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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86][SSE] Improve combining to ISD::MULHS/MULHU"
href="https://bugs.llvm.org/show_bug.cgi?id=45897">45897</a>
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<th>Summary</th>
<td>[X86][SSE] Improve combining to ISD::MULHS/MULHU
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
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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
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>llvm-dev@redking.me.uk
</td>
</tr>
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
</td>
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<p>
<div>
<pre>This is a general bug/brain dump to cover a few areas that we might want to
consider to encourage use of the vXi16 ISD::MULHS/MULHU opcodes where possible
as they can give notable perf improvements over vXi32 multiples.
1 - Move the PPC combines from <a href="https://reviews.llvm.org/D78272">https://reviews.llvm.org/D78272</a> into DAGCombiner
if x86 can benefit as well.
2 - X86ISelLowering.cpp - combinePMULH currently performs:
vXi16 trunc(srl(mul({sz}ext(x),{sz}ext(y)),16)) -> vXi16 mulh{sz}(x,y)
But it might be beneficial to combine any x/y with sufficient leading sign/zero
bits - it depends on how cheap the truncation will be (PACKS/VTRUNC/nop/???)
compared to the penalty of the wider multiply/shift:
vXi16 trunc(srl(mul(x,y),16)) -> vXi16 mulh{sz}(trunc(x),trunc(y))
3 - Try to perform more truncation style combines even after combining to
PACKS/VTRUNC ops. We can probably still see the truncation pattern behind the
op, so maybe a SelectionDAG::simplifyTrunc() would be useful or a x86-only
combineTruncLike? Or at the very least try to match the SimplifyDemandedBits
calls we have for ISD::TRUNCATE.
4 - See if [<a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86][SSE] Add ISD::MULHS/MULHU vXi64 support"
href="show_bug.cgi?id=38423">Bug #38423</a>] would still be useful - does
SimplifyDemandedBits/shuffle combining always simplify the vXi64 mul expansion
for us?</pre>
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</p>
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