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

[Amaury Séchet via llvm-dev]

That is a great first step. It makes things better indeed, but still causes
a few regression when nodes processing happens in a different order.

Roman patches seems to be the way to go.

Le mar. 27 août 2019 à 00:18, Craig Topper via llvm-dev <
llvm-dev at lists.llvm.org> a écrit :

> I pushed it to a later DAG combine in r369981. Let me know if you still
> have problems from it.
>
> ~Craig
>
>
> On Mon, Aug 26, 2019 at 1:02 PM Sanjay Patel via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> 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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