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 PrefBoth, ///< Block entry prefers both register and stack. 75 MustSpill ///< A register is impossible, variable must be spilled. 76 }; 77 78 /// BlockConstraint - Entry and exit constraints for a basic block. 79 struct BlockConstraint { 80 unsigned Number; ///< Basic block number (from MBB::getNumber()). 81 BorderConstraint Entry : 8; ///< Constraint on block entry. 82 BorderConstraint Exit : 8; ///< Constraint on block exit. 83 84 /// True when this block changes the value of the live range. This means 85 /// the block has a non-PHI def. When this is false, a live-in value on 86 /// the stack can be live-out on the stack without inserting a spill. 87 bool ChangesValue; 88 }; 89 90 /// prepare - Reset state and prepare for a new spill placement computation. 91 /// @param RegBundles Bit vector to receive the edge bundles where the 92 /// variable should be kept in a register. Each bit 93 /// corresponds to an edge bundle, a set bit means the 94 /// variable should be kept in a register through the 95 /// bundle. A clear bit means the variable should be 96 /// spilled. This vector is retained. 97 void prepare(BitVector &RegBundles); 98 99 /// addConstraints - Add constraints and biases. This method may be called 100 /// more than once to accumulate constraints. 101 /// @param LiveBlocks Constraints for blocks that have the variable live in or 102 /// live out. 103 void addConstraints(ArrayRef<BlockConstraint> LiveBlocks); 104 105 /// addPrefSpill - Add PrefSpill constraints to all blocks listed. This is 106 /// equivalent to calling addConstraint with identical BlockConstraints with 107 /// Entry = Exit = PrefSpill, and ChangesValue = false. 108 /// 109 /// @param Blocks Array of block numbers that prefer to spill in and out. 110 /// @param Strong When true, double the negative bias for these blocks. 111 void addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong); 112 113 /// addLinks - Add transparent blocks with the given numbers. 114 void addLinks(ArrayRef<unsigned> Links); 115 116 /// scanActiveBundles - Perform an initial scan of all bundles activated by 117 /// addConstraints and addLinks, updating their state. Add all the bundles 118 /// that now prefer a register to RecentPositive. 119 /// Prepare internal data structures for iterate. 120 /// Return true is there are any positive nodes. 121 bool scanActiveBundles(); 122 123 /// iterate - Update the network iteratively until convergence, or new bundles 124 /// are found. 125 void iterate(); 126 127 /// getRecentPositive - Return an array of bundles that became positive during 128 /// the previous call to scanActiveBundles or iterate. 129 ArrayRef<unsigned> getRecentPositive() { return RecentPositive; } 130 131 /// finish - Compute the optimal spill code placement given the 132 /// constraints. No MustSpill constraints will be violated, and the smallest 133 /// possible number of PrefX constraints will be violated, weighted by 134 /// expected execution frequencies. 135 /// The selected bundles are returned in the bitvector passed to prepare(). 136 /// @return True if a perfect solution was found, allowing the variable to be 137 /// in a register through all relevant bundles. 138 bool finish(); 139 140 /// getBlockFrequency - Return the estimated block execution frequency per 141 /// function invocation. 142 float getBlockFrequency(unsigned Number) const { 143 return BlockFrequency[Number]; 144 } 145 146 private: 147 virtual bool runOnMachineFunction(MachineFunction&); 148 virtual void getAnalysisUsage(AnalysisUsage&) const; 149 virtual void releaseMemory(); 150 151 void activate(unsigned); 152 }; 153 154 } // end namespace llvm 155 156 #endif 157
