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Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]6 <title>Accurate Garbage Collection with LLVM</title>
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14<body>
15
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]16<h1>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]17 Accurate Garbage Collection with LLVM
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]18</h1>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]19
20<ol>
21 <li><a href="#introduction">Introduction</a>
22 <ul>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]23 <li><a href="#feature">Goals and non-goals</a></li>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]24 </ul>
25 </li>
26
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]27 <li><a href="#quickstart">Getting started</a>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]28 <ul>
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]29 <li><a href="#quickstart-compiler">In your compiler</a></li>
30 <li><a href="#quickstart-runtime">In your runtime library</a></li>
31 <li><a href="#shadow-stack">About the shadow stack</a></li>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]32 </ul>
33 </li>
34
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]35 <li><a href="#core">Core support</a>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]36 <ul>
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]37 <li><a href="#gcattr">Specifying GC code generation:
38 <tt>gc "..."</tt></a></li>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]39 <li><a href="#gcroot">Identifying GC roots on the stack:
40 <tt>llvm.gcroot</tt></a></li>
41 <li><a href="#barriers">Reading and writing references in the heap</a>
42 <ul>
43 <li><a href="#gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a></li>
44 <li><a href="#gcread">Read barrier: <tt>llvm.gcread</tt></a></li>
45 </ul>
46 </li>
47 </ul>
48 </li>
49
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]50 <li><a href="#plugin">Compiler plugin interface</a>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]51 <ul>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]52 <li><a href="#collector-algos">Overview of available features</a></li>
53 <li><a href="#stack-map">Computing stack maps</a></li>
54 <li><a href="#init-roots">Initializing roots to null:
55 <tt>InitRoots</tt></a></li>
56 <li><a href="#custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>,
57 <tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a></li>
58 <li><a href="#safe-points">Generating safe points:
59 <tt>NeededSafePoints</tt></a></li>
60 <li><a href="#assembly">Emitting assembly code:
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]61 <tt>GCMetadataPrinter</tt></a></li>
Chris Lattner3e5d2a62004-07-09 05:03:54 +0000[diff] [blame]62 </ul>
Chris Lattner71679332004-05-27 05:52:10 +0000[diff] [blame]63 </li>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]64
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]65 <li><a href="#runtime-impl">Implementing a collector runtime</a>
66 <ul>
67 <li><a href="#gcdescriptors">Tracing GC pointers from heap
68 objects</a></li>
69 </ul>
70 </li>
71
72 <li><a href="#references">References</a></li>
73
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]74</ol>
75
76<div class="doc_author">
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]77 <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
78 Gordon Henriksen</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]79</div>
80
81<!-- *********************************************************************** -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]82<h2>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]83 <a name="introduction">Introduction</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]84</h2>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]85<!-- *********************************************************************** -->
86
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]87<div>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]88
89<p>Garbage collection is a widely used technique that frees the programmer from
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]90having to know the lifetimes of heap objects, making software easier to produce
91and maintain. Many programming languages rely on garbage collection for
92automatic memory management. There are two primary forms of garbage collection:
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]93conservative and accurate.</p>
94
95<p>Conservative garbage collection often does not require any special support
96from either the language or the compiler: it can handle non-type-safe
97programming languages (such as C/C++) and does not require any special
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]98information from the compiler. The
Jeff Cohen43a2fb72007-04-18 17:26:14 +0000[diff] [blame]99<a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/">Boehm collector</a> is
100an example of a state-of-the-art conservative collector.</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]101
102<p>Accurate garbage collection requires the ability to identify all pointers in
103the program at run-time (which requires that the source-language be type-safe in
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]104most cases). Identifying pointers at run-time requires compiler support to
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]105locate all places that hold live pointer variables at run-time, including the
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]106<a href="#gcroot">processor stack and registers</a>.</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]107
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]108<p>Conservative garbage collection is attractive because it does not require any
109special compiler support, but it does have problems. In particular, because the
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]110conservative garbage collector cannot <i>know</i> that a particular word in the
111machine is a pointer, it cannot move live objects in the heap (preventing the
112use of compacting and generational GC algorithms) and it can occasionally suffer
113from memory leaks due to integer values that happen to point to objects in the
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]114program. In addition, some aggressive compiler transformations can break
115conservative garbage collectors (though these seem rare in practice).</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]116
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]117<p>Accurate garbage collectors do not suffer from any of these problems, but
118they can suffer from degraded scalar optimization of the program. In particular,
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]119because the runtime must be able to identify and update all pointers active in
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]120the program, some optimizations are less effective. In practice, however, the
Chris Lattner784ae662009-05-13 18:02:09 +0000[diff] [blame]121locality and performance benefits of using aggressive garbage collection
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]122techniques dominates any low-level losses.</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]123
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]124<p>This document describes the mechanisms and interfaces provided by LLVM to
125support accurate garbage collection.</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]126
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]127<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]128<h3>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]129 <a name="feature">Goals and non-goals</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]130</h3>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]131
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]132<div>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]133
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]134<p>LLVM's intermediate representation provides <a href="#intrinsics">garbage
Chris Lattner851b7712008-04-24 05:59:56 +0000[diff] [blame]135collection intrinsics</a> that offer support for a broad class of
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]136collector models. For instance, the intrinsics permit:</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]137
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]138<ul>
139 <li>semi-space collectors</li>
140 <li>mark-sweep collectors</li>
141 <li>generational collectors</li>
142 <li>reference counting</li>
143 <li>incremental collectors</li>
144 <li>concurrent collectors</li>
145 <li>cooperative collectors</li>
146</ul>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]147
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]148<p>We hope that the primitive support built into the LLVM IR is sufficient to
149support a broad class of garbage collected languages including Scheme, ML, Java,
150C#, Perl, Python, Lua, Ruby, other scripting languages, and more.</p>
151
NAKAMURA Takumi6bd36d52011-04-09 09:51:57 +0000[diff] [blame]152<p>However, LLVM does not itself provide a garbage collector&mdash;this should
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]153be part of your language's runtime library. LLVM provides a framework for
154compile time <a href="#plugin">code generation plugins</a>. The role of these
155plugins is to generate code and data structures which conforms to the <em>binary
156interface</em> specified by the <em>runtime library</em>. This is similar to the
157relationship between LLVM and DWARF debugging info, for example. The
158difference primarily lies in the lack of an established standard in the domain
NAKAMURA Takumi6bd36d52011-04-09 09:51:57 +0000[diff] [blame]159of garbage collection&mdash;thus the plugins.</p>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]160
161<p>The aspects of the binary interface with which LLVM's GC support is
162concerned are:</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]163
164<ul>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]165 <li>Creation of GC-safe points within code where collection is allowed to
166 execute safely.</li>
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]167 <li>Computation of the stack map. For each safe point in the code, object
168 references within the stack frame must be identified so that the
169 collector may traverse and perhaps update them.</li>
170 <li>Write barriers when storing object references to the heap. These are
171 commonly used to optimize incremental scans in generational
172 collectors.</li>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]173 <li>Emission of read barriers when loading object references. These are
174 useful for interoperating with concurrent collectors.</li>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]175</ul>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]176
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]177<p>There are additional areas that LLVM does not directly address:</p>
178
179<ul>
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]180 <li>Registration of global roots with the runtime.</li>
181 <li>Registration of stack map entries with the runtime.</li>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]182 <li>The functions used by the program to allocate memory, trigger a
183 collection, etc.</li>
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]184 <li>Computation or compilation of type maps, or registration of them with
185 the runtime. These are used to crawl the heap for object
186 references.</li>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]187</ul>
188
189<p>In general, LLVM's support for GC does not include features which can be
190adequately addressed with other features of the IR and does not specify a
191particular binary interface. On the plus side, this means that you should be
192able to integrate LLVM with an existing runtime. On the other hand, it leaves
193a lot of work for the developer of a novel language. However, it's easy to get
194started quickly and scale up to a more sophisticated implementation as your
195compiler matures.</p>
196
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]197</div>
198
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]199</div>
200
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]201<!-- *********************************************************************** -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]202<h2>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]203 <a name="quickstart">Getting started</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]204</h2>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]205<!-- *********************************************************************** -->
206
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]207<div>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]208
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]209<p>Using a GC with LLVM implies many things, for example:</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]210
211<ul>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]212 <li>Write a runtime library or find an existing one which implements a GC
213 heap.<ol>
214 <li>Implement a memory allocator.</li>
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]215 <li>Design a binary interface for the stack map, used to identify
216 references within a stack frame on the machine stack.*</li>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]217 <li>Implement a stack crawler to discover functions on the call stack.*</li>
218 <li>Implement a registry for global roots.</li>
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]219 <li>Design a binary interface for type maps, used to identify references
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]220 within heap objects.</li>
221 <li>Implement a collection routine bringing together all of the above.</li>
222 </ol></li>
223 <li>Emit compatible code from your compiler.<ul>
224 <li>Initialization in the main function.</li>
225 <li>Use the <tt>gc "..."</tt> attribute to enable GC code generation
226 (or <tt>F.setGC("...")</tt>).</li>
227 <li>Use <tt>@llvm.gcroot</tt> to mark stack roots.</li>
228 <li>Use <tt>@llvm.gcread</tt> and/or <tt>@llvm.gcwrite</tt> to
229 manipulate GC references, if necessary.</li>
230 <li>Allocate memory using the GC allocation routine provided by the
231 runtime library.</li>
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]232 <li>Generate type maps according to your runtime's binary interface.</li>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]233 </ul></li>
234 <li>Write a compiler plugin to interface LLVM with the runtime library.*<ul>
235 <li>Lower <tt>@llvm.gcread</tt> and <tt>@llvm.gcwrite</tt> to appropriate
236 code sequences.*</li>
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]237 <li>Compile LLVM's stack map to the binary form expected by the
238 runtime.</li>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]239 </ul></li>
240 <li>Load the plugin into the compiler. Use <tt>llc -load</tt> or link the
241 plugin statically with your language's compiler.*</li>
242 <li>Link program executables with the runtime.</li>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]243</ul>
244
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]245<p>To help with several of these tasks (those indicated with a *), LLVM
246includes a highly portable, built-in ShadowStack code generator. It is compiled
247into <tt>llc</tt> and works even with the interpreter and C backends.</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]248
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]249<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]250<h3>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]251 <a name="quickstart-compiler">In your compiler</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]252</h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]253
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]254<div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]255
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]256<p>To turn the shadow stack on for your functions, first call:</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]257
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]258<div class="doc_code"><pre
259>F.setGC("shadow-stack");</pre></div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]260
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]261<p>for each function your compiler emits. Since the shadow stack is built into
262LLVM, you do not need to load a plugin.</p>
263
264<p>Your compiler must also use <tt>@llvm.gcroot</tt> as documented.
265Don't forget to create a root for each intermediate value that is generated
266when evaluating an expression. In <tt>h(f(), g())</tt>, the result of
267<tt>f()</tt> could easily be collected if evaluating <tt>g()</tt> triggers a
268collection.</p>
269
270<p>There's no need to use <tt>@llvm.gcread</tt> and <tt>@llvm.gcwrite</tt> over
271plain <tt>load</tt> and <tt>store</tt> for now. You will need them when
272switching to a more advanced GC.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]273
274</div>
275
276<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]277<h3>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]278 <a name="quickstart-runtime">In your runtime</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]279</h3>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]280
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]281<div>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]282
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]283<p>The shadow stack doesn't imply a memory allocation algorithm. A semispace
284collector or building atop <tt>malloc</tt> are great places to start, and can
285be implemented with very little code.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]286
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]287<p>When it comes time to collect, however, your runtime needs to traverse the
288stack roots, and for this it needs to integrate with the shadow stack. Luckily,
289doing so is very simple. (This code is heavily commented to help you
290understand the data structure, but there are only 20 lines of meaningful
291code.)</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]292
NAKAMURA Takumi0300d882011-08-12 06:17:17 +0000[diff] [blame]293<pre class="doc_code">
294/// @brief The map for a single function's stack frame. One of these is
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]295/// compiled as constant data into the executable for each function.
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]296///
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]297/// Storage of metadata values is elided if the %metadata parameter to
298/// @llvm.gcroot is null.
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]299struct FrameMap {
300 int32_t NumRoots; //&lt; Number of roots in stack frame.
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]301 int32_t NumMeta; //&lt; Number of metadata entries. May be &lt; NumRoots.
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]302 const void *Meta[0]; //&lt; Metadata for each root.
303};
304
305/// @brief A link in the dynamic shadow stack. One of these is embedded in the
306/// stack frame of each function on the call stack.
307struct StackEntry {
308 StackEntry *Next; //&lt; Link to next stack entry (the caller's).
309 const FrameMap *Map; //&lt; Pointer to constant FrameMap.
310 void *Roots[0]; //&lt; Stack roots (in-place array).
311};
312
313/// @brief The head of the singly-linked list of StackEntries. Functions push
314/// and pop onto this in their prologue and epilogue.
315///
316/// Since there is only a global list, this technique is not threadsafe.
317StackEntry *llvm_gc_root_chain;
318
319/// @brief Calls Visitor(root, meta) for each GC root on the stack.
320/// root and meta are exactly the values passed to
321/// <tt>@llvm.gcroot</tt>.
322///
323/// Visitor could be a function to recursively mark live objects. Or it
324/// might copy them to another heap or generation.
325///
326/// @param Visitor A function to invoke for every GC root on the stack.
327void visitGCRoots(void (*Visitor)(void **Root, const void *Meta)) {
328 for (StackEntry *R = llvm_gc_root_chain; R; R = R->Next) {
329 unsigned i = 0;
330
331 // For roots [0, NumMeta), the metadata pointer is in the FrameMap.
332 for (unsigned e = R->Map->NumMeta; i != e; ++i)
Benjamin Kramereaccdd32009-08-05 15:42:44 +0000[diff] [blame]333 Visitor(&amp;R->Roots[i], R->Map->Meta[i]);
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]334
335 // For roots [NumMeta, NumRoots), the metadata pointer is null.
336 for (unsigned e = R->Map->NumRoots; i != e; ++i)
Benjamin Kramereaccdd32009-08-05 15:42:44 +0000[diff] [blame]337 Visitor(&amp;R->Roots[i], NULL);
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]338 }
NAKAMURA Takumi0300d882011-08-12 06:17:17 +0000[diff] [blame]339}</pre>
340
341</div>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]342
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]343<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]344<h3>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]345 <a name="shadow-stack">About the shadow stack</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]346</h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]347
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]348<div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]349
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]350<p>Unlike many GC algorithms which rely on a cooperative code generator to
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]351compile stack maps, this algorithm carefully maintains a linked list of stack
352roots [<a href="#henderson02">Henderson2002</a>]. This so-called "shadow stack"
353mirrors the machine stack. Maintaining this data structure is slower than using
354a stack map compiled into the executable as constant data, but has a significant
355portability advantage because it requires no special support from the target
356code generator, and does not require tricky platform-specific code to crawl
357the machine stack.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]358
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]359<p>The tradeoff for this simplicity and portability is:</p>
360
361<ul>
362 <li>High overhead per function call.</li>
363 <li>Not thread-safe.</li>
364</ul>
365
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]366<p>Still, it's an easy way to get started. After your compiler and runtime are
367up and running, writing a <a href="#plugin">plugin</a> will allow you to take
368advantage of <a href="#collector-algos">more advanced GC features</a> of LLVM
369in order to improve performance.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]370
371</div>
372
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]373</div>
374
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]375<!-- *********************************************************************** -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]376<h2>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]377 <a name="core">IR features</a><a name="intrinsics"></a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]378</h2>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]379<!-- *********************************************************************** -->
380
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]381<div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]382
383<p>This section describes the garbage collection facilities provided by the
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]384<a href="LangRef.html">LLVM intermediate representation</a>. The exact behavior
385of these IR features is specified by the binary interface implemented by a
386<a href="#plugin">code generation plugin</a>, not by this document.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]387
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]388<p>These facilities are limited to those strictly necessary; they are not
389intended to be a complete interface to any garbage collector. A program will
390need to interface with the GC library using the facilities provided by that
391program.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]392
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]393<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]394<h3>
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]395 <a name="gcattr">Specifying GC code generation: <tt>gc "..."</tt></a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]396</h3>
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]397
NAKAMURA Takumi0300d882011-08-12 06:17:17 +0000[diff] [blame]398<div>
399
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]400<div class="doc_code"><tt>
Benjamin Kramereaccdd32009-08-05 15:42:44 +0000[diff] [blame]401 define <i>ty</i> @<i>name</i>(...) <span style="text-decoration: underline">gc "<i>name</i>"</span> { ...
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]402</tt></div>
403
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]404<p>The <tt>gc</tt> function attribute is used to specify the desired GC style
405to the compiler. Its programmatic equivalent is the <tt>setGC</tt> method of
406<tt>Function</tt>.</p>
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]407
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]408<p>Setting <tt>gc "<i>name</i>"</tt> on a function triggers a search for a
409matching code generation plugin "<i>name</i>"; it is that plugin which defines
410the exact nature of the code generated to support GC. If none is found, the
411compiler will raise an error.</p>
412
413<p>Specifying the GC style on a per-function basis allows LLVM to link together
414programs that use different garbage collection algorithms (or none at all).</p>
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]415
416</div>
417
418<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]419<h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]420 <a name="gcroot">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]421</h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]422
NAKAMURA Takumi0300d882011-08-12 06:17:17 +0000[diff] [blame]423<div>
424
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]425<div class="doc_code"><tt>
Chris Lattner0b9db522008-04-24 06:00:30 +0000[diff] [blame]426 void @llvm.gcroot(i8** %ptrloc, i8* %metadata)
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]427</tt></div>
428
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]429<p>The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM that a stack
430variable references an object on the heap and is to be tracked for garbage
431collection. The exact impact on generated code is specified by a <a
432href="#plugin">compiler plugin</a>.</p>
433
434<p>A compiler which uses mem2reg to raise imperative code using <tt>alloca</tt>
435into SSA form need only add a call to <tt>@llvm.gcroot</tt> for those variables
436which a pointers into the GC heap.</p>
437
438<p>It is also important to mark intermediate values with <tt>llvm.gcroot</tt>.
439For example, consider <tt>h(f(), g())</tt>. Beware leaking the result of
440<tt>f()</tt> in the case that <tt>g()</tt> triggers a collection.</p>
441
442<p>The first argument <b>must</b> be a value referring to an alloca instruction
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]443or a bitcast of an alloca. The second contains a pointer to metadata that
444should be associated with the pointer, and <b>must</b> be a constant or global
445value address. If your target collector uses tags, use a null pointer for
446metadata.</p>
447
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]448<p>The <tt>%metadata</tt> argument can be used to avoid requiring heap objects
449to have 'isa' pointers or tag bits. [<a href="#appel89">Appel89</a>, <a
450href="#goldberg91">Goldberg91</a>, <a href="#tolmach94">Tolmach94</a>] If
451specified, its value will be tracked along with the location of the pointer in
452the stack frame.</p>
453
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]454<p>Consider the following fragment of Java code:</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]455
NAKAMURA Takumi0300d882011-08-12 06:17:17 +0000[diff] [blame]456<pre class="doc_code">
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]457 {
458 Object X; // A null-initialized reference to an object
459 ...
460 }
461</pre>
462
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]463<p>This block (which may be located in the middle of a function or in a loop
464nest), could be compiled to this LLVM code:</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]465
NAKAMURA Takumi0300d882011-08-12 06:17:17 +0000[diff] [blame]466<pre class="doc_code">
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]467Entry:
468 ;; In the entry block for the function, allocate the
469 ;; stack space for X, which is an LLVM pointer.
470 %X = alloca %Object*
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]471
472 ;; Tell LLVM that the stack space is a stack root.
473 ;; Java has type-tags on objects, so we pass null as metadata.
474 %tmp = bitcast %Object** %X to i8**
Chris Lattner0b9db522008-04-24 06:00:30 +0000[diff] [blame]475 call void @llvm.gcroot(i8** %X, i8* null)
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]476 ...
477
478 ;; "CodeBlock" is the block corresponding to the start
Reid Spencerbbea18c2004-05-25 08:45:31 +0000[diff] [blame]479 ;; of the scope above.
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]480CodeBlock:
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]481 ;; Java null-initializes pointers.
482 store %Object* null, %Object** %X
483
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]484 ...
485
486 ;; As the pointer goes out of scope, store a null value into
487 ;; it, to indicate that the value is no longer live.
488 store %Object* null, %Object** %X
489 ...
490</pre>
491
492</div>
493
494<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]495<h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]496 <a name="barriers">Reading and writing references in the heap</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]497</h3>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]498
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]499<div>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]500
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]501<p>Some collectors need to be informed when the mutator (the program that needs
502garbage collection) either reads a pointer from or writes a pointer to a field
503of a heap object. The code fragments inserted at these points are called
504<em>read barriers</em> and <em>write barriers</em>, respectively. The amount of
505code that needs to be executed is usually quite small and not on the critical
506path of any computation, so the overall performance impact of the barrier is
507tolerable.</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]508
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]509<p>Barriers often require access to the <em>object pointer</em> rather than the
510<em>derived pointer</em> (which is a pointer to the field within the
511object). Accordingly, these intrinsics take both pointers as separate arguments
512for completeness. In this snippet, <tt>%object</tt> is the object pointer, and
513<tt>%derived</tt> is the derived pointer:</p>
514
Chris Lattner851b7712008-04-24 05:59:56 +0000[diff] [blame]515<blockquote><pre>
516 ;; An array type.
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]517 %class.Array = type { %class.Object, i32, [0 x %class.Object*] }
Chris Lattner851b7712008-04-24 05:59:56 +0000[diff] [blame]518 ...
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]519
520 ;; Load the object pointer from a gcroot.
521 %object = load %class.Array** %object_addr
522
523 ;; Compute the derived pointer.
Chris Lattner851b7712008-04-24 05:59:56 +0000[diff] [blame]524 %derived = getelementptr %object, i32 0, i32 2, i32 %n</pre></blockquote>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]525
Gordon Henriksen14248612009-03-06 02:42:47 +0000[diff] [blame]526<p>LLVM does not enforce this relationship between the object and derived
527pointer (although a <a href="#plugin">plugin</a> might). However, it would be
528an unusual collector that violated it.</p>
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]529
Gordon Henriksen14248612009-03-06 02:42:47 +0000[diff] [blame]530<p>The use of these intrinsics is naturally optional if the target GC does
531require the corresponding barrier. Such a GC plugin will replace the intrinsic
532calls with the corresponding <tt>load</tt> or <tt>store</tt> instruction if they
533are used.</p>
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]534
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]535<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]536<h4>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]537 <a name="gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]538</h4>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]539
NAKAMURA Takumi0300d882011-08-12 06:17:17 +0000[diff] [blame]540<div>
541
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]542<div class="doc_code"><tt>
543void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived)
544</tt></div>
545
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]546<p>For write barriers, LLVM provides the <tt>llvm.gcwrite</tt> intrinsic
547function. It has exactly the same semantics as a non-volatile <tt>store</tt> to
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]548the derived pointer (the third argument). The exact code generated is specified
549by a <a href="#plugin">compiler plugin</a>.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]550
551<p>Many important algorithms require write barriers, including generational
552and concurrent collectors. Additionally, write barriers could be used to
553implement reference counting.</p>
554
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]555</div>
556
557<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]558<h4>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]559 <a name="gcread">Read barrier: <tt>llvm.gcread</tt></a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]560</h4>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]561
NAKAMURA Takumi0300d882011-08-12 06:17:17 +0000[diff] [blame]562<div>
563
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]564<div class="doc_code"><tt>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]565i8* @llvm.gcread(i8* %object, i8** %derived)<br>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]566</tt></div>
567
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]568<p>For read barriers, LLVM provides the <tt>llvm.gcread</tt> intrinsic function.
569It has exactly the same semantics as a non-volatile <tt>load</tt> from the
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]570derived pointer (the second argument). The exact code generated is specified by
571a <a href="#plugin">compiler plugin</a>.</p>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]572
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]573<p>Read barriers are needed by fewer algorithms than write barriers, and may
574have a greater performance impact since pointer reads are more frequent than
575writes.</p>
576
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]577</div>
578
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]579</div>
580
581</div>
582
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]583<!-- *********************************************************************** -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]584<h2>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]585 <a name="plugin">Implementing a collector plugin</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]586</h2>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]587<!-- *********************************************************************** -->
588
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]589<div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]590
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]591<p>User code specifies which GC code generation to use with the <tt>gc</tt>
592function attribute or, equivalently, with the <tt>setGC</tt> method of
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]593<tt>Function</tt>.</p>
594
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]595<p>To implement a GC plugin, it is necessary to subclass
596<tt>llvm::GCStrategy</tt>, which can be accomplished in a few lines of
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]597boilerplate code. LLVM's infrastructure provides access to several important
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]598algorithms. For an uncontroversial collector, all that remains may be to
599compile LLVM's computed stack map to assembly code (using the binary
600representation expected by the runtime library). This can be accomplished in
601about 100 lines of code.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]602
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]603<p>This is not the appropriate place to implement a garbage collected heap or a
604garbage collector itself. That code should exist in the language's runtime
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]605library. The compiler plugin is responsible for generating code which
606conforms to the binary interface defined by library, most essentially the
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]607<a href="#stack-map">stack map</a>.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]608
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]609<p>To subclass <tt>llvm::GCStrategy</tt> and register it with the compiler:</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]610
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]611<blockquote><pre>// lib/MyGC/MyGC.cpp - Example LLVM GC plugin
612
613#include "llvm/CodeGen/GCStrategy.h"
614#include "llvm/CodeGen/GCMetadata.h"
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]615#include "llvm/Support/Compiler.h"
616
617using namespace llvm;
618
619namespace {
Duncan Sands6c5e4352010-05-11 20:16:09 +0000[diff] [blame]620 class LLVM_LIBRARY_VISIBILITY MyGC : public GCStrategy {
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]621 public:
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]622 MyGC() {}
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]623 };
624
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]625 GCRegistry::Add&lt;MyGC&gt;
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]626 X("mygc", "My bespoke garbage collector.");
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]627}</pre></blockquote>
628
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]629<p>This boilerplate collector does nothing. More specifically:</p>
630
631<ul>
632 <li><tt>llvm.gcread</tt> calls are replaced with the corresponding
633 <tt>load</tt> instruction.</li>
634 <li><tt>llvm.gcwrite</tt> calls are replaced with the corresponding
635 <tt>store</tt> instruction.</li>
636 <li>No safe points are added to the code.</li>
637 <li>The stack map is not compiled into the executable.</li>
638</ul>
639
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]640<p>Using the LLVM makefiles (like the <a
641href="http://llvm.org/viewvc/llvm-project/llvm/trunk/projects/sample/">sample
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]642project</a>), this code can be compiled as a plugin using a simple
643makefile:</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]644
645<blockquote><pre
646># lib/MyGC/Makefile
647
648LEVEL := ../..
649LIBRARYNAME = <var>MyGC</var>
650LOADABLE_MODULE = 1
651
652include $(LEVEL)/Makefile.common</pre></blockquote>
653
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]654<p>Once the plugin is compiled, code using it may be compiled using <tt>llc
655-load=<var>MyGC.so</var></tt> (though <var>MyGC.so</var> may have some other
656platform-specific extension):</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]657
658<blockquote><pre
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]659>$ cat sample.ll
660define void @f() gc "mygc" {
661entry:
662 ret void
663}
664$ llvm-as &lt; sample.ll | llc -load=MyGC.so</pre></blockquote>
665
666<p>It is also possible to statically link the collector plugin into tools, such
667as a language-specific compiler front-end.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]668
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]669<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]670<h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]671 <a name="collector-algos">Overview of available features</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]672</h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]673
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]674<div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]675
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]676<p><tt>GCStrategy</tt> provides a range of features through which a plugin
677may do useful work. Some of these are callbacks, some are algorithms that can
678be enabled, disabled, or customized. This matrix summarizes the supported (and
679planned) features and correlates them with the collection techniques which
680typically require them.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]681
682<table>
683 <tr>
684 <th>Algorithm</th>
685 <th>Done</th>
686 <th>shadow stack</th>
687 <th>refcount</th>
688 <th>mark-sweep</th>
689 <th>copying</th>
690 <th>incremental</th>
691 <th>threaded</th>
692 <th>concurrent</th>
693 </tr>
694 <tr>
695 <th class="rowhead"><a href="#stack-map">stack map</a></th>
696 <td>&#10004;</td>
697 <td></td>
698 <td></td>
699 <td>&#10008;</td>
700 <td>&#10008;</td>
701 <td>&#10008;</td>
702 <td>&#10008;</td>
703 <td>&#10008;</td>
704 </tr>
705 <tr>
706 <th class="rowhead"><a href="#init-roots">initialize roots</a></th>
707 <td>&#10004;</td>
708 <td>&#10008;</td>
709 <td>&#10008;</td>
710 <td>&#10008;</td>
711 <td>&#10008;</td>
712 <td>&#10008;</td>
713 <td>&#10008;</td>
714 <td>&#10008;</td>
715 </tr>
716 <tr class="doc_warning">
717 <th class="rowhead">derived pointers</th>
718 <td>NO</td>
719 <td></td>
720 <td></td>
721 <td></td>
722 <td></td>
723 <td></td>
724 <td>&#10008;*</td>
725 <td>&#10008;*</td>
726 </tr>
727 <tr>
728 <th class="rowhead"><em><a href="#custom">custom lowering</a></em></th>
729 <td>&#10004;</td>
730 <th></th>
731 <th></th>
732 <th></th>
733 <th></th>
734 <th></th>
735 <th></th>
736 <th></th>
737 </tr>
738 <tr>
739 <th class="rowhead indent">gcroot</th>
740 <td>&#10004;</td>
741 <td>&#10008;</td>
742 <td>&#10008;</td>
743 <td></td>
744 <td></td>
745 <td></td>
746 <td></td>
747 <td></td>
748 </tr>
749 <tr>
750 <th class="rowhead indent">gcwrite</th>
751 <td>&#10004;</td>
752 <td></td>
753 <td>&#10008;</td>
754 <td></td>
755 <td></td>
756 <td>&#10008;</td>
757 <td></td>
758 <td>&#10008;</td>
759 </tr>
760 <tr>
761 <th class="rowhead indent">gcread</th>
762 <td>&#10004;</td>
763 <td></td>
764 <td></td>
765 <td></td>
766 <td></td>
767 <td></td>
768 <td></td>
769 <td>&#10008;</td>
770 </tr>
771 <tr>
772 <th class="rowhead"><em><a href="#safe-points">safe points</a></em></th>
773 <td></td>
774 <th></th>
775 <th></th>
776 <th></th>
777 <th></th>
778 <th></th>
779 <th></th>
780 <th></th>
781 </tr>
782 <tr>
783 <th class="rowhead indent">in calls</th>
784 <td>&#10004;</td>
785 <td></td>
786 <td></td>
787 <td>&#10008;</td>
788 <td>&#10008;</td>
789 <td>&#10008;</td>
790 <td>&#10008;</td>
791 <td>&#10008;</td>
792 </tr>
793 <tr>
794 <th class="rowhead indent">before calls</th>
795 <td>&#10004;</td>
796 <td></td>
797 <td></td>
798 <td></td>
799 <td></td>
800 <td></td>
801 <td>&#10008;</td>
802 <td>&#10008;</td>
803 </tr>
804 <tr class="doc_warning">
805 <th class="rowhead indent">for loops</th>
806 <td>NO</td>
807 <td></td>
808 <td></td>
809 <td></td>
810 <td></td>
811 <td></td>
812 <td>&#10008;</td>
813 <td>&#10008;</td>
814 </tr>
815 <tr>
816 <th class="rowhead indent">before escape</th>
817 <td>&#10004;</td>
818 <td></td>
819 <td></td>
820 <td></td>
821 <td></td>
822 <td></td>
823 <td>&#10008;</td>
824 <td>&#10008;</td>
825 </tr>
826 <tr class="doc_warning">
827 <th class="rowhead">emit code at safe points</th>
828 <td>NO</td>
829 <td></td>
830 <td></td>
831 <td></td>
832 <td></td>
833 <td></td>
834 <td>&#10008;</td>
835 <td>&#10008;</td>
836 </tr>
837 <tr>
838 <th class="rowhead"><em>output</em></th>
839 <td></td>
840 <th></th>
841 <th></th>
842 <th></th>
843 <th></th>
844 <th></th>
845 <th></th>
846 <th></th>
847 </tr>
848 <tr>
849 <th class="rowhead indent"><a href="#assembly">assembly</a></th>
850 <td>&#10004;</td>
851 <td></td>
852 <td></td>
853 <td>&#10008;</td>
854 <td>&#10008;</td>
855 <td>&#10008;</td>
856 <td>&#10008;</td>
857 <td>&#10008;</td>
858 </tr>
859 <tr class="doc_warning">
860 <th class="rowhead indent">JIT</th>
861 <td>NO</td>
862 <td></td>
863 <td></td>
864 <td class="optl">&#10008;</td>
865 <td class="optl">&#10008;</td>
866 <td class="optl">&#10008;</td>
867 <td class="optl">&#10008;</td>
868 <td class="optl">&#10008;</td>
869 </tr>
870 <tr class="doc_warning">
871 <th class="rowhead indent">obj</th>
872 <td>NO</td>
873 <td></td>
874 <td></td>
875 <td class="optl">&#10008;</td>
876 <td class="optl">&#10008;</td>
877 <td class="optl">&#10008;</td>
878 <td class="optl">&#10008;</td>
879 <td class="optl">&#10008;</td>
880 </tr>
881 <tr class="doc_warning">
882 <th class="rowhead">live analysis</th>
883 <td>NO</td>
884 <td></td>
885 <td></td>
886 <td class="optl">&#10008;</td>
887 <td class="optl">&#10008;</td>
888 <td class="optl">&#10008;</td>
889 <td class="optl">&#10008;</td>
890 <td class="optl">&#10008;</td>
891 </tr>
892 <tr class="doc_warning">
893 <th class="rowhead">register map</th>
894 <td>NO</td>
895 <td></td>
896 <td></td>
897 <td class="optl">&#10008;</td>
898 <td class="optl">&#10008;</td>
899 <td class="optl">&#10008;</td>
900 <td class="optl">&#10008;</td>
901 <td class="optl">&#10008;</td>
902 </tr>
903 <tr>
904 <td colspan="10">
905 <div><span class="doc_warning">*</span> Derived pointers only pose a
906 hazard to copying collectors.</div>
907 <div><span class="optl">&#10008;</span> in gray denotes a feature which
908 could be utilized if available.</div>
909 </td>
910 </tr>
911</table>
912
913<p>To be clear, the collection techniques above are defined as:</p>
914
915<dl>
916 <dt>Shadow Stack</dt>
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]917 <dd>The mutator carefully maintains a linked list of stack roots.</dd>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]918 <dt>Reference Counting</dt>
919 <dd>The mutator maintains a reference count for each object and frees an
920 object when its count falls to zero.</dd>
921 <dt>Mark-Sweep</dt>
922 <dd>When the heap is exhausted, the collector marks reachable objects starting
923 from the roots, then deallocates unreachable objects in a sweep
924 phase.</dd>
925 <dt>Copying</dt>
926 <dd>As reachability analysis proceeds, the collector copies objects from one
927 heap area to another, compacting them in the process. Copying collectors
928 enable highly efficient "bump pointer" allocation and can improve locality
929 of reference.</dd>
930 <dt>Incremental</dt>
931 <dd>(Including generational collectors.) Incremental collectors generally have
932 all the properties of a copying collector (regardless of whether the
933 mature heap is compacting), but bring the added complexity of requiring
934 write barriers.</dd>
935 <dt>Threaded</dt>
936 <dd>Denotes a multithreaded mutator; the collector must still stop the mutator
937 ("stop the world") before beginning reachability analysis. Stopping a
938 multithreaded mutator is a complicated problem. It generally requires
939 highly platform specific code in the runtime, and the production of
940 carefully designed machine code at safe points.</dd>
941 <dt>Concurrent</dt>
942 <dd>In this technique, the mutator and the collector run concurrently, with
943 the goal of eliminating pause times. In a <em>cooperative</em> collector,
944 the mutator further aids with collection should a pause occur, allowing
945 collection to take advantage of multiprocessor hosts. The "stop the world"
946 problem of threaded collectors is generally still present to a limited
947 extent. Sophisticated marking algorithms are necessary. Read barriers may
948 be necessary.</dd>
949</dl>
950
951<p>As the matrix indicates, LLVM's garbage collection infrastructure is already
952suitable for a wide variety of collectors, but does not currently extend to
953multithreaded programs. This will be added in the future as there is
954interest.</p>
955
956</div>
957
958<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]959<h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]960 <a name="stack-map">Computing stack maps</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]961</h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]962
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]963<div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]964
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]965<p>LLVM automatically computes a stack map. One of the most important features
966of a <tt>GCStrategy</tt> is to compile this information into the executable in
967the binary representation expected by the runtime library.</p>
968
969<p>The stack map consists of the location and identity of each GC root in the
970each function in the module. For each root:</p>
971
972<ul>
973 <li><tt>RootNum</tt>: The index of the root.</li>
974 <li><tt>StackOffset</tt>: The offset of the object relative to the frame
975 pointer.</li>
976 <li><tt>RootMetadata</tt>: The value passed as the <tt>%metadata</tt>
977 parameter to the <a href="#gcroot"><tt>@llvm.gcroot</tt></a> intrinsic.</li>
978</ul>
979
980<p>Also, for the function as a whole:</p>
981
982<ul>
983 <li><tt>getFrameSize()</tt>: The overall size of the function's initial
984 stack frame, not accounting for any dynamic allocation.</li>
985 <li><tt>roots_size()</tt>: The count of roots in the function.</li>
986</ul>
987
988<p>To access the stack map, use <tt>GCFunctionMetadata::roots_begin()</tt> and
989-<tt>end()</tt> from the <tt><a
990href="#assembly">GCMetadataPrinter</a></tt>:</p>
991
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]992<blockquote><pre
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]993>for (iterator I = begin(), E = end(); I != E; ++I) {
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]994 GCFunctionInfo *FI = *I;
995 unsigned FrameSize = FI-&gt;getFrameSize();
996 size_t RootCount = FI-&gt;roots_size();
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]997
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]998 for (GCFunctionInfo::roots_iterator RI = FI-&gt;roots_begin(),
999 RE = FI-&gt;roots_end();
1000 RI != RE; ++RI) {
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1001 int RootNum = RI->Num;
1002 int RootStackOffset = RI->StackOffset;
1003 Constant *RootMetadata = RI->Metadata;
1004 }
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1005}</pre></blockquote>
1006
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]1007<p>If the <tt>llvm.gcroot</tt> intrinsic is eliminated before code generation by
1008a custom lowering pass, LLVM will compute an empty stack map. This may be useful
1009for collector plugins which implement reference counting or a shadow stack.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1010
1011</div>
1012
1013
1014<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]1015<h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1016 <a name="init-roots">Initializing roots to null: <tt>InitRoots</tt></a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]1017</h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1018
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]1019<div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1020
1021<blockquote><pre
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1022>MyGC::MyGC() {
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1023 InitRoots = true;
1024}</pre></blockquote>
1025
1026<p>When set, LLVM will automatically initialize each root to <tt>null</tt> upon
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1027entry to the function. This prevents the GC's sweep phase from visiting
1028uninitialized pointers, which will almost certainly cause it to crash. This
1029initialization occurs before custom lowering, so the two may be used
1030together.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1031
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1032<p>Since LLVM does not yet compute liveness information, there is no means of
1033distinguishing an uninitialized stack root from an initialized one. Therefore,
1034this feature should be used by all GC plugins. It is enabled by default.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1035
1036</div>
1037
1038
1039<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]1040<h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1041 <a name="custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>,
1042 <tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]1043</h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1044
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]1045<div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1046
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1047<p>For GCs which use barriers or unusual treatment of stack roots, these
1048flags allow the collector to perform arbitrary transformations of the LLVM
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1049IR:</p>
1050
1051<blockquote><pre
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1052>class MyGC : public GCStrategy {
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1053public:
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1054 MyGC() {
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1055 CustomRoots = true;
1056 CustomReadBarriers = true;
1057 CustomWriteBarriers = true;
1058 }
1059
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1060 virtual bool initializeCustomLowering(Module &amp;M);
1061 virtual bool performCustomLowering(Function &amp;F);
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1062};</pre></blockquote>
1063
1064<p>If any of these flags are set, then LLVM suppresses its default lowering for
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1065the corresponding intrinsics and instead calls
1066<tt>performCustomLowering</tt>.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1067
1068<p>LLVM's default action for each intrinsic is as follows:</p>
1069
1070<ul>
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]1071 <li><tt>llvm.gcroot</tt>: Leave it alone. The code generator must see it
1072 or the stack map will not be computed.</li>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1073 <li><tt>llvm.gcread</tt>: Substitute a <tt>load</tt> instruction.</li>
1074 <li><tt>llvm.gcwrite</tt>: Substitute a <tt>store</tt> instruction.</li>
1075</ul>
1076
1077<p>If <tt>CustomReadBarriers</tt> or <tt>CustomWriteBarriers</tt> are specified,
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1078then <tt>performCustomLowering</tt> <strong>must</strong> eliminate the
1079corresponding barriers.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1080
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1081<p><tt>performCustomLowering</tt> must comply with the same restrictions as <a
1082href="WritingAnLLVMPass.html#runOnFunction"><tt
1083>FunctionPass::runOnFunction</tt></a>.
1084Likewise, <tt>initializeCustomLowering</tt> has the same semantics as <a
1085href="WritingAnLLVMPass.html#doInitialization_mod"><tt
1086>Pass::doInitialization(Module&amp;)</tt></a>.</p>
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1087
1088<p>The following can be used as a template:</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1089
1090<blockquote><pre
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1091>#include "llvm/Module.h"
Gordon Henriksen8790a222007-12-22 23:32:32 +0000[diff] [blame]1092#include "llvm/IntrinsicInst.h"
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1093
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1094bool MyGC::initializeCustomLowering(Module &amp;M) {
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1095 return false;
1096}
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1097
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1098bool MyGC::performCustomLowering(Function &amp;F) {
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1099 bool MadeChange = false;
1100
1101 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
Gordon Henriksene09a4f82007-12-22 23:34:26 +0000[diff] [blame]1102 for (BasicBlock::iterator II = BB-&gt;begin(), E = BB-&gt;end(); II != E; )
1103 if (IntrinsicInst *CI = dyn_cast&lt;IntrinsicInst&gt;(II++))
Gordon Henriksen8790a222007-12-22 23:32:32 +0000[diff] [blame]1104 if (Function *F = CI-&gt;getCalledFunction())
1105 switch (F-&gt;getIntrinsicID()) {
1106 case Intrinsic::gcwrite:
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1107 // Handle llvm.gcwrite.
Gordon Henriksen8790a222007-12-22 23:32:32 +0000[diff] [blame]1108 CI-&gt;eraseFromParent();
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1109 MadeChange = true;
Gordon Henriksen8790a222007-12-22 23:32:32 +0000[diff] [blame]1110 break;
1111 case Intrinsic::gcread:
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1112 // Handle llvm.gcread.
Gordon Henriksen8790a222007-12-22 23:32:32 +0000[diff] [blame]1113 CI-&gt;eraseFromParent();
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1114 MadeChange = true;
Gordon Henriksen8790a222007-12-22 23:32:32 +0000[diff] [blame]1115 break;
1116 case Intrinsic::gcroot:
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1117 // Handle llvm.gcroot.
Gordon Henriksen8790a222007-12-22 23:32:32 +0000[diff] [blame]1118 CI-&gt;eraseFromParent();
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1119 MadeChange = true;
Gordon Henriksen8790a222007-12-22 23:32:32 +0000[diff] [blame]1120 break;
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1121 }
1122
1123 return MadeChange;
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1124}</pre></blockquote>
1125
1126</div>
1127
1128
1129<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]1130<h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1131 <a name="safe-points">Generating safe points: <tt>NeededSafePoints</tt></a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]1132</h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1133
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]1134<div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1135
1136<p>LLVM can compute four kinds of safe points:</p>
1137
1138<blockquote><pre
1139>namespace GC {
1140 /// PointKind - The type of a collector-safe point.
1141 ///
1142 enum PointKind {
1143 Loop, //&lt; Instr is a loop (backwards branch).
1144 Return, //&lt; Instr is a return instruction.
1145 PreCall, //&lt; Instr is a call instruction.
1146 PostCall //&lt; Instr is the return address of a call.
1147 };
1148}</pre></blockquote>
1149
1150<p>A collector can request any combination of the four by setting the
1151<tt>NeededSafePoints</tt> mask:</p>
1152
1153<blockquote><pre
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1154>MyGC::MyGC() {
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1155 NeededSafePoints = 1 &lt;&lt; GC::Loop
1156 | 1 &lt;&lt; GC::Return
1157 | 1 &lt;&lt; GC::PreCall
1158 | 1 &lt;&lt; GC::PostCall;
1159}</pre></blockquote>
1160
1161<p>It can then use the following routines to access safe points.</p>
1162
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1163<blockquote><pre
1164>for (iterator I = begin(), E = end(); I != E; ++I) {
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1165 GCFunctionInfo *MD = *I;
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1166 size_t PointCount = MD-&gt;size();
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1167
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1168 for (GCFunctionInfo::iterator PI = MD-&gt;begin(),
1169 PE = MD-&gt;end(); PI != PE; ++PI) {
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1170 GC::PointKind PointKind = PI-&gt;Kind;
1171 unsigned PointNum = PI-&gt;Num;
1172 }
1173}
1174</pre></blockquote>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1175
1176<p>Almost every collector requires <tt>PostCall</tt> safe points, since these
1177correspond to the moments when the function is suspended during a call to a
1178subroutine.</p>
1179
1180<p>Threaded programs generally require <tt>Loop</tt> safe points to guarantee
1181that the application will reach a safe point within a bounded amount of time,
1182even if it is executing a long-running loop which contains no function
1183calls.</p>
1184
1185<p>Threaded collectors may also require <tt>Return</tt> and <tt>PreCall</tt>
1186safe points to implement "stop the world" techniques using self-modifying code,
1187where it is important that the program not exit the function without reaching a
1188safe point (because only the topmost function has been patched).</p>
1189
1190</div>
1191
1192
1193<!-- ======================================================================= -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]1194<h3>
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1195 <a name="assembly">Emitting assembly code: <tt>GCMetadataPrinter</tt></a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]1196</h3>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1197
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]1198<div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1199
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]1200<p>LLVM allows a plugin to print arbitrary assembly code before and after the
1201rest of a module's assembly code. At the end of the module, the GC can compile
1202the LLVM stack map into assembly code. (At the beginning, this information is not
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1203yet computed.)</p>
1204
1205<p>Since AsmWriter and CodeGen are separate components of LLVM, a separate
1206abstract base class and registry is provided for printing assembly code, the
Gordon Henriksen29a25eb2009-03-02 03:47:20 +0000[diff] [blame]1207<tt>GCMetadaPrinter</tt> and <tt>GCMetadataPrinterRegistry</tt>. The AsmWriter
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1208will look for such a subclass if the <tt>GCStrategy</tt> sets
1209<tt>UsesMetadata</tt>:</p>
1210
1211<blockquote><pre
1212>MyGC::MyGC() {
1213 UsesMetadata = true;
1214}</pre></blockquote>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1215
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]1216<p>This separation allows JIT-only clients to be smaller.</p>
1217
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1218<p>Note that LLVM does not currently have analogous APIs to support code
1219generation in the JIT, nor using the object writers.</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1220
1221<blockquote><pre
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1222>// lib/MyGC/MyGCPrinter.cpp - Example LLVM GC printer
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1223
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1224#include "llvm/CodeGen/GCMetadataPrinter.h"
1225#include "llvm/Support/Compiler.h"
1226
1227using namespace llvm;
1228
1229namespace {
Duncan Sands6c5e4352010-05-11 20:16:09 +0000[diff] [blame]1230 class LLVM_LIBRARY_VISIBILITY MyGCPrinter : public GCMetadataPrinter {
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1231 public:
1232 virtual void beginAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
1233 const TargetAsmInfo &amp;TAI);
1234
1235 virtual void finishAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
1236 const TargetAsmInfo &amp;TAI);
1237 };
1238
1239 GCMetadataPrinterRegistry::Add&lt;MyGCPrinter&gt;
1240 X("mygc", "My bespoke garbage collector.");
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1241}</pre></blockquote>
1242
1243<p>The collector should use <tt>AsmPrinter</tt> and <tt>TargetAsmInfo</tt> to
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1244print portable assembly code to the <tt>std::ostream</tt>. The collector itself
1245contains the stack map for the entire module, and may access the
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1246<tt>GCFunctionInfo</tt> using its own <tt>begin()</tt> and <tt>end()</tt>
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1247methods. Here's a realistic example:</p>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1248
1249<blockquote><pre
1250>#include "llvm/CodeGen/AsmPrinter.h"
1251#include "llvm/Function.h"
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1252#include "llvm/Target/TargetMachine.h"
1253#include "llvm/Target/TargetData.h"
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1254#include "llvm/Target/TargetAsmInfo.h"
1255
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1256void MyGCPrinter::beginAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1257 const TargetAsmInfo &amp;TAI) {
1258 // Nothing to do.
1259}
1260
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1261void MyGCPrinter::finishAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1262 const TargetAsmInfo &amp;TAI) {
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1263 // Set up for emitting addresses.
1264 const char *AddressDirective;
1265 int AddressAlignLog;
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1266 if (AP.TM.getTargetData()->getPointerSize() == sizeof(int32_t)) {
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1267 AddressDirective = TAI.getData32bitsDirective();
1268 AddressAlignLog = 2;
1269 } else {
1270 AddressDirective = TAI.getData64bitsDirective();
1271 AddressAlignLog = 3;
1272 }
1273
1274 // Put this in the data section.
1275 AP.SwitchToDataSection(TAI.getDataSection());
1276
1277 // For each function...
Gordon Henriksen7843c162007-12-11 00:30:17 +0000[diff] [blame]1278 for (iterator FI = begin(), FE = end(); FI != FE; ++FI) {
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1279 GCFunctionInfo &amp;MD = **FI;
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1280
1281 // Emit this data structure:
1282 //
1283 // struct {
1284 // int32_t PointCount;
1285 // struct {
1286 // void *SafePointAddress;
1287 // int32_t LiveCount;
1288 // int32_t LiveOffsets[LiveCount];
1289 // } Points[PointCount];
1290 // } __gcmap_&lt;FUNCTIONNAME&gt;;
1291
1292 // Align to address width.
1293 AP.EmitAlignment(AddressAlignLog);
1294
Gordon Henriksen3519d5d2009-03-06 01:57:32 +0000[diff] [blame]1295 // Emit the symbol by which the stack map entry can be found.
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1296 std::string Symbol;
1297 Symbol += TAI.getGlobalPrefix();
1298 Symbol += "__gcmap_";
1299 Symbol += MD.getFunction().getName();
1300 if (const char *GlobalDirective = TAI.getGlobalDirective())
1301 OS &lt;&lt; GlobalDirective &lt;&lt; Symbol &lt;&lt; "\n";
1302 OS &lt;&lt; TAI.getGlobalPrefix() &lt;&lt; Symbol &lt;&lt; ":\n";
1303
1304 // Emit PointCount.
1305 AP.EmitInt32(MD.size());
1306 AP.EOL("safe point count");
1307
1308 // And each safe point...
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1309 for (GCFunctionInfo::iterator PI = MD.begin(),
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1310 PE = MD.end(); PI != PE; ++PI) {
1311 // Align to address width.
1312 AP.EmitAlignment(AddressAlignLog);
1313
1314 // Emit the address of the safe point.
1315 OS &lt;&lt; AddressDirective
1316 &lt;&lt; TAI.getPrivateGlobalPrefix() &lt;&lt; "label" &lt;&lt; PI-&gt;Num;
1317 AP.EOL("safe point address");
1318
1319 // Emit the stack frame size.
1320 AP.EmitInt32(MD.getFrameSize());
1321 AP.EOL("stack frame size");
1322
1323 // Emit the number of live roots in the function.
1324 AP.EmitInt32(MD.live_size(PI));
1325 AP.EOL("live root count");
1326
1327 // And for each live root...
Gordon Henriksen75609c72008-08-24 03:18:23 +0000[diff] [blame]1328 for (GCFunctionInfo::live_iterator LI = MD.live_begin(PI),
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1329 LE = MD.live_end(PI);
1330 LI != LE; ++LI) {
1331 // Print its offset within the stack frame.
1332 AP.EmitInt32(LI-&gt;StackOffset);
1333 AP.EOL("stack offset");
1334 }
1335 }
1336 }
1337}
1338</pre></blockquote>
1339
1340</div>
1341
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]1342</div>
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1343
1344<!-- *********************************************************************** -->
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]1345<h2>
Chris Lattner71679332004-05-27 05:52:10 +0000[diff] [blame]1346 <a name="references">References</a>
NAKAMURA Takumifc8d9302011-04-18 23:59:50 +0000[diff] [blame]1347</h2>
Chris Lattner71679332004-05-27 05:52:10 +0000[diff] [blame]1348<!-- *********************************************************************** -->
1349
NAKAMURA Takumiaa3d6242011-04-23 00:30:22 +0000[diff] [blame]1350<div>
Chris Lattner71679332004-05-27 05:52:10 +0000[diff] [blame]1351
1352<p><a name="appel89">[Appel89]</a> Runtime Tags Aren't Necessary. Andrew
1353W. Appel. Lisp and Symbolic Computation 19(7):703-705, July 1989.</p>
1354
1355<p><a name="goldberg91">[Goldberg91]</a> Tag-free garbage collection for
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1356strongly typed programming languages. Benjamin Goldberg. ACM SIGPLAN
Chris Lattner71679332004-05-27 05:52:10 +0000[diff] [blame]1357PLDI'91.</p>
1358
1359<p><a name="tolmach94">[Tolmach94]</a> Tag-free garbage collection using
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1360explicit type parameters. Andrew Tolmach. Proceedings of the 1994 ACM
Chris Lattner71679332004-05-27 05:52:10 +0000[diff] [blame]1361conference on LISP and functional programming.</p>
1362
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1363<p><a name="henderson02">[Henderson2002]</a> <a
1364href="http://citeseer.ist.psu.edu/henderson02accurate.html">
1365Accurate Garbage Collection in an Uncooperative Environment</a>.
1366Fergus Henderson. International Symposium on Memory Management 2002.</p>
1367
Chris Lattner71679332004-05-27 05:52:10 +0000[diff] [blame]1368</div>
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]1369
Gordon Henriksen0076a672007-09-27 19:31:36 +0000[diff] [blame]1370
Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]1371<!-- *********************************************************************** -->
1372
1373<hr>
1374<address>
1375 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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Chris Lattnerf689d572004-05-23 21:02:20 +0000[diff] [blame]1379
1380 <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
NAKAMURA Takumica46f5a2011-04-09 02:13:37 +0000[diff] [blame]1381 <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
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