[llvm-dev] LLVM 11 and trunk selecting 4 wide instead of 8 wide loop vectorization for AVX-enabled target
Neil Henning via llvm-dev
llvm-dev at lists.llvm.org
Thu Jul 16 11:54:38 PDT 2020
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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