Reassociate.cpp revision d8e1eea678833cc2b15e4ea69a5a403ba9c3b013
1//===- Reassociate.cpp - Reassociate binary expressions -------------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// This pass reassociates commutative expressions in an order that is designed 11// to promote better constant propagation, GCSE, LICM, PRE... 12// 13// For example: 4 + (x + 5) -> x + (4 + 5) 14// 15// Note that this pass works best if left shifts have been promoted to explicit 16// multiplies before this pass executes. 17// 18// In the implementation of this algorithm, constants are assigned rank = 0, 19// function arguments are rank = 1, and other values are assigned ranks 20// corresponding to the reverse post order traversal of current function 21// (starting at 2), which effectively gives values in deep loops higher rank 22// than values not in loops. 23// 24//===----------------------------------------------------------------------===// 25 26#include "llvm/Transforms/Scalar.h" 27#include "llvm/Function.h" 28#include "llvm/Instructions.h" 29#include "llvm/Type.h" 30#include "llvm/Pass.h" 31#include "llvm/Constant.h" 32#include "llvm/Support/CFG.h" 33#include "Support/Debug.h" 34#include "Support/PostOrderIterator.h" 35#include "Support/Statistic.h" 36using namespace llvm; 37 38namespace { 39 Statistic<> NumLinear ("reassociate","Number of insts linearized"); 40 Statistic<> NumChanged("reassociate","Number of insts reassociated"); 41 Statistic<> NumSwapped("reassociate","Number of insts with operands swapped"); 42 43 class Reassociate : public FunctionPass { 44 std::map<BasicBlock*, unsigned> RankMap; 45 std::map<Value*, unsigned> ValueRankMap; 46 public: 47 bool runOnFunction(Function &F); 48 49 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 50 AU.setPreservesCFG(); 51 } 52 private: 53 void BuildRankMap(Function &F); 54 unsigned getRank(Value *V); 55 bool ReassociateExpr(BinaryOperator *I); 56 bool ReassociateBB(BasicBlock *BB); 57 }; 58 59 RegisterOpt<Reassociate> X("reassociate", "Reassociate expressions"); 60} 61 62// Public interface to the Reassociate pass 63FunctionPass *llvm::createReassociatePass() { return new Reassociate(); } 64 65void Reassociate::BuildRankMap(Function &F) { 66 unsigned i = 2; 67 68 // Assign distinct ranks to function arguments 69 for (Function::aiterator I = F.abegin(), E = F.aend(); I != E; ++I) 70 ValueRankMap[I] = ++i; 71 72 ReversePostOrderTraversal<Function*> RPOT(&F); 73 for (ReversePostOrderTraversal<Function*>::rpo_iterator I = RPOT.begin(), 74 E = RPOT.end(); I != E; ++I) 75 RankMap[*I] = ++i << 16; 76} 77 78unsigned Reassociate::getRank(Value *V) { 79 if (isa<Argument>(V)) return ValueRankMap[V]; // Function argument... 80 81 if (Instruction *I = dyn_cast<Instruction>(V)) { 82 // If this is an expression, return the 1+MAX(rank(LHS), rank(RHS)) so that 83 // we can reassociate expressions for code motion! Since we do not recurse 84 // for PHI nodes, we cannot have infinite recursion here, because there 85 // cannot be loops in the value graph that do not go through PHI nodes. 86 // 87 if (I->getOpcode() == Instruction::PHI || 88 I->getOpcode() == Instruction::Alloca || 89 I->getOpcode() == Instruction::Malloc || isa<TerminatorInst>(I) || 90 I->mayWriteToMemory()) // Cannot move inst if it writes to memory! 91 return RankMap[I->getParent()]; 92 93 unsigned &CachedRank = ValueRankMap[I]; 94 if (CachedRank) return CachedRank; // Rank already known? 95 96 // If not, compute it! 97 unsigned Rank = 0, MaxRank = RankMap[I->getParent()]; 98 for (unsigned i = 0, e = I->getNumOperands(); 99 i != e && Rank != MaxRank; ++i) 100 Rank = std::max(Rank, getRank(I->getOperand(i))); 101 102 DEBUG(std::cerr << "Calculated Rank[" << V->getName() << "] = " 103 << Rank+1 << "\n"); 104 105 return CachedRank = Rank+1; 106 } 107 108 // Otherwise it's a global or constant, rank 0. 109 return 0; 110} 111 112 113bool Reassociate::ReassociateExpr(BinaryOperator *I) { 114 Value *LHS = I->getOperand(0); 115 Value *RHS = I->getOperand(1); 116 unsigned LHSRank = getRank(LHS); 117 unsigned RHSRank = getRank(RHS); 118 119 bool Changed = false; 120 121 // Make sure the LHS of the operand always has the greater rank... 122 if (LHSRank < RHSRank) { 123 bool Success = !I->swapOperands(); 124 assert(Success && "swapOperands failed"); 125 126 std::swap(LHS, RHS); 127 std::swap(LHSRank, RHSRank); 128 Changed = true; 129 ++NumSwapped; 130 DEBUG(std::cerr << "Transposed: " << *I 131 /* << " Result BB: " << I->getParent()*/); 132 } 133 134 // If the LHS is the same operator as the current one is, and if we are the 135 // only expression using it... 136 // 137 if (BinaryOperator *LHSI = dyn_cast<BinaryOperator>(LHS)) 138 if (LHSI->getOpcode() == I->getOpcode() && LHSI->hasOneUse()) { 139 // If the rank of our current RHS is less than the rank of the LHS's LHS, 140 // then we reassociate the two instructions... 141 142 unsigned TakeOp = 0; 143 if (BinaryOperator *IOp = dyn_cast<BinaryOperator>(LHSI->getOperand(0))) 144 if (IOp->getOpcode() == LHSI->getOpcode()) 145 TakeOp = 1; // Hoist out non-tree portion 146 147 if (RHSRank < getRank(LHSI->getOperand(TakeOp))) { 148 // Convert ((a + 12) + 10) into (a + (12 + 10)) 149 I->setOperand(0, LHSI->getOperand(TakeOp)); 150 LHSI->setOperand(TakeOp, RHS); 151 I->setOperand(1, LHSI); 152 153 // Move the LHS expression forward, to ensure that it is dominated by 154 // its operands. 155 LHSI->getParent()->getInstList().remove(LHSI); 156 I->getParent()->getInstList().insert(I, LHSI); 157 158 ++NumChanged; 159 DEBUG(std::cerr << "Reassociated: " << *I/* << " Result BB: " 160 << I->getParent()*/); 161 162 // Since we modified the RHS instruction, make sure that we recheck it. 163 ReassociateExpr(LHSI); 164 ReassociateExpr(I); 165 return true; 166 } 167 } 168 169 return Changed; 170} 171 172 173// NegateValue - Insert instructions before the instruction pointed to by BI, 174// that computes the negative version of the value specified. The negative 175// version of the value is returned, and BI is left pointing at the instruction 176// that should be processed next by the reassociation pass. 177// 178static Value *NegateValue(Value *V, BasicBlock::iterator &BI) { 179 // We are trying to expose opportunity for reassociation. One of the things 180 // that we want to do to achieve this is to push a negation as deep into an 181 // expression chain as possible, to expose the add instructions. In practice, 182 // this means that we turn this: 183 // X = -(A+12+C+D) into X = -A + -12 + -C + -D = -12 + -A + -C + -D 184 // so that later, a: Y = 12+X could get reassociated with the -12 to eliminate 185 // the constants. We assume that instcombine will clean up the mess later if 186 // we introduce tons of unnecessary negation instructions... 187 // 188 if (Instruction *I = dyn_cast<Instruction>(V)) 189 if (I->getOpcode() == Instruction::Add && I->hasOneUse()) { 190 Value *RHS = NegateValue(I->getOperand(1), BI); 191 Value *LHS = NegateValue(I->getOperand(0), BI); 192 193 // We must actually insert a new add instruction here, because the neg 194 // instructions do not dominate the old add instruction in general. By 195 // adding it now, we are assured that the neg instructions we just 196 // inserted dominate the instruction we are about to insert after them. 197 // 198 return BinaryOperator::create(Instruction::Add, LHS, RHS, 199 I->getName()+".neg", 200 cast<Instruction>(RHS)->getNext()); 201 } 202 203 // Insert a 'neg' instruction that subtracts the value from zero to get the 204 // negation. 205 // 206 return BI = BinaryOperator::createNeg(V, V->getName() + ".neg", BI); 207} 208 209 210bool Reassociate::ReassociateBB(BasicBlock *BB) { 211 bool Changed = false; 212 for (BasicBlock::iterator BI = BB->begin(); BI != BB->end(); ++BI) { 213 214 DEBUG(std::cerr << "Reassociating: " << *BI); 215 if (BI->getOpcode() == Instruction::Sub && !BinaryOperator::isNeg(BI)) { 216 // Convert a subtract into an add and a neg instruction... so that sub 217 // instructions can be commuted with other add instructions... 218 // 219 // Calculate the negative value of Operand 1 of the sub instruction... 220 // and set it as the RHS of the add instruction we just made... 221 // 222 std::string Name = BI->getName(); 223 BI->setName(""); 224 Instruction *New = 225 BinaryOperator::create(Instruction::Add, BI->getOperand(0), 226 BI->getOperand(1), Name, BI); 227 228 // Everyone now refers to the add instruction... 229 BI->replaceAllUsesWith(New); 230 231 // Put the new add in the place of the subtract... deleting the subtract 232 BB->getInstList().erase(BI); 233 234 BI = New; 235 New->setOperand(1, NegateValue(New->getOperand(1), BI)); 236 237 Changed = true; 238 DEBUG(std::cerr << "Negated: " << *New /*<< " Result BB: " << BB*/); 239 } 240 241 // If this instruction is a commutative binary operator, and the ranks of 242 // the two operands are sorted incorrectly, fix it now. 243 // 244 if (BI->isAssociative()) { 245 BinaryOperator *I = cast<BinaryOperator>(BI); 246 if (!I->use_empty()) { 247 // Make sure that we don't have a tree-shaped computation. If we do, 248 // linearize it. Convert (A+B)+(C+D) into ((A+B)+C)+D 249 // 250 Instruction *LHSI = dyn_cast<Instruction>(I->getOperand(0)); 251 Instruction *RHSI = dyn_cast<Instruction>(I->getOperand(1)); 252 if (LHSI && (int)LHSI->getOpcode() == I->getOpcode() && 253 RHSI && (int)RHSI->getOpcode() == I->getOpcode() && 254 RHSI->hasOneUse()) { 255 // Insert a new temporary instruction... (A+B)+C 256 BinaryOperator *Tmp = BinaryOperator::create(I->getOpcode(), LHSI, 257 RHSI->getOperand(0), 258 RHSI->getName()+".ra", 259 BI); 260 BI = Tmp; 261 I->setOperand(0, Tmp); 262 I->setOperand(1, RHSI->getOperand(1)); 263 264 // Process the temporary instruction for reassociation now. 265 I = Tmp; 266 ++NumLinear; 267 Changed = true; 268 DEBUG(std::cerr << "Linearized: " << *I/* << " Result BB: " << BB*/); 269 } 270 271 // Make sure that this expression is correctly reassociated with respect 272 // to it's used values... 273 // 274 Changed |= ReassociateExpr(I); 275 } 276 } 277 } 278 279 return Changed; 280} 281 282 283bool Reassociate::runOnFunction(Function &F) { 284 // Recalculate the rank map for F 285 BuildRankMap(F); 286 287 bool Changed = false; 288 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) 289 Changed |= ReassociateBB(FI); 290 291 // We are done with the rank map... 292 RankMap.clear(); 293 ValueRankMap.clear(); 294 return Changed; 295} 296 297