Fix GitHub #20389

[fiigii]

Fix #20389

@CarolEidt @AriNuer @Sunny-pu

[danmoseley]

Why does it need to check for Avx2 if its only using Avx?

[fiigii]

Integer SIMD addition instructions (please see Vector256Add<T>) are provided by AVX2, and AVX just has floating point instructions.

[danmoseley]

So if I understand right, public static Vector256<T> SetAllVector256<T>(T value) where T : struct (which is not on the Avx2 class) requires Avx2 when T is an integer type? If so, it seems to me this is likely to be a common mistake, which presumably will only show up when someone tests on a machine that does not support AVX2?

@tannergooding

[tannergooding]

The issue here is with the Vector256Add call (which calls Avx2.Add:

coreclr/tests/src/JIT/HardwareIntrinsics/X86/General/VectorHelpers.cs

Line 66 in 9bd2032

return Avx.StaticCast<byte, T>(Avx2.Add(Avx.StaticCast<T, byte>(left), Avx.StaticCast<T, byte>(right)));

).

SetAllVector256 is a helper method and is being moved to the Vector256<T> type as part of #20147 (as soon as I push the changes). It will be available always.

[danmoseley]

Oops, yes I quoted the wrong API. OK, after that change I should never need if (Avx2.IsSupported) unless I am using the Avx2 class?

[tannergooding]

Correct.

[fiigii]

@tannergooding Thanks for the reply 😄
@danmosemsft Can we merge this PR?

[danmoseley]

@tannergooding can sign off