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

[Ryan Taylor via llvm-dev]

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



On Wed, Feb 5, 2020, 7:47 AM Nicolai Hähnle <nhaehnle at gmail.com> wrote:

> 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
>
>
>
> --
> Lerne, wie die Welt wirklich ist,
> aber vergiss niemals, wie sie sein sollte.
>
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