Why we use CompareInfo.Invariant for Ordinal?

[iSazonov]

I am not so familiar with unicode in-depth and I am amazed to see that
public int IndexOf(char value, StringComparison comparisonType)
and all IndexOf overloads for strings like
public int IndexOf(string value, int startIndex, int count, StringComparison comparisonType)
call CompareInfo.Invariant.IndexOf() for both InvariantCulture (InvariantCultureIgnoreCase) and Ordinal (OrdinalIgnoreCase).

In the same time we have IndexOf overloads for chars which use really ordinal comparison from SpanHelpers with SIMD accelerations.
public int IndexOf(char value) => SpanHelpers.IndexOf(ref _firstChar, value, Length);

We even have [public static int CompareOrdinal(string strA, int indexA, string strB, int indexB, int length) }(https://source.dot.net/#System.Private.CoreLib/shared/System/String.Comparison.cs,341906879aa2feb9).
(internally SpanHelpers with SIMD accelerations.)

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I'd expect the same for strings too if StringComparison is Ordinal or OrdinalIgnoreCase.

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In the case we could implement very fast SIMD accelerated methods like IndexOf(), LastIndexOf(), Replace() and perhaps some more.

For OrdinalIgnoreCase we could implement unicode simple case folding (#17233). With this enhancement we could to get great wins in RegEx (see Rust RegEx lib) and simplify Boyer-Moore implementation (which is also useful in IndexOf() and Replace() - #20674 and https://github.com/dotnet/coreclr/issues/6918).
Some languages (like PowerShell) could get great wins in identifier comparisons.

[jkotas]

This is duplicate of https://github.com/dotnet/coreclr/issues/19672. The char case was fixed already.

[iSazonov]

@jkotas Is your link correct?
And my request is for strings not chars.

[jkotas]

@jkotas Is your link correct?

It is correct now.

And my request is for strings not chars.

The linked issue is for string.IndexOf(.... If you would like to submit PRs with more fixes towards this, we will be happy to take them.

[iSazonov]

Why the fix is not replicated to CoreFX? https://github.com/dotnet/corefx/blob/master/src/Common/src/CoreLib/System/String.Searching.cs#L55

I am not ready to submit PRs but I will.

[jkotas]

@Anipik The recent change in String.Searching.cs was not mirrored correctly to corefx. Could you please take a look?

[Anipik]

@jkotas there is no missing commit for this change. we will have to manually open the PR to correct it.
i can open PR to correct it in corefx and make sure the commit is not mirrored back to coreclr or corert(i.e mirror didnot get stuck due to merge conflicts)

[jkotas]

That would be great. Could you please do that?

[Anipik]

yeah i am on it