Accurate Garbage Collection with LLVM

1. Introduction
   - GC features provided and algorithms supported
2. Using the collectors
   - ShadowStack - A highly portable collector
   - SemiSpace - A simple copying collector runtime
   - Ocaml - An Objective Caml-compatible collector
3. Core support
   - Specifying GC code generation: `gc "..."`
   - Identifying GC roots on the stack: `llvm.gcroot`
   - Reading and writing references in the heap
     - Write barrier: `llvm.gcwrite`
     - Read barrier: `llvm.gcread`
4. Recommended runtime interface
   - Garbage collector startup and initialization
   - Allocating memory from the GC
   - Explicit invocation of the garbage collector
   - Tracing GC pointers from the program stack
   - Tracing GC pointers from static roots
5. Implementing a collector plugin
   - Overview of available features
   - Computing stack maps
   - Initializing roots to null: `InitRoots`
   - Custom lowering of intrinsics: `CustomRoots`, `CustomReadBarriers`, and `CustomWriteBarriers`
   - Generating safe points: `NeededSafePoints`
   - Emitting assembly code: `beginAssembly` and `finishAssembly`
6. Implementing a collector runtime
   - Tracing GC pointers from heap objects
7. References

Written by Chris Lattner and Gordon Henriksen

Introduction

Garbage collection is a widely used technique that frees the programmer from having to know the lifetimes of heap objects, making software easier to produce and maintain. Many programming languages rely on garbage collection for automatic memory management. There are two primary forms of garbage collection: conservative and accurate.

Conservative garbage collection often does not require any special support from either the language or the compiler: it can handle non-type-safe programming languages (such as C/C++) and does not require any special information from the compiler. The Boehm collector is an example of a state-of-the-art conservative collector.

Accurate garbage collection requires the ability to identify all pointers in the program at run-time (which requires that the source-language be type-safe in most cases). Identifying pointers at run-time requires compiler support to locate all places that hold live pointer variables at run-time, including the processor stack and registers.

Conservative garbage collection is attractive because it does not require any special compiler support, but it does have problems. In particular, because the conservative garbage collector cannot know that a particular word in the machine is a pointer, it cannot move live objects in the heap (preventing the use of compacting and generational GC algorithms) and it can occasionally suffer from memory leaks due to integer values that happen to point to objects in the program. In addition, some aggressive compiler transformations can break conservative garbage collectors (though these seem rare in practice).

Accurate garbage collectors do not suffer from any of these problems, but they can suffer from degraded scalar optimization of the program. In particular, because the runtime must be able to identify and update all pointers active in the program, some optimizations are less effective. In practice, however, the locality and performance benefits of using aggressive garbage allocation techniques dominates any low-level losses.

This document describes the mechanisms and interfaces provided by LLVM to support accurate garbage collection.

GC features provided and algorithms supported

LLVM's intermediate representation provides garbage collection intrinsics which offer support for a broad class of collector models. For instance, the intrinsics permit:

- semi-space collectors
- mark-sweep collectors
- generational collectors
- reference counting
- incremental collectors
- concurrent collectors
- cooperative collectors

We hope that the primitive support built into the LLVM IR is sufficient to support a broad class of garbage collected languages including Scheme, ML, Java, C#, Perl, Python, Lua, Ruby, other scripting languages, and more.

However, LLVM does not itself implement a garbage collector. This is because collectors are tightly coupled to object models, and LLVM is agnostic to object models. Since LLVM is agnostic to object models, it would be inappropriate for LLVM to dictate any particular collector. Instead, LLVM provides a framework for garbage collector implementations in two manners:

- At compile time with collector plugins for the compiler. Collector plugins have ready access to important garbage collector algorithms. Leveraging these tools, it is straightforward to emit type-accurate stack maps for your runtime in as little as ~100 lines of C++ code.
- At runtime with suggested runtime interfaces, which allow front-end compilers to support a range of collection runtimes.

Using the collectors

In general, using a collector implies:

- Emitting compatible code, including initialization in the main program if necessary.
- Loading a compiler plugin if the collector is not statically linked with your compiler. For `llc`, use the `-load` option.
- Selecting the collection algorithm by applying the `gc "..."` attribute to your garbage collected functions, or equivalently with the `setCollector` method.
- Linking your final executable with the garbage collector runtime.

This table summarizes the available runtimes.

Collector | `gc` attribute | Linkage | `gcroot` | `gcread` | `gcwrite`
--- | --- | --- | --- | --- | ---
SemiSpace | `gc "shadow-stack"` | TODO FIXME | required | optional | optional
Ocaml | `gc "ocaml"` | provided by ocamlopt | required | optional | optional

The sections for Collection intrinsics and Recommended runtime interface detail the interfaces that collectors may require user programs to utilize.

ShadowStack - A highly portable collector
`Collector *llvm::createShadowStackCollector();`

The ShadowStack backend is invoked with the `gc "shadow-stack"` function attribute. Unlike many collectors which rely on a cooperative code generator to generate stack maps, this algorithm carefully maintains a linked list of stack root descriptors [Henderson2002]. This so-called "shadow stack" mirrors the machine stack. Maintaining this data structure is slower than using stack maps, but has a significant portability advantage because it requires no special support from the target code generator.

The ShadowStack collector does not use read or write barriers, so the user program may use `load` and `store` instead of `llvm.gcread` and `llvm.gcwrite`.

ShadowStack is a code generator plugin only. It must be paired with a compatible runtime.

SemiSpace - A simple copying collector runtime

The SemiSpace runtime implements with the suggested runtime interface and is compatible the ShadowStack backend.

SemiSpace is a very simple copying collector. When it starts up, it allocates two blocks of memory for the heap. It uses a simple bump-pointer allocator to allocate memory from the first block until it runs out of space. When it runs out of space, it traces through all of the roots of the program, copying blocks to the other half of the memory space.

This runtime is highly experimental and has not been used in a real project. Enhancements would be welcomed.

Ocaml - An Objective Caml-compatible collector
`Collector *llvm::createOcamlCollector();`

The ocaml backend is invoked with the `gc "ocaml"` function attribute. It supports the Objective Caml language runtime by emitting a type-accurate stack map in the form of an ocaml 3.10.0-compatible frametable. The linkage requirements are satisfied automatically by the `ocamlopt` compiler when linking an executable.

The ocaml collector does not use read or write barriers, so the user program may use `load` and `store` instead of `llvm.gcread` and `llvm.gcwrite`.

Core support

This section describes the garbage collection facilities provided by the LLVM intermediate representation.

These facilities are limited to those strictly necessary for compilation. They are not intended to be a complete interface to any garbage collector. Notably, heap allocation is not among the supplied primitives. A user program will also need to interface with the runtime, using either the suggested runtime interface or another interface specified by the runtime.

Specifying GC code generation: `gc "..."`
`define ty @name(...) gc "collector" { ...`

The `gc` function attribute is used to specify the desired collector algorithm to the compiler. It is equivalent to specify the collector name programmatically using the `setCollector` method of `Function`.

Specifying the collector on a per-function basis allows LLVM to link together programs which use different garbage collection algorithms.

Identifying GC roots on the stack: `llvm.gcroot`
`void %llvm.gcroot(i8** %ptrloc, i8* %metadata)`

The `llvm.gcroot` intrinsic is used to inform LLVM of a pointer variable on the stack. The first argument must be an alloca instruction or a bitcast of an alloca. The second contains a pointer to metadata that should be associated with the pointer, and must be a constant or global value address. If your target collector uses tags, use a null pointer for metadata.

Consider the following fragment of Java code:

           {
             Object X;   // A null-initialized reference to an object
             ...
           }

This block (which may be located in the middle of a function or in a loop nest), could be compiled to this LLVM code:

    Entry:
       ;; In the entry block for the function, allocate the
       ;; stack space for X, which is an LLVM pointer.
       %X = alloca %Object*

       ;; Tell LLVM that the stack space is a stack root.
       ;; Java has type-tags on objects, so we pass null as metadata.
       %tmp = bitcast %Object** %X to i8**
       call void %llvm.gcroot(%i8** %X, i8* null)
       ...

       ;; "CodeBlock" is the block corresponding to the start
       ;;  of the scope above.
    CodeBlock:
       ;; Java null-initializes pointers.
       store %Object* null, %Object** %X

       ...

       ;; As the pointer goes out of scope, store a null value into
       ;; it, to indicate that the value is no longer live.
       store %Object* null, %Object** %X
       ...

Reading and writing references in the heap

Some collectors need to be informed when the mutator (the program that needs garbage collection) either reads a pointer from or writes a pointer to a field of a heap object. The code fragments inserted at these points are called read barriers and write barriers, respectively. The amount of code that needs to be executed is usually quite small and not on the critical path of any computation, so the overall performance impact of the barrier is tolerable.

Barriers often require access to the object pointer rather than the derived pointer (which is a pointer to the field within the object). Accordingly, these intrinsics take both pointers as separate arguments for completeness. In this snippet, `%object` is the object pointer, and `%derived` is the derived pointer:

>         ;; An array type.
>         %class.Array = type { %class.Object, i32, [0 x %class.Object*] }
>     ...

>         ;; Load the object pointer from a gcroot.
>         %object = load %class.Array** %object_addr

>         ;; Compute the derived pointer.
>         %derived = getelementptr %obj, i32 0, i32 2, i32 %n

Write barrier: `llvm.gcwrite`
`void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived)`

For write barriers, LLVM provides the `llvm.gcwrite` intrinsic function. It has exactly the same semantics as a non-volatile `store` to the derived pointer (the third argument).

Many important algorithms require write barriers, including generational and concurrent collectors. Additionally, write barriers could be used to implement reference counting.

The use of this intrinsic is optional if the target collector does use write barriers. If so, the collector will replace it with the corresponding `store`.

Read barrier: `llvm.gcread`
`i8* @llvm.gcread(i8* %object, i8** %derived)
`

For read barriers, LLVM provides the `llvm.gcread` intrinsic function. It has exactly the same semantics as a non-volatile `load` from the derived pointer (the second argument).

Read barriers are needed by fewer algorithms than write barriers, and may have a greater performance impact since pointer reads are more frequent than writes.

As with `llvm.gcwrite`, a target collector might not require the use of this intrinsic.

Recommended runtime interface

LLVM specifies the following recommended runtime interface to the garbage collection at runtime. A program should use these interfaces to accomplish the tasks not supported by the intrinsics.

Unlike the intrinsics, which are integral to LLVM's code generator, there is nothing unique about these interfaces; a front-end compiler and runtime are free to agree to a different specification.

Note: This interface is a work in progress.

Garbage collector startup and initialization
`void llvm_gc_initialize(unsigned InitialHeapSize);`

The `llvm_gc_initialize` function should be called once before any other garbage collection functions are called. This gives the garbage collector the chance to initialize itself and allocate the heap. The initial heap size to allocate should be specified as an argument.

Allocating memory from the GC
`void *llvm_gc_allocate(unsigned Size);`

The `llvm_gc_allocate` function is a global function defined by the garbage collector implementation to allocate memory. It returns a zeroed-out block of memory of the specified size, sufficiently aligned to store any object.

Explicit invocation of the garbage collector
`void llvm_gc_collect();`

The `llvm_gc_collect` function is exported by the garbage collector implementations to provide a full collection, even when the heap is not exhausted. This can be used by end-user code as a hint, and may be ignored by the garbage collector.

Tracing GC pointers from the program stack
`void llvm_cg_walk_gcroots(void (*FP)(void **Root, void *Meta));`

The `llvm_cg_walk_gcroots` function is a function provided by the code generator that iterates through all of the GC roots on the stack, calling the specified function pointer with each record. For each GC root, the address of the pointer and the meta-data (from the `llvm.gcroot` intrinsic) are provided.

Tracing GC pointers from static roots
TODO
Implementing a collector plugin

User code specifies which collector plugin to use with the `gc` function attribute or, equivalently, with the `setCollector` method of `Function`.

To implement a collector plugin, it is necessary to subclass `llvm::Collector`, which can be accomplished in a few lines of boilerplate code. LLVM's infrastructure provides access to several important algorithms. For an uncontroversial collector, all that remains may be to emit the assembly code for the collector's unique stack map data structure, which might be accomplished in as few as 100 LOC.

To subclass `llvm::Collector` and register a collector:

>     // lib/MyGC/MyGC.cpp - Example LLVM collector plugin

>     #include "llvm/CodeGen/Collector.h"
>     #include "llvm/CodeGen/Collectors.h"
>     #include "llvm/CodeGen/CollectorMetadata.h"
>     #include "llvm/Support/Compiler.h"

>     using namespace llvm;

>     namespace {
>       class VISIBILITY_HIDDEN MyCollector : public Collector {
>       public:
>         MyCollector() {}
>       };

>       CollectorRegistry::Add<MyCollector>
>       X("mygc", "My bespoke garbage collector.");
>     }

Using the LLVM makefiles (like the sample project), this can be built into a plugin using a simple makefile:

>     # lib/MyGC/Makefile

>     LEVEL := ../..
>     LIBRARYNAME = MyGC
>     LOADABLE_MODULE = 1

>     include $(LEVEL)/Makefile.common

Once the plugin is compiled, code using it may be compiled using `llc -load=MyGC.so` (though MyGC.so may have some other platform-specific extension):

>     $ cat sample.ll
>     define void @f() gc "mygc" {
>     entry:
>             ret void
>     }
>     $ llvm-as < sample.ll | llc -load=MyGC.so

It is also possible to statically link the collector plugin into tools, such as a language-specific compiler front-end.

Overview of available features

The boilerplate collector above does nothing. More specifically:

- `llvm.gcread` calls are replaced with the corresponding `load` instruction.
- `llvm.gcwrite` calls are replaced with the corresponding `store` instruction.
- No stack map is emitted, and no safe points are added.

`Collector` provides a range of features through which a plugin collector may do useful work. This matrix summarizes the supported (and planned) features and correlates them with the collection techniques which typically require them.

Algorithm | Done | shadow stack | refcount | mark-sweep | copying | incremental | threaded | concurrent
--- | --- | --- | --- | --- | --- | --- | --- | ---
stack map | ✔ |  |  | ✘ | ✘ | ✘ | ✘ | ✘
initialize roots | ✔ | ✘ | ✘ | ✘ | ✘ | ✘ | ✘ | ✘
derived pointers | NO |  |  |  |  |  | ✘* | ✘*
custom lowering | ✔ |  |  |  |  |  |  |
gcroot | ✔ | ✘ | ✘ |  |  |  |  |
gcwrite | ✔ |  | ✘ |  |  | ✘ |  | ✘
gcread | ✔ |  |  |  |  |  |  | ✘
safe points |  |  |  |  |  |  |  |
in calls | ✔ |  |  | ✘ | ✘ | ✘ | ✘ | ✘
before calls | ✔ |  |  |  |  |  | ✘ | ✘
for loops | NO |  |  |  |  |  | ✘ | ✘
before escape | ✔ |  |  |  |  |  | ✘ | ✘
emit code at safe points | NO |  |  |  |  |  | ✘ | ✘
output |  |  |  |  |  |  |  |
assembly | ✔ |  |  | ✘ | ✘ | ✘ | ✘ | ✘
JIT | NO |  |  | ✘ | ✘ | ✘ | ✘ | ✘
obj | NO |  |  | ✘ | ✘ | ✘ | ✘ | ✘
live analysis | NO |  |  | ✘ | ✘ | ✘ | ✘ | ✘
register map | NO |  |  | ✘ | ✘ | ✘ | ✘ | ✘
* Derived pointers only pose a hazard to copying collectors. ✘ in gray denotes a feature which could be utilized if available.

To be clear, the collection techniques above are defined as:

Shadow Stack
  The mutator carefully maintains a linked list of stack root descriptors.
Reference Counting
  The mutator maintains a reference count for each object and frees an object when its count falls to zero.
Mark-Sweep
  When the heap is exhausted, the collector marks reachable objects starting from the roots, then deallocates unreachable objects in a sweep phase.
Copying
  As reachability analysis proceeds, the collector copies objects from one heap area to another, compacting them in the process. Copying collectors enable highly efficient "bump pointer" allocation and can improve locality of reference.
Incremental
  (Including generational collectors.) Incremental collectors generally have all the properties of a copying collector (regardless of whether the mature heap is compacting), but bring the added complexity of requiring write barriers.
Threaded
  Denotes a multithreaded mutator; the collector must still stop the mutator ("stop the world") before beginning reachability analysis. Stopping a multithreaded mutator is a complicated problem. It generally requires highly platform specific code in the runtime, and the production of carefully designed machine code at safe points.
Concurrent
  In this technique, the mutator and the collector run concurrently, with the goal of eliminating pause times. In a cooperative collector, the mutator further aids with collection should a pause occur, allowing collection to take advantage of multiprocessor hosts. The "stop the world" problem of threaded collectors is generally still present to a limited extent. Sophisticated marking algorithms are necessary. Read barriers may be necessary.

As the matrix indicates, LLVM's garbage collection infrastructure is already suitable for a wide variety of collectors, but does not currently extend to multithreaded programs. This will be added in the future as there is interest.

Computing stack maps

>     for (iterator I = begin(), E = end(); I != E; ++I) {
>       CollectorMetadata *MD = *I;
>       unsigned FrameSize = MD->getFrameSize();
>       size_t RootCount = MD->roots_size();

>       for (CollectorMetadata::roots_iterator RI = MD->roots_begin(),
>                                              RE = MD->roots_end();
>                                              RI != RE; ++RI) {
>         int RootNum = RI->Num;
>         int RootStackOffset = RI->StackOffset;
>         Constant *RootMetadata = RI->Metadata;
>       }
>     }

LLVM automatically computes a stack map. All a `Collector` needs to do is access it using `CollectorMetadata::roots_begin()` and -`end()`. If the `llvm.gcroot` intrinsic is eliminated before code generation by a custom lowering pass, LLVM's stack map will be empty.

Initializing roots to null: `InitRoots`

>     MyCollector::MyCollector() {
>       InitRoots = true;
>     }

When set, LLVM will automatically initialize each root to `null` upon entry to the function. This prevents the reachability analysis from finding uninitialized values in stack roots at runtime, which will almost certainly cause it to segfault. This initialization occurs before custom lowering, so the two may be used together.

Since LLVM does not yet compute liveness information, this feature should be used by all collectors which do not custom lower `llvm.gcroot`, and even some that do.

Custom lowering of intrinsics: `CustomRoots`, `CustomReadBarriers`, and `CustomWriteBarriers`

For collectors with barriers or unusual treatment of stack roots, these flags allow the collector to perform any required transformation on the LLVM IR:

>     class MyCollector : public Collector {
>     public:
>       MyCollector() {
>         CustomRoots = true;
>         CustomReadBarriers = true;
>         CustomWriteBarriers = true;
>       }

>       virtual bool initializeCustomLowering(Module &M);
>       virtual bool performCustomLowering(Function &F);
>     };

If any of these flags are set, then LLVM suppresses its default lowering for the corresponding intrinsics and instead passes them on to a custom lowering pass specified by the collector.

LLVM's default action for each intrinsic is as follows:

- `llvm.gcroot`: Pass through to the code generator to generate a stack map.
- `llvm.gcread`: Substitute a `load` instruction.
- `llvm.gcwrite`: Substitute a `store` instruction.

If `CustomReadBarriers` or `CustomWriteBarriers` are specified, then `performCustomLowering` must eliminate the corresponding barriers.

`performCustomLowering`, must comply with the same restrictions as `runOnFunction`, and that `initializeCustomLowering` has the same semantics as `doInitialization(Module &)`.

The following can be used as a template:

>     #include "llvm/Module.h"
>     #include "llvm/Instructions.h"

>     bool MyCollector::initializeCustomLowering(Module &M) {
>       return false;
>     }

>     bool MyCollector::performCustomLowering(Function &F) {
>       const Module *M = F.getParent();

>       Function *GCReadInt  = M->getFunction("llvm.gcread"),
>                *GCWriteInt = M->getFunction("llvm.gcwrite"),
>                *GCRootInt  = M->getFunction("llvm.gcroot");

>       bool MadeChange = false;

>       for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
>         for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E;)
>           if (CallInst *CI = dyn_cast<CallInst>(II++))
>             if (Function *F = CI->getCalledFunction())
>               if (F == GCWriteInt) {
>                 // Handle llvm.gcwrite.
>                 CI->eraseFromParent();
>                 MadeChange = true;
>               } else if (F == GCReadInt) {
>                 // Handle llvm.gcread.
>                 CI->eraseFromParent();
>                 MadeChange = true;
>               } else if (F == GCRootInt) {
>                 // Handle llvm.gcroot.
>                 CI->eraseFromParent();
>                 MadeChange = true;
>               }

>       return MadeChange;
>     }

Generating safe points: `NeededSafePoints`

LLVM can compute four kinds of safe points:

>     namespace GC {
>       /// PointKind - The type of a collector-safe point.
>       ///
>       enum PointKind {
>         Loop,    //< Instr is a loop (backwards branch).
>         Return,  //< Instr is a return instruction.
>         PreCall, //< Instr is a call instruction.
>         PostCall //< Instr is the return address of a call.
>       };
>     }

A collector can request any combination of the four by setting the `NeededSafePoints` mask:

>     MyCollector::MyCollector() {
>       NeededSafePoints = 1 << GC::Loop
>                        | 1 << GC::Return
>                        | 1 << GC::PreCall
>                        | 1 << GC::PostCall;
>     }

It can then use the following routines to access safe points.

>     for (iterator I = begin(), E = end(); I != E; ++I) {
>       CollectorMetadata *MD = *I;
>       size_t PointCount = MD->size();

>       for (CollectorMetadata::iterator PI = MD->begin(),
>                                        PE = MD->end(); PI != PE; ++PI) {
>         GC::PointKind PointKind = PI->Kind;
>         unsigned PointNum = PI->Num;
>       }
>     }

Almost every collector requires `PostCall` safe points, since these correspond to the moments when the function is suspended during a call to a subroutine.

Threaded programs generally require `Loop` safe points to guarantee that the application will reach a safe point within a bounded amount of time, even if it is executing a long-running loop which contains no function calls.

Threaded collectors may also require `Return` and `PreCall` safe points to implement "stop the world" techniques using self-modifying code, where it is important that the program not exit the function without reaching a safe point (because only the topmost function has been patched).

Emitting assembly code: `beginAssembly` and `finishAssembly`

LLVM allows a collector to print arbitrary assembly code before and after the rest of a module's assembly code. From the latter callback, the collector can print stack maps built by the code generator.

Note that LLVM does not currently have analogous APIs to support code generation in the JIT, nor using the object writers.

>     class MyCollector : public Collector {
>     public:
>       virtual void beginAssembly(std::ostream &OS, AsmPrinter &AP,
>                                  const TargetAsmInfo &TAI);

>       virtual void finishAssembly(std::ostream &OS, AsmPrinter &AP,
>                                   const TargetAsmInfo &TAI);
>     }

The collector should use `AsmPrinter` and `TargetAsmInfo` to print portable assembly code to the `std::ostream`. The collector itself contains the stack map for the entire module, and may access the `CollectorMetadata` using its own `begin()` and `end()` methods. Here's a realistic example:

>     #include "llvm/CodeGen/AsmPrinter.h"
>     #include "llvm/Function.h"
>     #include "llvm/Target/TargetMachine.h"
>     #include "llvm/Target/TargetData.h"
>     #include "llvm/Target/TargetAsmInfo.h"

>     void MyCollector::beginAssembly(std::ostream &OS, AsmPrinter &AP,
>                                     const TargetAsmInfo &TAI) {
>       // Nothing to do.
>     }

>     void MyCollector::finishAssembly(std::ostream &OS, AsmPrinter &AP,
>                                      const TargetAsmInfo &TAI) {
>       // Set up for emitting addresses.
>       const char *AddressDirective;
>       int AddressAlignLog;
>       if (AP.TM.getTargetData()->getPointerSize() == sizeof(int32_t)) {
>         AddressDirective = TAI.getData32bitsDirective();
>         AddressAlignLog = 2;
>       } else {
>         AddressDirective = TAI.getData64bitsDirective();
>         AddressAlignLog = 3;
>       }

>       // Put this in the data section.
>       AP.SwitchToDataSection(TAI.getDataSection());

>       // For each function...
>       for (iterator FI = begin(), FE = end(); FI != FE; ++FI) {
>         CollectorMetadata &MD = **FI;

>         // Emit this data structure:
>         //
>         // struct {
>         //   int32_t PointCount;
>         //   struct {
>         //     void *SafePointAddress;
>         //     int32_t LiveCount;
>         //     int32_t LiveOffsets[LiveCount];
>         //   } Points[PointCount];
>         // } __gcmap_<FUNCTIONNAME>;

>         // Align to address width.
>         AP.EmitAlignment(AddressAlignLog);

>         // Emit the symbol by which the stack map can be found.
>         std::string Symbol;
>         Symbol += TAI.getGlobalPrefix();
>         Symbol += "__gcmap_";
>         Symbol += MD.getFunction().getName();
>         if (const char *GlobalDirective = TAI.getGlobalDirective())
>           OS << GlobalDirective << Symbol << "\n";
>         OS << TAI.getGlobalPrefix() << Symbol << ":\n";

>         // Emit PointCount.
>         AP.EmitInt32(MD.size());
>         AP.EOL("safe point count");

>         // And each safe point...
>         for (CollectorMetadata::iterator PI = MD.begin(),
>                                          PE = MD.end(); PI != PE; ++PI) {
>           // Align to address width.
>           AP.EmitAlignment(AddressAlignLog);

>           // Emit the address of the safe point.
>           OS << AddressDirective
>              << TAI.getPrivateGlobalPrefix() << "label" << PI->Num;
>           AP.EOL("safe point address");

>           // Emit the stack frame size.
>           AP.EmitInt32(MD.getFrameSize());
>           AP.EOL("stack frame size");

>           // Emit the number of live roots in the function.
>           AP.EmitInt32(MD.live_size(PI));
>           AP.EOL("live root count");

>           // And for each live root...
>           for (CollectorMetadata::live_iterator LI = MD.live_begin(PI),
>                                                 LE = MD.live_end(PI);
>                                                 LI != LE; ++LI) {
>             // Print its offset within the stack frame.
>             AP.EmitInt32(LI->StackOffset);
>             AP.EOL("stack offset");
>           }
>         }
>       }
>     }

Implementing a collector runtime

Implementing a garbage collector for LLVM is fairly straightforward. The LLVM garbage collectors are provided in a form that makes them easy to link into the language-specific runtime that a language front-end would use. They require functionality from the language-specific runtime to get information about where pointers are located in heap objects.

The implementation must include the `llvm_gc_allocate` and `llvm_gc_collect` functions. To do this, it will probably have to trace through the roots from the stack and understand the GC descriptors for heap objects. Luckily, there are some example implementations available.

Tracing GC pointers from heap objects

The three most common ways to keep track of where pointers live in heap objects are (listed in order of space overhead required):

1. In languages with polymorphic objects, pointers from an object header are usually used to identify the GC pointers in the heap object. This is common for object-oriented languages like Self, Smalltalk, Java, or C#.
2. If heap objects are not polymorphic, often the "shape" of the heap can be determined from the roots of the heap or from some other meta-data [Appel89, Goldberg91, Tolmach94]. In this case, the garbage collector can propagate the information around from meta data stored with the roots. This often eliminates the need to have a header on objects in the heap. This is common in the ML family.
3. If all heap objects have pointers in the same locations, or pointers can be distinguished just by looking at them (e.g., the low order bit is clear), no book-keeping is needed at all. This is common for Lisp-like languages.

The LLVM garbage collectors are capable of supporting all of these styles of language, including ones that mix various implementations. To do this, it allows the source-language to associate meta-data with the stack roots, and the heap tracing routines can propagate the information. In addition, LLVM allows the front-end to extract GC information in any form from a specific object pointer (this supports situations #1 and #3).

References

[Appel89] Runtime Tags Aren't Necessary. Andrew W. Appel. Lisp and Symbolic Computation 19(7):703-705, July 1989.

[Goldberg91] Tag-free garbage collection for strongly typed programming languages. Benjamin Goldberg. ACM SIGPLAN PLDI'91.

[Tolmach94] Tag-free garbage collection using explicit type parameters. Andrew Tolmach. Proceedings of the 1994 ACM conference on LISP and functional programming.

[Henderson2002] Accurate Garbage Collection in an Uncooperative Environment. Fergus Henderson. International Symposium on Memory Management 2002.