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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - failure to optimize a simple loop to a memset call (incorrect fear of aliasing?)"
href="https://llvm.org/bugs/show_bug.cgi?id=27209">27209</a>
</td>
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<th>Summary</th>
<td>failure to optimize a simple loop to a memset call (incorrect fear of aliasing?)
</td>
</tr>
<tr>
<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>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>richard-llvm@metafoo.co.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>LLVM fails to optimize this simple loop to a memset:
void f(char *p, char *q) {
for (char *r = q; r != q + 256; ++r) *r = *p;
}
Presumably the relevant optimization is not firing here because of fear that p
might alias some element of the array pointed at by q. However, that doesn't
matter, because *p is the only thing written to that array, and the store size
is the same as the alignment, so *p can never actually change as a result.
See also <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Make std::fill_n use memset_pattern*() for pod types of the right size"
href="show_bug.cgi?id=27206">https://llvm.org/bugs/show_bug.cgi?id=27206</a> -- LLVM should at least be
able to optimize that if the type of the field within struct S is char, and
should be able to hoist the load of s.i out of the loop for any type where size
== alignment. (And using C++ object lifetime rules, the hoisting is always
permitted regardless of alignment.)</pre>
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</p>
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