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 #include "llvm/Support/BlockFrequency.h" 34 35 namespace llvm { 36 37 class BitVector; 38 class EdgeBundles; 39 class MachineBasicBlock; 40 class MachineLoopInfo; 41 42 class SpillPlacement : public MachineFunctionPass { 43 struct Node; 44 const MachineFunction *MF; 45 const EdgeBundles *bundles; 46 const MachineLoopInfo *loops; 47 Node *nodes; 48 49 // Nodes that are active in the current computation. Owned by the prepare() 50 // caller. 51 BitVector *ActiveNodes; 52 53 // Nodes with active links. Populated by scanActiveBundles. 54 SmallVector<unsigned, 8> Linked; 55 56 // Nodes that went positive during the last call to scanActiveBundles or 57 // iterate. 58 SmallVector<unsigned, 8> RecentPositive; 59 60 // Block frequencies are computed once. Indexed by block number. 61 SmallVector<BlockFrequency, 4> BlockFrequencies; 62 63 public: 64 static char ID; // Pass identification, replacement for typeid. 65 66 SpillPlacement() : MachineFunctionPass(ID), nodes(0) {} 67 ~SpillPlacement() { releaseMemory(); } 68 69 /// BorderConstraint - A basic block has separate constraints for entry and 70 /// exit. 71 enum BorderConstraint { 72 DontCare, ///< Block doesn't care / variable not live. 73 PrefReg, ///< Block entry/exit prefers a register. 74 PrefSpill, ///< Block entry/exit prefers a stack slot. 75 PrefBoth, ///< Block entry prefers both register and stack. 76 MustSpill ///< A register is impossible, variable must be spilled. 77 }; 78 79 /// BlockConstraint - Entry and exit constraints for a basic block. 80 struct BlockConstraint { 81 unsigned Number; ///< Basic block number (from MBB::getNumber()). 82 BorderConstraint Entry : 8; ///< Constraint on block entry. 83 BorderConstraint Exit : 8; ///< Constraint on block exit. 84 85 /// True when this block changes the value of the live range. This means 86 /// the block has a non-PHI def. When this is false, a live-in value on 87 /// the stack can be live-out on the stack without inserting a spill. 88 bool ChangesValue; 89 }; 90 91 /// prepare - Reset state and prepare for a new spill placement computation. 92 /// @param RegBundles Bit vector to receive the edge bundles where the 93 /// variable should be kept in a register. Each bit 94 /// corresponds to an edge bundle, a set bit means the 95 /// variable should be kept in a register through the 96 /// bundle. A clear bit means the variable should be 97 /// spilled. This vector is retained. 98 void prepare(BitVector &RegBundles); 99 100 /// addConstraints - Add constraints and biases. This method may be called 101 /// more than once to accumulate constraints. 102 /// @param LiveBlocks Constraints for blocks that have the variable live in or 103 /// live out. 104 void addConstraints(ArrayRef<BlockConstraint> LiveBlocks); 105 106 /// addPrefSpill - Add PrefSpill constraints to all blocks listed. This is 107 /// equivalent to calling addConstraint with identical BlockConstraints with 108 /// Entry = Exit = PrefSpill, and ChangesValue = false. 109 /// 110 /// @param Blocks Array of block numbers that prefer to spill in and out. 111 /// @param Strong When true, double the negative bias for these blocks. 112 void addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong); 113 114 /// addLinks - Add transparent blocks with the given numbers. 115 void addLinks(ArrayRef<unsigned> Links); 116 117 /// scanActiveBundles - Perform an initial scan of all bundles activated by 118 /// addConstraints and addLinks, updating their state. Add all the bundles 119 /// that now prefer a register to RecentPositive. 120 /// Prepare internal data structures for iterate. 121 /// Return true is there are any positive nodes. 122 bool scanActiveBundles(); 123 124 /// iterate - Update the network iteratively until convergence, or new bundles 125 /// are found. 126 void iterate(); 127 128 /// getRecentPositive - Return an array of bundles that became positive during 129 /// the previous call to scanActiveBundles or iterate. 130 ArrayRef<unsigned> getRecentPositive() { return RecentPositive; } 131 132 /// finish - Compute the optimal spill code placement given the 133 /// constraints. No MustSpill constraints will be violated, and the smallest 134 /// possible number of PrefX constraints will be violated, weighted by 135 /// expected execution frequencies. 136 /// The selected bundles are returned in the bitvector passed to prepare(). 137 /// @return True if a perfect solution was found, allowing the variable to be 138 /// in a register through all relevant bundles. 139 bool finish(); 140 141 /// getBlockFrequency - Return the estimated block execution frequency per 142 /// function invocation. 143 BlockFrequency getBlockFrequency(unsigned Number) const { 144 return BlockFrequencies[Number]; 145 } 146 147 private: 148 virtual bool runOnMachineFunction(MachineFunction&); 149 virtual void getAnalysisUsage(AnalysisUsage&) const; 150 virtual void releaseMemory(); 151 152 void activate(unsigned); 153 }; 154 155 } // end namespace llvm 156 157 #endif 158
