Improved RuntimeHelpers.ConvertLength codegen

[Sergio0694]

Follow up from #3520

PR Type

What kind of change does this PR introduce?

• Optimization

What is the current behavior?

The codegen for the second branch in RuntimeHelpers.ConvertLength does a signed division:

https://github.com/windows-toolkit/WindowsCommunityToolkit/blob/9b75c9f910f999834c64bd00e522d7ae464c121b/Microsoft.Toolkit.HighPerformance/Helpers/Internals/RuntimeHelpers.cs#L43-L46

This is not the best for the codegen, as the JIT has to handle the sign in that division, resulting in the following:

; [System.Byte, System.Private.CoreLib],[System.Numerics.Vector4, System.Numerics.Vectors]
ConvertLength[TFrom, TTo](Int32)
    L0000: mov eax, ecx
    L0002: sar eax, 0x1f
    L0005: and eax, 0xf
    L0008: add eax, ecx
    L000a: sar eax, 4
    L000d: ret

What is the new behavior?

Avoided that with a cast to uint, since the length is guaranteed to be a positive value in [0, int.MaxValue] anyway:

; [System.Byte, System.Private.CoreLib],[System.Numerics.Vector4, System.Numerics.Vectors]
    L0000: mov eax, ecx
    L0002: shr eax, 4
    L0005: ret

Perfect! 😄🎉

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

[ghost]

Thanks Sergio0694 for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌

[ghost]

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[michael-hawker]

@RosarioPulella want to take a quick look? 😉

@Sergio0694 always another instruction to remove, eh?

[Sergio0694]

@michael-hawker Ahahah always! Trying to make the 7.0 release of the HighPerformance package as good as I possibly can! 😊

[Rosuavio]

Love it! I never knew that the division of uints has so many less instructions than ints

[Rosuavio]

Also I see he in progress label on the PR, is this PR ready to be merged? @Sergio0694

[Sergio0694]

@RosarioPulella Yup, it's ready to go! The bot added that label automatically when I put this PR into the 7.0 milestone 🙂