Optimize Guid formatting

[pentp]

Streamline the default path and use more powerful vector intrinsics for faster formatting.

Method	Mean (main)	Error	Mean (PR)	Error	Version w/ AVX-512 VBMI
GuidToString	15.938 ns	0.345 ns	15.438 ns	0.306 ns	14.087 ns
TryFormat	1.679 ns	0.043 ns	1.037 ns	0.032 ns	0.845 ns
Utf8TryFormat	1.926 ns	0.057 ns	0.574 ns	0.023 ns	0.461 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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[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.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

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[pentp]

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.

I dropped the AVX-512 part and removed comparsion operator changes also (will create a separate PR for that).
This PR is best viewed with whitespace diffs ignored.
The benefits from AVX-512 weren't actually that big compared to all the other changes, but I wouldn't say it's only a small portion of hardware - Steam hardware survey shows 29% of CPUs have AVX-512 currently.

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

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