1 //===- ScalarEvolutionNormalization.cpp - See below -------------*- 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 file implements utilities for working with "normalized" expressions. 11 // See the comments at the top of ScalarEvolutionNormalization.h for details. 12 // 13 //===----------------------------------------------------------------------===// 14 15 #include "llvm/Analysis/Dominators.h" 16 #include "llvm/Analysis/LoopInfo.h" 17 #include "llvm/Analysis/ScalarEvolutionExpressions.h" 18 #include "llvm/Analysis/ScalarEvolutionNormalization.h" 19 using namespace llvm; 20 21 /// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression 22 /// and now we need to decide whether the user should use the preinc or post-inc 23 /// value. If this user should use the post-inc version of the IV, return true. 24 /// 25 /// Choosing wrong here can break dominance properties (if we choose to use the 26 /// post-inc value when we cannot) or it can end up adding extra live-ranges to 27 /// the loop, resulting in reg-reg copies (if we use the pre-inc value when we 28 /// should use the post-inc value). 29 static bool IVUseShouldUsePostIncValue(Instruction *User, Value *Operand, 30 const Loop *L, DominatorTree *DT) { 31 // If the user is in the loop, use the preinc value. 32 if (L->contains(User)) return false; 33 34 BasicBlock *LatchBlock = L->getLoopLatch(); 35 if (!LatchBlock) 36 return false; 37 38 // Ok, the user is outside of the loop. If it is dominated by the latch 39 // block, use the post-inc value. 40 if (DT->dominates(LatchBlock, User->getParent())) 41 return true; 42 43 // There is one case we have to be careful of: PHI nodes. These little guys 44 // can live in blocks that are not dominated by the latch block, but (since 45 // their uses occur in the predecessor block, not the block the PHI lives in) 46 // should still use the post-inc value. Check for this case now. 47 PHINode *PN = dyn_cast<PHINode>(User); 48 if (!PN || !Operand) return false; // not a phi, not dominated by latch block. 49 50 // Look at all of the uses of Operand by the PHI node. If any use corresponds 51 // to a block that is not dominated by the latch block, give up and use the 52 // preincremented value. 53 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) 54 if (PN->getIncomingValue(i) == Operand && 55 !DT->dominates(LatchBlock, PN->getIncomingBlock(i))) 56 return false; 57 58 // Okay, all uses of Operand by PN are in predecessor blocks that really are 59 // dominated by the latch block. Use the post-incremented value. 60 return true; 61 } 62 63 const SCEV *llvm::TransformForPostIncUse(TransformKind Kind, 64 const SCEV *S, 65 Instruction *User, 66 Value *OperandValToReplace, 67 PostIncLoopSet &Loops, 68 ScalarEvolution &SE, 69 DominatorTree &DT) { 70 if (isa<SCEVConstant>(S) || isa<SCEVUnknown>(S)) 71 return S; 72 73 if (const SCEVCastExpr *X = dyn_cast<SCEVCastExpr>(S)) { 74 const SCEV *O = X->getOperand(); 75 const SCEV *N = TransformForPostIncUse(Kind, O, User, OperandValToReplace, 76 Loops, SE, DT); 77 if (O != N) 78 switch (S->getSCEVType()) { 79 case scZeroExtend: return SE.getZeroExtendExpr(N, S->getType()); 80 case scSignExtend: return SE.getSignExtendExpr(N, S->getType()); 81 case scTruncate: return SE.getTruncateExpr(N, S->getType()); 82 default: llvm_unreachable("Unexpected SCEVCastExpr kind!"); 83 } 84 return S; 85 } 86 87 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) { 88 // An addrec. This is the interesting part. 89 SmallVector<const SCEV *, 8> Operands; 90 const Loop *L = AR->getLoop(); 91 // The addrec conceptually uses its operands at loop entry. 92 Instruction *LUser = L->getHeader()->begin(); 93 // Transform each operand. 94 for (SCEVNAryExpr::op_iterator I = AR->op_begin(), E = AR->op_end(); 95 I != E; ++I) { 96 const SCEV *O = *I; 97 const SCEV *N = TransformForPostIncUse(Kind, O, LUser, 0, Loops, SE, DT); 98 Operands.push_back(N); 99 } 100 // Conservatively use AnyWrap until/unless we need FlagNW. 101 const SCEV *Result = SE.getAddRecExpr(Operands, L, SCEV::FlagAnyWrap); 102 switch (Kind) { 103 default: llvm_unreachable("Unexpected transform name!"); 104 case NormalizeAutodetect: 105 if (IVUseShouldUsePostIncValue(User, OperandValToReplace, L, &DT)) { 106 const SCEV *TransformedStep = 107 TransformForPostIncUse(Kind, AR->getStepRecurrence(SE), 108 User, OperandValToReplace, Loops, SE, DT); 109 Result = SE.getMinusSCEV(Result, TransformedStep); 110 Loops.insert(L); 111 } 112 #if 0 113 // This assert is conceptually correct, but ScalarEvolution currently 114 // sometimes fails to canonicalize two equal SCEVs to exactly the same 115 // form. It's possibly a pessimization when this happens, but it isn't a 116 // correctness problem, so disable this assert for now. 117 assert(S == TransformForPostIncUse(Denormalize, Result, 118 User, OperandValToReplace, 119 Loops, SE, DT) && 120 "SCEV normalization is not invertible!"); 121 #endif 122 break; 123 case Normalize: 124 if (Loops.count(L)) { 125 const SCEV *TransformedStep = 126 TransformForPostIncUse(Kind, AR->getStepRecurrence(SE), 127 User, OperandValToReplace, Loops, SE, DT); 128 Result = SE.getMinusSCEV(Result, TransformedStep); 129 } 130 #if 0 131 // See the comment on the assert above. 132 assert(S == TransformForPostIncUse(Denormalize, Result, 133 User, OperandValToReplace, 134 Loops, SE, DT) && 135 "SCEV normalization is not invertible!"); 136 #endif 137 break; 138 case Denormalize: 139 if (Loops.count(L)) 140 Result = cast<SCEVAddRecExpr>(Result)->getPostIncExpr(SE); 141 break; 142 } 143 return Result; 144 } 145 146 if (const SCEVNAryExpr *X = dyn_cast<SCEVNAryExpr>(S)) { 147 SmallVector<const SCEV *, 8> Operands; 148 bool Changed = false; 149 // Transform each operand. 150 for (SCEVNAryExpr::op_iterator I = X->op_begin(), E = X->op_end(); 151 I != E; ++I) { 152 const SCEV *O = *I; 153 const SCEV *N = TransformForPostIncUse(Kind, O, User, OperandValToReplace, 154 Loops, SE, DT); 155 Changed |= N != O; 156 Operands.push_back(N); 157 } 158 // If any operand actually changed, return a transformed result. 159 if (Changed) 160 switch (S->getSCEVType()) { 161 case scAddExpr: return SE.getAddExpr(Operands); 162 case scMulExpr: return SE.getMulExpr(Operands); 163 case scSMaxExpr: return SE.getSMaxExpr(Operands); 164 case scUMaxExpr: return SE.getUMaxExpr(Operands); 165 default: llvm_unreachable("Unexpected SCEVNAryExpr kind!"); 166 } 167 return S; 168 } 169 170 if (const SCEVUDivExpr *X = dyn_cast<SCEVUDivExpr>(S)) { 171 const SCEV *LO = X->getLHS(); 172 const SCEV *RO = X->getRHS(); 173 const SCEV *LN = TransformForPostIncUse(Kind, LO, User, OperandValToReplace, 174 Loops, SE, DT); 175 const SCEV *RN = TransformForPostIncUse(Kind, RO, User, OperandValToReplace, 176 Loops, SE, DT); 177 if (LO != LN || RO != RN) 178 return SE.getUDivExpr(LN, RN); 179 return S; 180 } 181 182 llvm_unreachable("Unexpected SCEV kind!"); 183 return 0; 184 } 185
