Lower regex MaxUnrollSize from 16 to 7 (#126092)

Author: stephentoub

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

@@ -1606,9 +1606,10 @@ private static void EmitTryMatchAtCurrentPosition(IndentedTextWriter writer, Reg
             // "doneLabel" is simply the final return location from the TryMatchAtCurrentPosition method that will undo any captures and exit, signaling to
             // the calling scan loop that nothing was matched.
 
-            // Arbitrary limit for unrolling vs creating a loop.  We want to balance size in the generated
-            // code with other costs, like the (small) overhead of slicing to create the temp span to iterate.
-            const int MaxUnrollSize = 16;
+            // Limit for unrolling vs creating a loop. Benchmarking shows vectorized operations
+            // (e.g. ContainsAnyExcept) beat unrolled scalar checks at counts above ~4-8, so
+            // we unroll up to/including this threshold and use a loop with vectorization beyond it.
+            const int MaxUnrollSize = 7;
 
             RegexOptions options = rm.Options;
             RegexTree regexTree = rm.Tree;

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexCompiler.cs

@@ -4585,9 +4585,10 @@ void EmitSingleCharRepeater(RegexNode node, bool emitLengthChecksIfRequired = tr
                     return;
                 }
 
-                // Arbitrary limit for unrolling vs creating a loop.  We want to balance size in the generated
-                // code with other costs, like the (small) overhead of slicing to create the temp span to iterate.
-                const int MaxUnrollSize = 16;
+                // Limit for unrolling vs creating a loop. Benchmarking shows vectorized operations
+                // (e.g. ContainsAnyExcept) beat unrolled scalar checks at counts above ~4-8, so
+                // we unroll up to/including this threshold and use a loop with vectorization beyond it.
+                const int MaxUnrollSize = 7;
 
                 if (iterations <= MaxUnrollSize)
                 {