[llvm-dev] LLVM X86 backend combineIncDecVector's transform

[Sanjay Patel via llvm-dev]

No objections from me to make it run later. I didn't see the potential
conflicts when I added that code. Delayed combine, custom lowering, or
DAGToDAGISel all seem like viable options to me.

On Mon, Aug 26, 2019 at 2:04 PM Roman Lebedev <lebedev.ri at gmail.com> wrote:

> I have previously posted these two patches:
>
> [X86][CodeGen][NFC] Delay `combineIncDecVector()` from DAGCombine to
> X86DAGToDAGISel
> https://reviews.llvm.org/D62327
>
> [DAGCombine][X86][AArch64][AMDGPU][MIPS][PPC] (sub x, c) -> (add x,
> -c) vector edition.
> https://reviews.llvm.org/D62341
>
> While they got stuck since i wasn't really interested in them
> (i'm mostly interested in scalars, not vectors...),
> i don't think there was any fatal roadblocks there,
> so i guess i should rebase them and see what's the status now.
>
> Roman.
>
> On Mon, Aug 26, 2019 at 8:56 PM Topper, Craig <craig.topper at intel.com>
> wrote:
> >
> > I think DAGToDAG is too late because the build_vector has already been
> turned into a constant pool load by then so it’s a little difficult to get
> back. Maybe we can delay it to !DCI.isBeforeLegalizeOps()? That would at
> least let the first DAG combine and the post type legalization DAG combine
> see the add, 1.
> >
> >
> >
> > +Sanjay as well
> >
> >
> >
> > From: Amaury Séchet <deadalnix at gmail.com>
> > Sent: Monday, August 26, 2019 10:48 AM
> > To: Topper, Craig <craig.topper at intel.com>; llvm-dev at redking.me.uk;
> efriedma at quicinc.com; lebedev.ri at gmail.com; llvm-dev <
> llvm-dev at lists.llvm.org>
> > Subject: LLVM X86 backend combineIncDecVector's transform
> >
> >
> >
> > Hi all,
> >
> > As you knwo already, I'm trying to change DAGCombiner so that it process
> the nodes in topological order. Doing so is not difficult per se, but this
> creates various improvements and regression to the existing test suite. I'd
> like to work through as many of the regressions as possible ahead of time.
> >
> > One source of such regressions is combineIncDecVector in the X86
> backend. It changes (add X, 1) into (sub X, -1) in order to be able to use
> the pcmpeq instruction.
> >
> > This is all well and good, but numerous paterns are matching an add
> rather than a sub, and in fact, DAGCombiner does the inverse transform by
> itself as it consider the add form to be canonical. An example of such
> pattern is the X86ISD::AVG node, but there are more.
> >
> > It seems to me like this transformation is useful, but doesn't happen at
> the right place in the pipeline. Doing so later on, for instance at the DAG
> to DAG level would likely give DAGCombiner  more opportunities to do its
> job, and also ensure that all instances of the pattern are detected.
> >
> > It would be great if someone familiar with the X86 backend could look
> into this.
> >
> > Thanks in advance,
> >
> > Amaury Séchet
>
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