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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 - unaligned access in 32-bit SSE PIC code"
href="https://bugs.llvm.org/show_bug.cgi?id=41612">41612</a>
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<th>Summary</th>
<td>unaligned access in 32-bit SSE PIC code
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
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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>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>jay.foad@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>I'm using Clang 8 on Ubuntu 19.04. I believe the bug affects at least clang 7
and 8 and trunk, according to godbolt.org.
$ clang --version
clang version 8.0.0-3 (tags/RELEASE_800/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
To reproduce:
cat h.c
typedef __attribute__((aligned)) struct { long long a[2]; } T;
T b(void);
int e(void);
void c(T *g) {
*g = b();
}
T h(T g) {
if (e())
c(&g);
return g;
}
$ cat main.c
typedef __attribute__((aligned)) struct { long long a[2]; } T;
T h(T);
T b(void) {
T x = {0};
return x;
}
int e(void) {
return 1;
}
int main() {
T x = {0};
(void)h(x);
return 0;
}
$ clang -m32 -O -march=pentium4 -mfpmath=sse -fPIC main.c h.c -o main -Wall
$ ./main
Segmentation fault (core dumped)
The problem seems to be that when c() is inlined into h(), it assigns to g
using a movapd instruction, even though g is an argument on the stack and is
not 16-byte aligned.
Here's a godbolt link: <a href="https://godbolt.org/z/LrgIXg">https://godbolt.org/z/LrgIXg</a>
You can see that the disassembly of h() includes:
lea edi, [esp + 52]
...
movaps xmmword ptr [edi], xmm0
Assuming that esp was 16-byte aligned at this point, this does an unaligned
store to esp + 52.</pre>
</div>
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