Optimize string.Split(char, ...)

[hamarb123]

In some cases string.Split(char, ...) was slower (significantly) than the single string seperator overload - this PR optimizes the char overload/s to have much closer performance (still a small gap in 1 case) - the single-char overloads are the most common case, so I have added the additional optimisation just for that for now - benchmarks below.

Main benchmark that was the focus - single char splitter with rare/no seperators (17% improvement on arm, 47% improvement on x64) - char almost matches or outperforms string now (could probably get it to match by adjusting the branches carefully - I can implement if wanted, but it will probably make it less readable):

X64 (AMD):

Arm:

Other cases have not regressed meaningfully (within margin of error) - some have even possibly improved slightly on x64: EgorBot/Benchmarks#28

[Copilot]

Pull request overview

This PR optimizes string.Split(char, ...) by adding a dedicated fast path for the most common scenario: splitting on a single separator character, including a new vectorized implementation for scanning separator positions.

Changes:

• Added a single-separator (separators.Length == 1) specialized path in MakeSeparatorListAny.
• Introduced a new MakeSeparatorListVectorized(..., char c) overload to vectorize scanning for a single separator char.
• Adjusted the existing 2–3 separator path to assume separators.Length >= 2 (since the 1-separator case is now handled earlier).

[hamarb123]

Seems to improve perf by ~25%, but there's still a substantial gap on X86 (would need another ~40% improvement to match string).

AMD:

Arm:

Intel:

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[hamarb123]

@MihuBot

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Comments suppressed due to low confidence (1)

src/libraries/System.Private.CoreLib/src/System/SpanHelpers.Packed.cs:1147

• These PackSources helpers were made internal to support new callers outside PackedSpanHelpers, but the file-level comment indicates this type exists to hide "private helpers". Consider updating the nearby commentary / documenting the intended usage constraints (e.g., only call when the relevant ISA is supported and CanUsePackedIndexOf has been validated) to avoid future misuse within the assembly.

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        [CompExactlyDependsOn(typeof(Avx512BW))]
        internal static Vector512<byte> PackSources(Vector512<short> source0, Vector512<short> source1)
        {
            Debug.Assert(Avx512BW.IsSupported);
            // Pack two vectors of characters into bytes. While the type is Vector256<short>, these are really UInt16 characters.
            // X86: Downcast every character using saturation.

[Copilot]

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Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.