Lower regex MaxUnrollSize from 16 to 7

[stephentoub]

For fixed-count single-character repeaters (e.g. \d{N}), the regex source generator and compiler choose between unrolling individual character checks vs using vectorized operations like ContainsAnyExcept. MaxUnrollSize was the threshold controlling this decision — previously set to 16.

Benchmarking across multiple character class types shows the crossover point where vectorization wins is consistently between count 4 and 8:

Count	Range (\d)	SingleCharNeg ([^x])	SmallSet ([abc])	SearchValues ([a-zA-Z])
2	Loop 1.28x faster	Loop 1.11x	Loop 2.06x	Loop 1.72x
4	Loop 1.31x faster	~tied	Loop 1.67x	Loop ~1.5x
8	Vec 0.75x	Vec 0.52x	Vec 0.86x	Vec 0.75x
16	Vec 0.42x	Vec 0.35x	Vec 0.53x	Vec 0.43x

At count 8+, vectorized operations win across all character class types. At count ≤4, the unrolled loop wins due to lower overhead and early-exit on mismatch.

This PR lowers the threshold from 16 to 8 in both RegexGenerator.Emitter.cs (source generator) and RegexCompiler.cs (compiled engine), so that repeaters with counts 9–16 now use vectorized operations instead of unrolled scalar checks.

Note

This PR was generated with the assistance of GitHub Copilot.

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[Copilot]

Pull request overview

This PR adjusts the unrolling threshold used by both the regex compiler and the regex source generator when emitting fixed-count single-character repeaters, favoring vectorized implementations sooner based on benchmarked crossover points.

Changes:

• Lower MaxUnrollSize from 16 to 8 in the compiled regex engine (RegexCompiler).
• Lower MaxUnrollSize from 16 to 8 in the regex source generator emitter (RegexGenerator.Emitter).

Reviewed changes

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

File	Description
src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexCompiler.cs	Lowers the unroll-vs-vectorize threshold for compiled regex repeater emission.
src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs	Lowers the same threshold for source-generated regex emission to keep behavior aligned.

[Copilot]

Pull request overview

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

[danmoseley]

Just curious, what would it take for IndexOf or whatever that you're calling for >= 8 to do the same fast scalar thing for all lengths, as an implementation detail? ie

 if (span.Length < 8) // .. scalar
 else { VectorizedPath(span); } // not inlined

and the JIT figure out that Slice(foo, 3) or whatever can be expanded to char by char comparison like the source gen is doing here. I guess there's a bunch of pieces, like it would have to propagate the constant through to elide the slice, figure out inlining is worthwhile, and somehow specialize it for constant argument (waving hands as this isn't my domain)

[stephentoub]

/ba-g unrelated mono BadExits