math/big: don't use Float in init to help linker discard 162 KiB

Removes 162 KiB from binaries that don't use math/big.Float:

-rwxr-xr-x 1 bradfitz bradfitz 1916590 Apr 14 12:21 x.after
-rwxr-xr-x 1 bradfitz bradfitz 2082575 Apr 14 12:21 x.before

No change in deps (this package already used sync).

No change in benchmarks:

name                 old time/op    new time/op    delta
FloatSqrt/64-8         1.06µs ±10%    1.03µs ± 6%   ~     (p=0.133 n=10+9)
FloatSqrt/128-8        2.26µs ± 9%    2.28µs ± 9%   ~     (p=0.460 n=10+8)
FloatSqrt/256-8        2.29µs ± 5%    2.31µs ± 3%   ~     (p=0.214 n=9+9)
FloatSqrt/1000-8       5.82µs ± 3%    5.87µs ± 7%   ~     (p=0.666 n=9+9)
FloatSqrt/10000-8      56.4µs ± 5%    57.0µs ± 6%   ~     (p=0.436 n=10+10)
FloatSqrt/100000-8     1.34ms ± 8%    1.31ms ± 3%   ~     (p=0.447 n=10+9)
FloatSqrt/1000000-8     106ms ± 5%     107ms ± 7%   ~     (p=0.315 n=10+10)

name                 old alloc/op   new alloc/op   delta
FloatSqrt/64-8           280B ± 0%      280B ± 0%   ~     (all equal)
FloatSqrt/128-8          504B ± 0%      504B ± 0%   ~     (all equal)
FloatSqrt/256-8          344B ± 0%      344B ± 0%   ~     (all equal)
FloatSqrt/1000-8       1.30kB ± 0%    1.30kB ± 0%   ~     (all equal)
FloatSqrt/10000-8      13.5kB ± 0%    13.5kB ± 0%   ~     (p=0.403 n=10+10)
FloatSqrt/100000-8      123kB ± 0%     123kB ± 0%   ~     (p=0.393 n=10+10)
FloatSqrt/1000000-8    1.84MB ± 7%    1.84MB ± 5%   ~     (p=0.739 n=10+10)

name                 old allocs/op  new allocs/op  delta
FloatSqrt/64-8           8.00 ± 0%      8.00 ± 0%   ~     (all equal)
FloatSqrt/128-8          11.0 ± 0%      11.0 ± 0%   ~     (all equal)
FloatSqrt/256-8          5.00 ± 0%      5.00 ± 0%   ~     (all equal)
FloatSqrt/1000-8         6.00 ± 0%      6.00 ± 0%   ~     (all equal)
FloatSqrt/10000-8        6.00 ± 0%      6.00 ± 0%   ~     (all equal)
FloatSqrt/100000-8       6.00 ± 0%      6.00 ± 0%   ~     (all equal)
FloatSqrt/1000000-8      10.9 ±10%      10.8 ±17%   ~     (p=0.974 n=10+10)

Change-Id: I3337f1f531bf7b4fae192b9d90cd24ff2be14fea
Reviewed-on: https://go-review.googlesource.com/c/go/+/228108
Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

2 files changed, 17 insertions, 5 deletions

diff --git a/src/math/big/floatconv.go b/src/math/big/floatconv.go
index 95e32d3319..57b7df3936 100644
--- a/src/math/big/floatconv.go
+++ b/src/math/big/floatconv.go
@@ -290,7 +290,7 @@ func ParseFloat(s string, base int, prec uint, mode RoundingMode) (f *Float, b i
 	return new(Float).SetPrec(prec).SetMode(mode).Parse(s, base)
 }
 
-var _ fmt.Scanner = &floatZero // *Float must implement fmt.Scanner
+var _ fmt.Scanner = (*Float)(nil) // *Float must implement fmt.Scanner
 
 // Scan is a support routine for fmt.Scanner; it sets z to the value of
 // the scanned number. It accepts formats whose verbs are supported by
diff --git a/src/math/big/sqrt.go b/src/math/big/sqrt.go
index ac2094f28e..e11504ad07 100644
--- a/src/math/big/sqrt.go
+++ b/src/math/big/sqrt.go
@@ -4,12 +4,23 @@
 
 package big
 
-import "math"
-
-var (
-	three = NewFloat(3.0)
+import (
+	"math"
+	"sync"
 )
 
+var threeOnce struct {
+	sync.Once
+	v *Float
+}
+
+func three() *Float {
+	threeOnce.Do(func() {
+		threeOnce.v = NewFloat(3.0)
+	})
+	return threeOnce.v
+}
+
 // Sqrt sets z to the rounded square root of x, and returns it.
 //
 // If z's precision is 0, it is changed to x's precision before the
@@ -129,6 +140,7 @@ func (z *Float) sqrtInverse(x *Float) {
 	//   t2 = t - g(t) = ½t(3 - xt²)
 	u := newFloat(z.prec)
 	v := newFloat(z.prec)
+	three := three()
 	ng := func(t *Float) *Float {
 		u.prec = t.prec
 		v.prec = t.prec