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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 - Need a function attribute for stack alignment"
href="https://bugs.llvm.org/show_bug.cgi?id=40635">40635</a>
</td>
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<tr>
<th>Summary</th>
<td>Need a function attribute for stack alignment
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>peter@pcc.me.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>The value of the -mstack-alignment argument is not preserved in a function
attribute, which means that it gets dropped during LTO. Reproducer:
$ cat align.c
void g(void *);
void f() {
char x;
g(&x);
}
$ ~/l/ra/bin/clang align.c -mstack-alignment=32 -shared -fuse-ld=lld -O &&
~/l2/ra/bin/llvm-objdump -d a.out | grep -A5 ' f:'
00000000000010c0 f:
10c0: 48 83 ec 18 subq $24, %rsp
10c4: 48 8d 7c 24 17 leaq 23(%rsp), %rdi
10c9: e8 52 00 00 00 callq 82 <g@plt>
10ce: 48 83 c4 18 addq $24, %rsp
10d2: c3 retq
$ ~/l/ra/bin/clang align.c -mstack-alignment=32 -shared -fuse-ld=lld -O -flto
&& ~/l2/ra/bin/llvm-objdump -d a.out | grep -A5 ' f:'
00000000000010c0 f:
10c0: 50 pushq %rax
10c1: 48 8d 7c 24 07 leaq 7(%rsp), %rdi
10c6: e8 55 00 00 00 callq 85 <g@plt>
10cb: 58 popq %rax
10cc: c3 retq
This is the variable in Clang that stores the value of -mstack-alignment:
<a href="http://llvm-cs.pcc.me.uk/tools/clang/include/clang/Basic/CodeGenOptions.def#247">http://llvm-cs.pcc.me.uk/tools/clang/include/clang/Basic/CodeGenOptions.def#247</a>
It's only used to set a "stackrealign" function attribute (which doesn't store
the alignment value) and a code generation option StackAlignmentOverride that
isn't preserved through LTO. The right fix would probably be to remove
StackAlignmentOverride and replace it with a function attribute.</pre>
</div>
</p>
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