Optimize System.Text.ASCIIUtility for arm64 using cross-platform intrinsics

[jeffhandley]

This item tracks the remaining System.Text.ASCIIUtility methods that can be optimized for ARM64 using intrinsics. Many were completed in 5.0.0 with #35034, but two methods remain. Both of them had prior attempts to apply ARM64 intrinsics, but the efforts were not completed.

• System.Text.ASCIIUtility.GetIndexOfFirstNonAsciiChar() - Closed/Draft PR Get index of first non ascii char #39507
• System.Text.ASCIIUtility.NarrowUtf16ToAscii() - Closed/Draft PR Narrow utf16 to ascii #39509

Update for .NET 7
Now that we have cross-platform intrinsics APIs (#49397), these optimizations should be completed using those APIs instead of adding ARM64-specific intrinsics code paths. The effort could optionally include first measuring performance of these methods with the ARM64-specific intrinsics in place, and then measuring the performance of these methods with the cross-platform intrinsics.

We previously attempted to implement these optimizations during .NET 5, but the draft PRs linked above were closed because they were introducing unexpected regressions. Therefore, the effort on these optimizations must include investigation of those pitfalls and finding an approach that simultaneously optimizes the code for ARM64 and avoids regressions elsewhere.

Example use of the new cross-platform intrinsics: #63722

[ghost]

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[tarekgh]

CC @GrabYourPitchforks

[a74nh]

Could you please assign this to @SwapnilGaikwad

[danmoseley]

@kunalspathak with the attached change is this now addressed for all architectures, or should this remain open?

[a74nh]

System.Text.ASCIIUtility.GetIndexOfFirstNonAsciiChar() is done for Arm64/X64.

Expecting a pull request for System.Text.ASCIIUtility.NarrowUtf16ToAscii() to be raised late this week/early next week.

[kunalspathak]

System.Text.ASCIIUtility.GetIndexOfFirstNonAsciiChar() is done for Arm64/X64.

Expecting a pull request for System.Text.ASCIIUtility.NarrowUtf16ToAscii() to be raised late this week/early next week.

It's other way round. NarrowUtf16ToAscii() is complete and System.Text.ASCIIUtility.GetIndexOfFirstNonAsciiChar() is not yet complete.

[a74nh]

It's other way round. NarrowUtf16ToAscii() is complete and System.Text.ASCIIUtility.GetIndexOfFirstNonAsciiChar() is not yet complete.

Oops, yes :)