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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Cost modelling for load-cast"
href="https://bugs.llvm.org/show_bug.cgi?id=51975">51975</a>
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<th>Summary</th>
<td>Cost modelling for load-cast
</td>
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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>Linux
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<th>Status</th>
<td>NEW
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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>lebedev.ri@gmail.com
</td>
</tr>
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, pengfei.wang@intel.com, spatel+llvm@rotateright.com
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<p>
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<pre>We currently don't costmodel the fact that some casts
(mainly sext/zext, but also nt<->fp) can be folded into the load:
<a href="https://godbolt.org/z/vs7sx4nWz">https://godbolt.org/z/vs7sx4nWz</a>
This leads to rather pessimistic cost estimates,
and e.g. potentially affects LV.
Preceding question: does `X86TTIImpl::getCastInstrCost()`
have the right costs for e.g. `sext <8 x i16> to <8 x i32>`?
All `ISD::SIGN_EXTEND` in `AVX2ConversionTbl` are 2+,
for which target is that true?
I'd expect at least the s/zext's to cost 1.
Roughly, i suppose the solution matches that of the LOAD+BSWAP modelling,
i.e. if the cast is a single user of the load, then model it as free,
and when modelling the load which has a single cast user, then do ${MAGIC},
Where ${MAGIC} depends on the answer to the "Preceding question":
if the casts should cost 1, then we only need to adjust the load op sizes,
without increasing the load cost,
else i guess we need to report `max(load cost, cast cost)`?
Does this make sense?</pre>
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