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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47061 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Wojciech Matyjewicz 2008-02-13 11:51:34 +00:00
parent cdbcfccece
commit 3a4cbe2a41
1 changed files with 14 additions and 6 deletions
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20
lib/Analysis/ScalarEvolution.cpp
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@ -2525,18 +2525,26 @@ HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L, bool isSigned) {
return UnknownValue;
if (AddRec->isAffine()) {
// The number of iterations for "{n,+,1} < m", is m-n. However, we don't
// know that m is >= n on input to the loop. If it is, the condition
// returns true zero times. To handle both cases, we return SMAX(m, n)-n.
// FORNOW: We only support unit strides.
SCEVHandle One = SE.getIntegerSCEV(1, RHS->getType());
if (AddRec->getOperand(1) != One)
return UnknownValue;
SCEVHandle Start = AddRec->getOperand(0);
SCEVHandle End = isSigned ? SE.getSMaxExpr(RHS, Start) : (SCEVHandle)RHS;
// We know the LHS is of the form {n,+,1} and the RHS is some loop-invariant
// m. So, we count the number of iterations in which {n,+,1} < m is true.
// Note that we cannot simply return max(m-n,0) because it's not safe to
// treat m-n as signed nor unsinged due to overflow possibility.
// First, we get the value of the LHS in the first iteration: n
SCEVHandle Start = AddRec->getOperand(0);
// Then, we get the value of the LHS in the first iteration in which the
// above condition doesn't hold. This equals to max(m,n).
// FIXME (PR2003): we should have an "umax" operator as well.
SCEVHandle End = isSigned ? SE.getSMaxExpr(RHS,Start) : (SCEVHandle)RHS;
// Finally, we subtract these two values to get the number of times the
// backedge is executed: max(m,n)-n.
return SE.getMinusSCEV(End, Start);
}