//===- llvm/CodeGen/GCStrategy.h - Garbage collection -----------*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// 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.
//
// The read and write barrier support can be used with either implementation.
//
//===----------------------------------------------------------------------===//
#ifndef LLVM_IR_GCSTRATEGY_H
#define LLVM_IR_GCSTRATEGY_H
#include "llvm/Support/Registry.h"
#include <optional>
#include <string>
namespace llvm {
class Type;
/// 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:
friend class GCModuleInfo;
std::string Name;
protected:
bool UseStatepoints = false; /// Uses gc.statepoints as opposed to gc.roots,
/// if set, NeededSafePoints and UsesMetadata
/// should be left at their default values.
bool UseRS4GC = false; /// If UseStatepoints is set, this determines whether
/// the RewriteStatepointsForGC pass should rewrite
/// this function's calls.
/// This should only be set if UseStatepoints is set.
bool NeededSafePoints = false; ///< if set, calls are inferred to be safepoints
bool UsesMetadata = false; ///< If set, backend must emit metadata tables.
public:
GCStrategy();
virtual ~GCStrategy() = default;
/// 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; }
/// 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 type 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 'std::nullopt'
/// (i.e. an empty optional indicating it can't reliably distinguish.
virtual std::optional<bool> isGCManagedPointer(const Type *Ty) const {
return std::nullopt;
}
/// Returns true if the RewriteStatepointsForGC pass should run on functions
/// using this GC.
bool useRS4GC() const {
assert(useStatepoints() &&
"GC strategy has useRS4GC but not useStatepoints set");
return UseRS4GC;
}
///@}
/// If set, appropriate metadata tables must be emitted by the back-end
/// (assembler, JIT, or otherwise). The default stackmap information can be
/// found in the StackMap section as described in the documentation.
bool usesMetadata() const { return UsesMetadata; }
/** @name GCRoot Specific Properties
* These properties and overrides only apply to collector strategies using
* GCRoot.
*/
///@{
/// True if safe points need to be inferred on call sites
bool needsSafePoints() const { return NeededSafePoints; }
///@}
};
/// 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, you must also
/// register your GCMetadataPrinter subclass with the
/// GCMetadataPrinterRegistery as well.
using GCRegistry = Registry<GCStrategy>;
/// Lookup the GCStrategy object associated with the given gc name.
std::unique_ptr<GCStrategy> getGCStrategy(const StringRef Name);
} // end namespace llvm
#endif // LLVM_IR_GCSTRATEGY_H