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