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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Clang generates stacksave/stackrestore incorrectly when a loop has VLA and alloca"
href="https://llvm.org/bugs/show_bug.cgi?id=30229">30229</a>
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<th>Summary</th>
<td>Clang generates stacksave/stackrestore incorrectly when a loop has VLA and alloca
</td>
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<th>Product</th>
<td>clang
</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>
</tr>
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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>LLVM Codegen
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>bluechristlove@163.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>Consider the following case:
#include <alloca.h>
#include <string.h>
#include <new>
int main(void) {
char *cp = 0;
for (int i = 1; i <= 100; ++i) {
char vla[i];
memset(vla, 0, static_cast<size_t>(i));
if (!cp) cp = new (alloca(1)) char('Q');
}
return *cp == 'Q' ? 55 : 66;
}
Expected return code 55.
Clang emit LLVM IR:
we sill generate @llvm.stackstore and @llvm.stacksave. However, from GCC 6, the
VLA scope is changed when the block has alloca function:
see: <a href="https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html">https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html</a>
Space allocated with alloca exists until the containing function returns. The
space for a variable-length array is deallocated as soon as the array name's
scope ends, unless you also use alloca in this scope.
Clang should consider this condition. GCC 6 passes this case but Clang failed.
Similar bug: <a href="https://llvm.org/bugs/show_bug.cgi?format=multiple&id=16099">https://llvm.org/bugs/show_bug.cgi?format=multiple&id=16099</a></pre>
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