[llvm] r329350 - [InstCombine] nsz: -(X - Y) --> Y - X
via llvm-commits
llvm-commits at lists.llvm.org
Sat Apr 7 07:55:59 PDT 2018
Thanks so much!
—escha
> On Apr 6, 2018, at 10:31 AM, Sanjay Patel <spatel at rotateright.com> wrote:
>
> Hopefully, fixed with:
> https://reviews.llvm.org/rL329429 <https://reviews.llvm.org/rL329429>
>
> Let me know if you see other problems - thanks!
>
> On Fri, Apr 6, 2018 at 11:08 AM, Sanjay Patel <spatel at rotateright.com <mailto:spatel at rotateright.com>> wrote:
>
>
> On Fri, Apr 6, 2018 at 10:56 AM, <escha at apple.com <mailto:escha at apple.com>> wrote:
>
>
>> On Apr 6, 2018, at 9:48 AM, Sanjay Patel <spatel at rotateright.com <mailto: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 <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 <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 <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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