[llvm-dev] Eliminate some two entry PHI nodes - SimplifyCFG

[Nicolai Hähnle via llvm-dev]

Hi Ryan,

On Mon, Feb 3, 2020 at 7:08 PM Ryan Taylor via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> SimplifyCFG FoldTwoEntryPhiNode looks to simplify all 2 entry phi nodes in a block, if it can't do them all then it won't do any and returns. There is a lot of code that is directly in this function geared toward this requirement. Is it possible currently to get this function (or pass) to simply fold "some" of the phis (without having to fold them all?). I understand that "Simplify"CFG isn't really meant for this type of thing but there is a lot of code that is re-usable to do this with some refactoring of this function and potentially some addition of target specifics.
>
> For some phis, it's beneficial to keep them as the generated COPYs can be coalesced away later in the pipe. For other phis (for example, phis with all or potentially partial imm ops), it would be nice to opt these to selects (cmovs). It would also be nice to have this opt prior to the rest of SimplifyCFG as it might enable SimplifyCFG to do a better job of simplifying by reducing the number of phis, etc...

I don't see a significant problem in adding this to
FoldTwoEntryPhiNodes. The relevant changes would be isolated to the
initial loop over the phis, and would be somewhat parallel to the use
of simplifyInstruction.

The main question is to how to make this conditional on a per-target
basis, i.e. how to properly define any relevant heuristics via
TargetTransformInfo.

Cheers,
Nicolai



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