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/IR/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, nullptr)); 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 // Normalize this SCEV by subtracting the expression for the final step. 123 // We only allow affine AddRecs to be normalized, otherwise we would not 124 // be able to correctly denormalize. 125 // e.g. {1,+,3,+,2} == {-2,+,1,+,2} + {3,+,2} 126 // Normalized form: {-2,+,1,+,2} 127 // Denormalized form: {1,+,3,+,2} 128 // 129 // However, denormalization would use the a different step expression than 130 // normalization (see getPostIncExpr), generating the wrong final 131 // expression: {-2,+,1,+,2} + {1,+,2} => {-1,+,3,+,2} 132 if (AR->isAffine() && 133 IVUseShouldUsePostIncValue(User, OperandValToReplace, L, &DT)) { 134 const SCEV *TransformedStep = 135 TransformSubExpr(AR->getStepRecurrence(SE), 136 User, OperandValToReplace); 137 Result = SE.getMinusSCEV(Result, TransformedStep); 138 Loops.insert(L); 139 } 140 #if 0 141 // This assert is conceptually correct, but ScalarEvolution currently 142 // sometimes fails to canonicalize two equal SCEVs to exactly the same 143 // form. It's possibly a pessimization when this happens, but it isn't a 144 // correctness problem, so disable this assert for now. 145 assert(S == TransformSubExpr(Result, User, OperandValToReplace) && 146 "SCEV normalization is not invertible!"); 147 #endif 148 break; 149 case Normalize: 150 // We want to normalize step expression, because otherwise we might not be 151 // able to denormalize to the original expression. 152 // 153 // Here is an example what will happen if we don't normalize step: 154 // ORIGINAL ISE: 155 // {(100 /u {1,+,1}<%bb16>),+,(100 /u {1,+,1}<%bb16>)}<%bb25> 156 // NORMALIZED ISE: 157 // {((-1 * (100 /u {1,+,1}<%bb16>)) + (100 /u {0,+,1}<%bb16>)),+, 158 // (100 /u {0,+,1}<%bb16>)}<%bb25> 159 // DENORMALIZED BACK ISE: 160 // {((2 * (100 /u {1,+,1}<%bb16>)) + (-1 * (100 /u {2,+,1}<%bb16>))),+, 161 // (100 /u {1,+,1}<%bb16>)}<%bb25> 162 // Note that the initial value changes after normalization + 163 // denormalization, which isn't correct. 164 if (Loops.count(L)) { 165 const SCEV *TransformedStep = 166 TransformSubExpr(AR->getStepRecurrence(SE), 167 User, OperandValToReplace); 168 Result = SE.getMinusSCEV(Result, TransformedStep); 169 } 170 #if 0 171 // See the comment on the assert above. 172 assert(S == TransformSubExpr(Result, User, OperandValToReplace) && 173 "SCEV normalization is not invertible!"); 174 #endif 175 break; 176 case Denormalize: 177 // Here we want to normalize step expressions for the same reasons, as 178 // stated above. 179 if (Loops.count(L)) { 180 const SCEV *TransformedStep = 181 TransformSubExpr(AR->getStepRecurrence(SE), 182 User, OperandValToReplace); 183 Result = SE.getAddExpr(Result, TransformedStep); 184 } 185 break; 186 } 187 return Result; 188 } 189 190 if (const SCEVNAryExpr *X = dyn_cast<SCEVNAryExpr>(S)) { 191 SmallVector<const SCEV *, 8> Operands; 192 bool Changed = false; 193 // Transform each operand. 194 for (SCEVNAryExpr::op_iterator I = X->op_begin(), E = X->op_end(); 195 I != E; ++I) { 196 const SCEV *O = *I; 197 const SCEV *N = TransformSubExpr(O, User, OperandValToReplace); 198 Changed |= N != O; 199 Operands.push_back(N); 200 } 201 // If any operand actually changed, return a transformed result. 202 if (Changed) 203 switch (S->getSCEVType()) { 204 case scAddExpr: return SE.getAddExpr(Operands); 205 case scMulExpr: return SE.getMulExpr(Operands); 206 case scSMaxExpr: return SE.getSMaxExpr(Operands); 207 case scUMaxExpr: return SE.getUMaxExpr(Operands); 208 default: llvm_unreachable("Unexpected SCEVNAryExpr kind!"); 209 } 210 return S; 211 } 212 213 if (const SCEVUDivExpr *X = dyn_cast<SCEVUDivExpr>(S)) { 214 const SCEV *LO = X->getLHS(); 215 const SCEV *RO = X->getRHS(); 216 const SCEV *LN = TransformSubExpr(LO, User, OperandValToReplace); 217 const SCEV *RN = TransformSubExpr(RO, User, OperandValToReplace); 218 if (LO != LN || RO != RN) 219 return SE.getUDivExpr(LN, RN); 220 return S; 221 } 222 223 llvm_unreachable("Unexpected SCEV kind!"); 224 } 225 226 /// Manage recursive transformation across an expression DAG. Revisiting 227 /// expressions would lead to exponential recursion. 228 const SCEV *PostIncTransform:: 229 TransformSubExpr(const SCEV *S, Instruction *User, Value *OperandValToReplace) { 230 231 if (isa<SCEVConstant>(S) || isa<SCEVUnknown>(S)) 232 return S; 233 234 const SCEV *Result = Transformed.lookup(S); 235 if (Result) 236 return Result; 237 238 Result = TransformImpl(S, User, OperandValToReplace); 239 Transformed[S] = Result; 240 return Result; 241 } 242 243 /// Top level driver for transforming an expression DAG into its requested 244 /// post-inc form (either "Normalized" or "Denormalized"). 245 const SCEV *llvm::TransformForPostIncUse(TransformKind Kind, 246 const SCEV *S, 247 Instruction *User, 248 Value *OperandValToReplace, 249 PostIncLoopSet &Loops, 250 ScalarEvolution &SE, 251 DominatorTree &DT) { 252 PostIncTransform Transform(Kind, Loops, SE, DT); 253 return Transform.TransformSubExpr(S, User, OperandValToReplace); 254 } 255
