1 //===- SpillPlacement.h - Optimal Spill Code Placement ---------*- C++ -*--===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This analysis computes the optimal spill code placement between basic blocks. 10 // 11 // The runOnMachineFunction() method only precomputes some profiling information 12 // about the CFG. The real work is done by prepare(), addConstraints(), and 13 // finish() which are called by the register allocator. 14 // 15 // Given a variable that is live across multiple basic blocks, and given 16 // constraints on the basic blocks where the variable is live, determine which 17 // edge bundles should have the variable in a register and which edge bundles 18 // should have the variable in a stack slot. 19 // 20 // The returned bit vector can be used to place optimal spill code at basic 21 // block entries and exits. Spill code placement inside a basic block is not 22 // considered. 23 // 24 //===----------------------------------------------------------------------===// 25 26 #ifndef LLVM_LIB_CODEGEN_SPILLPLACEMENT_H 27 #define LLVM_LIB_CODEGEN_SPILLPLACEMENT_H 28 29 #include "llvm/ADT/ArrayRef.h" 30 #include "llvm/ADT/SmallVector.h" 31 #include "llvm/ADT/SparseSet.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 MachineBlockFrequencyInfo; 40 class MachineFunction; 41 42 class SpillPlacement : public MachineFunctionPass { 43 struct Node; 44 const MachineFunction *MF = nullptr; 45 const EdgeBundles *bundles = nullptr; 46 const MachineBlockFrequencyInfo *MBFI = nullptr; 47 Node *nodes = nullptr; 48 49 // Nodes that are active in the current computation. Owned by the prepare() 50 // caller. 51 BitVector *ActiveNodes = nullptr; 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, 8> BlockFrequencies; 62 63 /// Decision threshold. A node gets the output value 0 if the weighted sum of 64 /// its inputs falls in the open interval (-Threshold;Threshold). 65 BlockFrequency Threshold; 66 67 /// List of nodes that need to be updated in ::iterate. 68 SparseSet<unsigned> TodoList; 69 70 public: 71 static char ID; // Pass identification, replacement for typeid. 72 73 SpillPlacement() : MachineFunctionPass(ID) {} 74 ~SpillPlacement() override { releaseMemory(); } 75 76 /// BorderConstraint - A basic block has separate constraints for entry and 77 /// exit. 78 enum BorderConstraint { 79 DontCare, ///< Block doesn't care / variable not live. 80 PrefReg, ///< Block entry/exit prefers a register. 81 PrefSpill, ///< Block entry/exit prefers a stack slot. 82 PrefBoth, ///< Block entry prefers both register and stack. 83 MustSpill ///< A register is impossible, variable must be spilled. 84 }; 85 86 /// BlockConstraint - Entry and exit constraints for a basic block. 87 struct BlockConstraint { 88 unsigned Number; ///< Basic block number (from MBB::getNumber()). 89 BorderConstraint Entry : 8; ///< Constraint on block entry. 90 BorderConstraint Exit : 8; ///< Constraint on block exit. 91 92 /// True when this block changes the value of the live range. This means 93 /// the block has a non-PHI def. When this is false, a live-in value on 94 /// the stack can be live-out on the stack without inserting a spill. 95 bool ChangesValue; 96 97 void print(raw_ostream &OS) const; 98 void dump() const; 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 &mf) override; 159 void getAnalysisUsage(AnalysisUsage &AU) const override; 160 void releaseMemory() override; 161 162 void activate(unsigned n); 163 void setThreshold(BlockFrequency Entry); 164 165 bool update(unsigned n); 166 }; 167 168 } // end namespace llvm 169 170 #endif // LLVM_LIB_CODEGEN_SPILLPLACEMENT_H 171