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 SpillPlacement()65 SpillPlacement() : MachineFunctionPass(ID), nodes(0) {} ~SpillPlacement()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. getRecentPositive()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. getBlockFrequency(unsigned Number)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 143