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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86] Improve handling of moving with AH/BH/CH/DH"
href="https://bugs.llvm.org/show_bug.cgi?id=46094">46094</a>
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<th>Summary</th>
<td>[X86] Improve handling of moving with AH/BH/CH/DH
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Windows NT
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>llvm-dev@redking.me.uk
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<th>CC</th>
<td>andrea.dibiagio@gmail.com, craig.topper@gmail.com, efriedma@quicinc.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
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<pre>Based off a discussion <a href="https://reviews.llvm.org/D80466">https://reviews.llvm.org/D80466</a>
For byte packing cases:
define i16 @pack(i8 %hi, i8 %lo) {
%lox = zext i8 %lo to i16
%hix = zext i8 %hi to i16
%hix_shift = shl i16 %hix, 8
%pack = or i16 %lox, %hix_shift
ret i16 %pack
}
we can either shift+or or use movb to move %hi's byte directly into upper byte
of %lo's register.
movzbl %sil, %eax
shll $8, %edi
orl %edi, %eax
retq
vs
movzbl %sil, %eax
movb %dl, %ah
retq
The pros/cons are incredibly specific to the target hardware, the registers we
have available and what instructions we're feeding the packing operation
outof/into.</pre>
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