forked from OSchip/llvm-project
Starting to cleanup the garbage collection documentation
In this change: - Put the getting started section first - Create a dedicated section to document the built in collector strategies - Move discuss of ShadowStack into new section - Add placeholders for erlang, ocaml, and statepoint-example collectors There will be many more changes following. I plan on full integrating the documentation for gc.statepoint and gc.root. I want to make it much clearer on how to get started and what users should expect in terms of effort. llvm-svn: 230359
This commit is contained in:
Philip Reames
parent
af48495130
commit
e78cf557a3
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@ -1,5 +1,5 @@
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=====================================
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Accurate Garbage Collection with LLVM
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Garbage Collection with LLVM
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=====================================
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.. contents::
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@ -8,6 +8,153 @@ Accurate Garbage Collection with LLVM
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Introduction
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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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Getting started
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===============
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Using a GC with LLVM implies many things, for example:
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* Write a runtime library or find an existing one which implements a GC heap.
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#. Implement a memory allocator.
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#. Design a binary interface for the stack map, used to identify references
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within a stack frame on the machine stack.\*
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#. Implement a stack crawler to discover functions on the call stack.\*
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#. Implement a registry for global roots.
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#. Design a binary interface for type maps, used to identify references
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within heap objects.
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#. Implement a collection routine bringing together all of the above.
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* Emit compatible code from your compiler.
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* Initialization in the main function.
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* Use the ``gc "..."`` attribute to enable GC code generation (or
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``F.setGC("...")``).
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* Use ``@llvm.gcroot`` to mark stack roots.
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* Use ``@llvm.gcread`` and/or ``@llvm.gcwrite`` to manipulate GC references,
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if necessary.
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* Allocate memory using the GC allocation routine provided by the runtime
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library.
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* Generate type maps according to your runtime's binary interface.
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* Write a compiler plugin to interface LLVM with the runtime library.\*
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* Lower ``@llvm.gcread`` and ``@llvm.gcwrite`` to appropriate code
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sequences.\*
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* Compile LLVM's stack map to the binary form expected by the runtime.
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* Load the plugin into the compiler. Use ``llc -load`` or link the plugin
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statically with your language's compiler.\*
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* Link program executables with the runtime.
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To help with several of these tasks (those indicated with a \*), LLVM includes a
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highly portable, built-in ShadowStack code generator. It is compiled into
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``llc`` and works even with the interpreter and C backends.
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In your compiler
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----------------
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To turn the shadow stack on for your functions, first call:
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.. code-block:: c++
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F.setGC(<collector description name>);
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for each function your compiler emits. Since the shadow stack is built into
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LLVM, you do not need to load a plugin.
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Your compiler must also use ``@llvm.gcroot`` as documented. Don't forget to
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create a root for each intermediate value that is generated when evaluating an
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expression. In ``h(f(), g())``, the result of ``f()`` could easily be collected
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if evaluating ``g()`` triggers a collection.
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There's no need to use ``@llvm.gcread`` and ``@llvm.gcwrite`` over plain
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``load`` and ``store`` for now. You will need them when switching to a more
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advanced GC.
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In your runtime
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---------------
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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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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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@ -116,164 +263,6 @@ lot of work for the developer of a novel language. However, it's easy to get
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started quickly and scale up to a more sophisticated implementation as your
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compiler matures.
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Getting started
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===============
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Using a GC with LLVM implies many things, for example:
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* Write a runtime library or find an existing one which implements a GC heap.
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#. Implement a memory allocator.
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#. Design a binary interface for the stack map, used to identify references
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within a stack frame on the machine stack.\*
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#. Implement a stack crawler to discover functions on the call stack.\*
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#. Implement a registry for global roots.
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#. Design a binary interface for type maps, used to identify references
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within heap objects.
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#. Implement a collection routine bringing together all of the above.
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* Emit compatible code from your compiler.
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* Initialization in the main function.
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* Use the ``gc "..."`` attribute to enable GC code generation (or
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``F.setGC("...")``).
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* Use ``@llvm.gcroot`` to mark stack roots.
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* Use ``@llvm.gcread`` and/or ``@llvm.gcwrite`` to manipulate GC references,
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if necessary.
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* Allocate memory using the GC allocation routine provided by the runtime
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library.
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* Generate type maps according to your runtime's binary interface.
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* Write a compiler plugin to interface LLVM with the runtime library.\*
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* Lower ``@llvm.gcread`` and ``@llvm.gcwrite`` to appropriate code
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sequences.\*
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* Compile LLVM's stack map to the binary form expected by the runtime.
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* Load the plugin into the compiler. Use ``llc -load`` or link the plugin
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statically with your language's compiler.\*
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* Link program executables with the runtime.
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To help with several of these tasks (those indicated with a \*), LLVM includes a
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highly portable, built-in ShadowStack code generator. It is compiled into
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``llc`` and works even with the interpreter and C backends.
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In your compiler
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----------------
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To turn the shadow stack on for your functions, first call:
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.. code-block:: c++
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F.setGC("shadow-stack");
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for each function your compiler emits. Since the shadow stack is built into
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LLVM, you do not need to load a plugin.
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Your compiler must also use ``@llvm.gcroot`` as documented. Don't forget to
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create a root for each intermediate value that is generated when evaluating an
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expression. In ``h(f(), g())``, the result of ``f()`` could easily be collected
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if evaluating ``g()`` triggers a collection.
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There's no need to use ``@llvm.gcread`` and ``@llvm.gcwrite`` over plain
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``load`` and ``store`` for now. You will need them when switching to a more
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advanced GC.
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In your runtime
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---------------
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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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About the shadow stack
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----------------------
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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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.. _gc_intrinsics:
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IR features
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@ -447,6 +436,64 @@ greater performance impact since pointer reads are more frequent than writes.
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.. _plugin:
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Built In Collectors
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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 '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
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only in tree examples of how to produce a custom binary stack map format using
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a ''gcroot'' strategy.
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As there names imply, the binary format produced is intended to model that
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used by the Erlang and OCaml compilers respectively.
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The Statepoint Example GC
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-------------------------
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.. code-block:: c++
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F.setGC("statepoint-example");
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This GC provides an example of how one might use the infrastructure provided
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by ''gc.statepoint''.
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Implementing a collector plugin
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===============================
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