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