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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 - Can't coalesce four consecutive byte loads into a single x86 mov"
href="https://bugs.llvm.org/show_bug.cgi?id=33498">33498</a>
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<th>Summary</th>
<td>Can't coalesce four consecutive byte loads into a single x86 mov
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
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<th>OS</th>
<td>All
</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>Backend: X86
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>justin.lebar@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, mkuper@google.com
</td>
</tr></table>
<p>
<div>
<pre>Consider
unsigned foo(unsigned char* d) {
return (static_cast<unsigned int>(d[0]) << 0) |
(static_cast<unsigned int>(d[1]) << 8) |
(static_cast<unsigned int>(d[2]) << 16) |
(static_cast<unsigned int>(d[3]) << 24);
}
gcc 5.1+ compiles this as
mov eax, DWORD PTR [rdi]
ret
but clang 4.0 -O2 naively does a series of shifts bitwise ORs:
movzx eax, byte ptr [rdi]
movzx ecx, byte ptr [rdi + 1]
shl ecx, 8
or ecx, eax
movzx edx, byte ptr [rdi + 2]
shl edx, 16
or edx, ecx
movzx eax, byte ptr [rdi + 3]
shl eax, 24
or eax, edx
ret
I'll test on clang at HEAD in a few minutes once I get a build up and running.</pre>
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</p>
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