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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - SCEV is too conservative around proving no-overflow for loops without computable backedge counts"
href="https://llvm.org/bugs/show_bug.cgi?id=27691">27691</a>
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<th>Summary</th>
<td>SCEV is too conservative around proving no-overflow for loops without computable backedge counts
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>sanjoy@playingwithpointers.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Currently the following logic is guarded under
!isa<SCEVCouldNotCompute>(MaxBECount), when I think it applies even if
MaxBECount is not computable.
// If the backedge is guarded by a comparison with the pre-inc value
// the addrec is safe. Also, if the entry is guarded by a comparison
// with the start value and the backedge is guarded by a comparison
// with the post-inc value, the addrec is safe.
ICmpInst::Predicate Pred;
const SCEV *OverflowLimit =
getSignedOverflowLimitForStep(Step, &Pred, this);
if (OverflowLimit &&
(isLoopBackedgeGuardedByCond(L, Pred, AR, OverflowLimit) ||
(isLoopEntryGuardedByCond(L, Pred, Start, OverflowLimit) &&
isLoopBackedgeGuardedByCond(L, Pred, AR->getPostIncExpr(*this),
OverflowLimit)))) {
// Cache knowledge of AR NSW, then propagate NSW to the wide AddRec.
const_cast<SCEVAddRecExpr *>(AR)->setNoWrapFlags(SCEV::FlagNSW);
return getAddRecExpr(
getExtendAddRecStart<SCEVSignExtendExpr>(AR, Ty, this),
getSignExtendExpr(Step, Ty), L, AR->getNoWrapFlags());
}</pre>
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</p>
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