Use IndexOfAnyWhiteSpace in Guid.EatAllWhitespace

[Copilot]

Description

Replace the manual for loop scanning for whitespace in Guid.EatAllWhitespace with the new vectorized IndexOfAnyWhiteSpace helper added in #111439.

-int i;
-for (i = 0; i < charSpan.Length && !char.IsWhiteSpace(charSpan[i]); i++) ;
-if (i == charSpan.Length)
+int i = charSpan.IndexOfAnyWhiteSpace();
+if (i < 0)

Method	Toolchain	Mean	Error
ParseExactX_NoWhitespace	main	87.085 ns	0.4839 ns
ParseExactX_NoWhitespace	PR #127123	60.859 ns	0.0391 ns
ParseExactX_WithWhitespace	main	107.613 ns	0.2588 ns
ParseExactX_WithWhitespace	PR #127123	113.060 ns	0.0679 ns

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[MihaZupan]

This seems like one of the only places where using a vectorized helper should be a perf improvement.
Other places like Trim commonly deal with very early exits, whereas the fast path in this case is when there are no matches.

[MihaZupan]

@copilot Use the benchmark skill to test the impact of this change to Guid parsing

[Copilot]

@copilot Use the benchmark skill to test the impact of this change to Guid parsing

Note

This comment was generated by Copilot.

@EgorBot -linux_amd -osx_arm64

using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(Bench).Assembly).Run(args);

[MemoryDiagnoser]
public class Bench
{
    private string _noWhitespace;
    private string _withWhitespace;
    private string _dFormat;

    [GlobalSetup]
    public void Setup()
    {
        var guid = Guid.NewGuid();
        // X format: {0x00000000,0x0000,0x0000,{0x00,0x00,0x00,0x00,0x00,0x00,0x00,0x00}}
        _noWhitespace = guid.ToString("X");
        // Same but with whitespace scattered throughout (EatAllWhitespace must strip it)
        _withWhitespace = "  " + _noWhitespace.Insert(10, " ").Insert(20, "  ") + "  ";
        // D format for comparison (doesn't use EatAllWhitespace)
        _dFormat = guid.ToString("D");
    }

    [Benchmark]
    public Guid ParseExactX_NoWhitespace() => Guid.ParseExact(_noWhitespace, "X");

    [Benchmark]
    public Guid ParseExactX_WithWhitespace() => Guid.ParseExact(_withWhitespace, "X");

    [Benchmark(Baseline = true)]
    public Guid ParseExactD() => Guid.ParseExact(_dFormat, "D");
}

[Copilot]

Pull request overview

This PR updates Guid.EatAllWhitespace to use the newer vectorized MemoryExtensions.IndexOfAnyWhiteSpace helper (introduced in #111439) instead of a manual loop when scanning ReadOnlySpan<char> for the first whitespace.

Changes:

• Replace manual whitespace scan loop with IndexOfAnyWhiteSpace() in the char fast-path.
• Update the “no whitespace found” check to use i < 0 (the IndexOf* not-found convention).

[EgorBo]

@copilot, benchmark results are ready:

• ❌ ubuntu24_azure_turin
• ✅ macos26_helix_arm64

Please analyze the results and act accordingly.
NOTE: some benchmarks may be flaky or bi-modal, so use your judgment when interpreting small differences.

[Copilot]

@copilot, benchmark results are ready:

• ❌ ubuntu24_azure_turin...

macOS ARM64 (Apple M4) results:

Method	main	PR	Change
ParseExactX_NoWhitespace	87.09 ns	60.86 ns	-30.1% ✅
ParseExactX_WithWhitespace	107.61 ns	113.06 ns	+5.1%
ParseExactD (baseline)	8.04 ns	8.11 ns	unchanged

The fast path (no whitespace) sees a ~30% improvement from vectorized IndexOfAnyWhiteSpace, which aligns with @MihaZupan's observation that this is one of the places where vectorization helps since the common case is scanning the entire input without finding any whitespace.

The WithWhitespace case shows a small ~5% regression which is marginal and expected — the vectorized search has slightly more setup overhead that doesn't pay off when whitespace is found early.

The linux_amd run timed out (infrastructure issue, not related to the change).

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