lib/CodeGen/SpillPlacement.h

   1 //===-- SpillPlacement.h - Optimal Spill Code Placement --------*- C++ -*--===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This analysis computes the optimal spill code placement between basic blocks.
  11 //
  12 // The runOnMachineFunction() method only precomputes some profiling information
  13 // about the CFG. The real work is done by placeSpills() which is called by the
  14 // register allocator.
  15 //
  16 // Given a variable that is live across multiple basic blocks, and given
  17 // constraints on the basic blocks where the variable is live, determine which
  18 // edge bundles should have the variable in a register and which edge bundles
  19 // should have the variable in a stack slot.
  20 //
  21 // The returned bit vector can be used to place optimal spill code at basic
  22 // block entries and exits. Spill code placement inside a basic block is not
  23 // considered.
  24 //
  25 //===----------------------------------------------------------------------===//
  26
  27 #ifndef LLVM_CODEGEN_SPILLPLACEMENT_H
  28 #define LLVM_CODEGEN_SPILLPLACEMENT_H
  29
  30 #include "llvm/CodeGen/MachineFunctionPass.h"
  31
  32 namespace llvm {
  33
  34 class BitVector;
  35 class EdgeBundles;
  36 class MachineBasicBlock;
  37 class MachineLoopInfo;
  38 template <typename> class SmallVectorImpl;
  39
  40 class SpillPlacement  : public MachineFunctionPass {
  41   struct Node;
  42   const MachineFunction *MF;
  43   const EdgeBundles *bundles;
  44   const MachineLoopInfo *loops;
  45   Node *nodes;
  46
  47   // Nodes that are active in the current computation. Owned by the placeSpills
  48   // caller.
  49   BitVector *ActiveNodes;
  50
  51 public:
  52   static char ID; // Pass identification, replacement for typeid.
  53
  54   SpillPlacement() : MachineFunctionPass(ID), nodes(0) {}
  55   ~SpillPlacement() { releaseMemory(); }
  56
  57   /// BorderConstraint - A basic block has separate constraints for entry and
  58   /// exit.
  59   enum BorderConstraint {
  60     DontCare,  ///< Block doesn't care / variable not live.
  61     PrefReg,   ///< Block entry/exit prefers a register.
  62     PrefSpill, ///< Block entry/exit prefers a stack slot.
  63     MustSpill  ///< A register is impossible, variable must be spilled.
  64   };
  65
  66   /// BlockConstraint - Entry and exit constraints for a basic block.
  67   struct BlockConstraint {
  68     unsigned Number;            ///< Basic block number (from MBB::getNumber()).
  69     BorderConstraint Entry : 8; ///< Constraint on block entry.
  70     BorderConstraint Exit : 8;  ///< Constraint on block exit.
  71   };
  72
  73   /// placeSpills - Compute the optimal spill code placement given the
  74   /// constraints. No MustSpill constraints will be violated, and the smallest
  75   /// possible number of PrefX constraints will be violated, weighted by
  76   /// expected execution frequencies.
  77   /// @param LiveBlocks Constraints for blocks that have the variable live in or
  78   ///                   live out. DontCare/DontCare means the variable is live
  79   ///                   through the block. DontCare/X means the variable is live
  80   ///                   out, but not live in.
  81   /// @param RegBundles Bit vector to receive the edge bundles where the
  82   ///                   variable should be kept in a register. Each bit
  83   ///                   corresponds to an edge bundle, a set bit means the
  84   ///                   variable should be kept in a register through the
  85   ///                   bundle. A clear bit means the variable should be
  86   ///                   spilled.
  87   /// @return True if a perfect solution was found, allowing the variable to be
  88   ///         in a register through all relevant bundles.
  89   bool placeSpills(const SmallVectorImpl<BlockConstraint> &LiveBlocks,
  90                    BitVector &RegBundles);
  91
  92   /// getBlockFrequency - Return the estimated block execution frequency per
  93   /// function invocation.
  94   float getBlockFrequency(const MachineBasicBlock*);
  95
  96 private:
  97   virtual bool runOnMachineFunction(MachineFunction&);
  98   virtual void getAnalysisUsage(AnalysisUsage&) const;
  99   virtual void releaseMemory();
 100
 101   void activate(unsigned);
 102   void prepareNodes(const SmallVectorImpl<BlockConstraint>&);
 103   void iterate(const SmallVectorImpl<unsigned>&);
 104 };
 105
 106 } // end namespace llvm
 107
 108 #endif