[llvm-dev] Question about canonicalizing cmp+select

[Craig Topper via llvm-dev]

While its true select convertSelectOfCosntantsToMath() creates add+sext i1,
there's a transform in DAGCombiner::visitADDLike that does "add (sext i1),
X -> sub X, (zext i1)". Why didn't that fire for your target?

~Craig


On Tue, Jul 3, 2018 at 3:44 PM Sanjay Patel <spatel at rotateright.com> wrote:

> I linked the wrong patch review. Here's the patch that was actually
> committed:
> https://reviews.llvm.org/D48508
> https://reviews.llvm.org/rL335433
>
> On Tue, Jul 3, 2018 at 4:39 PM, Sanjay Patel <spatel at rotateright.com>
> wrote:
>
>> [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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