GCStrategy.h   [plain text]

//===-- 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_IR_GCSTRATEGY_H
#define LLVM_IR_GCSTRATEGY_H

#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 {
private:
  std::string Name;
  friend class GCModuleInfo;

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, 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;
}

#endif