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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.

llvm-svn: 224086
This commit is contained in:
Philip Reames 2014-12-12 00:49:03 +00:00
parent b0274833dd
commit 5f7fe2e3ac
2 changed files with 83 additions and 58 deletions

View File

@ -12,9 +12,14 @@
// 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, the GCStrategy will be populated with data about each
// function which uses it. Specifically:
// 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.
@ -31,6 +36,15 @@
// 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
@ -42,13 +56,6 @@
#include <string>
namespace llvm {
class GCStrategy;
/// The GC strategy registry uses all the defaults from Registry.
///
typedef Registry<GCStrategy> GCRegistry;
/// 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
@ -56,8 +63,8 @@ namespace llvm {
/// pass and recreated every time that pass is invalidated.
class GCStrategy {
private:
friend class GCModuleInfo;
std::string Name;
friend class GCModuleInfo;
protected:
unsigned NeededSafePoints; ///< Bitmask of required safe points.
@ -71,65 +78,96 @@ namespace llvm {
public:
GCStrategy();
virtual ~GCStrategy() {}
/// getName - The name of the GC strategy, for debugging.
///
/// 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; }
/// needsSafePoitns - True if safe points of any kind are required. By
// default, none are recorded.
/// 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;
}
/// needsSafePoint(Kind) - True if the given kind of safe point is
// required. By default, none are recorded.
/// 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;
}
/// customWriteBarrier - By default, write barriers are replaced with simple
/// store instructions. If true, then
/// performCustomLowering must instead lower them.
bool customWriteBarrier() const { return CustomWriteBarriers; }
/// customReadBarrier - By default, read barriers are replaced with simple
/// load instructions. If true, then
/// performCustomLowering must instead lower them.
bool customReadBarrier() const { return CustomReadBarriers; }
/// customRoots - By default, roots are left for the code generator so it
/// can generate a stack map. If true, then
// performCustomLowering must delete them.
/// 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; }
/// customSafePoints - By default, the GC analysis will find safe
/// points according to NeededSafePoints. If true,
/// then findCustomSafePoints must create them.
/// By default, the GC analysis will find safe points according to
/// NeededSafePoints. If true, then findCustomSafePoints must create them.
bool customSafePoints() const { return CustomSafePoints; }
/// initializeRoots - 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.
/// 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; }
/// usesMetadata - If set, appropriate metadata tables must be emitted by
/// the back-end (assembler, JIT, or otherwise).
/// 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.
virtual bool initializeCustomLowering(Module &F);
virtual bool performCustomLowering(Function &F);
virtual bool findCustomSafePoints(GCFunctionInfo& FI, MachineFunction& MF);
/// 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

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@ -102,19 +102,6 @@ GCStrategy::GCStrategy() :
UsesMetadata(false)
{}
bool GCStrategy::initializeCustomLowering(Module &M) { return false; }
bool GCStrategy::performCustomLowering(Function &F) {
dbgs() << "gc " << getName() << " must override performCustomLowering.\n";
llvm_unreachable("must override performCustomLowering");
}
bool GCStrategy::findCustomSafePoints(GCFunctionInfo& FI, MachineFunction &F) {
dbgs() << "gc " << getName() << " must override findCustomSafePoints.\n";
llvm_unreachable(nullptr);
}
// -----------------------------------------------------------------------------
INITIALIZE_PASS_BEGIN(LowerIntrinsics, "gc-lowering", "GC Lowering",