[release/7.0] Fix bug in String.Equals unrolling for CJK single-char strings

[EgorBo]

Manual backport of #78190 to release/7.0

Customer Impact

Fixes#78169 regression introduced in the String.Equals/StartsWith unrolling optimization in NET 7.0.

constStringv1="鉄";varv2=v1;Console.WriteLine($"{v1==v2}, {v1.Equals(v2,StringComparison.Ordinal)}, {v1.Equals(v2,StringComparison.OrdinalIgnoreCase)}");

Prints different values depending on code being optimized or not (by jit). All comparisons against single-char literals of certain symbols (all two-bytes chars which pass this check: bool IsAffected(char c) => (((ushort)c) >> 15) == 1;) of CJK languages are affected so we probably want to land the fix it as soon as possible.

Testing

We had a pretty big test coverage for all sorts of inputs for that optimizations, unfortunately, for the non-ASCII one we didn't use any of CJK symbols. Added CJK and did a fuzzy testing locally via a hand-written test generator for this API.

Risk

Low

[carlossanlop]

@EgorBo when this is ready, please send the email to Tactics, add the servicing-consider label, and get a code review sign-off. The deadline for merging backports to include in the December release is Monday.

[ghost]

Tagging subscribers to this area: @JulieLeeMSFT, @jakobbotsch
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Backport of #78190 to release/7.0

Customer Impact

Fixes #78169 .NET 7.0 regression introduced in the String.Equals unrolling optimization.

constStringv1="鉄";varv2=v1;Console.WriteLine($"{v1==v2}, {v1.Equals(v2,StringComparison.Ordinal)}, {v1.Equals(v2,StringComparison.OrdinalIgnoreCase)}");

Prints different values depending on code being optimized or not (by jit). All comparisons against single-char literals of certain symbols of CJK languages are affected so we probably want to land the fix it as soon as possible.

Testing

We had a pretty big test coverage for all sorts of inputs for that optimizations, unfortunately, for the non-ASCII one we didn't use any of CJK symbols. Added CJK and did a fuzzy testing locally via a hand-written test generator for this API.

Risk

Low

Author:	EgorBo
Assignees:	-
Labels:	area-CodeGen-coreclr
Milestone:	-

[EgorBo]

@jakobbotsch PTAL

[jeffschwMSFT]

approved. we will take for consideration in 7.0.x

[carlossanlop]

Approved by Tactics via email.
Signed off by area owners.
CI is green.
No OOB package authoring changes needed.
Ready to merge.