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_LIB_CODEGEN_SPILLPLACEMENT_H 28 #define LLVM_LIB_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 class MachineBlockFrequencyInfo; 42 43 class SpillPlacement : public MachineFunctionPass { 44 struct Node; 45 const MachineFunction *MF; 46 const EdgeBundles *bundles; 47 const MachineLoopInfo *loops; 48 const MachineBlockFrequencyInfo *MBFI; 49 Node *nodes; 50 51 // Nodes that are active in the current computation. Owned by the prepare() 52 // caller. 53 BitVector *ActiveNodes; 54 55 // Nodes with active links. Populated by scanActiveBundles. 56 SmallVector<unsigned, 8> Linked; 57 58 // Nodes that went positive during the last call to scanActiveBundles or 59 // iterate. 60 SmallVector<unsigned, 8> RecentPositive; 61 62 // Block frequencies are computed once. Indexed by block number. 63 SmallVector<BlockFrequency, 8> BlockFrequencies; 64 65 /// Decision threshold. A node gets the output value 0 if the weighted sum of 66 /// its inputs falls in the open interval (-Threshold;Threshold). 67 BlockFrequency Threshold; 68 69 public: 70 static char ID; // Pass identification, replacement for typeid. 71 72 SpillPlacement() : MachineFunctionPass(ID), nodes(nullptr) {} 73 ~SpillPlacement() override { releaseMemory(); } 74 75 /// BorderConstraint - A basic block has separate constraints for entry and 76 /// exit. 77 enum BorderConstraint { 78 DontCare, ///< Block doesn't care / variable not live. 79 PrefReg, ///< Block entry/exit prefers a register. 80 PrefSpill, ///< Block entry/exit prefers a stack slot. 81 PrefBoth, ///< Block entry prefers both register and stack. 82 MustSpill ///< A register is impossible, variable must be spilled. 83 }; 84 85 /// BlockConstraint - Entry and exit constraints for a basic block. 86 struct BlockConstraint { 87 unsigned Number; ///< Basic block number (from MBB::getNumber()). 88 BorderConstraint Entry : 8; ///< Constraint on block entry. 89 BorderConstraint Exit : 8; ///< Constraint on block exit. 90 91 /// True when this block changes the value of the live range. This means 92 /// the block has a non-PHI def. When this is false, a live-in value on 93 /// the stack can be live-out on the stack without inserting a spill. 94 bool ChangesValue; 95 }; 96 97 /// prepare - Reset state and prepare for a new spill placement computation. 98 /// @param RegBundles Bit vector to receive the edge bundles where the 99 /// variable should be kept in a register. Each bit 100 /// corresponds to an edge bundle, a set bit means the 101 /// variable should be kept in a register through the 102 /// bundle. A clear bit means the variable should be 103 /// spilled. This vector is retained. 104 void prepare(BitVector &RegBundles); 105 106 /// addConstraints - Add constraints and biases. This method may be called 107 /// more than once to accumulate constraints. 108 /// @param LiveBlocks Constraints for blocks that have the variable live in or 109 /// live out. 110 void addConstraints(ArrayRef<BlockConstraint> LiveBlocks); 111 112 /// addPrefSpill - Add PrefSpill constraints to all blocks listed. This is 113 /// equivalent to calling addConstraint with identical BlockConstraints with 114 /// Entry = Exit = PrefSpill, and ChangesValue = false. 115 /// 116 /// @param Blocks Array of block numbers that prefer to spill in and out. 117 /// @param Strong When true, double the negative bias for these blocks. 118 void addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong); 119 120 /// addLinks - Add transparent blocks with the given numbers. 121 void addLinks(ArrayRef<unsigned> Links); 122 123 /// scanActiveBundles - Perform an initial scan of all bundles activated by 124 /// addConstraints and addLinks, updating their state. Add all the bundles 125 /// that now prefer a register to RecentPositive. 126 /// Prepare internal data structures for iterate. 127 /// Return true is there are any positive nodes. 128 bool scanActiveBundles(); 129 130 /// iterate - Update the network iteratively until convergence, or new bundles 131 /// are found. 132 void iterate(); 133 134 /// getRecentPositive - Return an array of bundles that became positive during 135 /// the previous call to scanActiveBundles or iterate. 136 ArrayRef<unsigned> getRecentPositive() { return RecentPositive; } 137 138 /// finish - Compute the optimal spill code placement given the 139 /// constraints. No MustSpill constraints will be violated, and the smallest 140 /// possible number of PrefX constraints will be violated, weighted by 141 /// expected execution frequencies. 142 /// The selected bundles are returned in the bitvector passed to prepare(). 143 /// @return True if a perfect solution was found, allowing the variable to be 144 /// in a register through all relevant bundles. 145 bool finish(); 146 147 /// getBlockFrequency - Return the estimated block execution frequency per 148 /// function invocation. 149 BlockFrequency getBlockFrequency(unsigned Number) const { 150 return BlockFrequencies[Number]; 151 } 152 153 private: 154 bool runOnMachineFunction(MachineFunction&) override; 155 void getAnalysisUsage(AnalysisUsage&) const override; 156 void releaseMemory() override; 157 158 void activate(unsigned); 159 void setThreshold(const BlockFrequency &Entry); 160 }; 161 162 } // end namespace llvm 163 164 #endif 165
