lib/Analysis/ScalarEvolutionNormalization.cpp

   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 }