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.
  // 
  // When requested, the GCStrategy will be populated with data about each
  // function which uses it. 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.
  //
  //===----------------------------------------------------------------------===//
  
  #ifndef LLVM_CODEGEN_GCSTRATEGY_H
  #define LLVM_CODEGEN_GCSTRATEGY_H
  
  #include "llvm/CodeGen/GCMetadata.h"
  #include "llvm/Support/Registry.h"
  #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.
    class GCStrategy {
    public:
      typedef std::vector<GCFunctionInfo*> list_type;
      typedef list_type::iterator iterator;
      
    private:
      friend class GCModuleInfo;
      const Module *M;
      std::string Name;
      
      list_type Functions;
      
    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 InitRoots;            //< If set, roots are nulled during lowering.
      bool UsesMetadata;         //< If set, backend must emit metadata tables.
      
    public:
      GCStrategy();
      
      virtual ~GCStrategy();
      
      
      /// getName - The name of the GC strategy, for debugging.
      /// 
      const std::string &getName() const { return Name; }
  
      /// getModule - The module within which the GC strategy is operating.
      /// 
      const Module &getModule() const { return *M; }
  
      /// needsSafePoitns - True if safe points of any kind are required. By
      //                    default, none are recorded.
      bool needsSafePoints() const { return NeededSafePoints != 0; }
      
      /// needsSafePoint(Kind) - 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.
      bool customRoots() const { return CustomRoots; }
      
      /// 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.
      bool initializeRoots() const { return InitRoots; }
      
      /// usesMetadata - If set, appropriate metadata tables must be emitted by
      ///                the back-end (assembler, JIT, or otherwise).
      bool usesMetadata() const { return UsesMetadata; }
      
      /// begin/end - Iterators for function metadata.
      /// 
      iterator begin() { return Functions.begin(); }
      iterator end()   { return Functions.end();   }
  
      /// insertFunctionMetadata - Creates metadata for a function.
      /// 
      GCFunctionInfo *insertFunctionInfo(const Function &F);
  
      /// 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);
    };
    
  }
  
  #endif
  