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<p class="MsoNormal">You’re being quite vague about what the extra state is. Is this because you haven’t yet settled on what it should be, or do you actually intend to let it be a template parameter:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">template <typename T><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">struct Cost {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> unsigned Val;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New""> T State;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">}<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal"> Christopher Tetreault<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> <b>On Behalf Of
</b>David Sherwood via llvm-dev<br>
<b>Sent:</b> Thursday, November 5, 2020 7:43 AM<br>
<b>To:</b> llvm-dev@lists.llvm.org<br>
<b>Subject:</b> [EXT] [llvm-dev] [Proposal] Introducing the concept of invalid costs to the IR cost model<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-GB">Hi,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">I'd like to propose a change to our cost interfaces so that instead of returning<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">an unsigned value from functions like getInstructionCost, getUserCost, etc., we<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">instead return a wrapper class that encodes an integer cost along with extra<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">state. The extra state can be used to express:<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">1. A cost as infinitely expensive in order to prevent certain optimisations<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">taking place. For example, there are already examples in LLVM where the cost is<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">set extremely high, but not so high that it would cause overflow. This might be to<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">prevent vectorisation in cases where we would have to scalarize the operation,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">which is particularly relevant for scalable vectors, where scalarisation is<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">not [yet] available. There isn't currently a standard value for something that<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">constitutes very expensive and we can replace all the magic numbers with a<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">single invalid state.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">2. A cost as unknown, where the user is simply unable to determine an accurate<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">cost for an operation.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">This new wrapper class would work almost seamlessly with existing code as it<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">would contain the full set of operators required for arithmetic and comparisons.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">This is in addition to the ability to create invalid costs and query the validity<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">of an existing cost. Once a cost becomes invalid or unknown it will remain in that<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">state regardless of any further arithmetic performed.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">Kind Regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB">David Sherwood.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-GB"><o:p> </o:p></span></p>
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