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llvm-6502/include/llvm/CodeGen/GCStrategy.h
Philip Reames b7dfa31ac8 Comment and minor code cleanup for GCStrategy (NFC)
Updating comments to reflect the current state of the world after my recent changes to ownership structure and generally better describe what a GCStrategy is and how it works.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224086 91177308-0d34-0410-b5e6-96231b3b80d8
2014-12-12 00:49:03 +00:00

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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.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_CODEGEN_GCSTRATEGY_H
#define LLVM_CODEGEN_GCSTRATEGY_H
#include "llvm/CodeGen/GCMetadata.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/Support/Registry.h"
#include <string>
namespace llvm {
/// GCStrategy describes a garbage collector algorithm's code generation
/// requirements, and provides overridable hooks for those needs which cannot
/// be abstractly described. GCStrategy objects currently must be looked up
/// through the GCModuleInfo analysis pass. They are owned by the analysis
/// pass and recreated every time that pass is invalidated.
class GCStrategy {
private:
std::string Name;
friend class GCModuleInfo;
protected:
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 CustomSafePoints; ///< Default is to use NeededSafePoints
///< to find safe points.
bool InitRoots; ///< If set, roots are nulled during lowering.
bool UsesMetadata; ///< If set, backend must emit metadata tables.
public:
GCStrategy();
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, then performCustomLowering must instead lower
/// them.
bool customWriteBarrier() const { return CustomWriteBarriers; }
/// By default, read barriers are replaced with simple load
/// instructions. If true, then performCustomLowering must instead lower
/// them.
bool customReadBarrier() const { return CustomReadBarriers; }
/** @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 CustomSafePoints || 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, then performCustomLowering must delete them.
bool customRoots() const { return CustomRoots; }
/// By default, the GC analysis will find safe points according to
/// NeededSafePoints. If true, then findCustomSafePoints must create them.
bool customSafePoints() const { return CustomSafePoints; }
/// 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).
bool usesMetadata() const { return UsesMetadata; }
///@}
/// initializeCustomLowering/performCustomLowering - If any of the actions
/// are set to custom, performCustomLowering must be overriden to transform
/// the corresponding actions to LLVM IR. initializeCustomLowering is
/// optional to override. These are the only GCStrategy methods through
/// which the LLVM IR can be modified. These methods apply mostly to
/// gc.root based implementations, but can be overriden to provide custom
/// barrier lowerings with gc.statepoint as well.
///@{
virtual bool initializeCustomLowering(Module &F) {
// No changes made
return false;
}
virtual bool performCustomLowering(Function &F) {
llvm_unreachable("GCStrategy subclass specified a configuration which"
"requires a custom lowering without providing one");
}
///@}
/// Called if customSafepoints returns true, used only by gc.root
/// implementations.
virtual bool findCustomSafePoints(GCFunctionInfo& FI, MachineFunction& MF) {
llvm_unreachable("GCStrategy subclass specified a configuration which"
"requests custom safepoint identification without"
"providing an implementation for such");
}
};
/// 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;
}
#endif