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 prepare(), addConstraints(), and
  14 // finish() which are called by the 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/ADT/ArrayRef.h"
  31 #include "llvm/ADT/SmallVector.h"
  32 #include "llvm/CodeGen/MachineFunctionPass.h"
  33
  34 namespace llvm {
  35
  36 class BitVector;
  37 class EdgeBundles;
  38 class MachineBasicBlock;
  39 class MachineLoopInfo;
  40
  41 class SpillPlacement  : public MachineFunctionPass {
  42   struct Node;
  43   const MachineFunction *MF;
  44   const EdgeBundles *bundles;
  45   const MachineLoopInfo *loops;
  46   Node *nodes;
  47
  48   // Nodes that are active in the current computation. Owned by the prepare()
  49   // caller.
  50   BitVector *ActiveNodes;
  51
  52   // The number of active nodes with a positive bias.
  53   unsigned PositiveNodes;
  54
  55   // Block frequencies are computed once. Indexed by block number.
  56   SmallVector<float, 4> BlockFrequency;
  57
  58 public:
  59   static char ID; // Pass identification, replacement for typeid.
  60
  61   SpillPlacement() : MachineFunctionPass(ID), nodes(0) {}
  62   ~SpillPlacement() { releaseMemory(); }
  63
  64   /// BorderConstraint - A basic block has separate constraints for entry and
  65   /// exit.
  66   enum BorderConstraint {
  67     DontCare,  ///< Block doesn't care / variable not live.
  68     PrefReg,   ///< Block entry/exit prefers a register.
  69     PrefSpill, ///< Block entry/exit prefers a stack slot.
  70     MustSpill  ///< A register is impossible, variable must be spilled.
  71   };
  72
  73   /// BlockConstraint - Entry and exit constraints for a basic block.
  74   struct BlockConstraint {
  75     unsigned Number;            ///< Basic block number (from MBB::getNumber()).
  76     BorderConstraint Entry : 8; ///< Constraint on block entry.
  77     BorderConstraint Exit : 8;  ///< Constraint on block exit.
  78   };
  79
  80   /// prepare - Reset state and prepare for a new spill placement computation.
  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. This vector is retained.
  87   void prepare(BitVector &RegBundles);
  88
  89   /// addConstraints - Add constraints and biases. This method may be called
  90   /// more than once to accumulate constraints.
  91   /// @param LiveBlocks Constraints for blocks that have the variable live in or
  92   ///                   live out.
  93   void addConstraints(ArrayRef<BlockConstraint> LiveBlocks);
  94
  95   /// addLinks - Add transparent blocks with the given numbers.
  96   void addLinks(ArrayRef<unsigned> Links);
  97
  98   /// getPositiveNodes - Return the total number of graph nodes with a positive
  99   /// bias after adding constraints.
 100   unsigned getPositiveNodes() const { return PositiveNodes; }
 101
 102   /// finish - Compute the optimal spill code placement given the
 103   /// constraints. No MustSpill constraints will be violated, and the smallest
 104   /// possible number of PrefX constraints will be violated, weighted by
 105   /// expected execution frequencies.
 106   /// The selected bundles are returned in the bitvector passed to prepare().
 107   /// @return True if a perfect solution was found, allowing the variable to be
 108   ///         in a register through all relevant bundles.
 109   bool finish();
 110
 111   /// getBlockFrequency - Return the estimated block execution frequency per
 112   /// function invocation.
 113   float getBlockFrequency(unsigned Number) const {
 114     return BlockFrequency[Number];
 115   }
 116
 117 private:
 118   virtual bool runOnMachineFunction(MachineFunction&);
 119   virtual void getAnalysisUsage(AnalysisUsage&) const;
 120   virtual void releaseMemory();
 121
 122   void activate(unsigned);
 123   void iterate(const SmallVectorImpl<unsigned>&);
 124 };
 125
 126 } // end namespace llvm
 127
 128 #endif