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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 - instcombine: doesn't remove redundant allocation / memory access with aligned_alloc"
href="https://bugs.llvm.org/show_bug.cgi?id=44062">44062</a>
</td>
</tr>
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<th>Summary</th>
<td>instcombine: doesn't remove redundant allocation / memory access with aligned_alloc
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</td>
</tr>
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<th>Version</th>
<td>8.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>opt
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>uday@polymagelabs.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>In the snippet below, the allocated storage is optimized away when a malloc is
used, but not when an aligned_alloc is used. (aligned_alloc has effectively
similar semantics for the purpose of relevant opts here, and it has been in
glibc since 2.16 and in the C11 standard.) When a malloc is used, once the
load/stores are hoisted out by LICM, -instcombine gets rid of the heap
allocation and *s is effectively in a virtual register. But the same thing
doesn't happen with aligned_alloc; the allocation and the store to it remain.
<a href="https://godbolt.org/z/WCUF9n">https://godbolt.org/z/WCUF9n</a>
------------------------------------------------
#include <stdlib.h>
#define N 100
void foo(int Y[N], int A[N][N], int X[N]) {
for (int i = 0; i < N; ++i) {
// Allocated storage optimized away when malloc is used.
// int *s = (int*)malloc((sizeof(int)));
int *s = (int *)aligned_alloc(sizeof(int), sizeof(int));
*s = Y[i];
for (int j = 0; j < N; ++j) {
*s += A[i][j] * X[j];
}
Y[i] += *s;
free(s);
}
}
--------------
On this note, this issue exists with posix_memalign as well. With
posix_memalign, in fact, the load/stores to *s in the loop aren't replaced by a
scalar. Although this is an artificial C/C++ example to reproduce this issue,
the more interesting/real use case is for example when a pass/utility has to
generate an allocation for a buffer of vector element types -- as such, an
alignment to vector size boundaries is desired, which malloc doesn't provide
for > 16 bytes.</pre>
</div>
</p>
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