Avoid using two-char search in IndexOfOrdinalIgnoreCase for 1-char values

[stephentoub]

Should fix dotnet/perf-autofiling-issues#17208

[ghost]

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Issue Details

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Should fix dotnet/perf-autofiling-issues#17208

Author:	stephentoub
Assignees:	-
Labels:	area-System.Globalization
Milestone:	-

[EgorBo]

And for the 1-char value we'll be using IndexOfAny(c.ToUpper(), c.ToLower()) path, right?

[stephentoub]

And for the 1-char value we'll be using IndexOfAny(c.ToUpper(), c.ToLower()) path, right?

Yes, the loop below does:

int relativeIndex = isLetter? 
    PackedSpanHelpers.PackedIndexOfIsSupported
                        ? PackedSpanHelpers.IndexOfAny(ref Unsafe.Add(ref searchSpace, offset), valueCharU, valueCharL, searchSpaceMinusValueTailLength)
                        : SpanHelpers.IndexOfAnyChar(ref Unsafe.Add(ref searchSpace, offset), valueCharU, valueCharL, searchSpaceMinusValueTailLength) :
                    SpanHelpers.IndexOfChar(ref Unsafe.Add(ref searchSpace, offset), valueChar, searchSpaceMinusValueTailLength);