strconv: squeezed a bit more out of int/uint formatting

- less code
- slightly better performance (0-4%)

R=r, rsc
CC=golang-dev
https://golang.org/cl/5448120

src/pkg/strconv/itoa.go

@@ -12,7 +12,7 @@ func FormatUint(i uint64, base int) string {
 
 // FormatInt returns the string representation of i in the given base.
 func FormatInt(i int64, base int) string {
-	_, s := formatBits(nil, uint64(i), base, true, false)
+	_, s := formatBits(nil, uint64(i), base, i < 0, false)
 	return s
 }
 
@@ -24,7 +24,7 @@ func Itoa(i int) string {
 // AppendInt appends the string form of the integer i,
 // as generated by FormatInt, to dst and returns the extended buffer.
 func AppendInt(dst []byte, i int64, base int) []byte {
-	dst, _ = formatBits(dst, uint64(i), base, true, true)
+	dst, _ = formatBits(dst, uint64(i), base, i < 0, true)
 	return dst
 }
 
@@ -46,31 +46,21 @@ var shifts = [len(digits) + 1]uint{
 }
 
 // formatBits computes the string representation of u in the given base.
-// If signed is set, u is treated as int64 value. If append_ is set, the
-// string is appended to dst and the resulting byte slice is returned as
-// the first result value; otherwise the string is simply returned as the
-// second result value.
+// If negative is set, u is treated as negative int64 value. If append_
+// is set, the string is appended to dst and the resulting byte slice is
+// returned as the first result value; otherwise the string is returned
+// as the second result value.
 //
-func formatBits(dst []byte, u uint64, base int, signed, append_ bool) (d []byte, s string) {
+func formatBits(dst []byte, u uint64, base int, negative, append_ bool) (d []byte, s string) {
 	if base < 2 || base > len(digits) {
 		panic("invalid base")
 	}
 	// 2 <= base && base <= len(digits)
 
-	if u == 0 {
-		if append_ {
-			d = append(dst, '0')
-			return
-		}
-		s = "0"
-		return
-	}
-
 	var a [64 + 1]byte // +1 for sign of 64bit value in base 2
 	i := len(a)
 
-	x := int64(u)
-	if x < 0 && signed {
+	if negative {
 		u = -u
 	}
 
@@ -78,7 +68,7 @@ func formatBits(dst []byte, u uint64, base int, signed, append_ bool) (d []byte,
 	if base == 10 {
 		// common case: use constant 10 for / and % because
 		// the compiler can optimize it into a multiply+shift
-		for u != 0 {
+		for u >= 10 {
 			i--
 			a[i] = digits[u%10]
 			u /= 10
@@ -86,8 +76,9 @@ func formatBits(dst []byte, u uint64, base int, signed, append_ bool) (d []byte,
 
 	} else if s := shifts[base]; s > 0 {
 		// base is power of 2: use shifts and masks instead of / and %
-		m := uintptr(1)<<s - 1
-		for u != 0 {
+		b := uint64(base)
+		m := uintptr(b) - 1 // == 1<<s - 1
+		for u >= b {
 			i--
 			a[i] = digits[uintptr(u)&m]
 			u >>= s
@@ -96,15 +87,19 @@ func formatBits(dst []byte, u uint64, base int, signed, append_ bool) (d []byte,
 	} else {
 		// general case
 		b := uint64(base)
-		for u != 0 {
+		for u >= b {
 			i--
 			a[i] = digits[u%b]
 			u /= b
 		}
 	}
 
+	// u < base
+	i--
+	a[i] = digits[uintptr(u)]
+
 	// add sign, if any
-	if x < 0 && signed {
+	if negative {
 		i--
 		a[i] = '-'
 	}