strconv: faster FormatFloat(x, *, -1, 64) using Grisu3 algorithm.
The implementation is similar to the one from the double-conversion
library used in the Chrome V8 engine.
old ns/op new ns/op speedup
BenchmarkAppendFloatDecimal 591 480 1.2x
BenchmarkAppendFloat 2956 486 6.1x
BenchmarkAppendFloatExp 10622 503 21.1x
BenchmarkAppendFloatNegExp 40343 483 83.5x
BenchmarkAppendFloatBig 2798 664 4.2x
See F. Loitsch, ``Printing Floating-Point Numbers Quickly and
Accurately with Integers'', Proceedings of the ACM, 2010.
R=rsc
CC=golang-dev, remy
https://golang.org/cl/5502079
diff --git a/src/pkg/strconv/ftoa.go b/src/pkg/strconv/ftoa.go
index ab8dd2b..8eefbee 100644
--- a/src/pkg/strconv/ftoa.go
+++ b/src/pkg/strconv/ftoa.go
@@ -98,29 +98,43 @@
return fmtB(dst, neg, mant, exp, flt)
}
- // Create exact decimal representation.
- // The shift is exp - flt.mantbits because mant is a 1-bit integer
- // followed by a flt.mantbits fraction, and we are treating it as
- // a 1+flt.mantbits-bit integer.
- d := new(decimal)
- d.Assign(mant)
- d.Shift(exp - int(flt.mantbits))
-
- // Round appropriately.
// Negative precision means "only as much as needed to be exact."
- shortest := false
- if prec < 0 {
- shortest = true
- roundShortest(d, mant, exp, flt)
- switch fmt {
- case 'e', 'E':
- prec = d.nd - 1
- case 'f':
- prec = max(d.nd-d.dp, 0)
- case 'g', 'G':
- prec = d.nd
+ shortest := prec < 0
+
+ d := new(decimal)
+ if shortest {
+ ok := false
+ if optimize && bitSize == 64 {
+ // Try Grisu3 algorithm.
+ f := new(extFloat)
+ lower, upper := f.AssignComputeBounds(val)
+ ok = f.ShortestDecimal(d, &lower, &upper)
+ }
+ if !ok {
+ // Create exact decimal representation.
+ // The shift is exp - flt.mantbits because mant is a 1-bit integer
+ // followed by a flt.mantbits fraction, and we are treating it as
+ // a 1+flt.mantbits-bit integer.
+ d.Assign(mant)
+ d.Shift(exp - int(flt.mantbits))
+ roundShortest(d, mant, exp, flt)
+ }
+ // Precision for shortest representation mode.
+ if prec < 0 {
+ switch fmt {
+ case 'e', 'E':
+ prec = d.nd - 1
+ case 'f':
+ prec = max(d.nd-d.dp, 0)
+ case 'g', 'G':
+ prec = d.nd
+ }
}
} else {
+ // Create exact decimal representation.
+ d.Assign(mant)
+ d.Shift(exp - int(flt.mantbits))
+ // Round appropriately.
switch fmt {
case 'e', 'E':
d.Round(prec + 1)
