regexp/syntax: optimize EmptyOpContext

Minor. Saw this in a profile at few percent of CPU and was
curious what it was. Improves overall regexp benchmarks
anywhere from 0 to 3%, but they're a pain to run. You need to
run them in isolation for long runs to get stable numbers.

benchmark                  old ns/op    new ns/op    delta
BenchmarkEmptyOpContext          537          473  -11.92%

R=golang-dev, crawshaw
CC=golang-dev
https://golang.org/cl/13407043

src/pkg/regexp/syntax/prog.go

@@ -56,23 +56,26 @@ const (
 // Passing r2 == -1 indicates that the position is
 // at the end of the text.
 func EmptyOpContext(r1, r2 rune) EmptyOp {
-	var op EmptyOp
-	if r1 < 0 {
-		op |= EmptyBeginText | EmptyBeginLine
-	}
-	if r1 == '\n' {
+	var op EmptyOp = EmptyNoWordBoundary
+	var boundary byte
+	switch {
+	case IsWordChar(r1):
+		boundary = 1
+	case r1 == '\n':
 		op |= EmptyBeginLine
+	case r1 < 0:
+		op |= EmptyBeginText | EmptyBeginLine
 	}
-	if r2 < 0 {
-		op |= EmptyEndText | EmptyEndLine
-	}
-	if r2 == '\n' {
+	switch {
+	case IsWordChar(r2):
+		boundary ^= 1
+	case r2 == '\n':
 		op |= EmptyEndLine
+	case r2 < 0:
+		op |= EmptyEndText | EmptyEndLine
 	}
-	if IsWordChar(r1) != IsWordChar(r2) {
-		op |= EmptyWordBoundary
-	} else {
-		op |= EmptyNoWordBoundary
+	if boundary != 0 { // IsWordChar(r1) != IsWordChar(r2)
+		op ^= (EmptyWordBoundary | EmptyNoWordBoundary)
 	}
 	return op
 }

src/pkg/regexp/syntax/prog_test.go

@@ -103,3 +103,14 @@ func TestCompile(t *testing.T) {
 		}
 	}
 }
+
+func BenchmarkEmptyOpContext(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		var r1 rune = -1
+		for _, r2 := range "foo, bar, baz\nsome input text.\n" {
+			EmptyOpContext(r1, r2)
+			r1 = r2
+		}
+		EmptyOpContext(r1, -1)
+	}
+}