C++ ScalarEvolution::isKnownPredicate方法代码示例

// Return the number of iterations to peel off that make conditions in the
// body true/false. For example, if we peel 2 iterations off the loop below,
// the condition i < 2 can be evaluated at compile time.
//  for (i = 0; i < n; i++)
//    if (i < 2)
//      ..
//    else
//      ..
//   }
static unsigned countToEliminateCompares(Loop &L, unsigned MaxPeelCount,
                                         ScalarEvolution &SE) {
  assert(L.isLoopSimplifyForm() && "Loop needs to be in loop simplify form");
  unsigned DesiredPeelCount = 0;

  for (auto *BB : L.blocks()) {
    auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
    if (!BI || BI->isUnconditional())
      continue;

    // Ignore loop exit condition.
    if (L.getLoopLatch() == BB)
      continue;

    Value *Condition = BI->getCondition();
    Value *LeftVal, *RightVal;
    CmpInst::Predicate Pred;
    if (!match(Condition, m_ICmp(Pred, m_Value(LeftVal), m_Value(RightVal))))
      continue;

    const SCEV *LeftSCEV = SE.getSCEV(LeftVal);
    const SCEV *RightSCEV = SE.getSCEV(RightVal);

    // Do not consider predicates that are known to be true or false
    // independently of the loop iteration.
    if (SE.isKnownPredicate(Pred, LeftSCEV, RightSCEV) ||
        SE.isKnownPredicate(ICmpInst::getInversePredicate(Pred), LeftSCEV,
                            RightSCEV))
      continue;

    // Check if we have a condition with one AddRec and one non AddRec
    // expression. Normalize LeftSCEV to be the AddRec.
    if (!isa<SCEVAddRecExpr>(LeftSCEV)) {
      if (isa<SCEVAddRecExpr>(RightSCEV)) {
        std::swap(LeftSCEV, RightSCEV);
        Pred = ICmpInst::getSwappedPredicate(Pred);
      } else
        continue;
    }

    const SCEVAddRecExpr *LeftAR = cast<SCEVAddRecExpr>(LeftSCEV);

    // Avoid huge SCEV computations in the loop below, make sure we only
    // consider AddRecs of the loop we are trying to peel and avoid
    // non-monotonic predicates, as we will not be able to simplify the loop
    // body.
    // FIXME: For the non-monotonic predicates ICMP_EQ and ICMP_NE we can
    //        simplify the loop, if we peel 1 additional iteration, if there
    //        is no wrapping.
    bool Increasing;
    if (!LeftAR->isAffine() || LeftAR->getLoop() != &L ||
        !SE.isMonotonicPredicate(LeftAR, Pred, Increasing))
      continue;
    (void)Increasing;

    // Check if extending the current DesiredPeelCount lets us evaluate Pred
    // or !Pred in the loop body statically.
    unsigned NewPeelCount = DesiredPeelCount;

    const SCEV *IterVal = LeftAR->evaluateAtIteration(
        SE.getConstant(LeftSCEV->getType(), NewPeelCount), SE);

    // If the original condition is not known, get the negated predicate
    // (which holds on the else branch) and check if it is known. This allows
    // us to peel of iterations that make the original condition false.
    if (!SE.isKnownPredicate(Pred, IterVal, RightSCEV))
      Pred = ICmpInst::getInversePredicate(Pred);

    const SCEV *Step = LeftAR->getStepRecurrence(SE);
    while (NewPeelCount < MaxPeelCount &&
           SE.isKnownPredicate(Pred, IterVal, RightSCEV)) {
      IterVal = SE.getAddExpr(IterVal, Step);
      NewPeelCount++;
    }

    // Only peel the loop if the monotonic predicate !Pred becomes known in the
    // first iteration of the loop body after peeling.
    if (NewPeelCount > DesiredPeelCount &&
        SE.isKnownPredicate(ICmpInst::getInversePredicate(Pred), IterVal,
                            RightSCEV))
      DesiredPeelCount = NewPeelCount;
  }

  return DesiredPeelCount;
}