Fix GetIndexOfFirstNonAsciiByte_Vector not taken on ARM64

[neon-sunset]

Fixes #89924

CC @EgorBo @anthonycanino

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Issue Details

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Fixes #89924

Author:	neon-sunset
Assignees:	-
Labels:	area-System.Text.Encoding
Milestone:	-

[stephentoub]

If we do this, then doesn't this change leave a bunch of dead code in GetIndexOfFirstNonAsciiByte_Intrinsified that should be cleaned up? It has multiple blocks dedicated to AdvSimd which would never be reached. At which point the method should also be renamed to be SSE-specific.

But I'd also explicitly asked about this:
#88532 (comment)
and was told the preferring of the intrinsics path was by design. Maybe it was only measured on x64 and not on Arm?

cc: @tannergooding, @anthonycanino

[neon-sunset]

If we do this, then doesn't this change leave a bunch of dead code in GetIndexOfFirstNonAsciiByte_Intrinsified that should be cleaned up? It has multiple blocks dedicated to AdvSimd which would never be reached. At which point the method should also be renamed to be SSE-specific.

Let me fix that.

#88532 (comment)

The _Vector path is estimated to be ~40% faster on osx-arm64, It is likely the switch was not measured on ARM64.

[tannergooding]

The _Vector path is estimated to be ~40% faster on osx-arm64, It is likely the switch was not measured on ARM64.

@neon-sunset can you clarify please? .NET 7 was using the _Intrinsified path for Arm64: https://github.com/dotnet/runtime/blob/v7.0.10/src/libraries/System.Private.CoreLib/src/System/Text/ASCIIUtility.cs#L81-L91

The PR that introduced the _Vector path (#88532) didn't change _Intrinsified at all (outside using the centralized Vector128.Size constant) and didn't change which Arm64 was calling.

This then wouldn't seem like a regression, but rather simply a case where the new _Vector code is faster for Arm64. If that's the case, could you share your own numbers showing the .NET 7 vs .NET 8 difference (without this PR) and the difference for _Vector vs _Intrinsified for .NET 8 (this PR vs without this PR)?

[neon-sunset]

@neon-sunset can you clarify please? .NET 7 was using the _Intrinsified path for Arm64: https://github.com/dotnet/runtime/blob/v7.0.10/src/libraries/System.Private.CoreLib/src/System/Text/ASCIIUtility.cs#L81-L91

You are correct, my mistake.

This then wouldn't seem like a regression, but rather simply a case where the new _Vector code is faster for Arm64. If that's the case, could you share your own numbers showing the .NET 7 vs .NET 8 difference (without this PR) and the difference for _Vector vs _Intrinsified for .NET 8 (this PR vs without this PR)?

This is the plan, I'm just waiting for the Release build to finish to post detailed numbers 😄

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