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

[Ryan Taylor via llvm-dev]

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.

On Wed, Feb 5, 2020 at 8:20 AM Ryan Taylor <ryta1203 at gmail.com> wrote:

> 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..).
>
> On Wed, Feb 5, 2020, 7:58 AM Nicolai Hähnle <nhaehnle at gmail.com> wrote:
>
>> On Wed, Feb 5, 2020 at 1:53 PM Ryan Taylor <ryta1203 at gmail.com> wrote:
>> > 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
>>
>>
>> >
>> >
>> >
>> > 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.
>>
>>
>>
>> --
>> Lerne, wie die Welt wirklich ist,
>> aber vergiss niemals, wie sie sein sollte.
>>
>
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