regexp/syntax: test for lowercase letters first in IsWordChar

Lowercase letters occur more frequently than uppercase letters
in English text.  In IsWordChar, evaluate the most common case
(lowercase letters) first to minimize the expected value of its
execution time. Code clarity does not suffer by rearranging the
order of the checks.

Add a benchmark on a sentence demonstrating the performance
improvement.

name           old time/op  new time/op  delta
IsWordChar-10   122ns ± 0%   114ns ± 1%  -6.68%  (p=0.000 n=8+10)

Change-Id: Ieee8126a4bd8ee8703905b4f75724623029f6fa2
Reviewed-on: https://go-review.googlesource.com/c/go/+/404100
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: thepudds <thepudds1460@gmail.com>

src/regexp/syntax/prog.go

@@ -106,7 +106,9 @@ func EmptyOpContext(r1, r2 rune) EmptyOp {
 // during the evaluation of the \b and \B zero-width assertions.
 // These assertions are ASCII-only: the word characters are [A-Za-z0-9_].
 func IsWordChar(r rune) bool {
-	return 'A' <= r && r <= 'Z' || 'a' <= r && r <= 'z' || '0' <= r && r <= '9' || r == '_'
+	// Test for lowercase letters first, as these occur more
+	// frequently than uppercase letters in common cases.
+	return 'a' <= r && r <= 'z' || 'A' <= r && r <= 'Z' || '0' <= r && r <= '9' || r == '_'
 }
 
 // An Inst is a single instruction in a regular expression program.

src/regexp/syntax/prog_test.go

@@ -127,3 +127,18 @@ func BenchmarkEmptyOpContext(b *testing.B) {
 		EmptyOpContext(r1, -1)
 	}
 }
+
+var sink any
+
+func BenchmarkIsWordChar(b *testing.B) {
+	const chars = "Don't communicate by sharing memory, share memory by communicating."
+	for i := 0; i < b.N; i++ {
+		for _, r := range chars {
+			sink = IsWordChar(r)
+		}
+	}
+	if sink == nil {
+		b.Fatal("Benchmark did not run")
+	}
+	sink = nil
+}