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//===-- llvm/CodeGen/GCStrategy.h - Garbage collection ----------*- C++ -*-===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// GCStrategy coordinates code generation algorithms and implements some itself
// in order to generate code compatible with a target code generator as
// specified in a function's 'gc' attribute. Algorithms are enabled by setting
// flags in a subclass's constructor, and some virtual methods can be
// overridden.
// GCStrategy is relevant for implementations using either gc.root or
// gc.statepoint based lowering strategies, but is currently focused mostly on
// options for gc.root. This will change over time.
// When requested by a subclass of GCStrategy, the gc.root implementation will
// populate GCModuleInfo and GCFunctionInfo with that about each Function in
// the Module that opts in to garbage collection. Specifically:
// - Safe points
// Garbage collection is generally only possible at certain points in code.
// GCStrategy can request that the collector insert such points:
// - At and after any call to a subroutine
// - Before returning from the current function
// - Before backwards branches (loops)
// - Roots
// When a reference to a GC-allocated object exists on the stack, it must be
// stored in an alloca registered with llvm.gcoot.
// This information can used to emit the metadata tables which are required by
// the target garbage collector runtime.
// When used with gc.statepoint, information about safepoint and roots can be
// found in the binary StackMap section after code generation. Safepoint
// placement is currently the responsibility of the frontend, though late
// insertion support is planned. gc.statepoint does not currently support
// custom stack map formats; such can be generated by parsing the standard
// stack map section if desired.
// The read and write barrier support can be used with either implementation.
#include "llvm/ADT/Optional.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/Registry.h"
#include <string>
namespace llvm {
namespace GC {
/// PointKind - Used to indicate whether the address of the call instruction
/// or the address after the call instruction is listed in the stackmap. For
/// most runtimes, PostCall safepoints are appropriate.
enum PointKind {
PreCall, ///< Instr is a call instruction.
PostCall ///< Instr is the return address of a call.
/// GCStrategy describes a garbage collector algorithm's code generation
/// requirements, and provides overridable hooks for those needs which cannot
/// be abstractly described. GCStrategy objects must be looked up through
/// the Function. The objects themselves are owned by the Context and must
/// be immutable.
class GCStrategy {
std::string Name;
friend class GCModuleInfo;
bool UseStatepoints; /// Uses gc.statepoints as opposed to gc.roots,
/// if set, none of the other options can be
/// anything but their default values.
unsigned NeededSafePoints; ///< Bitmask of required safe points.
bool CustomReadBarriers; ///< Default is to insert loads.
bool CustomWriteBarriers; ///< Default is to insert stores.
bool CustomRoots; ///< Default is to pass through to backend.
bool InitRoots; ///< If set, roots are nulled during lowering.
bool UsesMetadata; ///< If set, backend must emit metadata tables.
virtual ~GCStrategy() {}
/// Return the name of the GC strategy. This is the value of the collector
/// name string specified on functions which use this strategy.
const std::string &getName() const { return Name; }
/// By default, write barriers are replaced with simple store
/// instructions. If true, you must provide a custom pass to lower
/// calls to @llvm.gcwrite.
bool customWriteBarrier() const { return CustomWriteBarriers; }
/// By default, read barriers are replaced with simple load
/// instructions. If true, you must provide a custom pass to lower
/// calls to @llvm.gcread.
bool customReadBarrier() const { return CustomReadBarriers; }
/// Returns true if this strategy is expecting the use of gc.statepoints,
/// and false otherwise.
bool useStatepoints() const { return UseStatepoints; }
/** @name Statepoint Specific Properties */
/// If the value specified can be reliably distinguished, returns true for
/// pointers to GC managed locations and false for pointers to non-GC
/// managed locations. Note a GCStrategy can always return 'None' (i.e. an
/// empty optional indicating it can't reliably distinguish.
virtual Optional<bool> isGCManagedPointer(const Value *V) const {
return None;
/** @name GCRoot Specific Properties
* These properties and overrides only apply to collector strategies using
* GCRoot.
/// True if safe points of any kind are required. By default, none are
/// recorded.
bool needsSafePoints() const { return NeededSafePoints != 0; }
/// True if the given kind of safe point is required. By default, none are
/// recorded.
bool needsSafePoint(GC::PointKind Kind) const {
return (NeededSafePoints & 1 << Kind) != 0;
/// By default, roots are left for the code generator so it can generate a
/// stack map. If true, you must provide a custom pass to lower
/// calls to @llvm.gcroot.
bool customRoots() const { return CustomRoots; }
/// If set, gcroot intrinsics should initialize their allocas to null
/// before the first use. This is necessary for most GCs and is enabled by
/// default.
bool initializeRoots() const { return InitRoots; }
/// If set, appropriate metadata tables must be emitted by the back-end
/// (assembler, JIT, or otherwise). For statepoint, this method is
/// currently unsupported. The stackmap information can be found in the
/// StackMap section as described in the documentation.
bool usesMetadata() const { return UsesMetadata; }
/// Subclasses of GCStrategy are made available for use during compilation by
/// adding them to the global GCRegistry. This can done either within the
/// LLVM source tree or via a loadable plugin. An example registeration
/// would be:
/// static GCRegistry::Add<CustomGC> X("custom-name",
/// "my custom supper fancy gc strategy");
/// Note that to use a custom GCMetadataPrinter w/gc.roots, you must also
/// register your GCMetadataPrinter subclass with the
/// GCMetadataPrinterRegistery as well.
typedef Registry<GCStrategy> GCRegistry;