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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Unneeded stack alignment on x86"
href="https://bugs.llvm.org/show_bug.cgi?id=40844">40844</a>
</td>
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<th>Summary</th>
<td>Unneeded stack alignment on x86
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>7.0
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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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>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>yyc1992@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
</td>
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<p>
<div>
<pre>Ref <a class="bz_bug_link
bz_status_NEW "
title="NEW - Vector ABI does not respect target attribute"
href="show_bug.cgi?id=40843">https://bugs.llvm.org/show_bug.cgi?id=40843</a>. The useless stack operations
are likely caused by a different issue...
Reproducible with C code,
```
typedef double _vdouble __attribute__((vector_size(32)));
typedef struct {
_vdouble x;
int y;
} vdouble;
vdouble f(vdouble x, vdouble y)
{
vdouble res = {x.x + y.x, x.y + y.y};
return res;
}
```
Compiling with clang with `-mavx` emits very similar code to GCC
```
f: # @f
# %bb.0:
pushq %rbp
movq %rsp, %rbp
andq $-32, %rsp
subq $32, %rsp
vmovapd 16(%rbp), %ymm0
vaddpd 80(%rbp), %ymm0, %ymm0
vmovapd %ymm0, (%rdi)
movl 112(%rbp), %eax
addl 48(%rbp), %eax
movl %eax, 32(%rdi)
movq %rdi, %rax
movq %rbp, %rsp
popq %rbp
vzeroupper
retq
```
except that it emits 6 additional instructions for stack realignment without
actually using the stack pointer anywhere else in the function.
Seems to be a x86 backend issue since I can reproduce even without enabling avx
(so nothing in the backend should require 32bit alignment, only the frontend
should) and I can't reproduce on arm or aarch64.</pre>
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