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=====================================
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Garbage Collection with LLVM
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=====================================
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.. contents::
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:local:
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Abstract
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========
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This document covers how to integrate LLVM into a compiler for a language which
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supports garbage collection. **Note that LLVM itself does not provide a
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garbage collector.** You must provide your own.
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Quick Start
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============
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First, you should pick a collector strategy. LLVM includes a number of built
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in ones, but you can also implement a loadable plugin with a custom definition.
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Note that the collector strategy is a description of how LLVM should generate
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code such that it interacts with your collector and runtime, not a description
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of the collector itself.
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Next, mark your generated functions as using your chosen collector strategy.
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From c++, you can call:
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.. code-block:: c++
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F.setGC(<collector description name>);
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This will produce IR like the following fragment:
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.. code-block:: llvm
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define void @foo() gc "<collector description name>" { ... }
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When generating LLVM IR for your functions, you will need to:
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* Use ``@llvm.gcread`` and/or ``@llvm.gcwrite`` in place of standard load and
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store instructions. These intrinsics are used to represent load and store
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barriers. If you collector does not require such barriers, you can skip
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this step.
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* Use the memory allocation routines provided by your garbage collector's
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runtime library.
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* If your collector requires them, generate type maps according to your
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runtime's binary interface. LLVM is not involved in the process. In
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particular, the LLVM type system is not suitable for conveying such
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information though the compiler.
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* Insert any coordination code required for interacting with your collector.
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Many collectors require running application code to periodically check a
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flag and conditionally call a runtime function. This is often referred to
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as a safepoint poll.
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You will need to identify roots (i.e. references to heap objects your collector
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needs to know about) in your generated IR, so that LLVM can encode them into
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your final stack maps. Depending on the collector strategy chosen, this is
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accomplished by using either the ``@llvm.gcroot`` intrinsics or an
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``gc.statepoint`` relocation sequence.
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Don't forget to create a root for each intermediate value that is generated when
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evaluating an expression. In ``h(f(), g())``, the result of ``f()`` could
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easily be collected if evaluating ``g()`` triggers a collection.
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Finally, you need to link your runtime library with the generated program
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executable (for a static compiler) or ensure the appropriate symbols are
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available for the runtime linker (for a JIT compiler).
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Introduction
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============
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What is Garbage Collection?
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---------------------------
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Garbage collection is a widely used technique that frees the programmer from
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having to know the lifetimes of heap objects, making software easier to produce
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and maintain. Many programming languages rely on garbage collection for
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automatic memory management. There are two primary forms of garbage collection:
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conservative and accurate.
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Conservative garbage collection often does not require any special support from
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either the language or the compiler: it can handle non-type-safe programming
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languages (such as C/C++) and does not require any special information from the
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compiler. The `Boehm collector
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<http://www.hpl.hp.com/personal/Hans_Boehm/gc/>`__ is an example of a
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state-of-the-art conservative collector.
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Accurate garbage collection requires the ability to identify all pointers in the
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program at run-time (which requires that the source-language be type-safe in
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most cases). Identifying pointers at run-time requires compiler support to
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locate all places that hold live pointer variables at run-time, including the
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:ref:`processor stack and registers <gcroot>`.
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Conservative garbage collection is attractive because it does not require any
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special compiler support, but it does have problems. In particular, because the
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conservative garbage collector cannot *know* that a particular word in the
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machine is a pointer, it cannot move live objects in the heap (preventing the
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use of compacting and generational GC algorithms) and it can occasionally suffer
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from memory leaks due to integer values that happen to point to objects in the
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program. In addition, some aggressive compiler transformations can break
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conservative garbage collectors (though these seem rare in practice).
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Accurate garbage collectors do not suffer from any of these problems, but they
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can suffer from degraded scalar optimization of the program. In particular,
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because the runtime must be able to identify and update all pointers active in
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the program, some optimizations are less effective. In practice, however, the
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locality and performance benefits of using aggressive garbage collection
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techniques dominates any low-level losses.
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This document describes the mechanisms and interfaces provided by LLVM to
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support accurate garbage collection.
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Goals and non-goals
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-------------------
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LLVM's intermediate representation provides :ref:`garbage collection intrinsics
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<gc_intrinsics>` that offer support for a broad class of collector models. For
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instance, the intrinsics permit:
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* semi-space collectors
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* mark-sweep collectors
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* generational collectors
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* incremental collectors
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* concurrent collectors
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* cooperative collectors
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* reference counting
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We hope that the support built into the LLVM IR is sufficient to support a
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broad class of garbage collected languages including Scheme, ML, Java, C#,
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Perl, Python, Lua, Ruby, other scripting languages, and more.
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Note that LLVM **does not itself provide a garbage collector** --- this should
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be part of your language's runtime library. LLVM provides a framework for
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describing the garbage collectors requirements to the compiler. In particular,
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LLVM provides support for generating stack maps at call sites, polling for a
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safepoint, and emitting load and store barriers. You can also extend LLVM -
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possibly through a loadable :ref:`code generation plugins <plugin>` - to
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generate code and data structures which conforms to the *binary interface*
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specified by the *runtime library*. This is similar to the relationship between
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LLVM and DWARF debugging info, for example. The difference primarily lies in
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the lack of an established standard in the domain of garbage collection --- thus
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the need for a flexible extension mechanism.
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The aspects of the binary interface with which LLVM's GC support is
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concerned are:
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* Creation of GC safepoints within code where collection is allowed to execute
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safely.
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* Computation of the stack map. For each safe point in the code, object
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references within the stack frame must be identified so that the collector may
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traverse and perhaps update them.
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* Write barriers when storing object references to the heap. These are commonly
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used to optimize incremental scans in generational collectors.
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* Emission of read barriers when loading object references. These are useful
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for interoperating with concurrent collectors.
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There are additional areas that LLVM does not directly address:
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* Registration of global roots with the runtime.
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* Registration of stack map entries with the runtime.
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* The functions used by the program to allocate memory, trigger a collection,
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etc.
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* Computation or compilation of type maps, or registration of them with the
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runtime. These are used to crawl the heap for object references.
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In general, LLVM's support for GC does not include features which can be
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adequately addressed with other features of the IR and does not specify a
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particular binary interface. On the plus side, this means that you should be
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able to integrate LLVM with an existing runtime. On the other hand, it can
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have the effect of leaving a lot of work for the developer of a novel
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language. We try to mitigate this by providing built in collector strategy
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descriptions that can work with many common collector designs and easy
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extension points. If you don't already have a specific binary interface
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you need to support, we recommend trying to use one of these built in collector
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strategies.
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.. _gc_intrinsics:
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LLVM IR Features
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================
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This section describes the garbage collection facilities provided by the
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:doc:`LLVM intermediate representation <LangRef>`. The exact behavior of these
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IR features is specified by the selected :ref:`GC strategy description
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<plugin>`.
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Specifying GC code generation: ``gc "..."``
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-------------------------------------------
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.. code-block:: text
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define <returntype> @name(...) gc "name" { ... }
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The ``gc`` function attribute is used to specify the desired GC strategy to the
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compiler. Its programmatic equivalent is the ``setGC`` method of ``Function``.
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Setting ``gc "name"`` on a function triggers a search for a matching subclass
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of GCStrategy. Some collector strategies are built in. You can add others
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using either the loadable plugin mechanism, or by patching your copy of LLVM.
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It is the selected GC strategy which defines the exact nature of the code
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generated to support GC. If none is found, the compiler will raise an error.
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Specifying the GC style on a per-function basis allows LLVM to link together
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programs that use different garbage collection algorithms (or none at all).
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.. _gcroot:
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Identifying GC roots on the stack
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----------------------------------
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LLVM currently supports two different mechanisms for describing references in
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compiled code at safepoints. ``llvm.gcroot`` is the older mechanism;
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``gc.statepoint`` has been added more recently. At the moment, you can choose
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either implementation (on a per :ref:`GC strategy <plugin>` basis). Longer
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term, we will probably either migrate away from ``llvm.gcroot`` entirely, or
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substantially merge their implementations. Note that most new development
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work is focused on ``gc.statepoint``.
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Using ``gc.statepoint``
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^^^^^^^^^^^^^^^^^^^^^^^^
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:doc:`This page <Statepoints>` contains detailed documentation for
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``gc.statepoint``.
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Using ``llvm.gcwrite``
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.. code-block:: llvm
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void @llvm.gcroot(i8** %ptrloc, i8* %metadata)
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The ``llvm.gcroot`` intrinsic is used to inform LLVM that a stack variable
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references an object on the heap and is to be tracked for garbage collection.
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The exact impact on generated code is specified by the Function's selected
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:ref:`GC strategy <plugin>`. All calls to ``llvm.gcroot`` **must** reside
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inside the first basic block.
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The first argument **must** be a value referring to an alloca instruction or a
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bitcast of an alloca. The second contains a pointer to metadata that should be
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associated with the pointer, and **must** be a constant or global value
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address. If your target collector uses tags, use a null pointer for metadata.
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A compiler which performs manual SSA construction **must** ensure that SSA
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values representing GC references are stored in to the alloca passed to the
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respective ``gcroot`` before every call site and reloaded after every call.
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A compiler which uses mem2reg to raise imperative code using ``alloca`` into
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SSA form need only add a call to ``@llvm.gcroot`` for those variables which
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are pointers into the GC heap.
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It is also important to mark intermediate values with ``llvm.gcroot``. For
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example, consider ``h(f(), g())``. Beware leaking the result of ``f()`` in the
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case that ``g()`` triggers a collection. Note, that stack variables must be
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initialized and marked with ``llvm.gcroot`` in function's prologue.
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The ``%metadata`` argument can be used to avoid requiring heap objects to have
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'isa' pointers or tag bits. [Appel89_, Goldberg91_, Tolmach94_] If specified,
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its value will be tracked along with the location of the pointer in the stack
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frame.
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Consider the following fragment of Java code:
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.. code-block:: java
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{
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Object X; // A null-initialized reference to an object
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...
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}
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This block (which may be located in the middle of a function or in a loop nest),
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could be compiled to this LLVM code:
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.. code-block:: llvm
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Entry:
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;; In the entry block for the function, allocate the
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;; stack space for X, which is an LLVM pointer.
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%X = alloca %Object*
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;; Tell LLVM that the stack space is a stack root.
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;; Java has type-tags on objects, so we pass null as metadata.
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%tmp = bitcast %Object** %X to i8**
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call void @llvm.gcroot(i8** %tmp, i8* null)
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...
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;; "CodeBlock" is the block corresponding to the start
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;; of the scope above.
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CodeBlock:
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;; Java null-initializes pointers.
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store %Object* null, %Object** %X
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...
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;; As the pointer goes out of scope, store a null value into
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;; it, to indicate that the value is no longer live.
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store %Object* null, %Object** %X
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...
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Reading and writing references in the heap
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------------------------------------------
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Some collectors need to be informed when the mutator (the program that needs
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garbage collection) either reads a pointer from or writes a pointer to a field
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of a heap object. The code fragments inserted at these points are called *read
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barriers* and *write barriers*, respectively. The amount of code that needs to
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be executed is usually quite small and not on the critical path of any
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computation, so the overall performance impact of the barrier is tolerable.
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Barriers often require access to the *object pointer* rather than the *derived
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pointer* (which is a pointer to the field within the object). Accordingly,
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these intrinsics take both pointers as separate arguments for completeness. In
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this snippet, ``%object`` is the object pointer, and ``%derived`` is the derived
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pointer:
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.. code-block:: llvm
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;; An array type.
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%class.Array = type { %class.Object, i32, [0 x %class.Object*] }
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...
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;; Load the object pointer from a gcroot.
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%object = load %class.Array** %object_addr
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;; Compute the derived pointer.
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%derived = getelementptr %object, i32 0, i32 2, i32 %n
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LLVM does not enforce this relationship between the object and derived pointer
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(although a particular :ref:`collector strategy <plugin>` might). However, it
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would be an unusual collector that violated it.
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The use of these intrinsics is naturally optional if the target GC does not
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require the corresponding barrier. The GC strategy used with such a collector
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should replace the intrinsic calls with the corresponding ``load`` or
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``store`` instruction if they are used.
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One known deficiency with the current design is that the barrier intrinsics do
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not include the size or alignment of the underlying operation performed. It is
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currently assumed that the operation is of pointer size and the alignment is
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assumed to be the target machine's default alignment.
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Write barrier: ``llvm.gcwrite``
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.. code-block:: llvm
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void @llvm.gcwrite(i8* %value, i8* %object, i8** %derived)
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For write barriers, LLVM provides the ``llvm.gcwrite`` intrinsic function. It
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has exactly the same semantics as a non-volatile ``store`` to the derived
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pointer (the third argument). The exact code generated is specified by the
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Function's selected :ref:`GC strategy <plugin>`.
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Many important algorithms require write barriers, including generational and
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concurrent collectors. Additionally, write barriers could be used to implement
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reference counting.
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Read barrier: ``llvm.gcread``
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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.. code-block:: llvm
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i8* @llvm.gcread(i8* %object, i8** %derived)
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For read barriers, LLVM provides the ``llvm.gcread`` intrinsic function. It has
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exactly the same semantics as a non-volatile ``load`` from the derived pointer
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(the second argument). The exact code generated is specified by the Function's
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selected :ref:`GC strategy <plugin>`.
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Read barriers are needed by fewer algorithms than write barriers, and may have a
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greater performance impact since pointer reads are more frequent than writes.
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.. _plugin:
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.. _builtin-gc-strategies:
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Built In GC Strategies
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======================
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LLVM includes built in support for several varieties of garbage collectors.
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The Shadow Stack GC
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----------------------
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To use this collector strategy, mark your functions with:
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.. code-block:: c++
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F.setGC("shadow-stack");
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Unlike many GC algorithms which rely on a cooperative code generator to compile
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stack maps, this algorithm carefully maintains a linked list of stack roots
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[:ref:`Henderson2002 <henderson02>`]. This so-called "shadow stack" mirrors the
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machine stack. Maintaining this data structure is slower than using a stack map
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compiled into the executable as constant data, but has a significant portability
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advantage because it requires no special support from the target code generator,
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and does not require tricky platform-specific code to crawl the machine stack.
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The tradeoff for this simplicity and portability is:
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* High overhead per function call.
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* Not thread-safe.
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Still, it's an easy way to get started. After your compiler and runtime are up
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and running, writing a :ref:`plugin <plugin>` will allow you to take advantage
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of :ref:`more advanced GC features <collector-algos>` of LLVM in order to
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improve performance.
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The shadow stack doesn't imply a memory allocation algorithm. A semispace
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collector or building atop ``malloc`` are great places to start, and can be
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implemented with very little code.
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When it comes time to collect, however, your runtime needs to traverse the stack
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roots, and for this it needs to integrate with the shadow stack. Luckily, doing
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so is very simple. (This code is heavily commented to help you understand the
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data structure, but there are only 20 lines of meaningful code.)
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.. code-block:: c++
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/// @brief The map for a single function's stack frame. One of these is
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/// compiled as constant data into the executable for each function.
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///
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/// Storage of metadata values is elided if the %metadata parameter to
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/// @llvm.gcroot is null.
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struct FrameMap {
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int32_t NumRoots; //< Number of roots in stack frame.
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int32_t NumMeta; //< Number of metadata entries. May be < NumRoots.
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const void *Meta[0]; //< Metadata for each root.
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};
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/// @brief A link in the dynamic shadow stack. One of these is embedded in
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/// the stack frame of each function on the call stack.
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struct StackEntry {
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StackEntry *Next; //< Link to next stack entry (the caller's).
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const FrameMap *Map; //< Pointer to constant FrameMap.
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void *Roots[0]; //< Stack roots (in-place array).
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};
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/// @brief The head of the singly-linked list of StackEntries. Functions push
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/// and pop onto this in their prologue and epilogue.
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///
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/// Since there is only a global list, this technique is not threadsafe.
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StackEntry *llvm_gc_root_chain;
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/// @brief Calls Visitor(root, meta) for each GC root on the stack.
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/// root and meta are exactly the values passed to
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/// @llvm.gcroot.
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///
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/// Visitor could be a function to recursively mark live objects. Or it
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/// might copy them to another heap or generation.
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///
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/// @param Visitor A function to invoke for every GC root on the stack.
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void visitGCRoots(void (*Visitor)(void **Root, const void *Meta)) {
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for (StackEntry *R = llvm_gc_root_chain; R; R = R->Next) {
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unsigned i = 0;
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// For roots [0, NumMeta), the metadata pointer is in the FrameMap.
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for (unsigned e = R->Map->NumMeta; i != e; ++i)
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Visitor(&R->Roots[i], R->Map->Meta[i]);
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// For roots [NumMeta, NumRoots), the metadata pointer is null.
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for (unsigned e = R->Map->NumRoots; i != e; ++i)
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Visitor(&R->Roots[i], NULL);
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}
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}
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The 'Erlang' and 'Ocaml' GCs
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-----------------------------
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LLVM ships with two example collectors which leverage the ``gcroot``
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mechanisms. To our knowledge, these are not actually used by any language
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runtime, but they do provide a reasonable starting point for someone interested
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|
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.
|
|
|
|
.. _statepoint_example_gc:
|
|
|
|
The Statepoint Example GC
|
|
-------------------------
|
|
|
|
.. code-block:: c++
|
|
|
|
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
|
|
:ref:`PlaceSafepoints` and :ref:`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
|
|
:ref:`stackmap-section` using a format documented :ref:`here
|
|
<statepoint-stackmap-format>`. This format is intended to be the standard
|
|
format supported by LLVM going forward.
|
|
|
|
The CoreCLR GC
|
|
-------------------------
|
|
|
|
.. code-block:: c++
|
|
|
|
F.setGC("coreclr");
|
|
|
|
This GC leverages the ``gc.statepoint`` mechanism to support the
|
|
`CoreCLR <https://github.com/dotnet/coreclr>`__ runtime.
|
|
|
|
Support for this GC strategy is a work in progress. This strategy will
|
|
differ from
|
|
:ref:`statepoint-example GC<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:
|
|
|
|
#. A memory allocator which exposes an allocation function your compiled
|
|
code can call.
|
|
|
|
#. 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.
|
|
|
|
#. A stack crawler to discover functions on the call stack, and enumerate the
|
|
references listed in the stack map for each call site.
|
|
|
|
#. A mechanism for identifying references in global locations (e.g. global
|
|
variables).
|
|
|
|
#. 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 :ref:`stack map
|
|
<stack-map>`.
|
|
|
|
To subclass ``llvm::GCStrategy`` and register it with the compiler:
|
|
|
|
.. code-block:: c++
|
|
|
|
// 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:
|
|
|
|
.. code-block:: make
|
|
|
|
# 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.
|
|
|
|
.. _collector-algos:
|
|
|
|
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.
|
|
|
|
.. |v| unicode:: 0x2714
|
|
:trim:
|
|
|
|
.. |x| unicode:: 0x2718
|
|
:trim:
|
|
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| Algorithm | Done | Shadow | refcount | mark- | copying | incremental | threaded | concurrent |
|
|
| | | stack | | sweep | | | | |
|
|
+============+======+========+==========+=======+=========+=============+==========+============+
|
|
| stack map | |v| | | | |x| | |x| | |x| | |x| | |x| |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| initialize | |v| | |x| | |x| | |x| | |x| | |x| | |x| | |x| |
|
|
| roots | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| derived | NO | | | | | | **N**\* | **N**\* |
|
|
| pointers | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| **custom | |v| | | | | | | | |
|
|
| lowering** | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| *gcroot* | |v| | |x| | |x| | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| *gcwrite* | |v| | | |x| | | | |x| | | |x| |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| *gcread* | |v| | | | | | | | |x| |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| **safe | | | | | | | | |
|
|
| points** | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| *in | |v| | | | |x| | |x| | |x| | |x| | |x| |
|
|
| calls* | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| *before | |v| | | | | | | |x| | |x| |
|
|
| calls* | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| *for | NO | | | | | | **N** | **N** |
|
|
| loops* | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| *before | |v| | | | | | | |x| | |x| |
|
|
| escape* | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| emit code | NO | | | | | | **N** | **N** |
|
|
| at safe | | | | | | | | |
|
|
| points | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| **output** | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| *assembly* | |v| | | | |x| | |x| | |x| | |x| | |x| |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| *JIT* | NO | | | **?** | **?** | **?** | **?** | **?** |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| *obj* | NO | | | **?** | **?** | **?** | **?** | **?** |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| live | NO | | | **?** | **?** | **?** | **?** | **?** |
|
|
| analysis | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| register | NO | | | **?** | **?** | **?** | **?** | **?** |
|
|
| map | | | | | | | | |
|
|
+------------+------+--------+----------+-------+---------+-------------+----------+------------+
|
|
| \* 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.
|
|
|
|
.. _stack-map:
|
|
|
|
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 :ref:`GCMetadataPrinter <assembly>`:
|
|
|
|
.. code-block:: c++
|
|
|
|
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.
|
|
|
|
.. _init-roots:
|
|
|
|
Initializing roots to null: ``InitRoots``
|
|
-----------------------------------------
|
|
|
|
.. code-block:: c++
|
|
|
|
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:
|
|
|
|
.. code-block:: c++
|
|
|
|
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.
|
|
|
|
.. _safe-points:
|
|
|
|
Generating safe points: ``NeededSafePoints``
|
|
--------------------------------------------
|
|
|
|
LLVM can compute four kinds of safe points:
|
|
|
|
.. code-block:: c++
|
|
|
|
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:
|
|
|
|
.. code-block:: c++
|
|
|
|
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.
|
|
|
|
.. code-block:: c++
|
|
|
|
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).
|
|
|
|
.. _assembly:
|
|
|
|
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``:
|
|
|
|
.. code-block:: c++
|
|
|
|
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.
|
|
|
|
.. code-block:: c++
|
|
|
|
// 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
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a realistic example:
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.. code-block:: c++
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#include "llvm/CodeGen/AsmPrinter.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/DataLayout.h"
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#include "llvm/Target/TargetAsmInfo.h"
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#include "llvm/Target/TargetMachine.h"
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void MyGCPrinter::beginAssembly(AsmPrinter &AP) {
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// Nothing to do.
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}
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void MyGCPrinter::finishAssembly(AsmPrinter &AP) {
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MCStreamer &OS = AP.OutStreamer;
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unsigned IntPtrSize = AP.TM.getSubtargetImpl()->getDataLayout()->getPointerSize();
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// Put this in the data section.
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OS.SwitchSection(AP.getObjFileLowering().getDataSection());
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// For each function...
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for (iterator FI = begin(), FE = end(); FI != FE; ++FI) {
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GCFunctionInfo &MD = **FI;
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// A compact GC layout. Emit this data structure:
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//
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// struct {
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// int32_t PointCount;
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// void *SafePointAddress[PointCount];
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// int32_t StackFrameSize; // in words
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// int32_t StackArity;
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// int32_t LiveCount;
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// int32_t LiveOffsets[LiveCount];
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// } __gcmap_<FUNCTIONNAME>;
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// Align to address width.
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AP.EmitAlignment(IntPtrSize == 4 ? 2 : 3);
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// Emit PointCount.
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OS.AddComment("safe point count");
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AP.EmitInt32(MD.size());
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// And each safe point...
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for (GCFunctionInfo::iterator PI = MD.begin(),
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PE = MD.end(); PI != PE; ++PI) {
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// Emit the address of the safe point.
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OS.AddComment("safe point address");
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MCSymbol *Label = PI->Label;
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AP.EmitLabelPlusOffset(Label/*Hi*/, 0/*Offset*/, 4/*Size*/);
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}
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// Stack information never change in safe points! Only print info from the
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// first call-site.
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GCFunctionInfo::iterator PI = MD.begin();
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// Emit the stack frame size.
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OS.AddComment("stack frame size (in words)");
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AP.EmitInt32(MD.getFrameSize() / IntPtrSize);
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// Emit stack arity, i.e. the number of stacked arguments.
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unsigned RegisteredArgs = IntPtrSize == 4 ? 5 : 6;
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unsigned StackArity = MD.getFunction().arg_size() > RegisteredArgs ?
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MD.getFunction().arg_size() - RegisteredArgs : 0;
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OS.AddComment("stack arity");
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AP.EmitInt32(StackArity);
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// Emit the number of live roots in the function.
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OS.AddComment("live root count");
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AP.EmitInt32(MD.live_size(PI));
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// And for each live root...
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for (GCFunctionInfo::live_iterator LI = MD.live_begin(PI),
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LE = MD.live_end(PI);
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LI != LE; ++LI) {
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// Emit live root's offset within the stack frame.
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OS.AddComment("stack index (offset / wordsize)");
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AP.EmitInt32(LI->StackOffset);
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}
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}
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}
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References
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==========
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.. _appel89:
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[Appel89] Runtime Tags Aren't Necessary. Andrew W. Appel. Lisp and Symbolic
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Computation 19(7):703-705, July 1989.
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.. _goldberg91:
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[Goldberg91] Tag-free garbage collection for strongly typed programming
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|
languages. Benjamin Goldberg. ACM SIGPLAN PLDI'91.
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.. _tolmach94:
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[Tolmach94] Tag-free garbage collection using explicit type parameters. Andrew
|
|
Tolmach. Proceedings of the 1994 ACM conference on LISP and functional
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|
programming.
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.. _henderson02:
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[Henderson2002] `Accurate Garbage Collection in an Uncooperative Environment
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<http://citeseer.ist.psu.edu/henderson02accurate.html>`__
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