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

[via llvm-commits]

> 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?

> 
> So let me add the use check (I added the test already at https://reviews.llvm.org/rL329418 <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?

—escha

> 
> On Fri, Apr 6, 2018 at 9:33 AM, <escha at apple.com <mailto: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 <mailto: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 <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/Transforms/InstCombine/InstCombineAddSub.cpp?rev=329350&r1=329349&r2=329350&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/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/Transforms/InstCombine/fsub.ll?rev=329350&r1=329349&r2=329350&view=diff <http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/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
> >
> >
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> 
> 

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