llvm dev - Feb 2020 - 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.
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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.

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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