[llvm-dev] Question about canonicalizing cmp+select

[Sanjay Patel via llvm-dev]

[adding back llvm-dev and cc'ing Craig]

I think you are asking if we are missing a fold (or your target is missing
enabling another hook) to transform the sext+add into shift+or? I think the
answer is 'yes'. We probably should add that fold. This seems like a
similar case as the recent: https://reviews.llvm.org/D48466

Note that on x86, the sext+add becomes zext+sub:
        t20: i8 = setcc t3, Constant:i16<-1>, setgt:ch
      t24: i16 = zero_extend t20
    t17: i16 = sub Constant:i16<5>, t24

Would that transform help your target?

On Tue, Jul 3, 2018 at 3:55 PM, Yuan Lin <yualin at google.com> wrote:

> Hi, Roman and Sanjay,
>
>   Thank you for your reply!  We currently do run DAGCombiner, but didn't
> implement this specific transformation.  I just tried turning on
> convertSelectOfCosntantsToMath() in our ISelLowering, but that doesn't
> quite work because it generated a sign_extend op from i1 to i16, which our
> backend currently doesn't support.
>
>   Does the DAGCombiner already has this transformation implemented?
>
> Thanks,
> --Yuan
>
> On Tue, Jul 3, 2018 at 11:22 AM Sanjay Patel <spatel at rotateright.com>
> wrote:
>
>> Do you run DAGCombiner? And are you overriding TLI.
>> convertSelectOfConstantsToMath(VT) for your target?
>>
>> For the stated example (true val and false val constants in the select
>> differ by 1), that should already be converted.
>>
>>
>> On Tue, Jul 3, 2018 at 12:13 PM, Roman Lebedev <lebedev.ri at gmail.com>
>> wrote:
>>
>>> On Tue, Jul 3, 2018 at 9:06 PM, Yuan Lin via llvm-dev
>>> <llvm-dev at lists.llvm.org> wrote:
>>> > Hi, Sanjay/all,
>>> >
>>> >   I noticed in rL331486 that some compare-select optimizations are
>>> disabled
>>> > in favor of providing canonicalized cmp+select to the backend.
>>> >
>>> >   I am currently working on a private backend target, and the target
>>> has a
>>> > small code size limit.  With this change, some of the apps went over
>>> the
>>> > codesize limit.  As an example,
>>> >
>>> > C code:
>>> >   b = (a > -1) ? 4 : 5;
>>> >
>>> > ll code:
>>> > Before rL331486:
>>> >   %0 = lshr i16 %a.0.a.0., 15
>>> >   %1 = or i16 %0, 4
>>> >
>>> > After rL331486:
>>> >   %cmp = icmp sgt i16 %a.0.a.0., -1
>>> >   %cond = select i1 %cmp, i16 4, i16 5
>>> >
>>> >   With the various encoding restrictions of my particular target, the
>>> > cmp/select generated slightly larger code size.  However, because the
>>> apps
>>> > were very close to the code size limit, this slight change pushed them
>>> over
>>> > the limit.
>>> >
>>> >   If I still prefer to have this optimization performed, then is my
>>> best
>>> > strategy moving forward to implement this optimization as a peephole
>>> opt in
>>> > my backend?
>>> I personally think it should probably be a DAGCombine transform,
>>> driven by a Target Transform Info hook (e.g. like hasAndNot()).
>>>
>>> > Thanks,
>>> > --Yuan
>>> Roman.
>>>
>>> > _______________________________________________
>>> > LLVM Developers mailing list
>>> > llvm-dev at lists.llvm.org
>>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>> >
>>>
>>
>>
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