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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 - __attribute__((force_align_arg_pointer)) produces incorrect stack alignment"
href="https://bugs.llvm.org/show_bug.cgi?id=37162">37162</a>
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<th>Summary</th>
<td>__attribute__((force_align_arg_pointer)) produces incorrect stack alignment
</td>
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<th>Product</th>
<td>new-bugs
</td>
</tr>
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<th>Version</th>
<td>6.0
</td>
</tr>
<tr>
<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>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>henrik@gramner.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>__attribute__((force_align_arg_pointer)) uses a hardcoded alignment value of 16
which breaks when using a stack alignment that's larger than 16.
It should behave the same way as -mstackrealign (which works correctly) except
on a per-function basis.
The use case is a library that makes heavy use of AVX/AVX2 and uses 32-byte
stack alignment to be able to do aligned load/stores of ymm registers from/to
stack buffers without having to realign the stack in every function, in
combination with the force_align_arg_pointer attribute on API entry points.
GCC handles the alignment correctly.
Minimal test case:
void bar(void);
__attribute__((force_align_arg_pointer)) void foo(void)
{
bar();
}
-mstack-alignment=32:
<foo>:
push rbp
mov rbp,rsp
and rsp,0xfffffffffffffff0
sub rsp,0x10
call 11 <foo+0x11> d: R_X86_64_PC32 bar-0x4
mov rsp,rbp
pop rbp
ret
-mstack-alignment=32 -mstackrealign (with or without force_align_arg_pointer):
<foo>:
push rbp
mov rbp,rsp
and rsp,0xffffffffffffffe0
sub rsp,0x20
call 11 <foo+0x11> d: R_X86_64_PC32 bar-0x4
mov rsp,rbp
pop rbp
ret
(Also the "sub rsp, <alignment>" instructions seems pointless to me. And yes,
they're still there with -O3).</pre>
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