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      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     default: llvm_unreachable("Unexpected transform name!");
    122     case NormalizeAutodetect:
    123       if (IVUseShouldUsePostIncValue(User, OperandValToReplace, L, &DT)) {
    124         const SCEV *TransformedStep =
    125           TransformSubExpr(AR->getStepRecurrence(SE),
    126                            User, OperandValToReplace);
    127         Result = SE.getMinusSCEV(Result, TransformedStep);
    128         Loops.insert(L);
    129       }
    130 #if 0
    131       // This assert is conceptually correct, but ScalarEvolution currently
    132       // sometimes fails to canonicalize two equal SCEVs to exactly the same
    133       // form. It's possibly a pessimization when this happens, but it isn't a
    134       // correctness problem, so disable this assert for now.
    135       assert(S == TransformSubExpr(Result, User, OperandValToReplace) &&
    136              "SCEV normalization is not invertible!");
    137 #endif
    138       break;
    139     case Normalize:
    140       if (Loops.count(L)) {
    141         const SCEV *TransformedStep =
    142           TransformSubExpr(AR->getStepRecurrence(SE),
    143                            User, OperandValToReplace);
    144         Result = SE.getMinusSCEV(Result, TransformedStep);
    145       }
    146 #if 0
    147       // See the comment on the assert above.
    148       assert(S == TransformSubExpr(Result, User, OperandValToReplace) &&
    149              "SCEV normalization is not invertible!");
    150 #endif
    151       break;
    152     case Denormalize:
    153       if (Loops.count(L))
    154         Result = cast<SCEVAddRecExpr>(Result)->getPostIncExpr(SE);
    155       break;
    156     }
    157     return Result;
    158   }
    159 
    160   if (const SCEVNAryExpr *X = dyn_cast<SCEVNAryExpr>(S)) {
    161     SmallVector<const SCEV *, 8> Operands;
    162     bool Changed = false;
    163     // Transform each operand.
    164     for (SCEVNAryExpr::op_iterator I = X->op_begin(), E = X->op_end();
    165          I != E; ++I) {
    166       const SCEV *O = *I;
    167       const SCEV *N = TransformSubExpr(O, User, OperandValToReplace);
    168       Changed |= N != O;
    169       Operands.push_back(N);
    170     }
    171     // If any operand actually changed, return a transformed result.
    172     if (Changed)
    173       switch (S->getSCEVType()) {
    174       case scAddExpr: return SE.getAddExpr(Operands);
    175       case scMulExpr: return SE.getMulExpr(Operands);
    176       case scSMaxExpr: return SE.getSMaxExpr(Operands);
    177       case scUMaxExpr: return SE.getUMaxExpr(Operands);
    178       default: llvm_unreachable("Unexpected SCEVNAryExpr kind!");
    179       }
    180     return S;
    181   }
    182 
    183   if (const SCEVUDivExpr *X = dyn_cast<SCEVUDivExpr>(S)) {
    184     const SCEV *LO = X->getLHS();
    185     const SCEV *RO = X->getRHS();
    186     const SCEV *LN = TransformSubExpr(LO, User, OperandValToReplace);
    187     const SCEV *RN = TransformSubExpr(RO, User, OperandValToReplace);
    188     if (LO != LN || RO != RN)
    189       return SE.getUDivExpr(LN, RN);
    190     return S;
    191   }
    192 
    193   llvm_unreachable("Unexpected SCEV kind!");
    194   return 0;
    195 }
    196 
    197 /// Manage recursive transformation across an expression DAG. Revisiting
    198 /// expressions would lead to exponential recursion.
    199 const SCEV *PostIncTransform::
    200 TransformSubExpr(const SCEV *S, Instruction *User, Value *OperandValToReplace) {
    201 
    202   if (isa<SCEVConstant>(S) || isa<SCEVUnknown>(S))
    203     return S;
    204 
    205   const SCEV *Result = Transformed.lookup(S);
    206   if (Result)
    207     return Result;
    208 
    209   Result = TransformImpl(S, User, OperandValToReplace);
    210   Transformed[S] = Result;
    211   return Result;
    212 }
    213 
    214 /// Top level driver for transforming an expression DAG into its requested
    215 /// post-inc form (either "Normalized" or "Denormalized".
    216 const SCEV *llvm::TransformForPostIncUse(TransformKind Kind,
    217                                          const SCEV *S,
    218                                          Instruction *User,
    219                                          Value *OperandValToReplace,
    220                                          PostIncLoopSet &Loops,
    221                                          ScalarEvolution &SE,
    222                                          DominatorTree &DT) {
    223   PostIncTransform Transform(Kind, Loops, SE, DT);
    224   return Transform.TransformSubExpr(S, User, OperandValToReplace);
    225 }
    226