Reassociate.cpp revision e96fda3002dd0769d3dd758ac5008ba8cda92349
1//===- Reassociate.cpp - Reassociate binary expressions -------------------===// 2// 3// This pass reassociates commutative expressions in an order that is designed 4// to promote better constant propagation, GCSE, LICM, PRE... 5// 6// For example: 4 + (x + 5) -> x + (4 + 5) 7// 8// Note that this pass works best if left shifts have been promoted to explicit 9// multiplies before this pass executes. 10// 11// In the implementation of this algorithm, constants are assigned rank = 0, 12// function arguments are rank = 1, and other values are assigned ranks 13// corresponding to the reverse post order traversal of current function 14// (starting at 2), which effectively gives values in deep loops higher rank 15// than values not in loops. 16// 17// This code was originally written by Chris Lattner, and was then cleaned up 18// and perfected by Casey Carter. 19// 20//===----------------------------------------------------------------------===// 21 22#include "llvm/Transforms/Scalar.h" 23#include "llvm/Function.h" 24#include "llvm/iOperators.h" 25#include "llvm/Type.h" 26#include "llvm/Pass.h" 27#include "llvm/Constant.h" 28#include "llvm/Support/CFG.h" 29#include "Support/PostOrderIterator.h" 30#include "Support/Statistic.h" 31 32namespace { 33 Statistic<> NumLinear ("reassociate","Number of insts linearized"); 34 Statistic<> NumChanged("reassociate","Number of insts reassociated"); 35 Statistic<> NumSwapped("reassociate","Number of insts with operands swapped"); 36 37 class Reassociate : public FunctionPass { 38 std::map<BasicBlock*, unsigned> RankMap; 39 std::map<Instruction*, unsigned> InstRankMap; 40 public: 41 bool runOnFunction(Function &F); 42 43 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 44 AU.setPreservesCFG(); 45 } 46 private: 47 void BuildRankMap(Function &F); 48 unsigned getRank(Value *V); 49 bool ReassociateExpr(BinaryOperator *I); 50 bool ReassociateBB(BasicBlock *BB); 51 }; 52 53 RegisterOpt<Reassociate> X("reassociate", "Reassociate expressions"); 54} 55 56Pass *createReassociatePass() { return new Reassociate(); } 57 58void Reassociate::BuildRankMap(Function &F) { 59 unsigned i = 1; 60 ReversePostOrderTraversal<Function*> RPOT(&F); 61 for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(), 62 E = RPOT.end(); I != E; ++I) 63 RankMap[*I] = ++i; 64} 65 66unsigned Reassociate::getRank(Value *V) { 67 if (isa<Argument>(V)) return 1; // Function argument... 68 if (Instruction *I = dyn_cast<Instruction>(V)) { 69 // If this is an expression, return the MAX(rank(LHS), rank(RHS)) so that we 70 // can reassociate expressions for code motion! Since we do not recurse for 71 // PHI nodes, we cannot have infinite recursion here, because there cannot 72 // be loops in the value graph that do not go through PHI nodes. 73 // 74 if (I->getOpcode() == Instruction::PHINode || 75 I->getOpcode() == Instruction::Alloca || 76 I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) || 77 I->mayWriteToMemory()) // Cannot move inst if it writes to memory! 78 return RankMap[I->getParent()]; 79 80 unsigned &CachedRank = InstRankMap[I]; 81 if (CachedRank) return CachedRank; // Rank already known? 82 83 // If not, compute it! 84 unsigned Rank = 0, MaxRank = RankMap[I->getParent()]; 85 for (unsigned i = 0, e = I->getNumOperands(); 86 i != e && Rank != MaxRank; ++i) 87 Rank = std::max(Rank, getRank(I->getOperand(i))); 88 89 return CachedRank = Rank; 90 } 91 92 // Otherwise it's a global or constant, rank 0. 93 return 0; 94} 95 96 97bool Reassociate::ReassociateExpr(BinaryOperator *I) { 98 Value *LHS = I->getOperand(0); 99 Value *RHS = I->getOperand(1); 100 unsigned LHSRank = getRank(LHS); 101 unsigned RHSRank = getRank(RHS); 102 103 bool Changed = false; 104 105 // Make sure the LHS of the operand always has the greater rank... 106 if (LHSRank < RHSRank) { 107 bool Success = !I->swapOperands(); 108 assert(Success && "swapOperands failed"); 109 110 std::swap(LHS, RHS); 111 std::swap(LHSRank, RHSRank); 112 Changed = true; 113 ++NumSwapped; 114 DEBUG(std::cerr << "Transposed: " << I 115 /* << " Result BB: " << I->getParent()*/); 116 } 117 118 // If the LHS is the same operator as the current one is, and if we are the 119 // only expression using it... 120 // 121 if (BinaryOperator *LHSI = dyn_cast<BinaryOperator>(LHS)) 122 if (LHSI->getOpcode() == I->getOpcode() && LHSI->use_size() == 1) { 123 // If the rank of our current RHS is less than the rank of the LHS's LHS, 124 // then we reassociate the two instructions... 125 if (RHSRank < getRank(LHSI->getOperand(0))) { 126 unsigned TakeOp = 0; 127 if (BinaryOperator *IOp = dyn_cast<BinaryOperator>(LHSI->getOperand(0))) 128 if (IOp->getOpcode() == LHSI->getOpcode()) 129 TakeOp = 1; // Hoist out non-tree portion 130 131 // Convert ((a + 12) + 10) into (a + (12 + 10)) 132 I->setOperand(0, LHSI->getOperand(TakeOp)); 133 LHSI->setOperand(TakeOp, RHS); 134 I->setOperand(1, LHSI); 135 136 // Move the LHS expression forward, to ensure that it is dominated by 137 // its operands. 138 LHSI->getParent()->getInstList().remove(LHSI); 139 I->getParent()->getInstList().insert(I, LHSI); 140 141 ++NumChanged; 142 DEBUG(std::cerr << "Reassociated: " << I/* << " Result BB: " 143 << I->getParent()*/); 144 145 // Since we modified the RHS instruction, make sure that we recheck it. 146 ReassociateExpr(LHSI); 147 return true; 148 } 149 } 150 151 return Changed; 152} 153 154 155// NegateValue - Insert instructions before the instruction pointed to by BI, 156// that computes the negative version of the value specified. The negative 157// version of the value is returned, and BI is left pointing at the instruction 158// that should be processed next by the reassociation pass. 159// 160static Value *NegateValue(Value *V, BasicBlock::iterator &BI) { 161 // We are trying to expose opportunity for reassociation. One of the things 162 // that we want to do to achieve this is to push a negation as deep into an 163 // expression chain as possible, to expose the add instructions. In practice, 164 // this means that we turn this: 165 // X = -(A+12+C+D) into X = -A + -12 + -C + -D = -12 + -A + -C + -D 166 // so that later, a: Y = 12+X could get reassociated with the -12 to eliminate 167 // the constants. We assume that instcombine will clean up the mess later if 168 // we introduce tons of unneccesary negation instructions... 169 // 170 if (Instruction *I = dyn_cast<Instruction>(V)) 171 if (I->getOpcode() == Instruction::Add && I->use_size() == 1) { 172 Value *RHS = NegateValue(I->getOperand(1), BI); 173 Value *LHS = NegateValue(I->getOperand(0), BI); 174 175 // We must actually insert a new add instruction here, because the neg 176 // instructions do not dominate the old add instruction in general. By 177 // adding it now, we are assured that the neg instructions we just 178 // inserted dominate the instruction we are about to insert after them. 179 // 180 return BinaryOperator::create(Instruction::Add, LHS, RHS, 181 I->getName()+".neg", 182 cast<Instruction>(RHS)->getNext()); 183 } 184 185 // Insert a 'neg' instruction that subtracts the value from zero to get the 186 // negation. 187 // 188 return BI = BinaryOperator::createNeg(V, V->getName() + ".neg", BI); 189} 190 191 192bool Reassociate::ReassociateBB(BasicBlock *BB) { 193 bool Changed = false; 194 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) { 195 196 DEBUG(std::cerr << "Processing: " << *BI); 197 if (BI->getOpcode() == Instruction::Sub && !BinaryOperator::isNeg(BI)) { 198 // Convert a subtract into an add and a neg instruction... so that sub 199 // instructions can be commuted with other add instructions... 200 // 201 // Calculate the negative value of Operand 1 of the sub instruction... 202 // and set it as the RHS of the add instruction we just made... 203 // 204 std::string Name = BI->getName(); 205 BI->setName(""); 206 Instruction *New = 207 BinaryOperator::create(Instruction::Add, BI->getOperand(0), 208 BI->getOperand(1), Name, BI); 209 210 // Everyone now refers to the add instruction... 211 BI->replaceAllUsesWith(New); 212 213 // Put the new add in the place of the subtract... deleting the subtract 214 BB->getInstList().erase(BI); 215 216 BI = New; 217 New->setOperand(1, NegateValue(New->getOperand(1), BI)); 218 219 Changed = true; 220 DEBUG(std::cerr << "Negated: " << New /*<< " Result BB: " << BB*/); 221 } 222 223 // If this instruction is a commutative binary operator, and the ranks of 224 // the two operands are sorted incorrectly, fix it now. 225 // 226 if (BI->isAssociative()) { 227 BinaryOperator *I = cast<BinaryOperator>(BI); 228 if (!I->use_empty()) { 229 // Make sure that we don't have a tree-shaped computation. If we do, 230 // linearize it. Convert (A+B)+(C+D) into ((A+B)+C)+D 231 // 232 Instruction *LHSI = dyn_cast<Instruction>(I->getOperand(0)); 233 Instruction *RHSI = dyn_cast<Instruction>(I->getOperand(1)); 234 if (LHSI && (int)LHSI->getOpcode() == I->getOpcode() && 235 RHSI && (int)RHSI->getOpcode() == I->getOpcode() && 236 RHSI->use_size() == 1) { 237 // Insert a new temporary instruction... (A+B)+C 238 BinaryOperator *Tmp = BinaryOperator::create(I->getOpcode(), LHSI, 239 RHSI->getOperand(0), 240 RHSI->getName()+".ra", 241 BI); 242 BI = Tmp; 243 I->setOperand(0, Tmp); 244 I->setOperand(1, RHSI->getOperand(1)); 245 246 // Process the temporary instruction for reassociation now. 247 I = Tmp; 248 ++NumLinear; 249 Changed = true; 250 DEBUG(std::cerr << "Linearized: " << I/* << " Result BB: " << BB*/); 251 } 252 253 // Make sure that this expression is correctly reassociated with respect 254 // to it's used values... 255 // 256 Changed |= ReassociateExpr(I); 257 } 258 } 259 } 260 261 return Changed; 262} 263 264 265bool Reassociate::runOnFunction(Function &F) { 266 // Recalculate the rank map for F 267 BuildRankMap(F); 268 269 bool Changed = false; 270 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) 271 Changed |= ReassociateBB(FI); 272 273 // We are done with the rank map... 274 RankMap.clear(); 275 InstRankMap.clear(); 276 return Changed; 277} 278