Eliminate some two entry PHI nodes - SimplifyCFG

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…

We could fold the imm ops in the target backend but there are complications/complexities with doing that and we’d also potentially miss out on earlier optimizations.

Thanks.

Hi Ryan,

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

Conditional on the target supporting cmov? Though that’s probably not optimal.

Conditional on the target supporting cmov? Though that's probably not optimal.

No, it's not. Consider how the condition may be represented depending
on targets (e.g., flag registers....).

Cheers,
Nicolai

Just to be clear, I meant “cmov” as a generic definition of the target supporting some type of conditional mov, not explicitly ‘cmov’, so it would be target dependent, of course.

What I meant is that just because a target supports “cmov” it still might not be beneficial to the target to do the transform, and determining that could be quite complex depending on several things (the target info, target specific backend passes, etc…).

It might be better to simply add an option to SimplifyCFG that defaults to false for doing these individual phi nodes, then other targets wouldn’t be affected but could still choice to take advantage of it if they see fit.