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<p class="MsoNormal">Hi Chris,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks for the reply. My email was really to just ask if people were ok with the idea<o:p></o:p></p>
<p class="MsoNormal">in principle and I kind of deliberately avoided getting into too many details at this<o:p></o:p></p>
<p class="MsoNormal">point. If people are happy with the idea in principle I can push a patch up soon that<o:p></o:p></p>
<p class="MsoNormal">shows what such a class could like so that people can comment on it?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Regards,<o:p></o:p></p>
<p class="MsoNormal">David Sherwood.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Chris Tetreault <ctetreau@quicinc.com>
<br>
<b>Sent:</b> 05 November 2020 19:23<br>
<b>To:</b> David Sherwood <David.Sherwood@arm.com>; LLVM Dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> RE: [llvm-dev] [Proposal] Introducing the concept of invalid costs to the IR cost model<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span lang="EN-US">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></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">template <typename T><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">struct Cost {<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New""> unsigned Val;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New""> T State;<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-family:"Courier New"">}<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US">Thanks,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"> Christopher Tetreault<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> llvm-dev <<a href="mailto:llvm-dev-bounces@lists.llvm.org">llvm-dev-bounces@lists.llvm.org</a>>
<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> <a href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a><br>
<b>Subject:</b> [EXT] [llvm-dev] [Proposal] Introducing the concept of invalid costs to the IR cost model<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal">Hi,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I'd like to propose a change to our cost interfaces so that instead of returning<o:p></o:p></p>
<p class="MsoNormal">an unsigned value from functions like getInstructionCost, getUserCost, etc., we<o:p></o:p></p>
<p class="MsoNormal">instead return a wrapper class that encodes an integer cost along with extra<o:p></o:p></p>
<p class="MsoNormal">state. The extra state can be used to express:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">1. A cost as infinitely expensive in order to prevent certain optimisations<o:p></o:p></p>
<p class="MsoNormal">taking place. For example, there are already examples in LLVM where the cost is<o:p></o:p></p>
<p class="MsoNormal">set extremely high, but not so high that it would cause overflow. This might be to<o:p></o:p></p>
<p class="MsoNormal">prevent vectorisation in cases where we would have to scalarize the operation,<o:p></o:p></p>
<p class="MsoNormal">which is particularly relevant for scalable vectors, where scalarisation is<o:p></o:p></p>
<p class="MsoNormal">not [yet] available. There isn't currently a standard value for something that<o:p></o:p></p>
<p class="MsoNormal">constitutes very expensive and we can replace all the magic numbers with a<o:p></o:p></p>
<p class="MsoNormal">single invalid state.<o:p></o:p></p>
<p class="MsoNormal">2. A cost as unknown, where the user is simply unable to determine an accurate<o:p></o:p></p>
<p class="MsoNormal">cost for an operation.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This new wrapper class would work almost seamlessly with existing code as it<o:p></o:p></p>
<p class="MsoNormal">would contain the full set of operators required for arithmetic and comparisons.<o:p></o:p></p>
<p class="MsoNormal">This is in addition to the ability to create invalid costs and query the validity<o:p></o:p></p>
<p class="MsoNormal">of an existing cost. Once a cost becomes invalid or unknown it will remain in that<o:p></o:p></p>
<p class="MsoNormal">state regardless of any further arithmetic performed.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Kind Regards,<o:p></o:p></p>
<p class="MsoNormal">David Sherwood.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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