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Clarify HowFarToZero computation when the step is a positive power of two. Functionally this should be identical to the existing code except for the case where Step is maximally negative (eg, INT_MIN). We now punt in that one corner case to make reasoning about the code easier.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@224274 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Mark Heffernan 2014-12-15 21:19:53 +00:00
parent 1eb5ad1ecd
commit b10837d169
1 changed files with 13 additions and 8 deletions
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21
lib/Analysis/ScalarEvolution.cpp
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@ -6094,14 +6094,19 @@ ScalarEvolution::HowFarToZero(const SCEV *V, const Loop *L, bool ControlsExit) {
return ExitLimit(Distance, MaxBECount);
}
// If Step is a power of two that evenly divides Start we know that the loop
// will always terminate. Start may not be a constant so we just have the
// number of trailing zeros available. This is safe even in presence of
// overflow as the recurrence will overflow to exactly 0.
const APInt &StepV = StepC->getValue()->getValue();
if (StepV.isPowerOf2() &&
GetMinTrailingZeros(getNegativeSCEV(Start)) >= StepV.countTrailingZeros())
return getUDivExactExpr(Distance, CountDown ? getNegativeSCEV(Step) : Step);
// As a special case, handle the instance where Step is a positive power of
// two. In this case, determining whether Step divides Distance evenly can be
// done by counting and comparing the number of trailing zeros of Step and
// Distance.
if (!CountDown) {
const APInt &StepV = StepC->getValue()->getValue();
// StepV.isPowerOf2() returns true if StepV is an positive power of two. It
// also returns true if StepV is maximally negative (eg, INT_MIN), but that
// case is not handled as this code is guarded by !CountDown.
if (StepV.isPowerOf2() &&
GetMinTrailingZeros(Distance) >= StepV.countTrailingZeros())
return getUDivExactExpr(Distance, Step);
}
// If the condition controls loop exit (the loop exits only if the expression
// is true) and the addition is no-wrap we can use unsigned divide to