Optimize Guid formatting

[pentp]

Streamline the default path and use AVX-512 VBMI+VL for even faster formatting.

Method	Mean (main)	Error	Mean (PR)	Error
GuidToString	15.938 ns	0.345 ns	14.087 ns	0.320 ns
TryFormat	1.679 ns	0.043 ns	0.845 ns	0.021 ns
Utf8TryFormat	1.926 ns	0.057 ns	0.461 ns	0.025 ns

Benchmark code

public class Bench
{
    private Guid _guid = new("a8a110d5-fc49-43c5-bf46-802db8f843ff");
    private readonly byte[] _buffer = new byte[100];
    private readonly char[] _chars = new char[100];

    [Benchmark]
    public string GuidToString() => _guid.ToString();

    [Benchmark]
    public bool TryFormat() => _guid.TryFormat(_chars, out _);

    [Benchmark]
    public bool Utf8TryFormat() => Utf8Formatter.TryFormat(_guid, _buffer, out _);
}

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[Copilot]

Pull request overview

Optimizes Guid formatting by streamlining the default ToString path and adding an AVX-512 VBMI+VL vectorized implementation for fast hex formatting, with related tweaks in Utf8Formatter for Guid.

Changes:

• Reworks Guid.ToString / TryFormatCore to use a flags-based fast path and adds an AVX-512 VBMI+VL implementation for D/N/B/P formats.
• Refactors Guid comparison operators / CompareTo to use unsigned comparisons for _b / _c and a big-endian ulong compare for the trailing bytes.
• Updates Utf8Formatter.TryFormat(Guid, ...) to compute and pass the new Guid formatting flags directly.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/System.Private.CoreLib/src/System/Guid.cs	Adds new vectorized formatting path and adjusts comparison logic and ToString fast path to use flags-based formatting.
src/libraries/System.Private.CoreLib/src/System/Buffers/Text/Utf8Formatter/Utf8Formatter.Guid.cs	Updates Utf8Formatter’s Guid formatting to use flags directly and streamline default-format handling.

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[Copilot]

Utf8Formatter.TryFormat(Guid, ..., StandardFormat) now treats format.Symbol == '\0' as the default 'D' format without also checking that the StandardFormat is actually default (i.e., Precision == 0). This changes behavior for values like new StandardFormat('\0', 1), which previously would have thrown FormatException but will now format as 'D'.

Use format.IsDefault (or replicate FormattingHelpers.GetSymbolOrDefault logic) so that only the true default StandardFormat maps to 'D'.

[Copilot]

The comparison logic now treats _b / _c as ushort (and the tail as a big-endian ulong). This changes ordering vs the previous implementation for GUIDs where _b or _c have the high bit set (e.g., values corresponding to negative shorts).

There are GuidTests.CompareTo_TestData cases for _b and _c, but they only cover small positive values. Please add test cases covering boundary values (e.g., short.MinValue, -1, 0x8000) to lock in the intended ordering and ensure this behavior change is validated.

[tannergooding]

This seems like a lot of churn for what amounts to about a nanosecond of savings and which will only light up on a small portion of available hardware today.