strconv: simplify code for binary exponent float format

Use optimized formatBits function to format mantissa and exponent.
Add benchmark for binary exponent float format.

on darwin/386

benchmark                         old ns/op     new ns/op     delta
BenchmarkAppendFloatBinaryExp     520           122           -76.54%

on darwin/amd64

benchmark                         old ns/op     new ns/op     delta
BenchmarkAppendFloatBinaryExp     76.9          84.3          +9.62%

Change-Id: If543552f1960e1655bed3a4130914e5eaa3aac69
Reviewed-on: https://go-review.googlesource.com/5600
Reviewed-by: Robert Griesemer <gri@golang.org>

diff --git a/src/strconv/ftoa.go b/src/strconv/ftoa.go
index f885d96e9cd73f2f8dfcd4ec92a9831f313c42c0..d59c78e4936518ca0b37f5ef5c607aa0be37a670 100644 (file)
--- a/src/strconv/ftoa.go
+++ b/src/strconv/ftoa.go
@@ -47,7 +47,7 @@ func FormatFloat(f float64, fmt byte, prec, bitSize int) string {
 
 // AppendFloat appends the string form of the floating-point number f,
 // as generated by FormatFloat, to dst and returns the extended buffer.
-func AppendFloat(dst []byte, f float64, fmt byte, prec int, bitSize int) []byte {
+func AppendFloat(dst []byte, f float64, fmt byte, prec, bitSize int) []byte {
        return genericFtoa(dst, f, fmt, prec, bitSize)
 }
 
@@ -418,39 +418,27 @@ func fmtF(dst []byte, neg bool, d decimalSlice, prec int) []byte {
        return dst
 }
 
-// %b: -ddddddddp+ddd
+// %b: -ddddddddp±ddd
 func fmtB(dst []byte, neg bool, mant uint64, exp int, flt *floatInfo) []byte {
-       var buf [50]byte
-       w := len(buf)
-       exp -= int(flt.mantbits)
-       esign := byte('+')
-       if exp < 0 {
-               esign = '-'
-               exp = -exp
-       }
-       n := 0
-       for exp > 0 || n < 1 {
-               n++
-               w--
-               buf[w] = byte(exp%10 + '0')
-               exp /= 10
-       }
-       w--
-       buf[w] = esign
-       w--
-       buf[w] = 'p'
-       n = 0
-       for mant > 0 || n < 1 {
-               n++
-               w--
-               buf[w] = byte(mant%10 + '0')
-               mant /= 10
-       }
+       // sign
        if neg {
-               w--
-               buf[w] = '-'
+               dst = append(dst, '-')
        }
-       return append(dst, buf[w:]...)
+
+       // mantissa
+       dst, _ = formatBits(dst, mant, 10, false, true)
+
+       // p
+       dst = append(dst, 'p')
+
+       // ±exponent
+       exp -= int(flt.mantbits)
+       if exp >= 0 {
+               dst = append(dst, '+')
+       }
+       dst, _ = formatBits(dst, uint64(exp), 10, exp < 0, true)
+
+       return dst
 }
 
 func min(a, b int) int {

diff --git a/src/strconv/ftoa_test.go b/src/strconv/ftoa_test.go
index 39b861547eae437b255c6f8c4c47abf41ed85504..1b4dcd945b8a35d34f4af3dec2e1b6da3383ff9a 100644 (file)
--- a/src/strconv/ftoa_test.go
+++ b/src/strconv/ftoa_test.go
@@ -227,6 +227,7 @@ func BenchmarkAppendFloatNegExp(b *testing.B)  { benchmarkAppendFloat(b, -5.11e-
 func BenchmarkAppendFloatBig(b *testing.B) {
        benchmarkAppendFloat(b, 123456789123456789123456789, 'g', -1, 64)
 }
+func BenchmarkAppendFloatBinaryExp(b *testing.B) { benchmarkAppendFloat(b, -1, 'b', -1, 64) }
 
 func BenchmarkAppendFloat32Integer(b *testing.B)       { benchmarkAppendFloat(b, 33909, 'g', -1, 32) }
 func BenchmarkAppendFloat32ExactFraction(b *testing.B) { benchmarkAppendFloat(b, 3.375, 'g', -1, 32) }