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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [DA] Aliasing checks in DA are incorrect"
href="https://bugs.llvm.org/show_bug.cgi?id=42143">42143</a>
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<th>Summary</th>
<td>[DA] Aliasing checks in DA are incorrect
</td>
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<th>Product</th>
<td>libraries
</td>
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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>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
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<th>Assignee</th>
<td>david.green@arm.com
</td>
</tr>
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<th>Reporter</th>
<td>david.green@arm.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>The Aliasing checks in DA currently do this:
// Check the original locations (minus size) for noalias, which can happen
for
// tbaa, incompatible underlying object locations, etc.
MemoryLocation LocAS(LocA.Ptr, LocationSize::unknown(), LocA.AATags);
MemoryLocation LocBS(LocB.Ptr, LocationSize::unknown(), LocB.AATags);
if (AA->alias(LocAS, LocBS) == NoAlias)
return NoAlias;
// Check the underlying objects are the same
const Value *AObj = GetUnderlyingObject(LocA.Ptr, DL);
const Value *BObj = GetUnderlyingObject(LocB.Ptr, DL);
// If the underlying objects are the same, they must alias
if (AObj == BObj)
return MustAlias;
// We may have hit the recursion limit for underlying objects, or have
// underlying objects where we don't know they will alias.
if (!isIdentifiedObject(AObj) || !isIdentifiedObject(BObj))
return MayAlias;
// Otherwise we know the objects are different and both identified objects so
// must not alias.
return NoAlias;
The alias check is invalid for cases like this, which will return noalias for
individual loop iterations, but we care about cross-loop dependencies:
define float @f() {
entry:
%g = alloca float, align 4
%h = alloca float, align 4
br label %for.body
for.body: ; preds = %for.body, %entry
%p = phi float* [ %g, %entry ], [ %q, %for.body ]
%q = phi float* [ %h, %entry ], [ %p, %for.body ]
%0 = load float, float* %p, align 4
store float undef, float* %q, align 4
%branch_cond = fcmp ugt float %0, 0.0
br i1 %branch_cond, label %for.cond.cleanup, label %for.body
for.cond.cleanup: ; preds = %for.body
ret float undef
}</pre>
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