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Indvars: guard against exponential behavior in isHighCostExpansion.

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This should always be done as a matter of principal. I don't have a
case that exposes the problem. I just noticed this recently while
scanning the code and realized I meant to fix it long ago.

llvm-svn: 146438
This commit is contained in:
Andrew Trick 2011-12-12 22:46:16 +00:00
parent a16a63f190
commit 67432b451b
1 changed files with 7 additions and 2 deletions
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9
lib/Transforms/Scalar/IndVarSimplify.cpp
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@ -1238,7 +1238,11 @@ void IndVarSimplify::SimplifyAndExtend(Loop *L,
/// BackedgeTakenInfo. If these expressions have not been reduced, then
/// expanding them may incur additional cost (albeit in the loop preheader).
static bool isHighCostExpansion(const SCEV *S, BranchInst *BI,
SmallPtrSet<const SCEV*, 8> &Processed,
ScalarEvolution *SE) {
if (!Processed.insert(S))
return false;
// If the backedge-taken count is a UDiv, it's very likely a UDiv that
// ScalarEvolution's HowFarToZero or HowManyLessThans produced to compute a
// precise expression, rather than a UDiv from the user's code. If we can't
@ -1266,7 +1270,7 @@ static bool isHighCostExpansion(const SCEV *S, BranchInst *BI,
if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
I != E; ++I) {
if (isHighCostExpansion(*I, BI, SE))
if (isHighCostExpansion(*I, BI, Processed, SE))
return true;
}
return false;
@ -1309,7 +1313,8 @@ static bool canExpandBackedgeTakenCount(Loop *L, ScalarEvolution *SE) {
if (!BI)
return false;
if (isHighCostExpansion(BackedgeTakenCount, BI, SE))
SmallPtrSet<const SCEV*, 8> Processed;
if (isHighCostExpansion(BackedgeTakenCount, BI, Processed, SE))
return false;
return true;