[llvm-dev] LLVM 11 and trunk selecting 4 wide instead of 8 wide loop vectorization for AVX-enabled target

[Neil Henning via llvm-dev]

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
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