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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 avoid emitting a useless instruction when doing combined shits and rotates with a smaller result"
href="https://bugs.llvm.org/show_bug.cgi?id=46138">46138</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Failure to avoid emitting a useless instruction when doing combined shits and rotates with a smaller result
</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>
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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>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</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>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>uint32_t f()
{
static uint64_t x = 0;
x++;
x = (x>>32) | (x<<32);
return x;
}
With -O3, GCC outputs this :
f():
mov rax, QWORD PTR msws()::x[rip]
inc rax
rorx rax, rax, 32
mov QWORD PTR msws()::x[rip], rax
ret
LLVM outputs this :
msws(): # @msws()
mov rax, qword ptr [rip + msws()::x]
inc rax
rorx rcx, rax, 32
shr rax, 32
mov qword ptr [rip + msws()::x], rcx
ret
The `shr` could be elimiated and replaced with part of the result of the
preceding rotate</pre>
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