docs/GarbageCollection.html

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  16 <div class="doc_title">
  17   Accurate Garbage Collection with LLVM
  18 </div>
  19
  20 <ol>
  21   <li><a href="#introduction">Introduction</a>
  22     <ul>
  23     <li><a href="#feature">Goals and non-goals</a></li>
  24     </ul>
  25   </li>
  26
  27   <li><a href="#quickstart">Getting started</a>
  28     <ul>
  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>
  32     </ul>
  33   </li>
  34
  35   <li><a href="#core">Core support</a>
  36     <ul>
  37     <li><a href="#gcattr">Specifying GC code generation:
  38       <tt>gc "..."</tt></a></li>
  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
  50   <li><a href="#plugin">Compiler plugin interface</a>
  51     <ul>
  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:
  61       <tt>GCMetadataPrinter</tt></a></li>
  62     </ul>
  63   </li>
  64
  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
  74 </ol>
  75
  76 <div class="doc_author">
  77   <p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a> and
  78      Gordon Henriksen</p>
  79 </div>
  80
  81 <!-- *********************************************************************** -->
  82 <div class="doc_section">
  83   <a name="introduction">Introduction</a>
  84 </div>
  85 <!-- *********************************************************************** -->
  86
  87 <div class="doc_text">
  88
  89 <p>Garbage collection is a widely used technique that frees the programmer from
  90 having to know the lifetimes of heap objects, making software easier to produce
  91 and maintain. Many programming languages rely on garbage collection for
  92 automatic memory management. There are two primary forms of garbage collection:
  93 conservative and accurate.</p>
  94
  95 <p>Conservative garbage collection often does not require any special support
  96 from either the language or the compiler: it can handle non-type-safe
  97 programming languages (such as C/C++) and does not require any special
  98 information from the compiler. The
  99 <a href="http://www.hpl.hp.com/personal/Hans_Boehm/gc/">Boehm collector</a> is
 100 an example of a state-of-the-art conservative collector.</p>
 101
 102 <p>Accurate garbage collection requires the ability to identify all pointers in
 103 the program at run-time (which requires that the source-language be type-safe in
 104 most cases). Identifying pointers at run-time requires compiler support to
 105 locate all places that hold live pointer variables at run-time, including the
 106 <a href="#gcroot">processor stack and registers</a>.</p>
 107
 108 <p>Conservative garbage collection is attractive because it does not require any
 109 special compiler support, but it does have problems. In particular, because the
 110 conservative garbage collector cannot <i>know</i> that a particular word in the
 111 machine is a pointer, it cannot move live objects in the heap (preventing the
 112 use of compacting and generational GC algorithms) and it can occasionally suffer
 113 from memory leaks due to integer values that happen to point to objects in the
 114 program. In addition, some aggressive compiler transformations can break
 115 conservative garbage collectors (though these seem rare in practice).</p>
 116
 117 <p>Accurate garbage collectors do not suffer from any of these problems, but
 118 they can suffer from degraded scalar optimization of the program. In particular,
 119 because the runtime must be able to identify and update all pointers active in
 120 the program, some optimizations are less effective. In practice, however, the
 121 locality and performance benefits of using aggressive garbage collection
 122 techniques dominates any low-level losses.</p>
 123
 124 <p>This document describes the mechanisms and interfaces provided by LLVM to
 125 support accurate garbage collection.</p>
 126
 127 </div>
 128
 129 <!-- ======================================================================= -->
 130 <div class="doc_subsection">
 131   <a name="feature">Goals and non-goals</a>
 132 </div>
 133
 134 <div class="doc_text">
 135
 136 <p>LLVM's intermediate representation provides <a href="#intrinsics">garbage
 137 collection intrinsics</a> that offer support for a broad class of
 138 collector models. For instance, the intrinsics permit:</p>
 139
 140 <ul>
 141   <li>semi-space collectors</li>
 142   <li>mark-sweep collectors</li>
 143   <li>generational collectors</li>
 144   <li>reference counting</li>
 145   <li>incremental collectors</li>
 146   <li>concurrent collectors</li>
 147   <li>cooperative collectors</li>
 148 </ul>
 149
 150 <p>We hope that the primitive support built into the LLVM IR is sufficient to
 151 support a broad class of garbage collected languages including Scheme, ML, Java,
 152 C#, Perl, Python, Lua, Ruby, other scripting languages, and more.</p>
 153
 154 <p>However, LLVM does not itself provide a garbage collector&#151;this should
 155 be part of your language's runtime library. LLVM provides a framework for
 156 compile time <a href="#plugin">code generation plugins</a>. The role of these
 157 plugins is to generate code and data structures which conforms to the <em>binary
 158 interface</em> specified by the <em>runtime library</em>. This is similar to the
 159 relationship between LLVM and DWARF debugging info, for example. The
 160 difference primarily lies in the lack of an established standard in the domain
 161 of garbage collection&#151;thus the plugins.</p>
 162
 163 <p>The aspects of the binary interface with which LLVM's GC support is
 164 concerned are:</p>
 165
 166 <ul>
 167   <li>Creation of GC-safe points within code where collection is allowed to
 168       execute safely.</li>
 169   <li>Computation of the stack map. For each safe point in the code, object
 170       references within the stack frame must be identified so that the
 171       collector may traverse and perhaps update them.</li>
 172   <li>Write barriers when storing object references to the heap. These are
 173       commonly used to optimize incremental scans in generational
 174       collectors.</li>
 175   <li>Emission of read barriers when loading object references. These are
 176       useful for interoperating with concurrent collectors.</li>
 177 </ul>
 178
 179 <p>There are additional areas that LLVM does not directly address:</p>
 180
 181 <ul>
 182   <li>Registration of global roots with the runtime.</li>
 183   <li>Registration of stack map entries with the runtime.</li>
 184   <li>The functions used by the program to allocate memory, trigger a
 185       collection, etc.</li>
 186   <li>Computation or compilation of type maps, or registration of them with
 187       the runtime. These are used to crawl the heap for object
 188       references.</li>
 189 </ul>
 190
 191 <p>In general, LLVM's support for GC does not include features which can be
 192 adequately addressed with other features of the IR and does not specify a
 193 particular binary interface. On the plus side, this means that you should be
 194 able to integrate LLVM with an existing runtime. On the other hand, it leaves
 195 a lot of work for the developer of a novel language. However, it's easy to get
 196 started quickly and scale up to a more sophisticated implementation as your
 197 compiler matures.</p>
 198
 199 </div>
 200
 201 <!-- *********************************************************************** -->
 202 <div class="doc_section">
 203   <a name="quickstart">Getting started</a>
 204 </div>
 205 <!-- *********************************************************************** -->
 206
 207 <div class="doc_text">
 208
 209 <p>Using a GC with LLVM implies many things, for example:</p>
 210
 211 <ul>
 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>
 215     <li>Design a binary interface for the stack map, used to identify
 216         references within a stack frame on the machine stack.*</li>
 217     <li>Implement a stack crawler to discover functions on the call stack.*</li>
 218     <li>Implement a registry for global roots.</li>
 219     <li>Design a binary interface for type maps, used to identify references
 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>
 232     <li>Generate type maps according to your runtime's binary interface.</li>
 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>
 237     <li>Compile LLVM's stack map to the binary form expected by the
 238         runtime.</li>
 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>
 243 </ul>
 244
 245 <p>To help with several of these tasks (those indicated with a *), LLVM
 246 includes a highly portable, built-in ShadowStack code generator. It is compiled
 247 into <tt>llc</tt> and works even with the interpreter and C backends.</p>
 248
 249 </div>
 250
 251 <!-- ======================================================================= -->
 252 <div class="doc_subsection">
 253   <a name="quickstart-compiler">In your compiler</a>
 254 </div>
 255
 256 <div class="doc_text">
 257
 258 <p>To turn the shadow stack on for your functions, first call:</p>
 259
 260 <div class="doc_code"><pre
 261 >F.setGC("shadow-stack");</pre></div>
 262
 263 <p>for each function your compiler emits. Since the shadow stack is built into
 264 LLVM, you do not need to load a plugin.</p>
 265
 266 <p>Your compiler must also use <tt>@llvm.gcroot</tt> as documented.
 267 Don't forget to create a root for each intermediate value that is generated
 268 when evaluating an expression. In <tt>h(f(), g())</tt>, the result of
 269 <tt>f()</tt> could easily be collected if evaluating <tt>g()</tt> triggers a
 270 collection.</p>
 271
 272 <p>There's no need to use <tt>@llvm.gcread</tt> and <tt>@llvm.gcwrite</tt> over
 273 plain <tt>load</tt> and <tt>store</tt> for now. You will need them when
 274 switching to a more advanced GC.</p>
 275
 276 </div>
 277
 278 <!-- ======================================================================= -->
 279 <div class="doc_subsection">
 280   <a name="quickstart-runtime">In your runtime</a>
 281 </div>
 282
 283 <div class="doc_text">
 284
 285 <p>The shadow stack doesn't imply a memory allocation algorithm. A semispace
 286 collector or building atop <tt>malloc</tt> are great places to start, and can
 287 be implemented with very little code.</p>
 288
 289 <p>When it comes time to collect, however, your runtime needs to traverse the
 290 stack roots, and for this it needs to integrate with the shadow stack. Luckily,
 291 doing so is very simple. (This code is heavily commented to help you
 292 understand the data structure, but there are only 20 lines of meaningful
 293 code.)</p>
 294
 295 </div>
 296
 297 <div class="doc_code"><pre
 298 >/// @brief The map for a single function's stack frame. One of these is
 299 ///        compiled as constant data into the executable for each function.
 300 ///
 301 /// Storage of metadata values is elided if the %metadata parameter to
 302 /// @llvm.gcroot is null.
 303 struct FrameMap {
 304   int32_t NumRoots;    //&lt; Number of roots in stack frame.
 305   int32_t NumMeta;     //&lt; Number of metadata entries. May be &lt; NumRoots.
 306   const void *Meta[0]; //&lt; Metadata for each root.
 307 };
 308
 309 /// @brief A link in the dynamic shadow stack. One of these is embedded in the
 310 ///        stack frame of each function on the call stack.
 311 struct StackEntry {
 312   StackEntry *Next;    //&lt; Link to next stack entry (the caller's).
 313   const FrameMap *Map; //&lt; Pointer to constant FrameMap.
 314   void *Roots[0];      //&lt; Stack roots (in-place array).
 315 };
 316
 317 /// @brief The head of the singly-linked list of StackEntries. Functions push
 318 ///        and pop onto this in their prologue and epilogue.
 319 ///
 320 /// Since there is only a global list, this technique is not threadsafe.
 321 StackEntry *llvm_gc_root_chain;
 322
 323 /// @brief Calls Visitor(root, meta) for each GC root on the stack.
 324 ///        root and meta are exactly the values passed to
 325 ///        <tt>@llvm.gcroot</tt>.
 326 ///
 327 /// Visitor could be a function to recursively mark live objects. Or it
 328 /// might copy them to another heap or generation.
 329 ///
 330 /// @param Visitor A function to invoke for every GC root on the stack.
 331 void visitGCRoots(void (*Visitor)(void **Root, const void *Meta)) {
 332   for (StackEntry *R = llvm_gc_root_chain; R; R = R->Next) {
 333     unsigned i = 0;
 334
 335     // For roots [0, NumMeta), the metadata pointer is in the FrameMap.
 336     for (unsigned e = R->Map->NumMeta; i != e; ++i)
 337       Visitor(&amp;R->Roots[i], R->Map->Meta[i]);
 338
 339     // For roots [NumMeta, NumRoots), the metadata pointer is null.
 340     for (unsigned e = R->Map->NumRoots; i != e; ++i)
 341       Visitor(&amp;R->Roots[i], NULL);
 342   }
 343 }</pre></div>
 344
 345 <!-- ======================================================================= -->
 346 <div class="doc_subsection">
 347   <a name="shadow-stack">About the shadow stack</a>
 348 </div>
 349
 350 <div class="doc_text">
 351
 352 <p>Unlike many GC algorithms which rely on a cooperative code generator to
 353 compile stack maps, this algorithm carefully maintains a linked list of stack
 354 roots [<a href="#henderson02">Henderson2002</a>]. This so-called "shadow stack"
 355 mirrors the machine stack. Maintaining this data structure is slower than using
 356 a stack map compiled into the executable as constant data, but has a significant
 357 portability advantage because it requires no special support from the target
 358 code generator, and does not require tricky platform-specific code to crawl
 359 the machine stack.</p>
 360
 361 <p>The tradeoff for this simplicity and portability is:</p>
 362
 363 <ul>
 364   <li>High overhead per function call.</li>
 365   <li>Not thread-safe.</li>
 366 </ul>
 367
 368 <p>Still, it's an easy way to get started. After your compiler and runtime are
 369 up and running, writing a <a href="#plugin">plugin</a> will allow you to take
 370 advantage of <a href="#collector-algos">more advanced GC features</a> of LLVM
 371 in order to improve performance.</p>
 372
 373 </div>
 374
 375 <!-- *********************************************************************** -->
 376 <div class="doc_section">
 377   <a name="core">IR features</a><a name="intrinsics"></a>
 378 </div>
 379 <!-- *********************************************************************** -->
 380
 381 <div class="doc_text">
 382
 383 <p>This section describes the garbage collection facilities provided by the
 384 <a href="LangRef.html">LLVM intermediate representation</a>. The exact behavior
 385 of 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>
 387
 388 <p>These facilities are limited to those strictly necessary; they are not
 389 intended to be a complete interface to any garbage collector. A program will
 390 need to interface with the GC library using the facilities provided by that
 391 program.</p>
 392
 393 </div>
 394
 395 <!-- ======================================================================= -->
 396 <div class="doc_subsection">
 397   <a name="gcattr">Specifying GC code generation: <tt>gc "..."</tt></a>
 398 </div>
 399
 400 <div class="doc_code"><tt>
 401   define <i>ty</i> @<i>name</i>(...) <span style="text-decoration: underline">gc "<i>name</i>"</span> { ...
 402 </tt></div>
 403
 404 <div class="doc_text">
 405
 406 <p>The <tt>gc</tt> function attribute is used to specify the desired GC style
 407 to the compiler. Its programmatic equivalent is the <tt>setGC</tt> method of
 408 <tt>Function</tt>.</p>
 409
 410 <p>Setting <tt>gc "<i>name</i>"</tt> on a function triggers a search for a
 411 matching code generation plugin "<i>name</i>"; it is that plugin which defines
 412 the exact nature of the code generated to support GC. If none is found, the
 413 compiler will raise an error.</p>
 414
 415 <p>Specifying the GC style on a per-function basis allows LLVM to link together
 416 programs that use different garbage collection algorithms (or none at all).</p>
 417
 418 </div>
 419
 420 <!-- ======================================================================= -->
 421 <div class="doc_subsection">
 422   <a name="gcroot">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
 423 </div>
 424
 425 <div class="doc_code"><tt>
 426   void @llvm.gcroot(i8** %ptrloc, i8* %metadata)
 427 </tt></div>
 428
 429 <div class="doc_text">
 430
 431 <p>The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM that a stack
 432 variable references an object on the heap and is to be tracked for garbage
 433 collection. The exact impact on generated code is specified by a <a
 434 href="#plugin">compiler plugin</a>.</p>
 435
 436 <p>A compiler which uses mem2reg to raise imperative code using <tt>alloca</tt>
 437 into SSA form need only add a call to <tt>@llvm.gcroot</tt> for those variables
 438 which a pointers into the GC heap.</p>
 439
 440 <p>It is also important to mark intermediate values with <tt>llvm.gcroot</tt>.
 441 For example, consider <tt>h(f(), g())</tt>. Beware leaking the result of
 442 <tt>f()</tt> in the case that <tt>g()</tt> triggers a collection.</p>
 443
 444 <p>The first argument <b>must</b> be a value referring to an alloca instruction
 445 or a bitcast of an alloca. The second contains a pointer to metadata that
 446 should be associated with the pointer, and <b>must</b> be a constant or global
 447 value address. If your target collector uses tags, use a null pointer for
 448 metadata.</p>
 449
 450 <p>The <tt>%metadata</tt> argument can be used to avoid requiring heap objects
 451 to have 'isa' pointers or tag bits. [<a href="#appel89">Appel89</a>, <a
 452 href="#goldberg91">Goldberg91</a>, <a href="#tolmach94">Tolmach94</a>] If
 453 specified, its value will be tracked along with the location of the pointer in
 454 the stack frame.</p>
 455
 456 <p>Consider the following fragment of Java code:</p>
 457
 458 <pre>
 459        {
 460          Object X;   // A null-initialized reference to an object
 461          ...
 462        }
 463 </pre>
 464
 465 <p>This block (which may be located in the middle of a function or in a loop
 466 nest), could be compiled to this LLVM code:</p>
 467
 468 <pre>
 469 Entry:
 470    ;; In the entry block for the function, allocate the
 471    ;; stack space for X, which is an LLVM pointer.
 472    %X = alloca %Object*
 473
 474    ;; Tell LLVM that the stack space is a stack root.
 475    ;; Java has type-tags on objects, so we pass null as metadata.
 476    %tmp = bitcast %Object** %X to i8**
 477    call void @llvm.gcroot(i8** %X, i8* null)
 478    ...
 479
 480    ;; "CodeBlock" is the block corresponding to the start
 481    ;;  of the scope above.
 482 CodeBlock:
 483    ;; Java null-initializes pointers.
 484    store %Object* null, %Object** %X
 485
 486    ...
 487
 488    ;; As the pointer goes out of scope, store a null value into
 489    ;; it, to indicate that the value is no longer live.
 490    store %Object* null, %Object** %X
 491    ...
 492 </pre>
 493
 494 </div>
 495
 496 <!-- ======================================================================= -->
 497 <div class="doc_subsection">
 498   <a name="barriers">Reading and writing references in the heap</a>
 499 </div>
 500
 501 <div class="doc_text">
 502
 503 <p>Some collectors need to be informed when the mutator (the program that needs
 504 garbage collection) either reads a pointer from or writes a pointer to a field
 505 of a heap object. The code fragments inserted at these points are called
 506 <em>read barriers</em> and <em>write barriers</em>, respectively. The amount of
 507 code that needs to be executed is usually quite small and not on the critical
 508 path of any computation, so the overall performance impact of the barrier is
 509 tolerable.</p>
 510
 511 <p>Barriers often require access to the <em>object pointer</em> rather than the
 512 <em>derived pointer</em> (which is a pointer to the field within the
 513 object). Accordingly, these intrinsics take both pointers as separate arguments
 514 for completeness. In this snippet, <tt>%object</tt> is the object pointer, and
 515 <tt>%derived</tt> is the derived pointer:</p>
 516
 517 <blockquote><pre>
 518     ;; An array type.
 519     %class.Array = type { %class.Object, i32, [0 x %class.Object*] }
 520     ...
 521
 522     ;; Load the object pointer from a gcroot.
 523     %object = load %class.Array** %object_addr
 524
 525     ;; Compute the derived pointer.
 526     %derived = getelementptr %object, i32 0, i32 2, i32 %n</pre></blockquote>
 527
 528 <p>LLVM does not enforce this relationship between the object and derived
 529 pointer (although a <a href="#plugin">plugin</a> might). However, it would be
 530 an unusual collector that violated it.</p>
 531
 532 <p>The use of these intrinsics is naturally optional if the target GC does
 533 require the corresponding barrier. Such a GC plugin will replace the intrinsic
 534 calls with the corresponding <tt>load</tt> or <tt>store</tt> instruction if they
 535 are used.</p>
 536
 537 </div>
 538
 539 <!-- ======================================================================= -->
 540 <div class="doc_subsubsection">
 541   <a name="gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a>
 542 </div>
 543
 544 <div class="doc_code"><tt>
 545 void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived)
 546 </tt></div>
 547
 548 <div class="doc_text">
 549
 550 <p>For write barriers, LLVM provides the <tt>llvm.gcwrite</tt> intrinsic
 551 function. It has exactly the same semantics as a non-volatile <tt>store</tt> to
 552 the derived pointer (the third argument). The exact code generated is specified
 553 by a <a href="#plugin">compiler plugin</a>.</p>
 554
 555 <p>Many important algorithms require write barriers, including generational
 556 and concurrent collectors. Additionally, write barriers could be used to
 557 implement reference counting.</p>
 558
 559 </div>
 560
 561 <!-- ======================================================================= -->
 562 <div class="doc_subsubsection">
 563   <a name="gcread">Read barrier: <tt>llvm.gcread</tt></a>
 564 </div>
 565
 566 <div class="doc_code"><tt>
 567 i8* @llvm.gcread(i8* %object, i8** %derived)<br>
 568 </tt></div>
 569
 570 <div class="doc_text">
 571
 572 <p>For read barriers, LLVM provides the <tt>llvm.gcread</tt> intrinsic function.
 573 It has exactly the same semantics as a non-volatile <tt>load</tt> from the
 574 derived pointer (the second argument). The exact code generated is specified by
 575 a <a href="#plugin">compiler plugin</a>.</p>
 576
 577 <p>Read barriers are needed by fewer algorithms than write barriers, and may
 578 have a greater performance impact since pointer reads are more frequent than
 579 writes.</p>
 580
 581 </div>
 582
 583 <!-- *********************************************************************** -->
 584 <div class="doc_section">
 585   <a name="plugin">Implementing a collector plugin</a>
 586 </div>
 587 <!-- *********************************************************************** -->
 588
 589 <div class="doc_text">
 590
 591 <p>User code specifies which GC code generation to use with the <tt>gc</tt>
 592 function attribute or, equivalently, with the <tt>setGC</tt> method of
 593 <tt>Function</tt>.</p>
 594
 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
 597 boilerplate code. LLVM's infrastructure provides access to several important
 598 algorithms. For an uncontroversial collector, all that remains may be to
 599 compile LLVM's computed stack map to assembly code (using the binary
 600 representation expected by the runtime library). This can be accomplished in
 601 about 100 lines of code.</p>
 602
 603 <p>This is not the appropriate place to implement a garbage collected heap or a
 604 garbage collector itself. That code should exist in the language's runtime
 605 library. The compiler plugin is responsible for generating code which
 606 conforms to the binary interface defined by library, most essentially the
 607 <a href="#stack-map">stack map</a>.</p>
 608
 609 <p>To subclass <tt>llvm::GCStrategy</tt> and register it with the compiler:</p>
 610
 611 <blockquote><pre>// lib/MyGC/MyGC.cpp - Example LLVM GC plugin
 612
 613 #include "llvm/CodeGen/GCStrategy.h"
 614 #include "llvm/CodeGen/GCMetadata.h"
 615 #include "llvm/Support/Compiler.h"
 616
 617 using namespace llvm;
 618
 619 namespace {
 620   class LLVM_LIBRARY_VISIBILITY MyGC : public GCStrategy {
 621   public:
 622     MyGC() {}
 623   };
 624
 625   GCRegistry::Add&lt;MyGC&gt;
 626   X("mygc", "My bespoke garbage collector.");
 627 }</pre></blockquote>
 628
 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
 640 <p>Using the LLVM makefiles (like the <a
 641 href="http://llvm.org/viewvc/llvm-project/llvm/trunk/projects/sample/">sample
 642 project</a>), this code can be compiled as a plugin using a simple
 643 makefile:</p>
 644
 645 <blockquote><pre
 646 ># lib/MyGC/Makefile
 647
 648 LEVEL := ../..
 649 LIBRARYNAME = <var>MyGC</var>
 650 LOADABLE_MODULE = 1
 651
 652 include $(LEVEL)/Makefile.common</pre></blockquote>
 653
 654 <p>Once the plugin is compiled, code using it may be compiled using <tt>llc
 655 -load=<var>MyGC.so</var></tt> (though <var>MyGC.so</var> may have some other
 656 platform-specific extension):</p>
 657
 658 <blockquote><pre
 659 >$ cat sample.ll
 660 define void @f() gc "mygc" {
 661 entry:
 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
 667 as a language-specific compiler front-end.</p>
 668
 669 </div>
 670
 671 <!-- ======================================================================= -->
 672 <div class="doc_subsection">
 673   <a name="collector-algos">Overview of available features</a>
 674 </div>
 675
 676 <div class="doc_text">
 677
 678 <p><tt>GCStrategy</tt> provides a range of features through which a plugin
 679 may do useful work. Some of these are callbacks, some are algorithms that can
 680 be enabled, disabled, or customized. This matrix summarizes the supported (and
 681 planned) features and correlates them with the collection techniques which
 682 typically require them.</p>
 683
 684 <table>
 685   <tr>
 686     <th>Algorithm</th>
 687     <th>Done</th>
 688     <th>shadow stack</th>
 689     <th>refcount</th>
 690     <th>mark-sweep</th>
 691     <th>copying</th>
 692     <th>incremental</th>
 693     <th>threaded</th>
 694     <th>concurrent</th>
 695   </tr>
 696   <tr>
 697     <th class="rowhead"><a href="#stack-map">stack map</a></th>
 698     <td>&#10004;</td>
 699     <td></td>
 700     <td></td>
 701     <td>&#10008;</td>
 702     <td>&#10008;</td>
 703     <td>&#10008;</td>
 704     <td>&#10008;</td>
 705     <td>&#10008;</td>
 706   </tr>
 707   <tr>
 708     <th class="rowhead"><a href="#init-roots">initialize roots</a></th>
 709     <td>&#10004;</td>
 710     <td>&#10008;</td>
 711     <td>&#10008;</td>
 712     <td>&#10008;</td>
 713     <td>&#10008;</td>
 714     <td>&#10008;</td>
 715     <td>&#10008;</td>
 716     <td>&#10008;</td>
 717   </tr>
 718   <tr class="doc_warning">
 719     <th class="rowhead">derived pointers</th>
 720     <td>NO</td>
 721     <td></td>
 722     <td></td>
 723     <td></td>
 724     <td></td>
 725     <td></td>
 726     <td>&#10008;*</td>
 727     <td>&#10008;*</td>
 728   </tr>
 729   <tr>
 730     <th class="rowhead"><em><a href="#custom">custom lowering</a></em></th>
 731     <td>&#10004;</td>
 732     <th></th>
 733     <th></th>
 734     <th></th>
 735     <th></th>
 736     <th></th>
 737     <th></th>
 738     <th></th>
 739   </tr>
 740   <tr>
 741     <th class="rowhead indent">gcroot</th>
 742     <td>&#10004;</td>
 743     <td>&#10008;</td>
 744     <td>&#10008;</td>
 745     <td></td>
 746     <td></td>
 747     <td></td>
 748     <td></td>
 749     <td></td>
 750   </tr>
 751   <tr>
 752     <th class="rowhead indent">gcwrite</th>
 753     <td>&#10004;</td>
 754     <td></td>
 755     <td>&#10008;</td>
 756     <td></td>
 757     <td></td>
 758     <td>&#10008;</td>
 759     <td></td>
 760     <td>&#10008;</td>
 761   </tr>
 762   <tr>
 763     <th class="rowhead indent">gcread</th>
 764     <td>&#10004;</td>
 765     <td></td>
 766     <td></td>
 767     <td></td>
 768     <td></td>
 769     <td></td>
 770     <td></td>
 771     <td>&#10008;</td>
 772   </tr>
 773   <tr>
 774     <th class="rowhead"><em><a href="#safe-points">safe points</a></em></th>
 775     <td></td>
 776     <th></th>
 777     <th></th>
 778     <th></th>
 779     <th></th>
 780     <th></th>
 781     <th></th>
 782     <th></th>
 783   </tr>
 784   <tr>
 785     <th class="rowhead indent">in calls</th>
 786     <td>&#10004;</td>
 787     <td></td>
 788     <td></td>
 789     <td>&#10008;</td>
 790     <td>&#10008;</td>
 791     <td>&#10008;</td>
 792     <td>&#10008;</td>
 793     <td>&#10008;</td>
 794   </tr>
 795   <tr>
 796     <th class="rowhead indent">before calls</th>
 797     <td>&#10004;</td>
 798     <td></td>
 799     <td></td>
 800     <td></td>
 801     <td></td>
 802     <td></td>
 803     <td>&#10008;</td>
 804     <td>&#10008;</td>
 805   </tr>
 806   <tr class="doc_warning">
 807     <th class="rowhead indent">for loops</th>
 808     <td>NO</td>
 809     <td></td>
 810     <td></td>
 811     <td></td>
 812     <td></td>
 813     <td></td>
 814     <td>&#10008;</td>
 815     <td>&#10008;</td>
 816   </tr>
 817   <tr>
 818     <th class="rowhead indent">before escape</th>
 819     <td>&#10004;</td>
 820     <td></td>
 821     <td></td>
 822     <td></td>
 823     <td></td>
 824     <td></td>
 825     <td>&#10008;</td>
 826     <td>&#10008;</td>
 827   </tr>
 828   <tr class="doc_warning">
 829     <th class="rowhead">emit code at safe points</th>
 830     <td>NO</td>
 831     <td></td>
 832     <td></td>
 833     <td></td>
 834     <td></td>
 835     <td></td>
 836     <td>&#10008;</td>
 837     <td>&#10008;</td>
 838   </tr>
 839   <tr>
 840     <th class="rowhead"><em>output</em></th>
 841     <td></td>
 842     <th></th>
 843     <th></th>
 844     <th></th>
 845     <th></th>
 846     <th></th>
 847     <th></th>
 848     <th></th>
 849   </tr>
 850   <tr>
 851     <th class="rowhead indent"><a href="#assembly">assembly</a></th>
 852     <td>&#10004;</td>
 853     <td></td>
 854     <td></td>
 855     <td>&#10008;</td>
 856     <td>&#10008;</td>
 857     <td>&#10008;</td>
 858     <td>&#10008;</td>
 859     <td>&#10008;</td>
 860   </tr>
 861   <tr class="doc_warning">
 862     <th class="rowhead indent">JIT</th>
 863     <td>NO</td>
 864     <td></td>
 865     <td></td>
 866     <td class="optl">&#10008;</td>
 867     <td class="optl">&#10008;</td>
 868     <td class="optl">&#10008;</td>
 869     <td class="optl">&#10008;</td>
 870     <td class="optl">&#10008;</td>
 871   </tr>
 872   <tr class="doc_warning">
 873     <th class="rowhead indent">obj</th>
 874     <td>NO</td>
 875     <td></td>
 876     <td></td>
 877     <td class="optl">&#10008;</td>
 878     <td class="optl">&#10008;</td>
 879     <td class="optl">&#10008;</td>
 880     <td class="optl">&#10008;</td>
 881     <td class="optl">&#10008;</td>
 882   </tr>
 883   <tr class="doc_warning">
 884     <th class="rowhead">live analysis</th>
 885     <td>NO</td>
 886     <td></td>
 887     <td></td>
 888     <td class="optl">&#10008;</td>
 889     <td class="optl">&#10008;</td>
 890     <td class="optl">&#10008;</td>
 891     <td class="optl">&#10008;</td>
 892     <td class="optl">&#10008;</td>
 893   </tr>
 894   <tr class="doc_warning">
 895     <th class="rowhead">register map</th>
 896     <td>NO</td>
 897     <td></td>
 898     <td></td>
 899     <td class="optl">&#10008;</td>
 900     <td class="optl">&#10008;</td>
 901     <td class="optl">&#10008;</td>
 902     <td class="optl">&#10008;</td>
 903     <td class="optl">&#10008;</td>
 904   </tr>
 905   <tr>
 906     <td colspan="10">
 907       <div><span class="doc_warning">*</span> Derived pointers only pose a
 908            hazard to copying collectors.</div>
 909       <div><span class="optl">&#10008;</span> in gray denotes a feature which
 910            could be utilized if available.</div>
 911     </td>
 912   </tr>
 913 </table>
 914
 915 <p>To be clear, the collection techniques above are defined as:</p>
 916
 917 <dl>
 918   <dt>Shadow Stack</dt>
 919   <dd>The mutator carefully maintains a linked list of stack roots.</dd>
 920   <dt>Reference Counting</dt>
 921   <dd>The mutator maintains a reference count for each object and frees an
 922       object when its count falls to zero.</dd>
 923   <dt>Mark-Sweep</dt>
 924   <dd>When the heap is exhausted, the collector marks reachable objects starting
 925       from the roots, then deallocates unreachable objects in a sweep
 926       phase.</dd>
 927   <dt>Copying</dt>
 928   <dd>As reachability analysis proceeds, the collector copies objects from one
 929       heap area to another, compacting them in the process. Copying collectors
 930       enable highly efficient "bump pointer" allocation and can improve locality
 931       of reference.</dd>
 932   <dt>Incremental</dt>
 933   <dd>(Including generational collectors.) Incremental collectors generally have
 934       all the properties of a copying collector (regardless of whether the
 935       mature heap is compacting), but bring the added complexity of requiring
 936       write barriers.</dd>
 937   <dt>Threaded</dt>
 938   <dd>Denotes a multithreaded mutator; the collector must still stop the mutator
 939       ("stop the world") before beginning reachability analysis. Stopping a
 940       multithreaded mutator is a complicated problem. It generally requires
 941       highly platform specific code in the runtime, and the production of
 942       carefully designed machine code at safe points.</dd>
 943   <dt>Concurrent</dt>
 944   <dd>In this technique, the mutator and the collector run concurrently, with
 945       the goal of eliminating pause times. In a <em>cooperative</em> collector,
 946       the mutator further aids with collection should a pause occur, allowing
 947       collection to take advantage of multiprocessor hosts. The "stop the world"
 948       problem of threaded collectors is generally still present to a limited
 949       extent. Sophisticated marking algorithms are necessary. Read barriers may
 950       be necessary.</dd>
 951 </dl>
 952
 953 <p>As the matrix indicates, LLVM's garbage collection infrastructure is already
 954 suitable for a wide variety of collectors, but does not currently extend to
 955 multithreaded programs. This will be added in the future as there is
 956 interest.</p>
 957
 958 </div>
 959
 960 <!-- ======================================================================= -->
 961 <div class="doc_subsection">
 962   <a name="stack-map">Computing stack maps</a>
 963 </div>
 964
 965 <div class="doc_text">
 966
 967 <p>LLVM automatically computes a stack map. One of the most important features
 968 of a <tt>GCStrategy</tt> is to compile this information into the executable in
 969 the binary representation expected by the runtime library.</p>
 970
 971 <p>The stack map consists of the location and identity of each GC root in the
 972 each function in the module. For each root:</p>
 973
 974 <ul>
 975   <li><tt>RootNum</tt>: The index of the root.</li>
 976   <li><tt>StackOffset</tt>: The offset of the object relative to the frame
 977       pointer.</li>
 978   <li><tt>RootMetadata</tt>: The value passed as the <tt>%metadata</tt>
 979       parameter to the <a href="#gcroot"><tt>@llvm.gcroot</tt></a> intrinsic.</li>
 980 </ul>
 981
 982 <p>Also, for the function as a whole:</p>
 983
 984 <ul>
 985   <li><tt>getFrameSize()</tt>: The overall size of the function's initial
 986       stack frame, not accounting for any dynamic allocation.</li>
 987   <li><tt>roots_size()</tt>: The count of roots in the function.</li>
 988 </ul>
 989
 990 <p>To access the stack map, use <tt>GCFunctionMetadata::roots_begin()</tt> and
 991 -<tt>end()</tt> from the <tt><a
 992 href="#assembly">GCMetadataPrinter</a></tt>:</p>
 993
 994 <blockquote><pre
 995 >for (iterator I = begin(), E = end(); I != E; ++I) {
 996   GCFunctionInfo *FI = *I;
 997   unsigned FrameSize = FI-&gt;getFrameSize();
 998   size_t RootCount = FI-&gt;roots_size();
 999
1000   for (GCFunctionInfo::roots_iterator RI = FI-&gt;roots_begin(),
1001                                       RE = FI-&gt;roots_end();
1002                                       RI != RE; ++RI) {
1003     int RootNum = RI->Num;
1004     int RootStackOffset = RI->StackOffset;
1005     Constant *RootMetadata = RI->Metadata;
1006   }
1007 }</pre></blockquote>
1008
1009 <p>If the <tt>llvm.gcroot</tt> intrinsic is eliminated before code generation by
1010 a custom lowering pass, LLVM will compute an empty stack map. This may be useful
1011 for collector plugins which implement reference counting or a shadow stack.</p>
1012
1013 </div>
1014
1015
1016 <!-- ======================================================================= -->
1017 <div class="doc_subsection">
1018   <a name="init-roots">Initializing roots to null: <tt>InitRoots</tt></a>
1019 </div>
1020
1021 <div class="doc_text">
1022
1023 <blockquote><pre
1024 >MyGC::MyGC() {
1025   InitRoots = true;
1026 }</pre></blockquote>
1027
1028 <p>When set, LLVM will automatically initialize each root to <tt>null</tt> upon
1029 entry to the function. This prevents the GC's sweep phase from visiting
1030 uninitialized pointers, which will almost certainly cause it to crash. This
1031 initialization occurs before custom lowering, so the two may be used
1032 together.</p>
1033
1034 <p>Since LLVM does not yet compute liveness information, there is no means of
1035 distinguishing an uninitialized stack root from an initialized one. Therefore,
1036 this feature should be used by all GC plugins. It is enabled by default.</p>
1037
1038 </div>
1039
1040
1041 <!-- ======================================================================= -->
1042 <div class="doc_subsection">
1043   <a name="custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>,
1044     <tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a>
1045 </div>
1046
1047 <div class="doc_text">
1048
1049 <p>For GCs which use barriers or unusual treatment of stack roots, these
1050 flags allow the collector to perform arbitrary transformations of the LLVM
1051 IR:</p>
1052
1053 <blockquote><pre
1054 >class MyGC : public GCStrategy {
1055 public:
1056   MyGC() {
1057     CustomRoots = true;
1058     CustomReadBarriers = true;
1059     CustomWriteBarriers = true;
1060   }
1061
1062   virtual bool initializeCustomLowering(Module &amp;M);
1063   virtual bool performCustomLowering(Function &amp;F);
1064 };</pre></blockquote>
1065
1066 <p>If any of these flags are set, then LLVM suppresses its default lowering for
1067 the corresponding intrinsics and instead calls
1068 <tt>performCustomLowering</tt>.</p>
1069
1070 <p>LLVM's default action for each intrinsic is as follows:</p>
1071
1072 <ul>
1073   <li><tt>llvm.gcroot</tt>: Leave it alone. The code generator must see it
1074                             or the stack map will not be computed.</li>
1075   <li><tt>llvm.gcread</tt>: Substitute a <tt>load</tt> instruction.</li>
1076   <li><tt>llvm.gcwrite</tt>: Substitute a <tt>store</tt> instruction.</li>
1077 </ul>
1078
1079 <p>If <tt>CustomReadBarriers</tt> or <tt>CustomWriteBarriers</tt> are specified,
1080 then <tt>performCustomLowering</tt> <strong>must</strong> eliminate the
1081 corresponding barriers.</p>
1082
1083 <p><tt>performCustomLowering</tt> must comply with the same restrictions as <a
1084 href="WritingAnLLVMPass.html#runOnFunction"><tt
1085 >FunctionPass::runOnFunction</tt></a>.
1086 Likewise, <tt>initializeCustomLowering</tt> has the same semantics as <a
1087 href="WritingAnLLVMPass.html#doInitialization_mod"><tt
1088 >Pass::doInitialization(Module&amp;)</tt></a>.</p>
1089
1090 <p>The following can be used as a template:</p>
1091
1092 <blockquote><pre
1093 >#include "llvm/Module.h"
1094 #include "llvm/IntrinsicInst.h"
1095
1096 bool MyGC::initializeCustomLowering(Module &amp;M) {
1097   return false;
1098 }
1099
1100 bool MyGC::performCustomLowering(Function &amp;F) {
1101   bool MadeChange = false;
1102
1103   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
1104     for (BasicBlock::iterator II = BB-&gt;begin(), E = BB-&gt;end(); II != E; )
1105       if (IntrinsicInst *CI = dyn_cast&lt;IntrinsicInst&gt;(II++))
1106         if (Function *F = CI-&gt;getCalledFunction())
1107           switch (F-&gt;getIntrinsicID()) {
1108           case Intrinsic::gcwrite:
1109             // Handle llvm.gcwrite.
1110             CI-&gt;eraseFromParent();
1111             MadeChange = true;
1112             break;
1113           case Intrinsic::gcread:
1114             // Handle llvm.gcread.
1115             CI-&gt;eraseFromParent();
1116             MadeChange = true;
1117             break;
1118           case Intrinsic::gcroot:
1119             // Handle llvm.gcroot.
1120             CI-&gt;eraseFromParent();
1121             MadeChange = true;
1122             break;
1123           }
1124
1125   return MadeChange;
1126 }</pre></blockquote>
1127
1128 </div>
1129
1130
1131 <!-- ======================================================================= -->
1132 <div class="doc_subsection">
1133   <a name="safe-points">Generating safe points: <tt>NeededSafePoints</tt></a>
1134 </div>
1135
1136 <div class="doc_text">
1137
1138 <p>LLVM can compute four kinds of safe points:</p>
1139
1140 <blockquote><pre
1141 >namespace GC {
1142   /// PointKind - The type of a collector-safe point.
1143   ///
1144   enum PointKind {
1145     Loop,    //&lt; Instr is a loop (backwards branch).
1146     Return,  //&lt; Instr is a return instruction.
1147     PreCall, //&lt; Instr is a call instruction.
1148     PostCall //&lt; Instr is the return address of a call.
1149   };
1150 }</pre></blockquote>
1151
1152 <p>A collector can request any combination of the four by setting the
1153 <tt>NeededSafePoints</tt> mask:</p>
1154
1155 <blockquote><pre
1156 >MyGC::MyGC() {
1157   NeededSafePoints = 1 &lt;&lt; GC::Loop
1158                    | 1 &lt;&lt; GC::Return
1159                    | 1 &lt;&lt; GC::PreCall
1160                    | 1 &lt;&lt; GC::PostCall;
1161 }</pre></blockquote>
1162
1163 <p>It can then use the following routines to access safe points.</p>
1164
1165 <blockquote><pre
1166 >for (iterator I = begin(), E = end(); I != E; ++I) {
1167   GCFunctionInfo *MD = *I;
1168   size_t PointCount = MD-&gt;size();
1169
1170   for (GCFunctionInfo::iterator PI = MD-&gt;begin(),
1171                                 PE = MD-&gt;end(); PI != PE; ++PI) {
1172     GC::PointKind PointKind = PI-&gt;Kind;
1173     unsigned PointNum = PI-&gt;Num;
1174   }
1175 }
1176 </pre></blockquote>
1177
1178 <p>Almost every collector requires <tt>PostCall</tt> safe points, since these
1179 correspond to the moments when the function is suspended during a call to a
1180 subroutine.</p>
1181
1182 <p>Threaded programs generally require <tt>Loop</tt> safe points to guarantee
1183 that the application will reach a safe point within a bounded amount of time,
1184 even if it is executing a long-running loop which contains no function
1185 calls.</p>
1186
1187 <p>Threaded collectors may also require <tt>Return</tt> and <tt>PreCall</tt>
1188 safe points to implement "stop the world" techniques using self-modifying code,
1189 where it is important that the program not exit the function without reaching a
1190 safe point (because only the topmost function has been patched).</p>
1191
1192 </div>
1193
1194
1195 <!-- ======================================================================= -->
1196 <div class="doc_subsection">
1197   <a name="assembly">Emitting assembly code: <tt>GCMetadataPrinter</tt></a>
1198 </div>
1199
1200 <div class="doc_text">
1201
1202 <p>LLVM allows a plugin to print arbitrary assembly code before and after the
1203 rest of a module's assembly code. At the end of the module, the GC can compile
1204 the LLVM stack map into assembly code. (At the beginning, this information is not
1205 yet computed.)</p>
1206
1207 <p>Since AsmWriter and CodeGen are separate components of LLVM, a separate
1208 abstract base class and registry is provided for printing assembly code, the
1209 <tt>GCMetadaPrinter</tt> and <tt>GCMetadataPrinterRegistry</tt>. The AsmWriter
1210 will look for such a subclass if the <tt>GCStrategy</tt> sets
1211 <tt>UsesMetadata</tt>:</p>
1212
1213 <blockquote><pre
1214 >MyGC::MyGC() {
1215   UsesMetadata = true;
1216 }</pre></blockquote>
1217
1218 <p>This separation allows JIT-only clients to be smaller.</p>
1219
1220 <p>Note that LLVM does not currently have analogous APIs to support code
1221 generation in the JIT, nor using the object writers.</p>
1222
1223 <blockquote><pre
1224 >// lib/MyGC/MyGCPrinter.cpp - Example LLVM GC printer
1225
1226 #include "llvm/CodeGen/GCMetadataPrinter.h"
1227 #include "llvm/Support/Compiler.h"
1228
1229 using namespace llvm;
1230
1231 namespace {
1232   class LLVM_LIBRARY_VISIBILITY MyGCPrinter : public GCMetadataPrinter {
1233   public:
1234     virtual void beginAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
1235                                const TargetAsmInfo &amp;TAI);
1236
1237     virtual void finishAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
1238                                 const TargetAsmInfo &amp;TAI);
1239   };
1240
1241   GCMetadataPrinterRegistry::Add&lt;MyGCPrinter&gt;
1242   X("mygc", "My bespoke garbage collector.");
1243 }</pre></blockquote>
1244
1245 <p>The collector should use <tt>AsmPrinter</tt> and <tt>TargetAsmInfo</tt> to
1246 print portable assembly code to the <tt>std::ostream</tt>. The collector itself
1247 contains the stack map for the entire module, and may access the
1248 <tt>GCFunctionInfo</tt> using its own <tt>begin()</tt> and <tt>end()</tt>
1249 methods. Here's a realistic example:</p>
1250
1251 <blockquote><pre
1252 >#include "llvm/CodeGen/AsmPrinter.h"
1253 #include "llvm/Function.h"
1254 #include "llvm/Target/TargetMachine.h"
1255 #include "llvm/Target/TargetData.h"
1256 #include "llvm/Target/TargetAsmInfo.h"
1257
1258 void MyGCPrinter::beginAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
1259                                 const TargetAsmInfo &amp;TAI) {
1260   // Nothing to do.
1261 }
1262
1263 void MyGCPrinter::finishAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
1264                                  const TargetAsmInfo &amp;TAI) {
1265   // Set up for emitting addresses.
1266   const char *AddressDirective;
1267   int AddressAlignLog;
1268   if (AP.TM.getTargetData()->getPointerSize() == sizeof(int32_t)) {
1269     AddressDirective = TAI.getData32bitsDirective();
1270     AddressAlignLog = 2;
1271   } else {
1272     AddressDirective = TAI.getData64bitsDirective();
1273     AddressAlignLog = 3;
1274   }
1275
1276   // Put this in the data section.
1277   AP.SwitchToDataSection(TAI.getDataSection());
1278
1279   // For each function...
1280   for (iterator FI = begin(), FE = end(); FI != FE; ++FI) {
1281     GCFunctionInfo &amp;MD = **FI;
1282
1283     // Emit this data structure:
1284     //
1285     // struct {
1286     //   int32_t PointCount;
1287     //   struct {
1288     //     void *SafePointAddress;
1289     //     int32_t LiveCount;
1290     //     int32_t LiveOffsets[LiveCount];
1291     //   } Points[PointCount];
1292     // } __gcmap_&lt;FUNCTIONNAME&gt;;
1293
1294     // Align to address width.
1295     AP.EmitAlignment(AddressAlignLog);
1296
1297     // Emit the symbol by which the stack map entry can be found.
1298     std::string Symbol;
1299     Symbol += TAI.getGlobalPrefix();
1300     Symbol += "__gcmap_";
1301     Symbol += MD.getFunction().getName();
1302     if (const char *GlobalDirective = TAI.getGlobalDirective())
1303       OS &lt;&lt; GlobalDirective &lt;&lt; Symbol &lt;&lt; "\n";
1304     OS &lt;&lt; TAI.getGlobalPrefix() &lt;&lt; Symbol &lt;&lt; ":\n";
1305
1306     // Emit PointCount.
1307     AP.EmitInt32(MD.size());
1308     AP.EOL("safe point count");
1309
1310     // And each safe point...
1311     for (GCFunctionInfo::iterator PI = MD.begin(),
1312                                      PE = MD.end(); PI != PE; ++PI) {
1313       // Align to address width.
1314       AP.EmitAlignment(AddressAlignLog);
1315
1316       // Emit the address of the safe point.
1317       OS &lt;&lt; AddressDirective
1318          &lt;&lt; TAI.getPrivateGlobalPrefix() &lt;&lt; "label" &lt;&lt; PI-&gt;Num;
1319       AP.EOL("safe point address");
1320
1321       // Emit the stack frame size.
1322       AP.EmitInt32(MD.getFrameSize());
1323       AP.EOL("stack frame size");
1324
1325       // Emit the number of live roots in the function.
1326       AP.EmitInt32(MD.live_size(PI));
1327       AP.EOL("live root count");
1328
1329       // And for each live root...
1330       for (GCFunctionInfo::live_iterator LI = MD.live_begin(PI),
1331                                             LE = MD.live_end(PI);
1332                                             LI != LE; ++LI) {
1333         // Print its offset within the stack frame.
1334         AP.EmitInt32(LI-&gt;StackOffset);
1335         AP.EOL("stack offset");
1336       }
1337     }
1338   }
1339 }
1340 </pre></blockquote>
1341
1342 </div>
1343
1344
1345 <!-- *********************************************************************** -->
1346 <div class="doc_section">
1347   <a name="references">References</a>
1348 </div>
1349 <!-- *********************************************************************** -->
1350
1351 <div class="doc_text">
1352
1353 <p><a name="appel89">[Appel89]</a> Runtime Tags Aren't Necessary. Andrew
1354 W. Appel. Lisp and Symbolic Computation 19(7):703-705, July 1989.</p>
1355
1356 <p><a name="goldberg91">[Goldberg91]</a> Tag-free garbage collection for
1357 strongly typed programming languages. Benjamin Goldberg. ACM SIGPLAN
1358 PLDI'91.</p>
1359
1360 <p><a name="tolmach94">[Tolmach94]</a> Tag-free garbage collection using
1361 explicit type parameters. Andrew Tolmach. Proceedings of the 1994 ACM
1362 conference on LISP and functional programming.</p>
1363
1364 <p><a name="henderson02">[Henderson2002]</a> <a
1365 href="http://citeseer.ist.psu.edu/henderson02accurate.html">
1366 Accurate Garbage Collection in an Uncooperative Environment</a>.
1367 Fergus Henderson. International Symposium on Memory Management 2002.</p>
1368
1369 </div>
1370
1371
1372 <!-- *********************************************************************** -->
1373
1374 <hr>
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1381   <a href="mailto:sabre@nondot.org">Chris Lattner</a><br>
1382   <a href="http://llvm.org">LLVM Compiler Infrastructure</a><br>
1383   Last modified: $Date$
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