fmt: add ascii fast path for decoding verbs

name                    old time/op  new time/op  delta
SprintfSlowParsingPath   108ns ± 4%   103ns ± 4%  -4.53%  (p=0.000 n=18+18)

Change-Id: I174463f303d1857e8d5b8a6283c025b3546e7b39
Reviewed-on: https://go-review.googlesource.com/44450
Run-TryBot: Martin Möhrmann <moehrmann@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rob Pike <r@golang.org>

src/fmt/fmt_test.go

@@ -1201,6 +1201,14 @@ func BenchmarkSprintfTruncateString(b *testing.B) {
 	})
 }
 
+func BenchmarkSprintfSlowParsingPath(b *testing.B) {
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			Sprintf("%.v", nil)
+		}
+	})
+}
+
 func BenchmarkSprintfQuoteString(b *testing.B) {
 	b.RunParallel(func(pb *testing.PB) {
 		for pb.Next() {

src/fmt/print.go

@@ -1067,8 +1067,11 @@ formatLoop:
 			break
 		}
 
-		verb, w := utf8.DecodeRuneInString(format[i:])
-		i += w
+		verb, size := rune(format[i]), 1
+		if verb >= utf8.RuneSelf {
+			verb, size = utf8.DecodeRuneInString(format[i:])
+		}
+		i += size
 
 		switch {
 		case verb == '%': // Percent does not absorb operands and ignores f.wid and f.prec.