Don't try to analyze this "backward" case. This is overly conservative

pending a correct solution.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@61589 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Nick Lewycky
2009-01-02 18:54:17 +00:00
parent 98117cdbd6
commit ae285bf5a6
2 changed files with 52 additions and 0 deletions

View File

@ -2905,6 +2905,11 @@ bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
return false;
}
static bool isNegative(SCEVHandle X) {
if (SCEVConstant *C = dyn_cast<SCEVConstant>(X))
return C->getValue()->getValue().isNegative();
}
/// potentialInfiniteLoop - Test whether the loop might jump over the exit value
/// due to wrapping around 2^n.
bool ScalarEvolutionsImpl::potentialInfiniteLoop(SCEV *Stride, SCEV *RHS,
@ -2958,6 +2963,13 @@ HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L,
if (potentialInfiniteLoop(Stride, RHS, isSigned, trueWhenEqual))
return UnknownValue;
// We don't handle this correctly at the moment. The problem is that when
// the stride is negative, we're not counting how many times 'less-than' is
// true as we approach it, we're counting how far away we are from wrapping
// around the backside.
if (isSigned && isNegative(Stride))
return UnknownValue;
// We know the LHS is of the form {n,+,s} and the RHS is some loop-invariant
// m. So, we count the number of iterations in which {n,+,s} < m is true.
// Note that we cannot simply return max(m-n,0)/s because it's not safe to