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<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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