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

[Craig Topper via llvm-dev]

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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