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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 - target attribute "cx16" is ignored"
href="https://bugs.llvm.org/show_bug.cgi?id=45651">45651</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>target attribute "cx16" is ignored
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>myriachan@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre>Compile the following with -O2 on x86_64:
__attribute__((__target__("cx16")))
void write128(_Atomic(__int128)* p, __int128 v)
{
__c11_atomic_store(p, v, __ATOMIC_SEQ_CST);
}
That generates:
<source>:4:5: warning: large atomic operation may incur significant performance
penalty [-Watomic-alignment]
__c11_atomic_store(p, v, __ATOMIC_SEQ_CST);
^
1 warning generated.
_Z8write128Pnn: # @_Z8write128Pnn
sub rsp, 24
mov rax, rdi
mov qword ptr [rsp + 8], rdx
mov qword ptr [rsp], rsi
mov rdx, rsp
mov edi, 16
mov rsi, rax
mov ecx, 5
call __atomic_store
add rsp, 24
ret
If you add -mcx16 to the command line, Clang generates the intended code:
_Z8write128Pnn: # @_Z8write128Pnn
push rbx
mov rcx, rdx
mov rbx, rsi
mov rax, qword ptr [rdi]
mov rdx, qword ptr [rdi + 8]
.LBB0_1: # =>This Inner Loop Header: Depth=1
lock cmpxchg16b xmmword ptr [rdi]
jne .LBB0_1
pop rbx
ret</pre>
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