvm.gcroot</tt></a> intrinsic) are provided.
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]392</p>
393</div>
394
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]395<!--_________________________________________________________________________-->
396<div class="doc_subsubsection">
397 <a name="staticroots">Tracing GC pointers from static roots</a>
398</div>
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]399
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]400<div class="doc_text">
401TODO
402</div>
403
404
405<!--_________________________________________________________________________-->
406<div class="doc_subsubsection">
407 <a name="gcdescriptors">Tracing GC pointers from heap objects</a>
408</div>
409
410<div class="doc_text">
411<p>
412The three most common ways to keep track of where pointers live in heap objects
413are (listed in order of space overhead required):</p>
414
415<ol>
416<li>In languages with polymorphic objects, pointers from an object header are
417usually used to identify the GC pointers in the heap object. This is common for
418object-oriented languages like Self, Smalltalk, Java, or C#.</li>
419
420<li>If heap objects are not polymorphic, often the "shape" of the heap can be
421determined from the roots of the heap or from some other meta-data [<a
422href="#appel89">Appel89</a>, <a href="#goldberg91">Goldberg91</a>, <a
423href="#tolmach94">Tolmach94</a>]. In this case, the garbage collector can
424propagate the information around from meta data stored with the roots. This
425often eliminates the need to have a header on objects in the heap. This is
426common in the ML family.</li>
427
428<li>If all heap objects have pointers in the same locations, or pointers can be
429distinguished just by looking at them (e.g., the low order bit is clear), no
430book-keeping is needed at all. This is common for Lisp-like languages.</li>
431</ol>
432
433<p>The LLVM garbage collectors are capable of supporting all of these styles of
434language, including ones that mix various implementations. To do this, it
435allows the source-language to associate meta-data with the <a
436href="#roots">stack roots</a>, and the heap tracing routines can propagate the
437information. In addition, LLVM allows the front-end to extract GC information
438from in any form from a specific object pointer (this supports situations #1 and
439#3).
440</p>
441
442<p><b>Making this efficient</b></p>
443
444
445
446</div>
447
448
449
450<!-- *********************************************************************** -->
451<div class="doc_section">
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]452 <a name="gcimpls">GC implementations available</a>
453</div>
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]454<!-- *********************************************************************** -->
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]455
456<div class="doc_text">
457
458<p>
459To make this more concrete, the currently implemented LLVM garbage collectors
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]460all live in the <tt>llvm/runtime/GC/*</tt> directories in the LLVM source-base.
461If you are interested in implementing an algorithm, there are many interesting
462possibilities (mark/sweep, a generational collector, a reference counting
463collector, etc), or you could choose to improve one of the existing algorithms.
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]464</p>
465
466</div>
467
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]468<!-- ======================================================================= -->
469<div class="doc_subsection">
Chris Lattner0b02dbc2004-07-09 05:03:54 +0000[diff] [blame]470 <a name="semispace">SemiSpace - A simple copying garbage collector</a>
Chris Lattner9b2a1842004-05-27 05:52:10 +0000[diff] [blame]471</div>
472
473<div class="doc_text">
474<p>
475SemiSpace is a very simple copying collector. When it starts up, it allocates
476two blocks of memory for the heap. It uses a simple bump-pointer allocator to
477allocate memory from the first block until it runs out of space. When it runs
478out of space, it traces through all of the roots of the program, copying blocks
479to the other half of the memory space.
480</p>
481
482</div>
483
484<!--_________________________________________________________________________-->
485<div class="doc_subsubsection">
486 Possible Improvements
487</div>
488
489<div class="doc_text">
490
491<p>
492If a collection cycle happens and the heap is not compacted very much (say less
493than 25% of the allocated memory was freed), the memory regions should be
494doubled in size.</p>
495
496</div>
497
498<!-- *********************************************************************** -->
499<div class="doc_section">
500 <a name="references">References</a>
501</div>
502<!-- *********************************************************************** -->
503
504<div class="doc_text">
505
506<p><a name="appel89">[Appel89]</a> Runtime Tags Aren't Necessary. Andrew
507W. Appel. Lisp and Symbolic Computation 19(7):703-705, July 1989.</p>
508
509<p><a name="goldberg91">[Goldberg91]</a> Tag-free garbage collection for
510strongly typed programming languages. Benjamin Goldberg. ACM SIGPLAN
511PLDI'91.</p>
512
513<p><a name="tolmach94">[Tolmach94]</a> Tag-free garbage collection using
514explicit type parameters. Andrew Tolmach. Proceedings of the 1994 ACM
515conference on LISP and functional programming.</p>
516
517</div>
Chris Lattner0d8c2db2004-05-23 21:02:20 +0000[diff] [blame]518
519<!-- *********************************************************************** -->
520
521<hr>
522<address>
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527
528 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
Reid Spencer05fe4b02006-03-14 05:39:39 +0000[diff] [blame]529 <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
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