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     15 
     16 <h1>
     17   Accurate Garbage Collection with LLVM
     18 </h1>
     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 (a] nondot.org">Chris Lattner</a> and
     78      Gordon Henriksen</p>
     79 </div>
     80 
     81 <!-- *********************************************************************** -->
     82 <h2>
     83   <a name="introduction">Introduction</a>
     84 </h2>
     85 <!-- *********************************************************************** -->
     86 
     87 <div>
     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 <!-- ======================================================================= -->
    128 <h3>
    129   <a name="feature">Goals and non-goals</a>
    130 </h3>
    131 
    132 <div>
    133 
    134 <p>LLVM's intermediate representation provides <a href="#intrinsics">garbage
    135 collection intrinsics</a> that offer support for a broad class of
    136 collector models. For instance, the intrinsics permit:</p>
    137 
    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>
    147 
    148 <p>We hope that the primitive support built into the LLVM IR is sufficient to
    149 support a broad class of garbage collected languages including Scheme, ML, Java,
    150 C#, Perl, Python, Lua, Ruby, other scripting languages, and more.</p>
    151 
    152 <p>However, LLVM does not itself provide a garbage collector&mdash;this should
    153 be part of your language's runtime library. LLVM provides a framework for
    154 compile time <a href="#plugin">code generation plugins</a>. The role of these
    155 plugins is to generate code and data structures which conforms to the <em>binary
    156 interface</em> specified by the <em>runtime library</em>. This is similar to the
    157 relationship between LLVM and DWARF debugging info, for example. The
    158 difference primarily lies in the lack of an established standard in the domain
    159 of garbage collection&mdash;thus the plugins.</p>
    160 
    161 <p>The aspects of the binary interface with which LLVM's GC support is
    162 concerned are:</p>
    163 
    164 <ul>
    165   <li>Creation of GC-safe points within code where collection is allowed to
    166       execute safely.</li>
    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>
    173   <li>Emission of read barriers when loading object references. These are
    174       useful for interoperating with concurrent collectors.</li>
    175 </ul>
    176 
    177 <p>There are additional areas that LLVM does not directly address:</p>
    178 
    179 <ul>
    180   <li>Registration of global roots with the runtime.</li>
    181   <li>Registration of stack map entries with the runtime.</li>
    182   <li>The functions used by the program to allocate memory, trigger a
    183       collection, etc.</li>
    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>
    187 </ul>
    188 
    189 <p>In general, LLVM's support for GC does not include features which can be
    190 adequately addressed with other features of the IR and does not specify a
    191 particular binary interface. On the plus side, this means that you should be
    192 able to integrate LLVM with an existing runtime. On the other hand, it leaves
    193 a lot of work for the developer of a novel language. However, it's easy to get
    194 started quickly and scale up to a more sophisticated implementation as your
    195 compiler matures.</p>
    196 
    197 </div>
    198 
    199 </div>
    200 
    201 <!-- *********************************************************************** -->
    202 <h2>
    203   <a name="quickstart">Getting started</a>
    204 </h2>
    205 <!-- *********************************************************************** -->
    206 
    207 <div>
    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 <!-- ======================================================================= -->
    250 <h3>
    251   <a name="quickstart-compiler">In your compiler</a>
    252 </h3>
    253 
    254 <div>
    255 
    256 <p>To turn the shadow stack on for your functions, first call:</p>
    257 
    258 <div class="doc_code"><pre
    259 >F.setGC("shadow-stack");</pre></div>
    260 
    261 <p>for each function your compiler emits. Since the shadow stack is built into
    262 LLVM, you do not need to load a plugin.</p>
    263 
    264 <p>Your compiler must also use <tt>@llvm.gcroot</tt> as documented.
    265 Don't forget to create a root for each intermediate value that is generated
    266 when 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
    268 collection.</p>
    269 
    270 <p>There's no need to use <tt>@llvm.gcread</tt> and <tt>@llvm.gcwrite</tt> over
    271 plain <tt>load</tt> and <tt>store</tt> for now. You will need them when
    272 switching to a more advanced GC.</p>
    273 
    274 </div>
    275 
    276 <!-- ======================================================================= -->
    277 <h3>
    278   <a name="quickstart-runtime">In your runtime</a>
    279 </h3>
    280 
    281 <div>
    282 
    283 <p>The shadow stack doesn't imply a memory allocation algorithm. A semispace
    284 collector or building atop <tt>malloc</tt> are great places to start, and can
    285 be implemented with very little code.</p>
    286 
    287 <p>When it comes time to collect, however, your runtime needs to traverse the
    288 stack roots, and for this it needs to integrate with the shadow stack. Luckily,
    289 doing so is very simple. (This code is heavily commented to help you
    290 understand the data structure, but there are only 20 lines of meaningful
    291 code.)</p>
    292 
    293 <pre class="doc_code">
    294 /// @brief The map for a single function's stack frame. One of these is
    295 ///        compiled as constant data into the executable for each function.
    296 /// 
    297 /// Storage of metadata values is elided if the %metadata parameter to
    298 /// @llvm.gcroot is null.
    299 struct FrameMap {
    300   int32_t NumRoots;    //&lt; Number of roots in stack frame.
    301   int32_t NumMeta;     //&lt; Number of metadata entries. May be &lt; NumRoots.
    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.
    307 struct 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.
    317 StackEntry *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.
    327 void 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)
    333       Visitor(&amp;R->Roots[i], R->Map->Meta[i]);
    334     
    335     // For roots [NumMeta, NumRoots), the metadata pointer is null.
    336     for (unsigned e = R->Map->NumRoots; i != e; ++i)
    337       Visitor(&amp;R->Roots[i], NULL);
    338   }
    339 }</pre>
    340 
    341 </div>
    342 
    343 <!-- ======================================================================= -->
    344 <h3>
    345   <a name="shadow-stack">About the shadow stack</a>
    346 </h3>
    347 
    348 <div>
    349 
    350 <p>Unlike many GC algorithms which rely on a cooperative code generator to
    351 compile stack maps, this algorithm carefully maintains a linked list of stack
    352 roots [<a href="#henderson02">Henderson2002</a>]. This so-called "shadow stack"
    353 mirrors the machine stack. Maintaining this data structure is slower than using
    354 a stack map compiled into the executable as constant data, but has a significant
    355 portability advantage because it requires no special support from the target
    356 code generator, and does not require tricky platform-specific code to crawl
    357 the machine stack.</p>
    358 
    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 
    366 <p>Still, it's an easy way to get started. After your compiler and runtime are
    367 up and running, writing a <a href="#plugin">plugin</a> will allow you to take
    368 advantage of <a href="#collector-algos">more advanced GC features</a> of LLVM
    369 in order to improve performance.</p>
    370 
    371 </div>
    372 
    373 </div>
    374 
    375 <!-- *********************************************************************** -->
    376 <h2>
    377   <a name="core">IR features</a><a name="intrinsics"></a>
    378 </h2>
    379 <!-- *********************************************************************** -->
    380 
    381 <div>
    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 <!-- ======================================================================= -->
    394 <h3>
    395   <a name="gcattr">Specifying GC code generation: <tt>gc "..."</tt></a>
    396 </h3>
    397 
    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 <p>The <tt>gc</tt> function attribute is used to specify the desired GC style
    405 to the compiler. Its programmatic equivalent is the <tt>setGC</tt> method of
    406 <tt>Function</tt>.</p>
    407 
    408 <p>Setting <tt>gc "<i>name</i>"</tt> on a function triggers a search for a
    409 matching code generation plugin "<i>name</i>"; it is that plugin which defines
    410 the exact nature of the code generated to support GC. If none is found, the
    411 compiler will raise an error.</p>
    412 
    413 <p>Specifying the GC style on a per-function basis allows LLVM to link together
    414 programs that use different garbage collection algorithms (or none at all).</p>
    415 
    416 </div>
    417 
    418 <!-- ======================================================================= -->
    419 <h3>
    420   <a name="gcroot">Identifying GC roots on the stack: <tt>llvm.gcroot</tt></a>
    421 </h3>
    422 
    423 <div>
    424 
    425 <div class="doc_code"><tt>
    426   void @llvm.gcroot(i8** %ptrloc, i8* %metadata)
    427 </tt></div>
    428 
    429 <p>The <tt>llvm.gcroot</tt> intrinsic is used to inform LLVM that a stack
    430 variable references an object on the heap and is to be tracked for garbage
    431 collection. The exact impact on generated code is specified by a <a
    432 href="#plugin">compiler plugin</a>.</p>
    433 
    434 <p>A compiler which uses mem2reg to raise imperative code using <tt>alloca</tt>
    435 into SSA form need only add a call to <tt>@llvm.gcroot</tt> for those variables
    436 which a pointers into the GC heap.</p>
    437 
    438 <p>It is also important to mark intermediate values with <tt>llvm.gcroot</tt>.
    439 For 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
    443 or a bitcast of an alloca. The second contains a pointer to metadata that
    444 should be associated with the pointer, and <b>must</b> be a constant or global
    445 value address. If your target collector uses tags, use a null pointer for
    446 metadata.</p>
    447 
    448 <p>The <tt>%metadata</tt> argument can be used to avoid requiring heap objects
    449 to have 'isa' pointers or tag bits. [<a href="#appel89">Appel89</a>, <a
    450 href="#goldberg91">Goldberg91</a>, <a href="#tolmach94">Tolmach94</a>] If
    451 specified, its value will be tracked along with the location of the pointer in
    452 the stack frame.</p>
    453 
    454 <p>Consider the following fragment of Java code:</p>
    455 
    456 <pre class="doc_code">
    457        {
    458          Object X;   // A null-initialized reference to an object
    459          ...
    460        }
    461 </pre>
    462 
    463 <p>This block (which may be located in the middle of a function or in a loop
    464 nest), could be compiled to this LLVM code:</p>
    465 
    466 <pre class="doc_code">
    467 Entry:
    468    ;; In the entry block for the function, allocate the
    469    ;; stack space for X, which is an LLVM pointer.
    470    %X = alloca %Object*
    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**
    475    call void @llvm.gcroot(i8** %X, i8* null)
    476    ...
    477 
    478    ;; "CodeBlock" is the block corresponding to the start
    479    ;;  of the scope above.
    480 CodeBlock:
    481    ;; Java null-initializes pointers.
    482    store %Object* null, %Object** %X
    483 
    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 <!-- ======================================================================= -->
    495 <h3>
    496   <a name="barriers">Reading and writing references in the heap</a>
    497 </h3>
    498 
    499 <div>
    500 
    501 <p>Some collectors need to be informed when the mutator (the program that needs
    502 garbage collection) either reads a pointer from or writes a pointer to a field
    503 of 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
    505 code that needs to be executed is usually quite small and not on the critical
    506 path of any computation, so the overall performance impact of the barrier is
    507 tolerable.</p>
    508 
    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
    511 object). Accordingly, these intrinsics take both pointers as separate arguments
    512 for completeness. In this snippet, <tt>%object</tt> is the object pointer, and 
    513 <tt>%derived</tt> is the derived pointer:</p>
    514 
    515 <blockquote><pre>
    516     ;; An array type.
    517     %class.Array = type { %class.Object, i32, [0 x %class.Object*] }
    518     ...
    519 
    520     ;; Load the object pointer from a gcroot.
    521     %object = load %class.Array** %object_addr
    522 
    523     ;; Compute the derived pointer.
    524     %derived = getelementptr %object, i32 0, i32 2, i32 %n</pre></blockquote>
    525 
    526 <p>LLVM does not enforce this relationship between the object and derived
    527 pointer (although a <a href="#plugin">plugin</a> might). However, it would be
    528 an unusual collector that violated it.</p>
    529 
    530 <p>The use of these intrinsics is naturally optional if the target GC does
    531 require the corresponding barrier. Such a GC plugin will replace the intrinsic
    532 calls with the corresponding <tt>load</tt> or <tt>store</tt> instruction if they
    533 are used.</p>
    534 
    535 <!-- ======================================================================= -->
    536 <h4>
    537   <a name="gcwrite">Write barrier: <tt>llvm.gcwrite</tt></a>
    538 </h4>
    539 
    540 <div>
    541 
    542 <div class="doc_code"><tt>
    543 void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived)
    544 </tt></div>
    545 
    546 <p>For write barriers, LLVM provides the <tt>llvm.gcwrite</tt> intrinsic
    547 function. It has exactly the same semantics as a non-volatile <tt>store</tt> to
    548 the derived pointer (the third argument). The exact code generated is specified
    549 by a <a href="#plugin">compiler plugin</a>.</p>
    550 
    551 <p>Many important algorithms require write barriers, including generational
    552 and concurrent collectors. Additionally, write barriers could be used to
    553 implement reference counting.</p>
    554 
    555 </div>
    556 
    557 <!-- ======================================================================= -->
    558 <h4>
    559   <a name="gcread">Read barrier: <tt>llvm.gcread</tt></a>
    560 </h4>
    561 
    562 <div>
    563 
    564 <div class="doc_code"><tt>
    565 i8* @llvm.gcread(i8* %object, i8** %derived)<br>
    566 </tt></div>
    567 
    568 <p>For read barriers, LLVM provides the <tt>llvm.gcread</tt> intrinsic function.
    569 It has exactly the same semantics as a non-volatile <tt>load</tt> from the
    570 derived pointer (the second argument). The exact code generated is specified by
    571 a <a href="#plugin">compiler plugin</a>.</p>
    572 
    573 <p>Read barriers are needed by fewer algorithms than write barriers, and may
    574 have a greater performance impact since pointer reads are more frequent than
    575 writes.</p>
    576 
    577 </div>
    578 
    579 </div>
    580 
    581 </div>
    582 
    583 <!-- *********************************************************************** -->
    584 <h2>
    585   <a name="plugin">Implementing a collector plugin</a>
    586 </h2>
    587 <!-- *********************************************************************** -->
    588 
    589 <div>
    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 <!-- ======================================================================= -->
    670 <h3>
    671   <a name="collector-algos">Overview of available features</a>
    672 </h3>
    673 
    674 <div>
    675 
    676 <p><tt>GCStrategy</tt> provides a range of features through which a plugin
    677 may do useful work. Some of these are callbacks, some are algorithms that can
    678 be enabled, disabled, or customized. This matrix summarizes the supported (and
    679 planned) features and correlates them with the collection techniques which
    680 typically require them.</p>
    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>
    917   <dd>The mutator carefully maintains a linked list of stack roots.</dd>
    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
    952 suitable for a wide variety of collectors, but does not currently extend to
    953 multithreaded programs. This will be added in the future as there is
    954 interest.</p>
    955 
    956 </div>
    957 
    958 <!-- ======================================================================= -->
    959 <h3>
    960   <a name="stack-map">Computing stack maps</a>
    961 </h3>
    962 
    963 <div>
    964 
    965 <p>LLVM automatically computes a stack map. One of the most important features
    966 of a <tt>GCStrategy</tt> is to compile this information into the executable in
    967 the 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
    970 each 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
    990 href="#assembly">GCMetadataPrinter</a></tt>:</p>
    991 
    992 <blockquote><pre
    993 >for (iterator I = begin(), E = end(); I != E; ++I) {
    994   GCFunctionInfo *FI = *I;
    995   unsigned FrameSize = FI-&gt;getFrameSize();
    996   size_t RootCount = FI-&gt;roots_size();
    997 
    998   for (GCFunctionInfo::roots_iterator RI = FI-&gt;roots_begin(),
    999                                       RE = FI-&gt;roots_end();
   1000                                       RI != RE; ++RI) {
   1001     int RootNum = RI->Num;
   1002     int RootStackOffset = RI->StackOffset;
   1003     Constant *RootMetadata = RI->Metadata;
   1004   }
   1005 }</pre></blockquote>
   1006 
   1007 <p>If the <tt>llvm.gcroot</tt> intrinsic is eliminated before code generation by
   1008 a custom lowering pass, LLVM will compute an empty stack map. This may be useful
   1009 for collector plugins which implement reference counting or a shadow stack.</p>
   1010 
   1011 </div>
   1012 
   1013 
   1014 <!-- ======================================================================= -->
   1015 <h3>
   1016   <a name="init-roots">Initializing roots to null: <tt>InitRoots</tt></a>
   1017 </h3>
   1018 
   1019 <div>
   1020 
   1021 <blockquote><pre
   1022 >MyGC::MyGC() {
   1023   InitRoots = true;
   1024 }</pre></blockquote>
   1025 
   1026 <p>When set, LLVM will automatically initialize each root to <tt>null</tt> upon
   1027 entry to the function. This prevents the GC's sweep phase from visiting
   1028 uninitialized pointers, which will almost certainly cause it to crash. This
   1029 initialization occurs before custom lowering, so the two may be used
   1030 together.</p>
   1031 
   1032 <p>Since LLVM does not yet compute liveness information, there is no means of
   1033 distinguishing an uninitialized stack root from an initialized one. Therefore,
   1034 this feature should be used by all GC plugins. It is enabled by default.</p>
   1035 
   1036 </div>
   1037 
   1038 
   1039 <!-- ======================================================================= -->
   1040 <h3>
   1041   <a name="custom">Custom lowering of intrinsics: <tt>CustomRoots</tt>, 
   1042     <tt>CustomReadBarriers</tt>, and <tt>CustomWriteBarriers</tt></a>
   1043 </h3>
   1044 
   1045 <div>
   1046 
   1047 <p>For GCs which use barriers or unusual treatment of stack roots, these
   1048 flags allow the collector to perform arbitrary transformations of the LLVM
   1049 IR:</p>
   1050 
   1051 <blockquote><pre
   1052 >class MyGC : public GCStrategy {
   1053 public:
   1054   MyGC() {
   1055     CustomRoots = true;
   1056     CustomReadBarriers = true;
   1057     CustomWriteBarriers = true;
   1058   }
   1059   
   1060   virtual bool initializeCustomLowering(Module &amp;M);
   1061   virtual bool performCustomLowering(Function &amp;F);
   1062 };</pre></blockquote>
   1063 
   1064 <p>If any of these flags are set, then LLVM suppresses its default lowering for
   1065 the corresponding intrinsics and instead calls
   1066 <tt>performCustomLowering</tt>.</p>
   1067 
   1068 <p>LLVM's default action for each intrinsic is as follows:</p>
   1069 
   1070 <ul>
   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>
   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,
   1078 then <tt>performCustomLowering</tt> <strong>must</strong> eliminate the
   1079 corresponding barriers.</p>
   1080 
   1081 <p><tt>performCustomLowering</tt> must comply with the same restrictions as <a
   1082 href="WritingAnLLVMPass.html#runOnFunction"><tt
   1083 >FunctionPass::runOnFunction</tt></a>.
   1084 Likewise, <tt>initializeCustomLowering</tt> has the same semantics as <a
   1085 href="WritingAnLLVMPass.html#doInitialization_mod"><tt
   1086 >Pass::doInitialization(Module&amp;)</tt></a>.</p>
   1087 
   1088 <p>The following can be used as a template:</p>
   1089 
   1090 <blockquote><pre
   1091 >#include "llvm/Module.h"
   1092 #include "llvm/IntrinsicInst.h"
   1093 
   1094 bool MyGC::initializeCustomLowering(Module &amp;M) {
   1095   return false;
   1096 }
   1097 
   1098 bool MyGC::performCustomLowering(Function &amp;F) {
   1099   bool MadeChange = false;
   1100   
   1101   for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
   1102     for (BasicBlock::iterator II = BB-&gt;begin(), E = BB-&gt;end(); II != E; )
   1103       if (IntrinsicInst *CI = dyn_cast&lt;IntrinsicInst&gt;(II++))
   1104         if (Function *F = CI-&gt;getCalledFunction())
   1105           switch (F-&gt;getIntrinsicID()) {
   1106           case Intrinsic::gcwrite:
   1107             // Handle llvm.gcwrite.
   1108             CI-&gt;eraseFromParent();
   1109             MadeChange = true;
   1110             break;
   1111           case Intrinsic::gcread:
   1112             // Handle llvm.gcread.
   1113             CI-&gt;eraseFromParent();
   1114             MadeChange = true;
   1115             break;
   1116           case Intrinsic::gcroot:
   1117             // Handle llvm.gcroot.
   1118             CI-&gt;eraseFromParent();
   1119             MadeChange = true;
   1120             break;
   1121           }
   1122   
   1123   return MadeChange;
   1124 }</pre></blockquote>
   1125 
   1126 </div>
   1127 
   1128 
   1129 <!-- ======================================================================= -->
   1130 <h3>
   1131   <a name="safe-points">Generating safe points: <tt>NeededSafePoints</tt></a>
   1132 </h3>
   1133 
   1134 <div>
   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
   1154 >MyGC::MyGC() {
   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 
   1163 <blockquote><pre
   1164 >for (iterator I = begin(), E = end(); I != E; ++I) {
   1165   GCFunctionInfo *MD = *I;
   1166   size_t PointCount = MD-&gt;size();
   1167 
   1168   for (GCFunctionInfo::iterator PI = MD-&gt;begin(),
   1169                                 PE = MD-&gt;end(); PI != PE; ++PI) {
   1170     GC::PointKind PointKind = PI-&gt;Kind;
   1171     unsigned PointNum = PI-&gt;Num;
   1172   }
   1173 }
   1174 </pre></blockquote>
   1175 
   1176 <p>Almost every collector requires <tt>PostCall</tt> safe points, since these
   1177 correspond to the moments when the function is suspended during a call to a
   1178 subroutine.</p>
   1179 
   1180 <p>Threaded programs generally require <tt>Loop</tt> safe points to guarantee
   1181 that the application will reach a safe point within a bounded amount of time,
   1182 even if it is executing a long-running loop which contains no function
   1183 calls.</p>
   1184 
   1185 <p>Threaded collectors may also require <tt>Return</tt> and <tt>PreCall</tt>
   1186 safe points to implement "stop the world" techniques using self-modifying code,
   1187 where it is important that the program not exit the function without reaching a
   1188 safe point (because only the topmost function has been patched).</p>
   1189 
   1190 </div>
   1191 
   1192 
   1193 <!-- ======================================================================= -->
   1194 <h3>
   1195   <a name="assembly">Emitting assembly code: <tt>GCMetadataPrinter</tt></a>
   1196 </h3>
   1197 
   1198 <div>
   1199 
   1200 <p>LLVM allows a plugin to print arbitrary assembly code before and after the
   1201 rest of a module's assembly code. At the end of the module, the GC can compile
   1202 the LLVM stack map into assembly code. (At the beginning, this information is not
   1203 yet computed.)</p>
   1204 
   1205 <p>Since AsmWriter and CodeGen are separate components of LLVM, a separate
   1206 abstract base class and registry is provided for printing assembly code, the
   1207 <tt>GCMetadaPrinter</tt> and <tt>GCMetadataPrinterRegistry</tt>. The AsmWriter
   1208 will 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>
   1215 
   1216 <p>This separation allows JIT-only clients to be smaller.</p>
   1217 
   1218 <p>Note that LLVM does not currently have analogous APIs to support code
   1219 generation in the JIT, nor using the object writers.</p>
   1220 
   1221 <blockquote><pre
   1222 >// lib/MyGC/MyGCPrinter.cpp - Example LLVM GC printer
   1223 
   1224 #include "llvm/CodeGen/GCMetadataPrinter.h"
   1225 #include "llvm/Support/Compiler.h"
   1226 
   1227 using namespace llvm;
   1228 
   1229 namespace {
   1230   class LLVM_LIBRARY_VISIBILITY MyGCPrinter : public GCMetadataPrinter {
   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.");
   1241 }</pre></blockquote>
   1242 
   1243 <p>The collector should use <tt>AsmPrinter</tt> and <tt>TargetAsmInfo</tt> to
   1244 print portable assembly code to the <tt>std::ostream</tt>. The collector itself
   1245 contains the stack map for the entire module, and may access the
   1246 <tt>GCFunctionInfo</tt> using its own <tt>begin()</tt> and <tt>end()</tt>
   1247 methods. Here's a realistic example:</p>
   1248 
   1249 <blockquote><pre
   1250 >#include "llvm/CodeGen/AsmPrinter.h"
   1251 #include "llvm/Function.h"
   1252 #include "llvm/Target/TargetMachine.h"
   1253 #include "llvm/Target/TargetData.h"
   1254 #include "llvm/Target/TargetAsmInfo.h"
   1255 
   1256 void MyGCPrinter::beginAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
   1257                                 const TargetAsmInfo &amp;TAI) {
   1258   // Nothing to do.
   1259 }
   1260 
   1261 void MyGCPrinter::finishAssembly(std::ostream &amp;OS, AsmPrinter &amp;AP,
   1262                                  const TargetAsmInfo &amp;TAI) {
   1263   // Set up for emitting addresses.
   1264   const char *AddressDirective;
   1265   int AddressAlignLog;
   1266   if (AP.TM.getTargetData()->getPointerSize() == sizeof(int32_t)) {
   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...
   1278   for (iterator FI = begin(), FE = end(); FI != FE; ++FI) {
   1279     GCFunctionInfo &amp;MD = **FI;
   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     
   1295     // Emit the symbol by which the stack map entry can be found.
   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...
   1309     for (GCFunctionInfo::iterator PI = MD.begin(),
   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...
   1328       for (GCFunctionInfo::live_iterator LI = MD.live_begin(PI),
   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 
   1342 </div>
   1343 
   1344 <!-- *********************************************************************** -->
   1345 <h2>
   1346   <a name="references">References</a>
   1347 </h2>
   1348 <!-- *********************************************************************** -->
   1349 
   1350 <div>
   1351 
   1352 <p><a name="appel89">[Appel89]</a> Runtime Tags Aren't Necessary. Andrew
   1353 W. 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
   1356 strongly typed programming languages. Benjamin Goldberg. ACM SIGPLAN
   1357 PLDI'91.</p>
   1358 
   1359 <p><a name="tolmach94">[Tolmach94]</a> Tag-free garbage collection using
   1360 explicit type parameters. Andrew Tolmach. Proceedings of the 1994 ACM
   1361 conference on LISP and functional programming.</p>
   1362 
   1363 <p><a name="henderson02">[Henderson2002]</a> <a
   1364 href="http://citeseer.ist.psu.edu/henderson02accurate.html">
   1365 Accurate Garbage Collection in an Uncooperative Environment</a>.
   1366 Fergus Henderson. International Symposium on Memory Management 2002.</p>
   1367 
   1368 </div>
   1369 
   1370 
   1371 <!-- *********************************************************************** -->
   1372 
   1373 <hr>
   1374 <address>
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   1379 
   1380   <a href="mailto:sabre (a] nondot.org">Chris Lattner</a><br>
   1381   <a href="http://llvm.org/">LLVM Compiler Infrastructure</a><br>
   1382   Last modified: $Date: 2011-08-12 02:17:17 -0400 (Fri, 12 Aug 2011) $
   1383 </address>
   1384 
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