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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - InstCombine: objectsize of alloca optimized to undef returns unknown"
href="https://llvm.org/bugs/show_bug.cgi?id=23301">23301</a>
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<th>Summary</th>
<td>InstCombine: objectsize of alloca optimized to undef returns unknown
</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>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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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>ahmed.bougacha@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Reported by Daniel Micay in PR23280:
#include <stdio.h>
#include <stdlib.h>
static inline __attribute__((always_inline)) size_t object_size(char *p) {
return __builtin_object_size(p, 0);
}
int main(int argc, char **argv) {
char buf[2];
printf("%zu\n", object_size(buf));
return 0;
}
This returns -1 because buf is optimized to undef by InstCombine.
When InstCombine tries to erase an alloca, the isAllocSiteRemovable logic is
smarter than the RAUW-undef logic: when there's a (objectsize (GEP/bitcast
(alloca))), we should look through the alloca users to properly lower the
objectsize calls. We currently just do something like alloca->RAUW(undef).
Perhaps the RAUW-undef logic and isAllocSiteRemovable should match? This would
let the above example print 2 instead of the current -1.</pre>
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