JIT: Re-enable acceleration of Vector512<long>.op_Multiply

[saucecontrol]

This was a regression in 9.0, from #103555

https://godbolt.org/z/11hs3Kqdd

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[EgorBo]

Thanks!

[EgorBo]

actually, I think this needs Avx512DQ ISA check

[saucecontrol]

If we're treating any of the Vector512 methods other than IsSupported as intrinsic, that implies we have the full baseline AVX-512 set (F,DQ,BW,CD,VL). It's a bit confusing because some of the import paths assert or check that, but most don't. I'm actually cleaning up some of those redundant asserts in a different branch now.

[EgorBo]

Ah ok, I thought that Vector512.IsHardwareAccelerated only relies on AVX512F, but looks like DOTNET_EnableAVX512DQ=0 turns it off so it's ok.

[saucecontrol]

Right, IsHardwareAccelerated 😄

The rule is Vector512.IsHardwareAccelerated will return false unless all of the following are satisfied:

1. Baseline AVX-512 support (F,CD,DW,BW,VL)
2. Not throttling based on CPUID check for Skylake-X and others with severe downclocking for 512-bit vector instructions, or DOTNET_PreferredVectorBitWidth: >= 512
3. No DOTNET_PreferredVectorBitWidth: < 512

The rule for whether Vector512 methods actually import as intrinsic is only that we have the baseline AVX-512 set, meaning IsHardwareAccelerated may return false, but all methods may actually be accelerated anyway.

So the fact that we're importing the methods for Vector512 as intrinsic in the first place means the ISA requirements have already been met.

Vector128 and Vector256 are a bit different, because the baseline ISA requirement may not be enough to accelerate all methods.

Vector256.IsHardwareAccelerated returns true only if AVX2 is supported, but we attempt to import methods as intrinsic as long as AVX is supported. Since many of the methods require AVX2 for acceleration, they have an extra check for AVX2 and then fall back to managed if it's not available. Hence all the (simdSize != 32) || compOpportunisticallyDependsOn(InstructionSet_AVX2) checks.

Similar checks are not included for Vector128, because the base requirement is SSE2, so almost all methods can be accelerated, minus a few that require SSE4.1 and check for it explicitly.

Clear as mud, I know...