[llvm] r329350 - [InstCombine] nsz: -(X - Y) --> Y - X

Fri Apr 6 10:08:23 PDT 2018

On Fri, Apr 6, 2018 at 10:56 AM, <escha at apple.com> wrote:

>
>
> On Apr 6, 2018, at 9:48 AM, Sanjay Patel <spatel at rotateright.com> wrote:
>
> Ah, thanks for letting me know. I did consider that case, but I was in an
> x86 mindset (and IIRC, your target has free or close-to-free fneg?), so the
> asm diff looks something like this:
>
> define float @neg_sub_nsz_extra_use(float %x, float %y) {
>   %t1 = fsub float %x, %y
>   %t2 = fsub nsz float -0.0, %t1
>   %t3 = fdiv float %t1, %t2 ; just for illustration of extra use of the
> 1st fsub...
>   ret float %t3
> }
>
> With fneg:
>     vsubss    %xmm1, %xmm0, %xmm0
>     vxorps    LCPI0_0(%rip), %xmm0, %xmm1 ; load a constant to fneg...sigh
>     vdivss    %xmm1, %xmm0, %xmm0
>
> Replace with fsub:
>     vsubss    %xmm1, %xmm0, %xmm2
>     vsubss    %xmm0, %xmm1, %xmm0
>     vdivss    %xmm0, %xmm2, %xmm0
>
>
> Yeah, we have free fneg (as do all GPUs that I know of, including AMDGPU).
>
> Even on x86, I’m nervous about avoiding a xorps by that method in general;
> i’d expect xorps to be faster because it uses a different execution pipe,
> at least in some cases.
>
> This transform, if done at all, should probably be done in target-specific
> DAG code, since it’s only profitable on a fairly specific subset of
> machines.
>
> I also haven’t looked, but can you double check that your Z - (X - Y) -->
> Z + (Y - X) transform also checks oneuse?
>
>
Yes - that one has m_OneUse:
https://reviews.llvm.org/rL329362

(also note that we already do that transform in instcombine, but requiring
the full set of FMF)

I can merge these to reduce the code.

>
>
>

> So let me add the use check (I added the test already at
> https://reviews.llvm.org/rL329418 ) to avoid the problem immediately. But
> maybe I should look at later target-specific (DAG) transforms that can
> change one to the other based on prefs?
>
>
> I’m not sure. The category of CPUs where “fsub is faster than xor” seems
> like a fairly Weird, Special Category. maybe just put it in the x86 backend
> alone?
>

Yeah, it's likely a uarch- and code-specific corner-case.

>
> —escha
>
>
> On Fri, Apr 6, 2018 at 9:33 AM, <escha at apple.com> wrote:
>
>> This patch is giving us fairly significant instruction count regressions
>> on a number of shaders. The regressions go away if we gate it with
>> hasOneUse, with makes sense; the patch as committed is duplicating
>> arithmetic!  Is it possible to add hasOneUse checks to this? I don’t feel
>> like it’s reasonable to duplicate fsubs just because they have a negated
>> use, especially since the compiler as a whole is very bad at deduplicating
>> them later.
>>
>> —escha
>>
>> > On Apr 5, 2018, at 2:37 PM, Sanjay Patel via llvm-commits <
>> llvm-commits at lists.llvm.org> wrote:
>> >
>> > Author: spatel
>> > Date: Thu Apr  5 14:37:17 2018
>> > New Revision: 329350
>> >
>> > URL: http://llvm.org/viewvc/llvm-project?rev=329350&view=rev
>> > Log:
>> > [InstCombine] nsz: -(X - Y) --> Y - X
>> >
>> > This restores part of the fold that was removed with rL73243 (PR4374).
>> >
>> > Modified:
>> >    llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
>> >    llvm/trunk/test/Transforms/InstCombine/fsub.ll
>> >
>> > Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
>> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transform
>> s/InstCombine/InstCombineAddSub.cpp?rev=329350&r1=329349&r2=
>> 329350&view=diff
>> > ============================================================
>> ==================
>> > --- llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp
>> (original)
>> > +++ llvm/trunk/lib/Transforms/InstCombine/InstCombineAddSub.cpp Thu
>> Apr  5 14:37:17 2018
>> > @@ -1698,10 +1698,17 @@ Instruction *InstCombiner::visitFSub(Bin
>> >                                   SQ.getWithInstruction(&I)))
>> >     return replaceInstUsesWith(I, V);
>> >
>> > -  // Subtraction from -0.0 is the canonical form of fneg.
>> > -  // fsub nsz 0, X ==> fsub nsz -0.0, X
>> > -  if (I.getFastMathFlags().noSignedZeros() && match(Op0,
>> m_PosZeroFP()))
>> > -    return BinaryOperator::CreateFNegFMF(Op1, &I);
>> > +  if (I.hasNoSignedZeros()) {
>> > +    // Subtraction from -0.0 is the canonical form of fneg.
>> > +    // fsub nsz 0, X ==> fsub nsz -0.0, X
>> > +    if (match(Op0, m_PosZeroFP()))
>> > +      return BinaryOperator::CreateFNegFMF(Op1, &I);
>> > +
>> > +    // With no-signed-zeros: -(X - Y) --> Y - X
>> > +    Value *X, *Y;
>> > +    if (match(Op0, m_NegZeroFP()) && match(Op1, m_FSub(m_Value(X),
>> m_Value(Y))))
>> > +      return BinaryOperator::CreateFSubFMF(Y, X, &I);
>> > +  }
>> >
>> >   if (isa<Constant>(Op0))
>> >     if (SelectInst *SI = dyn_cast<SelectInst>(Op1))
>> >
>> > Modified: llvm/trunk/test/Transforms/InstCombine/fsub.ll
>> > URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transfor
>> ms/InstCombine/fsub.ll?rev=329350&r1=329349&r2=329350&view=diff
>> > ============================================================
>> ==================
>> > --- llvm/trunk/test/Transforms/InstCombine/fsub.ll (original)
>> > +++ llvm/trunk/test/Transforms/InstCombine/fsub.ll Thu Apr  5 14:37:17
>> 2018
>> > @@ -19,8 +19,7 @@ define float @test1(float %x, float %y)
>> >
>> > define float @neg_sub(float %x, float %y) {
>> > ; CHECK-LABEL: @neg_sub(
>> > -; CHECK-NEXT:    [[T1:%.*]] = fsub float [[X:%.*]], [[Y:%.*]]
>> > -; CHECK-NEXT:    [[T2:%.*]] = fsub nsz float -0.000000e+00, [[T1]]
>> > +; CHECK-NEXT:    [[T2:%.*]] = fsub nsz float [[Y:%.*]], [[X:%.*]]
>> > ; CHECK-NEXT:    ret float [[T2]]
>> > ;
>> >   %t1 = fsub float %x, %y
>> >
>> >
>> > _______________________________________________
>> > llvm-commits mailing list
>> > llvm-commits at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>
>>
>
>
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