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   // Nodes with active links. Populated by scanActiveBundles.
  53   SmallVector<unsigned, 8> Linked;
  54
  55   // Nodes that went positive during the last call to scanActiveBundles or
  56   // iterate.
  57   SmallVector<unsigned, 8> RecentPositive;
  58
  59   // Block frequencies are computed once. Indexed by block number.
  60   SmallVector<float, 4> BlockFrequency;
  61
  62 public:
  63   static char ID; // Pass identification, replacement for typeid.
  64
  65   SpillPlacement() : MachineFunctionPass(ID), nodes(0) {}
  66   ~SpillPlacement() { releaseMemory(); }
  67
  68   /// BorderConstraint - A basic block has separate constraints for entry and
  69   /// exit.
  70   enum BorderConstraint {
  71     DontCare,  ///< Block doesn't care / variable not live.
  72     PrefReg,   ///< Block entry/exit prefers a register.
  73     PrefSpill, ///< Block entry/exit prefers a stack slot.
  74     MustSpill  ///< A register is impossible, variable must be spilled.
  75   };
  76
  77   /// BlockConstraint - Entry and exit constraints for a basic block.
  78   struct BlockConstraint {
  79     unsigned Number;            ///< Basic block number (from MBB::getNumber()).
  80     BorderConstraint Entry : 8; ///< Constraint on block entry.
  81     BorderConstraint Exit : 8;  ///< Constraint on block exit.
  82   };
  83
  84   /// prepare - Reset state and prepare for a new spill placement computation.
  85   /// @param RegBundles Bit vector to receive the edge bundles where the
  86   ///                   variable should be kept in a register. Each bit
  87   ///                   corresponds to an edge bundle, a set bit means the
  88   ///                   variable should be kept in a register through the
  89   ///                   bundle. A clear bit means the variable should be
  90   ///                   spilled. This vector is retained.
  91   void prepare(BitVector &RegBundles);
  92
  93   /// addConstraints - Add constraints and biases. This method may be called
  94   /// more than once to accumulate constraints.
  95   /// @param LiveBlocks Constraints for blocks that have the variable live in or
  96   ///                   live out.
  97   void addConstraints(ArrayRef<BlockConstraint> LiveBlocks);
  98
  99   /// addLinks - Add transparent blocks with the given numbers.
 100   void addLinks(ArrayRef<unsigned> Links);
 101
 102   /// scanActiveBundles - Perform an initial scan of all bundles activated by
 103   /// addConstraints and addLinks, updating their state. Add all the bundles
 104   /// that now prefer a register to RecentPositive.
 105   /// Prepare internal data structures for iterate.
 106   /// Return true is there are any positive nodes.
 107   bool scanActiveBundles();
 108
 109   /// iterate - Update the network iteratively until convergence, or new bundles
 110   /// are found.
 111   void iterate();
 112
 113   /// getRecentPositive - Return an array of bundles that became positive during
 114   /// the previous call to scanActiveBundles or iterate.
 115   ArrayRef<unsigned> getRecentPositive() { return RecentPositive; }
 116
 117   /// finish - Compute the optimal spill code placement given the
 118   /// constraints. No MustSpill constraints will be violated, and the smallest
 119   /// possible number of PrefX constraints will be violated, weighted by
 120   /// expected execution frequencies.
 121   /// The selected bundles are returned in the bitvector passed to prepare().
 122   /// @return True if a perfect solution was found, allowing the variable to be
 123   ///         in a register through all relevant bundles.
 124   bool finish();
 125
 126   /// getBlockFrequency - Return the estimated block execution frequency per
 127   /// function invocation.
 128   float getBlockFrequency(unsigned Number) const {
 129     return BlockFrequency[Number];
 130   }
 131
 132 private:
 133   virtual bool runOnMachineFunction(MachineFunction&);
 134   virtual void getAnalysisUsage(AnalysisUsage&) const;
 135   virtual void releaseMemory();
 136
 137   void activate(unsigned);
 138 };
 139
 140 } // end namespace llvm
 141
 142 #endif