fmt: speed up 10-20%

The check for Stringer etc. can only fire if the test is not a builtin, so avoid
the expensive check if we know there's no chance.
Also put in a fast path for pad, which saves a more modest amount.

benchmark                      old ns/op    new ns/op    delta
BenchmarkSprintfEmpty                148          152   +2.70%
BenchmarkSprintfString               585          497  -15.04%
BenchmarkSprintfInt                  441          396  -10.20%
BenchmarkSprintfIntInt               718          603  -16.02%
BenchmarkSprintfPrefixedInt          676          621   -8.14%
BenchmarkSprintfFloat               1003          953   -4.99%
BenchmarkManyArgs                   2945         2312  -21.49%
BenchmarkScanInts                1704152      1734441   +1.78%
BenchmarkScanRecursiveInt        1837397      1828920   -0.46%

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6245068

src/pkg/fmt/fmt_test.go

@@ -527,6 +527,14 @@ func BenchmarkSprintfFloat(b *testing.B) {
 	}
 }
 
+func BenchmarkManyArgs(b *testing.B) {
+	var buf bytes.Buffer
+	for i := 0; i < b.N; i++ {
+		buf.Reset()
+		Fprintf(&buf, "%2d/%2d/%2d %d:%d:%d %s %s\n", 3, 4, 5, 11, 12, 13, "hello", "world")
+	}
+}
+
 var mallocBuf bytes.Buffer
 
 var mallocTest = []struct {

src/pkg/fmt/format.go

@@ -110,11 +110,11 @@ func (f *fmt) writePadding(n int, padding []byte) {
 // Append b to f.buf, padded on left (w > 0) or right (w < 0 or f.minus)
 // clear flags afterwards.
 func (f *fmt) pad(b []byte) {
-	var padding []byte
+	if !f.widPresent || f.wid == 0 {
-	var left, right int
+		f.buf.Write(b)
-	if f.widPresent && f.wid != 0 {
+		return
-		padding, left, right = f.computePadding(len(b))
 	}
+	padding, left, right := f.computePadding(len(b))
 	if left > 0 {
 		f.writePadding(left, padding)
 	}
@@ -127,11 +127,11 @@ func (f *fmt) pad(b []byte) {
 // append s to buf, padded on left (w > 0) or right (w < 0 or f.minus).
 // clear flags afterwards.
 func (f *fmt) padString(s string) {
-	var padding []byte
+	if !f.widPresent || f.wid == 0 {
-	var left, right int
+		f.buf.WriteString(s)
-	if f.widPresent && f.wid != 0 {
+		return
-		padding, left, right = f.computePadding(utf8.RuneCountInString(s))
 	}
+	padding, left, right := f.computePadding(utf8.RuneCountInString(s))
 	if left > 0 {
 		f.writePadding(left, padding)
 	}

src/pkg/fmt/print.go

@@ -734,10 +734,6 @@ func (p *pp) printField(field interface{}, verb rune, plus, goSyntax bool, depth
 		return false
 	}
 
-	if wasString, handled := p.handleMethods(verb, plus, goSyntax, depth); handled {
-		return wasString
-	}
-
 	// Some types can be done without reflection.
 	switch f := field.(type) {
 	case bool:
@@ -779,6 +775,10 @@ func (p *pp) printField(field interface{}, verb rune, plus, goSyntax bool, depth
 		p.fmtBytes(f, verb, goSyntax, depth)
 		wasString = verb == 's'
 	default:
+		// If the type is not simple, it might have methods.
+		if wasString, handled := p.handleMethods(verb, plus, goSyntax, depth); handled {
+			return wasString
+		}
 		// Need to use reflection
 		return p.printReflectValue(reflect.ValueOf(field), verb, plus, goSyntax, depth)
 	}