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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 - Failure to use x86 array arithmetic instead of manual add"
href="https://bugs.llvm.org/show_bug.cgi?id=46724">46724</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Failure to use x86 array arithmetic instead of manual add
</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>gabravier@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>
</tr></table>
<p>
<div>
<pre>struct as
{
as* a;
as* b;
};
struct b
{
as* a;
as* b;
};
void f(b* a, as* s)
{
if (a->a)
{
s->a = a->a;
a->a->b = s;
}
else
{
a->b = s;
}
}
With -O3, GCC outputs this :
f(b*, as*):
mov rax, QWORD PTR [rdi]
test rax, rax
je .L2
mov QWORD PTR [rsi], rax
mov QWORD PTR [rax+8], rsi
ret
.L2:
mov QWORD PTR [rdi+8], rsi
ret
LLVM outputs this :
f(b*, as*): # @f(b*, as*)
mov rax, qword ptr [rdi]
test rax, rax
je .LBB0_2
mov qword ptr [rsi], rax
add rax, 8
mov qword ptr [rax], rsi
ret
.LBB0_2:
add rdi, 8
mov qword ptr [rdi], rsi
ret</pre>
</div>
</p>
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