+//===-- llvm/IR/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_IR_GCSTRATEGY_H
+#define LLVM_IR_GCSTRATEGY_H
+
+#include "llvm/ADT/Optional.h"
+#include "llvm/IR/Value.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/Module.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/Registry.h"
+#include <string>
+
+namespace llvm {
+ 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.
+ };
+ }
+
+
+ /// 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 {
+ private:
+ std::string Name;
+ friend class LLVMContextImpl;
+
+ protected:
+ 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.
+
+ 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; }
+
+ /// 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, then performCustomLowering must delete them.
+ 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; }
+
+ ///@}
+
+ /// 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");
+ }
+ };
+
+ /// 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