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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - 64-bit atomic_load"
href="https://bugs.llvm.org/show_bug.cgi?id=38515">38515</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>64-bit atomic_load
</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>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nruslan_devel@yahoo.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I have noticed that clang/llvm generates atomic_load (64-bit) for i686+ as
cmpxchg8b, whereas gcc avoids it by using FPU instructions.
#include <stdatomic.h>
unsigned long long func(_Atomic(unsigned long long) *a)
{
return atomic_load(a);
}
clang -Wall -O2 -m32 -S test.c
clang/llvm:
func: # @func
# %bb.0:
pushl %ebx
pushl %esi
movl 12(%esp), %esi
xorl %eax, %eax
xorl %edx, %edx
xorl %ecx, %ecx
xorl %ebx, %ebx
lock cmpxchg8b (%esi)
popl %esi
popl %ebx
retl
.Lfunc_end0:
whereas gcc generates:
func:
.LFB0:
.cfi_startproc
subl $12, %esp
.cfi_def_cfa_offset 16
movl 16(%esp), %eax
fildq (%eax)
fistpq (%esp)
movl (%esp), %eax
movl 4(%esp), %edx
addl $12, %esp
.cfi_def_cfa_offset 4
ret</pre>
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