Garbage Collection with LLVM — LLVM 5 documentation

# Garbage Collection with LLVM

- Abstract
- Quick Start
- Introduction
  - What is Garbage Collection?
  - Goals and non-goals
- LLVM IR Features
  - Specifying GC code generation: `gc "..."`
  - Identifying GC roots on the stack
    - Using `gc.statepoint`
    - Using `llvm.gcwrite`
  - Reading and writing references in the heap
    - Write barrier: `llvm.gcwrite`
    - Read barrier: `llvm.gcread`
- Built In GC Strategies
  - The Shadow Stack GC
  - The ‘Erlang’ and ‘Ocaml’ GCs
  - The Statepoint Example GC
  - The CoreCLR GC
- Custom GC Strategies
  - Collector Requirements
  - 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: `GCMetadataPrinter`
- References

## Abstract

This document covers how to integrate LLVM into a compiler for a language which supports garbage collection. Note that LLVM itself does not provide a garbage collector. You must provide your own.

## Quick Start

First, you should pick a collector strategy. LLVM includes a number of built in ones, but you can also implement a loadable plugin with a custom definition. Note that the collector strategy is a description of how LLVM should generate code such that it interacts with your collector and runtime, not a description of the collector itself.

Next, mark your generated functions as using your chosen collector strategy. From c++, you can call:

    F.setGC(<collector description name>);

This will produce IR like the following fragment:

    define void @foo() gc "<collector description name>" { ... }

When generating LLVM IR for your functions, you will need to:

- Use `@llvm.gcread` and/or `@llvm.gcwrite` in place of standard load and store instructions. These intrinsics are used to represent load and store barriers. If you collector does not require such barriers, you can skip this step.
- Use the memory allocation routines provided by your garbage collector’s runtime library.
- If your collector requires them, generate type maps according to your runtime’s binary interface. LLVM is not involved in the process. In particular, the LLVM type system is not suitable for conveying such information though the compiler.
- Insert any coordination code required for interacting with your collector. Many collectors require running application code to periodically check a flag and conditionally call a runtime function. This is often referred to as a safepoint poll.

You will need to identify roots (i.e. references to heap objects your collector needs to know about) in your generated IR, so that LLVM can encode them into your final stack maps. Depending on the collector strategy chosen, this is accomplished by using either the `@llvm.gcroot` intrinsics or an `gc.statepoint` relocation sequence.

Don’t forget to create a root for each intermediate value that is generated when evaluating an expression. In `h(f(), g())`, the result of `f()` could easily be collected if evaluating `g()` triggers a collection.

Finally, you need to link your runtime library with the generated program executable (for a static compiler) or ensure the appropriate symbols are available for the runtime linker (for a JIT compiler).

## Introduction

### What is Garbage Collection?

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 collection techniques dominates any low-level losses.

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

### Goals and non-goals

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

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

We hope that the 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.

Note that LLVM does not itself provide a garbage collector — this should be part of your language’s runtime library. LLVM provides a framework for describing the garbage collectors requirements to the compiler. In particular, LLVM provides support for generating stack maps at call sites, polling for a safepoint, and emitting load and store barriers. You can also extend LLVM - possibly through a loadable code generation plugins - to generate code and data structures which conforms to the binary interface specified by the runtime library. This is similar to the relationship between LLVM and DWARF debugging info, for example. The difference primarily lies in the lack of an established standard in the domain of garbage collection — thus the need for a flexible extension mechanism.

The aspects of the binary interface with which LLVM’s GC support is concerned are:

- Creation of GC safepoints within code where collection is allowed to execute safely.
- Computation of the stack map. For each safe point in the code, object references within the stack frame must be identified so that the collector may traverse and perhaps update them.
- Write barriers when storing object references to the heap. These are commonly used to optimize incremental scans in generational collectors.
- Emission of read barriers when loading object references. These are useful for interoperating with concurrent collectors.

There are additional areas that LLVM does not directly address:

- Registration of global roots with the runtime.
- Registration of stack map entries with the runtime.
- The functions used by the program to allocate memory, trigger a collection, etc.
- Computation or compilation of type maps, or registration of them with the runtime. These are used to crawl the heap for object references.

In general, LLVM’s support for GC does not include features which can be adequately addressed with other features of the IR and does not specify a particular binary interface. On the plus side, this means that you should be able to integrate LLVM with an existing runtime. On the other hand, it can have the effect of leaving a lot of work for the developer of a novel language. We try to mitigate this by providing built in collector strategy descriptions that can work with many common collector designs and easy extension points. If you don’t already have a specific binary interface you need to support, we recommend trying to use one of these built in collector strategies.

## LLVM IR Features

This section describes the garbage collection facilities provided by the LLVM intermediate representation. The exact behavior of these IR features is specified by the selected GC strategy description.

### Specifying GC code generation: `gc "..."`

    define <returntype> @name(...) gc "name" { ... }

The `gc` function attribute is used to specify the desired GC strategy to the compiler. Its programmatic equivalent is the `setGC` method of `Function`.

Setting `gc "name"` on a function triggers a search for a matching subclass of GCStrategy. Some collector strategies are built in. You can add others using either the loadable plugin mechanism, or by patching your copy of LLVM. It is the selected GC strategy which defines the exact nature of the code generated to support GC. If none is found, the compiler will raise an error.

Specifying the GC style on a per-function basis allows LLVM to link together programs that use different garbage collection algorithms (or none at all).

### Identifying GC roots on the stack

LLVM currently supports two different mechanisms for describing references in compiled code at safepoints. `llvm.gcroot` is the older mechanism; `gc.statepoint` has been added more recently. At the moment, you can choose either implementation (on a per GC strategy basis). Longer term, we will probably either migrate away from `llvm.gcroot` entirely, or substantially merge their implementations. Note that most new development work is focused on `gc.statepoint`.

#### Using `gc.statepoint`

This page contains detailed documentation for `gc.statepoint`.

#### Using `llvm.gcwrite`

    void @llvm.gcroot(i8** %ptrloc, i8* %metadata)

The `llvm.gcroot` intrinsic is used to inform LLVM that a stack variable references an object on the heap and is to be tracked for garbage collection. The exact impact on generated code is specified by the Function’s selected GC strategy. All calls to `llvm.gcroot` must reside inside the first basic block.

The first argument must be a value referring to 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.

A compiler which performs manual SSA construction must ensure that SSA values representing GC references are stored in to the alloca passed to the respective `gcroot` before every call site and reloaded after every call. A compiler which uses mem2reg to raise imperative code using `alloca` into SSA form need only add a call to `@llvm.gcroot` for those variables which are pointers into the GC heap.

It is also important to mark intermediate values with `llvm.gcroot`. For example, consider `h(f(), g())`. Beware leaking the result of `f()` in the case that `g()` triggers a collection. Note, that stack variables must be initialized and marked with `llvm.gcroot` in function’s prologue.

The `%metadata` argument can be used to avoid requiring heap objects to have ‘isa’ pointers or tag bits. [Appel89, Goldberg91, Tolmach94] If specified, its value will be tracked along with the location of the pointer in the stack frame.

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** %tmp, 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 %object, i32 0, i32 2, i32 %n

LLVM does not enforce this relationship between the object and derived pointer (although a particular collector strategy might). However, it would be an unusual collector that violated it.

The use of these intrinsics is naturally optional if the target GC does not require the corresponding barrier. The GC strategy used with such a collector should replace the intrinsic calls with the corresponding `load` or `store` instruction if they are used.

One known deficiency with the current design is that the barrier intrinsics do not include the size or alignment of the underlying operation performed. It is currently assumed that the operation is of pointer size and the alignment is assumed to be the target machine’s default alignment.

#### 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). The exact code generated is specified by the Function’s selected GC strategy.

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

#### 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). The exact code generated is specified by the Function’s selected GC strategy.

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.

## Built In GC Strategies

LLVM includes built in support for several varieties of garbage collectors.

### The Shadow Stack GC

To use this collector strategy, mark your functions with:

    F.setGC("shadow-stack");

Unlike many GC algorithms which rely on a cooperative code generator to compile stack maps, this algorithm carefully maintains a linked list of stack roots [Henderson2002]. This so-called “shadow stack” mirrors the machine stack. Maintaining this data structure is slower than using a stack map compiled into the executable as constant data, but has a significant portability advantage because it requires no special support from the target code generator, and does not require tricky platform-specific code to crawl the machine stack.

The tradeoff for this simplicity and portability is:

- High overhead per function call.
- Not thread-safe.

Still, it’s an easy way to get started. After your compiler and runtime are up and running, writing a plugin will allow you to take advantage of more advanced GC features of LLVM in order to improve performance.

The shadow stack doesn’t imply a memory allocation algorithm. A semispace collector or building atop `malloc` are great places to start, and can be implemented with very little code.

When it comes time to collect, however, your runtime needs to traverse the stack roots, and for this it needs to integrate with the shadow stack. Luckily, doing so is very simple. (This code is heavily commented to help you understand the data structure, but there are only 20 lines of meaningful code.)

    /// @brief The map for a single function's stack frame.  One of these is
    ///        compiled as constant data into the executable for each function.
    ///
    /// Storage of metadata values is elided if the %metadata parameter to
    /// @llvm.gcroot is null.
    struct FrameMap {
      int32_t NumRoots;    //< Number of roots in stack frame.
      int32_t NumMeta;     //< Number of metadata entries.  May be < NumRoots.
      const void *Meta[0]; //< Metadata for each root.
    };

    /// @brief A link in the dynamic shadow stack.  One of these is embedded in
    ///        the stack frame of each function on the call stack.
    struct StackEntry {
      StackEntry *Next;    //< Link to next stack entry (the caller's).
      const FrameMap *Map; //< Pointer to constant FrameMap.
      void *Roots[0];      //< Stack roots (in-place array).
    };

    /// @brief The head of the singly-linked list of StackEntries.  Functions push
    ///        and pop onto this in their prologue and epilogue.
    ///
    /// Since there is only a global list, this technique is not threadsafe.
    StackEntry *llvm_gc_root_chain;

    /// @brief Calls Visitor(root, meta) for each GC root on the stack.
    ///        root and meta are exactly the values passed to
    ///        @llvm.gcroot.
    ///
    /// Visitor could be a function to recursively mark live objects.  Or it
    /// might copy them to another heap or generation.
    ///
    /// @param Visitor A function to invoke for every GC root on the stack.
    void visitGCRoots(void (*Visitor)(void **Root, const void *Meta)) {
      for (StackEntry *R = llvm_gc_root_chain; R; R = R->Next) {
        unsigned i = 0;

        // For roots [0, NumMeta), the metadata pointer is in the FrameMap.
        for (unsigned e = R->Map->NumMeta; i != e; ++i)
          Visitor(&R->Roots[i], R->Map->Meta[i]);

        // For roots [NumMeta, NumRoots), the metadata pointer is null.
        for (unsigned e = R->Map->NumRoots; i != e; ++i)
          Visitor(&R->Roots[i], NULL);
      }
    }

### The ‘Erlang’ and ‘Ocaml’ GCs

LLVM ships with two example collectors which leverage the `gcroot` mechanisms. To our knowledge, these are not actually used by any language runtime, but they do provide a reasonable starting point for someone interested in writing an `gcroot` compatible GC plugin. In particular, these are the only in tree examples of how to produce a custom binary stack map format using a `gcroot` strategy.

As there names imply, the binary format produced is intended to model that used by the Erlang and OCaml compilers respectively.

### The Statepoint Example GC

    F.setGC("statepoint-example");

This GC provides an example of how one might use the infrastructure provided by `gc.statepoint`. This example GC is compatible with the PlaceSafepoints and RewriteStatepointsForGC utility passes which simplify `gc.statepoint` sequence insertion. If you need to build a custom GC strategy around the `gc.statepoints` mechanisms, it is recommended that you use this one as a starting point.

This GC strategy does not support read or write barriers. As a result, these intrinsics are lowered to normal loads and stores.

The stack map format generated by this GC strategy can be found in the Stack Map Section using a format documented here. This format is intended to be the standard format supported by LLVM going forward.

### The CoreCLR GC

    F.setGC("coreclr");

This GC leverages the `gc.statepoint` mechanism to support the CoreCLR runtime.

Support for this GC strategy is a work in progress. This strategy will differ from statepoint-example GC strategy in certain aspects like:

- Base-pointers of interior pointers are not explicitly tracked and reported.
- A different format is used for encoding stack maps.
- Safe-point polls are only needed before loop-back edges and before tail-calls (not needed at function-entry).

## Custom GC Strategies

If none of the built in GC strategy descriptions met your needs above, you will need to define a custom GCStrategy and possibly, a custom LLVM pass to perform lowering. Your best example of where to start defining a custom GCStrategy would be to look at one of the built in strategies.

You may be able to structure this additional code as a loadable plugin library. Loadable plugins are sufficient if all you need is to enable a different combination of built in functionality, but if you need to provide a custom lowering pass, you will need to build a patched version of LLVM. If you think you need a patched build, please ask for advice on llvm-dev. There may be an easy way we can extend the support to make it work for your use case without requiring a custom build.

### Collector Requirements

You should be able to leverage any existing collector library that includes the following elements:

1. A memory allocator which exposes an allocation function your compiled code can call.
2. A binary format for the stack map. A stack map describes the location of references at a safepoint and is used by precise collectors to identify references within a stack frame on the machine stack. Note that collectors which conservatively scan the stack don’t require such a structure.
3. A stack crawler to discover functions on the call stack, and enumerate the references listed in the stack map for each call site.
4. A mechanism for identifying references in global locations (e.g. global variables).
5. If you collector requires them, an LLVM IR implementation of your collectors load and store barriers. Note that since many collectors don’t require barriers at all, LLVM defaults to lowering such barriers to normal loads and stores unless you arrange otherwise.

### Implementing a collector plugin

User code specifies which GC code generation to use with the `gc` function attribute or, equivalently, with the `setGC` method of `Function`.

To implement a GC plugin, it is necessary to subclass `llvm::GCStrategy`, 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 compile LLVM’s computed stack map to assembly code (using the binary representation expected by the runtime library). This can be accomplished in about 100 lines of code.

This is not the appropriate place to implement a garbage collected heap or a garbage collector itself. That code should exist in the language’s runtime library. The compiler plugin is responsible for generating code which conforms to the binary interface defined by library, most essentially the stack map.

To subclass `llvm::GCStrategy` and register it with the compiler:

    // lib/MyGC/MyGC.cpp - Example LLVM GC plugin

    #include "llvm/CodeGen/GCStrategy.h"
    #include "llvm/CodeGen/GCMetadata.h"
    #include "llvm/Support/Compiler.h"

    using namespace llvm;

    namespace {
      class LLVM_LIBRARY_VISIBILITY MyGC : public GCStrategy {
      public:
        MyGC() {}
      };

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

This boilerplate collector 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 safe points are added to the code.
- The stack map is not compiled into the executable.

Using the LLVM makefiles, this code can be compiled as 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

`GCStrategy` provides a range of features through which a plugin may do useful work. Some of these are callbacks, some are algorithms that can be enabled, disabled, or customized. 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 |  |  |  |  |  | N* | N*
custom lowering | ✔ |  |  |  |  |  |  |
gcroot | ✔ | ✘ | ✘ |  |  |  |  |
gcwrite | ✔ |  | ✘ |  |  | ✘ |  | ✘
gcread | ✔ |  |  |  |  |  |  | ✘
safe points |  |  |  |  |  |  |  |
in calls | ✔ |  |  | ✘ | ✘ | ✘ | ✘ | ✘
before calls | ✔ |  |  |  |  |  | ✘ | ✘
for loops | NO |  |  |  |  |  | N | N
before escape | ✔ |  |  |  |  |  | ✘ | ✘
emit code at safe points | NO |  |  |  |  |  | N | N
output |  |  |  |  |  |  |  |
assembly | ✔ |  |  | ✘ | ✘ | ✘ | ✘ | ✘
JIT | NO |  |  | ? | ? | ? | ? | ?
obj | NO |  |  | ? | ? | ? | ? | ?
live analysis | NO |  |  | ? | ? | ? | ? | ?
register map | NO |  |  | ? | ? | ? | ? | ?
* Derived pointers only pose a hasard to copying collections.
? 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 roots.
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

LLVM automatically computes a stack map. One of the most important features of a `GCStrategy` is to compile this information into the executable in the binary representation expected by the runtime library.

The stack map consists of the location and identity of each GC root in the each function in the module. For each root:

- `RootNum`: The index of the root.
- `StackOffset`: The offset of the object relative to the frame pointer.
- `RootMetadata`: The value passed as the `%metadata` parameter to the `@llvm.gcroot` intrinsic.

Also, for the function as a whole:

- `getFrameSize()`: The overall size of the function’s initial stack frame,
    not accounting for any dynamic allocation.
- `roots_size()`: The count of roots in the function.

To access the stack map, use `GCFunctionMetadata::roots_begin()` and -`end()` from the GCMetadataPrinter:

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

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

If the `llvm.gcroot` intrinsic is eliminated before code generation by a custom lowering pass, LLVM will compute an empty stack map. This may be useful for collector plugins which implement reference counting or a shadow stack.

### Initializing roots to null: `InitRoots`

    MyGC::MyGC() {
      InitRoots = true;
    }

When set, LLVM will automatically initialize each root to `null` upon entry to the function. This prevents the GC’s sweep phase from visiting uninitialized pointers, which will almost certainly cause it to crash. This initialization occurs before custom lowering, so the two may be used together.

Since LLVM does not yet compute liveness information, there is no means of distinguishing an uninitialized stack root from an initialized one. Therefore, this feature should be used by all GC plugins. It is enabled by default.

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

For GCs which use barriers or unusual treatment of stack roots, these flags allow the collector to perform arbitrary transformations of the LLVM IR:

    class MyGC : public GCStrategy {
    public:
      MyGC() {
        CustomRoots = true;
        CustomReadBarriers = true;
        CustomWriteBarriers = true;
      }
    };

If any of these flags are set, LLVM suppresses its default lowering for the corresponding intrinsics. Instead, you must provide a custom Pass which lowers the intrinsics as desired. If you have opted in to custom lowering of a particular intrinsic your pass must eliminate all instances of the corresponding intrinsic in functions which opt in to your GC. The best example of such a pass is the ShadowStackGC and it’s ShadowStackGCLowering pass.

There is currently no way to register such a custom lowering pass without building a custom copy of LLVM.

### 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:

    MyGC::MyGC()  {
      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) {
      GCFunctionInfo *MD = *I;
      size_t PointCount = MD->size();

      for (GCFunctionInfo::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: `GCMetadataPrinter`

LLVM allows a plugin to print arbitrary assembly code before and after the rest of a module’s assembly code. At the end of the module, the GC can compile the LLVM stack map into assembly code. (At the beginning, this information is not yet computed.)

Since AsmWriter and CodeGen are separate components of LLVM, a separate abstract base class and registry is provided for printing assembly code, the `GCMetadaPrinter` and `GCMetadataPrinterRegistry`. The AsmWriter will look for such a subclass if the `GCStrategy` sets `UsesMetadata`:

    MyGC::MyGC() {
      UsesMetadata = true;
    }

This separation allows JIT-only clients to be smaller.

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

    // lib/MyGC/MyGCPrinter.cpp - Example LLVM GC printer

    #include "llvm/CodeGen/GCMetadataPrinter.h"
    #include "llvm/Support/Compiler.h"

    using namespace llvm;

    namespace {
      class LLVM_LIBRARY_VISIBILITY MyGCPrinter : public GCMetadataPrinter {
      public:
        virtual void beginAssembly(AsmPrinter &AP);

        virtual void finishAssembly(AsmPrinter &AP);
      };

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

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

    #include "llvm/CodeGen/AsmPrinter.h"
    #include "llvm/IR/Function.h"
    #include "llvm/IR/DataLayout.h"
    #include "llvm/Target/TargetAsmInfo.h"
    #include "llvm/Target/TargetMachine.h"

    void MyGCPrinter::beginAssembly(AsmPrinter &AP) {
      // Nothing to do.
    }

    void MyGCPrinter::finishAssembly(AsmPrinter &AP) {
      MCStreamer &OS = AP.OutStreamer;
      unsigned IntPtrSize = AP.getPointerSize();

      // Put this in the data section.
      OS.SwitchSection(AP.getObjFileLowering().getDataSection());

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

        // A compact GC layout. Emit this data structure:
        //
        // struct {
        //   int32_t PointCount;
        //   void *SafePointAddress[PointCount];
        //   int32_t StackFrameSize; // in words
        //   int32_t StackArity;
        //   int32_t LiveCount;
        //   int32_t LiveOffsets[LiveCount];
        // } __gcmap_<FUNCTIONNAME>;

        // Align to address width.
        AP.EmitAlignment(IntPtrSize == 4 ? 2 : 3);

        // Emit PointCount.
        OS.AddComment("safe point count");
        AP.EmitInt32(MD.size());

        // And each safe point...
        for (GCFunctionInfo::iterator PI = MD.begin(),
                                      PE = MD.end(); PI != PE; ++PI) {
          // Emit the address of the safe point.
          OS.AddComment("safe point address");
          MCSymbol *Label = PI->Label;
          AP.EmitLabelPlusOffset(Label/*Hi*/, 0/*Offset*/, 4/*Size*/);
        }

        // Stack information never change in safe points! Only print info from the
        // first call-site.
        GCFunctionInfo::iterator PI = MD.begin();

        // Emit the stack frame size.
        OS.AddComment("stack frame size (in words)");
        AP.EmitInt32(MD.getFrameSize() / IntPtrSize);

        // Emit stack arity, i.e. the number of stacked arguments.
        unsigned RegisteredArgs = IntPtrSize == 4 ? 5 : 6;
        unsigned StackArity = MD.getFunction().arg_size() > RegisteredArgs ?
                              MD.getFunction().arg_size() - RegisteredArgs : 0;
        OS.AddComment("stack arity");
        AP.EmitInt32(StackArity);

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

        // And for each live root...
        for (GCFunctionInfo::live_iterator LI = MD.live_begin(PI),
                                           LE = MD.live_end(PI);
                                           LI != LE; ++LI) {
          // Emit live root's offset within the stack frame.
          OS.AddComment("stack index (offset / wordsize)");
          AP.EmitInt32(LI->StackOffset);
        }
      }
    }

## 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