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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86] Avoid RMW ADC/SBB operations on some targets"
href="https://bugs.llvm.org/show_bug.cgi?id=40830">40830</a>
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<th>Summary</th>
<td>[X86] Avoid RMW ADC/SBB operations on some targets
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>Windows NT
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
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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, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, peter@cordes.ca, spatel+llvm@rotateright.com
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<pre>As discussed on [<a class="bz_bug_link
bz_status_CONFIRMED "
title="CONFIRMED - Unusual code generation for addcarryx (ADCX and ADOX)"
href="show_bug.cgi?id=34292">Bug #34292</a>], some targets have particularly slow RMW
instructions and we should be avoiding their generation.
e.g. <a href="https://godbolt.org/z/YHDQPO">https://godbolt.org/z/YHDQPO</a> shows that ADC/SBB RMW on Skylake is much
slower, with a higher uop count, than using folded loads and separate stores
instead, despite the latter having a higher instruction count.
On other targets, btver2 for instance, the RMW is a better option, as it
doesn't appear to generate extra uops and the weaker frontend means that a
higher instruction count will impact performance much more.</pre>
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