Add missing fast path for DateTime format "G"

[danmoseley]

Adds a fast path for DateTime.ToString("G", CultureInfo.InvariantCulture) similar to existing single-char fast path that 'o', 'r', 's', and 'u' already have in DateTimeFormat.Format / TryFormat<TChar>, and identical to the one the null/empty-format case (dt.ToString(CultureInfo.InvariantCulture)) already takes via TryFormatInvariantG.

The fast path is gated by offset.Ticks == NullOffset && ReferenceEquals(dtfi, DateTimeFormatInfo.InvariantInfo), so DateTimeOffset.ToString("G", InvariantCulture) and any custom/cloned DateTimeFormatInfo fall through to the existing pattern-based path unchanged.

Perf Results

5.5x faster

ResultsComparer, threshold 1%, noise 1 ns, 5 launches × 15 iterations × 16384 invocations, P-core pinned on i9-14900K:

summary: better: 1, geomean: 5.511, total diff: 1
No Slower results for the provided threshold = 1% and noise filter = 1 ns.

Faster	base/diff	Base Median (ns)	Diff Median (ns)
System.Tests.Perf_DateTime.ToStringInvariant(format: "G")	5.51	41.58	7.54

The remaining 19 Perf_DateTime.ToString* cases (other formats, current-culture, DateTimeOffset) are unchanged. Allocation unchanged everywhere.

ToStringInvariant(format) is a new benchmark in Perf_DateTime.cs that calls date.ToString(format, CultureInfo.InvariantCulture).

Tests

Output is byte-identical to the slow path for DateTime + invariant culture ("MM/dd/yyyy HH:mm:ss", 19 chars). Existing DateTime / DateTimeOffset formatting tests pass. In addition, an ad-hoc correctness harness compared baseline vs. modified outputs over 10 DateTimes (incl. MinValue, MaxValue, year 1, year 9999, Feb 29 2000, all DateTimeKinds) × 5 cultures × 16 formats × {ToString, TryFormat} plus DateTimeOffset equivalents — 1040 byte-identical outputs. Separately verified TryFormat with too-small destinations (sizes 0..20 char, 0..25 UTF-8), composite formatting $"{dt:G}", cloned DTFI, and DTFI with customized LongTimePattern — all behave identically to baseline.

Note

This PR description was drafted by GitHub Copilot.

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[Copilot]

Pull request overview

Adds an invariant-culture fast path for the standard "G" format when formatting a DateTime (i.e., offset.Ticks == NullOffset) by directly using the existing TryFormatInvariantG implementation. This aligns "G" with the existing single-character fast paths (e.g., "o", "r", "s", "u") and the already-existing null/empty-format invariant fast path.

Changes:

• Add a "G" switch case in DateTimeFormat.Format that fast-paths DateTime.ToString("G", InvariantCulture) to TryFormatInvariantG.
• Add the corresponding "G" fast path in DateTimeFormat.TryFormat<TChar> for invariant culture + DateTime (NullOffset) scenarios.

Show a summary per file

File	Description
src/libraries/System.Private.CoreLib/src/System/Globalization/DateTimeFormat.cs	Adds an invariant "G" fast path in both Format and TryFormat<TChar> using TryFormatInvariantG when offset is NullOffset.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0