http://llvm.org/bugs/show_bug.cgi?id=2607
Summary: SCEV miscomputes trip count for loop with SGT condition
Product: new-bugs
Version: unspecified
Platform: PC
OS/Version: Linux
Status: NEW
Severity: normal
Priority: P2
Component: new bugs
AssignedTo: [EMAIL PROTECTED]
ReportedBy: [EMAIL PROTECTED]
CC: [email protected]
Testcase:
#include <stdio.h>
int a(unsigned j) {unsigned i; for (i = 0; (int)i > (int)j; i--) {} return i;}
int main() {printf("%d", a(0x80000000));}
Compare the output with llvm-gcc with "-O0" vs. "-O3 -fno-inline" to see the
issue.
Relevant chunk from ScalarEvolutionsImpl::ComputeIterationCount:
case ICmpInst::ICMP_SGT: {
SCEVHandle TC = HowManyLessThans(SE.getNegativeSCEV(LHS),
SE.getNegativeSCEV(RHS), L, true);
if (!isa<SCEVCouldNotCompute>(TC)) return TC;
break;
}
The calls to getNegativeSCEV should be calls to getNotSCEV; negation doesn't
properly flip the comparison when one side is INT_MIN. (It looked suspicious
when I saw it, although it took a little while to come up with a testcase.)
I'll throw together a patch later.
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