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/ADT/SparseSet.h" 33 #include "llvm/CodeGen/MachineFunctionPass.h" 34 #include "llvm/Support/BlockFrequency.h" 35 36 namespace llvm { 37 38 class BitVector; 39 class EdgeBundles; 40 class MachineBasicBlock; 41 class MachineLoopInfo; 42 class MachineBlockFrequencyInfo; 43 44 class SpillPlacement : public MachineFunctionPass { 45 struct Node; 46 const MachineFunction *MF; 47 const EdgeBundles *bundles; 48 const MachineLoopInfo *loops; 49 const MachineBlockFrequencyInfo *MBFI; 50 Node *nodes; 51 52 // Nodes that are active in the current computation. Owned by the prepare() 53 // caller. 54 BitVector *ActiveNodes; 55 56 // Nodes with active links. Populated by scanActiveBundles. 57 SmallVector<unsigned, 8> Linked; 58 59 // Nodes that went positive during the last call to scanActiveBundles or 60 // iterate. 61 SmallVector<unsigned, 8> RecentPositive; 62 63 // Block frequencies are computed once. Indexed by block number. 64 SmallVector<BlockFrequency, 8> BlockFrequencies; 65 66 /// Decision threshold. A node gets the output value 0 if the weighted sum of 67 /// its inputs falls in the open interval (-Threshold;Threshold). 68 BlockFrequency Threshold; 69 70 /// List of nodes that need to be updated in ::iterate. 71 SparseSet<unsigned> TodoList; 72 73 public: 74 static char ID; // Pass identification, replacement for typeid. 75 76 SpillPlacement() : MachineFunctionPass(ID), nodes(nullptr) {} 77 ~SpillPlacement() override { releaseMemory(); } 78 79 /// BorderConstraint - A basic block has separate constraints for entry and 80 /// exit. 81 enum BorderConstraint { 82 DontCare, ///< Block doesn't care / variable not live. 83 PrefReg, ///< Block entry/exit prefers a register. 84 PrefSpill, ///< Block entry/exit prefers a stack slot. 85 PrefBoth, ///< Block entry prefers both register and stack. 86 MustSpill ///< A register is impossible, variable must be spilled. 87 }; 88 89 /// BlockConstraint - Entry and exit constraints for a basic block. 90 struct BlockConstraint { 91 unsigned Number; ///< Basic block number (from MBB::getNumber()). 92 BorderConstraint Entry : 8; ///< Constraint on block entry. 93 BorderConstraint Exit : 8; ///< Constraint on block exit. 94 95 /// True when this block changes the value of the live range. This means 96 /// the block has a non-PHI def. When this is false, a live-in value on 97 /// the stack can be live-out on the stack without inserting a spill. 98 bool ChangesValue; 99 }; 100 101 /// prepare - Reset state and prepare for a new spill placement computation. 102 /// @param RegBundles Bit vector to receive the edge bundles where the 103 /// variable should be kept in a register. Each bit 104 /// corresponds to an edge bundle, a set bit means the 105 /// variable should be kept in a register through the 106 /// bundle. A clear bit means the variable should be 107 /// spilled. This vector is retained. 108 void prepare(BitVector &RegBundles); 109 110 /// addConstraints - Add constraints and biases. This method may be called 111 /// more than once to accumulate constraints. 112 /// @param LiveBlocks Constraints for blocks that have the variable live in or 113 /// live out. 114 void addConstraints(ArrayRef<BlockConstraint> LiveBlocks); 115 116 /// addPrefSpill - Add PrefSpill constraints to all blocks listed. This is 117 /// equivalent to calling addConstraint with identical BlockConstraints with 118 /// Entry = Exit = PrefSpill, and ChangesValue = false. 119 /// 120 /// @param Blocks Array of block numbers that prefer to spill in and out. 121 /// @param Strong When true, double the negative bias for these blocks. 122 void addPrefSpill(ArrayRef<unsigned> Blocks, bool Strong); 123 124 /// addLinks - Add transparent blocks with the given numbers. 125 void addLinks(ArrayRef<unsigned> Links); 126 127 /// scanActiveBundles - Perform an initial scan of all bundles activated by 128 /// addConstraints and addLinks, updating their state. Add all the bundles 129 /// that now prefer a register to RecentPositive. 130 /// Prepare internal data structures for iterate. 131 /// Return true is there are any positive nodes. 132 bool scanActiveBundles(); 133 134 /// iterate - Update the network iteratively until convergence, or new bundles 135 /// are found. 136 void iterate(); 137 138 /// getRecentPositive - Return an array of bundles that became positive during 139 /// the previous call to scanActiveBundles or iterate. 140 ArrayRef<unsigned> getRecentPositive() { return RecentPositive; } 141 142 /// finish - Compute the optimal spill code placement given the 143 /// constraints. No MustSpill constraints will be violated, and the smallest 144 /// possible number of PrefX constraints will be violated, weighted by 145 /// expected execution frequencies. 146 /// The selected bundles are returned in the bitvector passed to prepare(). 147 /// @return True if a perfect solution was found, allowing the variable to be 148 /// in a register through all relevant bundles. 149 bool finish(); 150 151 /// getBlockFrequency - Return the estimated block execution frequency per 152 /// function invocation. 153 BlockFrequency getBlockFrequency(unsigned Number) const { 154 return BlockFrequencies[Number]; 155 } 156 157 private: 158 bool runOnMachineFunction(MachineFunction&) override; 159 void getAnalysisUsage(AnalysisUsage&) const override; 160 void releaseMemory() override; 161 162 void activate(unsigned); 163 void setThreshold(const BlockFrequency &Entry); 164 165 bool update(unsigned); 166 }; 167 168 } // end namespace llvm 169 170 #endif 171
