math/big: add benchmarks for big.Float String

In addition to the DecimalConversion benchmark, that exercises the
String method of the internal decimal type on a range of small shifts,
add a few benchmarks for the big.Float String method. They can be used
to obtain more realistic data on the real-world performance of
big.Float printing.

Change-Id: I7ada324e7603cb1ce7492ccaf3382db0096223ba
Reviewed-on: https://go-review.googlesource.com/31275
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/big/decimal_test.go

@@ -4,7 +4,10 @@
 
 package big
 
-import "testing"
+import (
+	"fmt"
+	"testing"
+)
 
 func TestDecimalString(t *testing.T) {
 	for _, test := range []struct {
@@ -116,3 +119,16 @@ func BenchmarkDecimalConversion(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkFloatString(b *testing.B) {
+	x := new(Float)
+	for _, prec := range []uint{1e2, 1e3, 1e4, 1e5} {
+		x.SetPrec(prec).SetRat(NewRat(1, 3))
+		b.Run(fmt.Sprintf("%v", prec), func(b *testing.B) {
+			b.ReportAllocs()
+			for i := 0; i < b.N; i++ {
+				sink = x.String()
+			}
+		})
+	}
+}