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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Poor code generation for rotates"
href="https://bugs.llvm.org/show_bug.cgi?id=35487">35487</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Poor code generation for rotates
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>5.0
</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>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>fabiang@radgametools.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>This: <a href="https://godbolt.org/g/b2t1a9">https://godbolt.org/g/b2t1a9</a>
unsigned long f(unsigned long x, int amt)
{
x += (x << amt) | (x >> (64 - amt));
return x & 0xffffffffu;
}
unsigned long g(unsigned long x, int amt)
{
x += (x << amt) | (x >> (64 - amt));
return x;
}
produces:
f(unsigned long, int): # @f(unsigned long, int)
mov rax, rdi
mov ecx, esi
shl rax, cl
mov ecx, 64
sub ecx, esi
mov rdx, rdi
shr rdx, cl
or edx, eax
add edi, edx
mov rax, rdi
ret
g(unsigned long, int): # @g(unsigned long, int)
mov rax, rdi
mov ecx, esi
rol rax, cl
lea rax, [rax + rdi]
ret
Found while investigating a significant perf difference between LLVM and gcc on
a hash function. (Yes, "amt" is variable in the code in question.)</pre>
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