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 namespace { 64 65 /// Hold the state used during post-inc expression transformation, including a 66 /// map of transformed expressions. 67 class PostIncTransform { 68 TransformKind Kind; 69 PostIncLoopSet &Loops; 70 ScalarEvolution &SE; 71 DominatorTree &DT; 72 73 DenseMap<const SCEV*, const SCEV*> Transformed; 74 75 public: 76 PostIncTransform(TransformKind kind, PostIncLoopSet &loops, 77 ScalarEvolution &se, DominatorTree &dt): 78 Kind(kind), Loops(loops), SE(se), DT(dt) {} 79 80 const SCEV *TransformSubExpr(const SCEV *S, Instruction *User, 81 Value *OperandValToReplace); 82 83 protected: 84 const SCEV *TransformImpl(const SCEV *S, Instruction *User, 85 Value *OperandValToReplace); 86 }; 87 88 } // namespace 89 90 /// Implement post-inc transformation for all valid expression types. 91 const SCEV *PostIncTransform:: 92 TransformImpl(const SCEV *S, Instruction *User, Value *OperandValToReplace) { 93 94 if (const SCEVCastExpr *X = dyn_cast<SCEVCastExpr>(S)) { 95 const SCEV *O = X->getOperand(); 96 const SCEV *N = TransformSubExpr(O, User, OperandValToReplace); 97 if (O != N) 98 switch (S->getSCEVType()) { 99 case scZeroExtend: return SE.getZeroExtendExpr(N, S->getType()); 100 case scSignExtend: return SE.getSignExtendExpr(N, S->getType()); 101 case scTruncate: return SE.getTruncateExpr(N, S->getType()); 102 default: llvm_unreachable("Unexpected SCEVCastExpr kind!"); 103 } 104 return S; 105 } 106 107 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) { 108 // An addrec. This is the interesting part. 109 SmallVector<const SCEV *, 8> Operands; 110 const Loop *L = AR->getLoop(); 111 // The addrec conceptually uses its operands at loop entry. 112 Instruction *LUser = L->getHeader()->begin(); 113 // Transform each operand. 114 for (SCEVNAryExpr::op_iterator I = AR->op_begin(), E = AR->op_end(); 115 I != E; ++I) { 116 Operands.push_back(TransformSubExpr(*I, LUser, 0)); 117 } 118 // Conservatively use AnyWrap until/unless we need FlagNW. 119 const SCEV *Result = SE.getAddRecExpr(Operands, L, SCEV::FlagAnyWrap); 120 switch (Kind) { 121 case NormalizeAutodetect: 122 if (IVUseShouldUsePostIncValue(User, OperandValToReplace, L, &DT)) { 123 const SCEV *TransformedStep = 124 TransformSubExpr(AR->getStepRecurrence(SE), 125 User, OperandValToReplace); 126 Result = SE.getMinusSCEV(Result, TransformedStep); 127 Loops.insert(L); 128 } 129 #if 0 130 // This assert is conceptually correct, but ScalarEvolution currently 131 // sometimes fails to canonicalize two equal SCEVs to exactly the same 132 // form. It's possibly a pessimization when this happens, but it isn't a 133 // correctness problem, so disable this assert for now. 134 assert(S == TransformSubExpr(Result, User, OperandValToReplace) && 135 "SCEV normalization is not invertible!"); 136 #endif 137 break; 138 case Normalize: 139 if (Loops.count(L)) { 140 const SCEV *TransformedStep = 141 TransformSubExpr(AR->getStepRecurrence(SE), 142 User, OperandValToReplace); 143 Result = SE.getMinusSCEV(Result, TransformedStep); 144 } 145 #if 0 146 // See the comment on the assert above. 147 assert(S == TransformSubExpr(Result, User, OperandValToReplace) && 148 "SCEV normalization is not invertible!"); 149 #endif 150 break; 151 case Denormalize: 152 if (Loops.count(L)) 153 Result = cast<SCEVAddRecExpr>(Result)->getPostIncExpr(SE); 154 break; 155 } 156 return Result; 157 } 158 159 if (const SCEVNAryExpr *X = dyn_cast<SCEVNAryExpr>(S)) { 160 SmallVector<const SCEV *, 8> Operands; 161 bool Changed = false; 162 // Transform each operand. 163 for (SCEVNAryExpr::op_iterator I = X->op_begin(), E = X->op_end(); 164 I != E; ++I) { 165 const SCEV *O = *I; 166 const SCEV *N = TransformSubExpr(O, User, OperandValToReplace); 167 Changed |= N != O; 168 Operands.push_back(N); 169 } 170 // If any operand actually changed, return a transformed result. 171 if (Changed) 172 switch (S->getSCEVType()) { 173 case scAddExpr: return SE.getAddExpr(Operands); 174 case scMulExpr: return SE.getMulExpr(Operands); 175 case scSMaxExpr: return SE.getSMaxExpr(Operands); 176 case scUMaxExpr: return SE.getUMaxExpr(Operands); 177 default: llvm_unreachable("Unexpected SCEVNAryExpr kind!"); 178 } 179 return S; 180 } 181 182 if (const SCEVUDivExpr *X = dyn_cast<SCEVUDivExpr>(S)) { 183 const SCEV *LO = X->getLHS(); 184 const SCEV *RO = X->getRHS(); 185 const SCEV *LN = TransformSubExpr(LO, User, OperandValToReplace); 186 const SCEV *RN = TransformSubExpr(RO, User, OperandValToReplace); 187 if (LO != LN || RO != RN) 188 return SE.getUDivExpr(LN, RN); 189 return S; 190 } 191 192 llvm_unreachable("Unexpected SCEV kind!"); 193 } 194 195 /// Manage recursive transformation across an expression DAG. Revisiting 196 /// expressions would lead to exponential recursion. 197 const SCEV *PostIncTransform:: 198 TransformSubExpr(const SCEV *S, Instruction *User, Value *OperandValToReplace) { 199 200 if (isa<SCEVConstant>(S) || isa<SCEVUnknown>(S)) 201 return S; 202 203 const SCEV *Result = Transformed.lookup(S); 204 if (Result) 205 return Result; 206 207 Result = TransformImpl(S, User, OperandValToReplace); 208 Transformed[S] = Result; 209 return Result; 210 } 211 212 /// Top level driver for transforming an expression DAG into its requested 213 /// post-inc form (either "Normalized" or "Denormalized". 214 const SCEV *llvm::TransformForPostIncUse(TransformKind Kind, 215 const SCEV *S, 216 Instruction *User, 217 Value *OperandValToReplace, 218 PostIncLoopSet &Loops, 219 ScalarEvolution &SE, 220 DominatorTree &DT) { 221 PostIncTransform Transform(Kind, Loops, SE, DT); 222 return Transform.TransformSubExpr(S, User, OperandValToReplace); 223 } 224