[PATCH] D113872: [CGP] Handle select instructions relying on the same condition

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

In D113872#3247529 <https://reviews.llvm.org/D113872#3247529>, @lkail wrote:

>> Basic idea is to rely on MCSchedModel's MispredictPenalty instead of "hardcoded" TwoEntryPHINodeFoldingThreshold.
>
> Is it appropriate to introduce `MCSchedModel` in simplifycfg?

If it's properly exposed via TTI, should be fine i guess.
I'll try to look into simplifycfg stuff.

>> because it is not generally correct to consider that the latency of two instructions is a sum of their latencies, because they could be independent and could have been executed in parallel.
>
> Good point.



> Another point I have missed in the patch is what if some `select`s can't be lowered to corresponding target's conditional move finally, like `cmove` in x86, `isel` in ppc. In ppc, we don't have legal operation for `ISD::Select` on float types. This looks breaking the consistency between costmodel in LLVM IR and final assembly. Does this also imply we can't handle branch -> select precisely in simplifycfg?

I would guess `TTI::getSelectCost()` should have a parameter with type of the operands being selected.

But regardless of what happens in simplifycfg, it sounds like ppc should have it's own version of X86CmovConversion, if it doesn't already.


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https://reviews.llvm.org/D113872