[llvm-dev] LLVM 11 and trunk selecting 4 wide instead of 8 wide loop vectorization for AVX-enabled target
Sanjay Patel via llvm-dev
llvm-dev at lists.llvm.org
Thu Jul 16 12:11:04 PDT 2020
Right - the vectorizer doesn't know that we have SLEEF, so the cost model
is assuming the pow gets expanded.
I'm not familiar with status on SLEEF, but we have support for other
veclibs in TargetLibraryInfo. So we can see that we are willing to generate
an <8 x float> call if that is known supported:
$ ./opt -loop-vectorize vec4.ll -S -vector-library=SVML -mattr=avx | grep
pow
%50 = call <8 x float> @__svml_powf8(<8 x float> %wide.load, <8 x float>
%wide.load4)
On Thu, Jul 16, 2020 at 2:55 PM Neil Henning <neil.henning at unity3d.com>
wrote:
> So for us we use SLEEF to actually implement the libcalls (LLVM
> intrinsics) that LLVM by default would generate - and since SLEEF has
> highly optimal 8-wide pow, optimized for AVX and AVX2, we really want to
> use that.
>
> So we would not see 4/8 libcalls and instead see 1 call to something that
> lights up the ymm registers. I guess the problem then is that the default
> expectation is that pow would be implemented using N scalar libcalls?
>
> Cheers,
> -Neil.
>
> On Thu, Jul 16, 2020 at 6:08 PM Sanjay Patel <spatel at rotateright.com>
> wrote:
>
>> The debug spew for loop vectorization shows:
>> LV: Found an estimated cost of 49 for VF 4 For instruction: %14 = tail
>> call float @llvm.pow.f32(float %10, float %13)
>> LV: Vector loop of width 4 costs: 13.
>>
>> LV: Found an estimated cost of 107 for VF 8 For instruction: %14 = tail
>> call float @llvm.pow.f32(float %10, float %13)
>> LV: Vector loop of width 8 costs: 14.
>> LV: Selecting VF: 4.
>>
>> So rounding of the integer division could be to blame?
>>
>> But before we focus on that, there's a lot of hand-waving involved in
>> creating these costs beginning with the base cost implementation:
>> unsigned SingleCallCost = 10; // Library call cost. Make it expensive.
>>
>> But before we focus on that... :)
>>
>> Are we modeling the right thing? Ie, are you not expecting to see 4 or 8
>> libcalls when the vector pow call gets expanded on this example? If we are
>> doing those libcalls, then it's not clear to me how anything else in the
>> loop matters for performance.
>>
>> On Thu, Jul 16, 2020 at 10:20 AM Neil Henning via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Tried a bunch of them there (x86-64, haswell, znver2) and they all
>>> defaulted to 4-wide - haswell additionally caused some extra loop unrolling
>>> but still with 8-wide pows.
>>>
>>> Cheers,
>>> -Neil.
>>>
>>> On Thu, Jul 16, 2020 at 2:39 PM Roman Lebedev <lebedev.ri at gmail.com>
>>> wrote:
>>>
>>>> Did you specify the target CPU the code should be optimized for?
>>>> For clang that is -march=native/znver2/... / -mtune=<same>
>>>> For opt/llc that is --mcpu=<same>
>>>> I would expect that by default, some generic baseline is picked.
>>>>
>>>> On Thu, Jul 16, 2020 at 4:25 PM Neil Henning via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> Hey list,
>>>>>
>>>>> I've recently done the first test run of bumping our Burst compiler
>>>>> from LLVM 10 -> 11 now that the branch has been cut, and have noticed an
>>>>> apparent loop vectorization codegen regression for X86 with AVX or AVX2
>>>>> enabled. The following IR example is vectorized to 4 wide with LLVM 11 and
>>>>> trunk whereas in LLVM 10 it (correctly as per what we want) vectorized it 8
>>>>> wide matching the ymm registers.
>>>>>
>>>>> ; ModuleID = '../test.ll'
>>>>> source_filename = "main"
>>>>> target datalayout =
>>>>> "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
>>>>> target triple = "x86_64-pc-windows-msvc-coff"
>>>>>
>>>>> %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0" = type {
>>>>> float*, i32, [4 x i8] }
>>>>>
>>>>> ; Function Attrs: nofree
>>>>> define dllexport void @func(float* noalias nocapture %output,
>>>>> %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0"* nocapture nonnull
>>>>> readonly dereferenceable(16) %a,
>>>>> %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0"* nocapture nonnull
>>>>> readonly dereferenceable(16) %b) local_unnamed_addr #0 !ubaa. !1 {
>>>>> entry:
>>>>> %0 = getelementptr
>>>>> %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0",
>>>>> %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0"* %a, i64 0, i32 1
>>>>> %1 = load i32, i32* %0, align 1
>>>>> %.not = icmp eq i32 %1, 0
>>>>> br i1 %.not, label %BL.0042, label %BL.0005.lr.ph
>>>>>
>>>>> BL.0005.lr.ph: ; preds = %entry
>>>>> %2 = bitcast %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0"*
>>>>> %a to i8**
>>>>> %3 = load i8*, i8** %2, align 1
>>>>> %4 = bitcast %"Burst.Compiler.IL.Tests.VectorsMaths/FloatPointer.0"*
>>>>> %b to i8**
>>>>> %5 = load i8*, i8** %4, align 1
>>>>> %wide.trip.count = zext i32 %1 to i64
>>>>> br label %BL.0005
>>>>>
>>>>> BL.0005: ; preds = %BL.0005, %
>>>>> BL.0005.lr.ph
>>>>> %indvars.iv = phi i64 [ 0, %BL.0005.lr.ph ], [ %indvars.iv.next,
>>>>> %BL.0005 ]
>>>>> %6 = shl nuw nsw i64 %indvars.iv, 2
>>>>> %7 = getelementptr float, float* %output, i64 %indvars.iv
>>>>> %8 = getelementptr i8, i8* %3, i64 %6
>>>>> %9 = bitcast i8* %8 to float*
>>>>> %10 = load float, float* %9, align 4
>>>>> %11 = getelementptr i8, i8* %5, i64 %6
>>>>> %12 = bitcast i8* %11 to float*
>>>>> %13 = load float, float* %12, align 4
>>>>> %14 = tail call float @llvm.pow.f32(float %10, float %13)
>>>>> store float %14, float* %7, align 4
>>>>> %indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
>>>>> %exitcond.not = icmp eq i64 %indvars.iv.next, %wide.trip.count
>>>>> br i1 %exitcond.not, label %BL.0042, label %BL.0005
>>>>>
>>>>> BL.0042: ; preds = %BL.0005,
>>>>> %entry
>>>>> ret void
>>>>> }
>>>>>
>>>>> ; Function Attrs: norecurse readnone
>>>>> define dllexport void @burst.initialize(i8* (i8*)* nocapture readnone
>>>>> %callback) local_unnamed_addr #1 !ubaa. !0 {
>>>>> entry:
>>>>> ret void
>>>>> }
>>>>>
>>>>> ; Function Attrs: nounwind readnone speculatable willreturn
>>>>> declare float @llvm.pow.f32(float, float) #2
>>>>>
>>>>> attributes #0 = { nofree }
>>>>> attributes #1 = { norecurse readnone }
>>>>> attributes #2 = { nounwind readnone speculatable willreturn }
>>>>>
>>>>> !ubaa.Burst.Compiler.IL.Tests.VectorsMaths\2FFloatPointer.0 = !{!0,
>>>>> !0, !0, !0}
>>>>>
>>>>> !0 = !{i1 false}
>>>>> !1 = !{i1 true, i1 false, i1 false}
>>>>>
>>>>> If I run this with ../llvm-project/llvm/build/bin/opt.exe -o - -S -O3
>>>>> ../avx_sad_4.ll -mattr=avx -debug, I can see that the loop vectorizer
>>>>> correctly considers using 8-wide ymm registers for this, but has decided
>>>>> that the 4-wide variant is cheaper based on some cost modelling I don't
>>>>> understand.
>>>>>
>>>>> So is this expected behaviour? I know there was some cost model
>>>>> changes in the 10->11 timeframe.
>>>>>
>>>>> Thanks for any help,
>>>>>
>>>>> Cheers,
>>>>> -Neil.
>>>>>
>>>> Roman
>>>>
>>>>
>>>>> --
>>>>> Neil Henning
>>>>> Senior Software Engineer Compiler
>>>>> unity.com
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>
>>>
>>> --
>>> Neil Henning
>>> Senior Software Engineer Compiler
>>> unity.com
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
>
> --
> Neil Henning
> Senior Software Engineer Compiler
> unity.com
>
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