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Fix for PR2607: SCEV miscomputing the loop count for loops with an

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SGT exit condition.  Essentially, the correct way to flip an inequality 
in 2's complement is the not operator, not the negation operator.  
That said, the difference only affects cases involving INT_MIN.

Also, enhance the pre-test search logic to be a bit smarter about 
inequalities flipped with a not operator, so it can eliminate the smax 
from the iteration count for simple loops.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54184 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eli Friedman
2008-07-30 00:04:08 +00:00
parent 83ea1f8e19
commit 068acc3caa
2 changed files with 34 additions and 3 deletions
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10
lib/Analysis/ScalarEvolution.cpp
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@ -1976,8 +1976,8 @@ SCEVHandle ScalarEvolutionsImpl::ComputeIterationCount(const Loop *L) {
break;
}
case ICmpInst::ICMP_SGT: {
SCEVHandle TC = HowManyLessThans(SE.getNegativeSCEV(LHS),
SE.getNegativeSCEV(RHS), L, true);
SCEVHandle TC = HowManyLessThans(SE.getNotSCEV(LHS),
SE.getNotSCEV(RHS), L, true);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
break;
}
@ -2724,7 +2724,11 @@ bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
if (!PreCondLHS->getType()->isInteger()) return false;
return LHS == getSCEV(PreCondLHS) && RHS == getSCEV(PreCondRHS);
SCEVHandle PreCondLHSSCEV = getSCEV(PreCondLHS);
SCEVHandle PreCondRHSSCEV = getSCEV(PreCondRHS);
return (LHS == PreCondLHSSCEV && RHS == PreCondRHSSCEV) ||
(LHS == SE.getNotSCEV(PreCondRHSSCEV) &&
RHS == SE.getNotSCEV(PreCondLHSSCEV));
}
/// HowManyLessThans - Return the number of times a backedge containing the