expvar: swap Float sync. from mutex to atomic.

Float type from a mutex to atomic bit array in a manner akin to
Google Guava's AtomicDouble[0], including adding a benchmark for the
type (benchcmp included below) along with some expvar_test.go cruft
being fixed.

benchmark             old ns/op     new ns/op     delta
BenchmarkFloatSet     115           9.37          -91.85%
BenchmarkFloatAdd     114           17.1          -85.00%

benchmark             old allocs     new allocs     delta
BenchmarkFloatSet     0              0              +0.00%
BenchmarkFloatAdd     0              0              +0.00%

benchmark             old bytes     new bytes     delta
BenchmarkFloatSet     0             0             +0.00%
BenchmarkFloatAdd     0             0             +0.00%

[0] - http://goo.gl/m4dtlI

Change-Id: I4ce6a913734ec692e3ed243f6e6f7c11da4c6036
Reviewed-on: https://go-review.googlesource.com/3687
Reviewed-by: Rob Pike <r@golang.org>

src/expvar/expvar.go

@@ -26,6 +26,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"math"
 	"net/http"
 	"os"
 	"runtime"
@@ -59,28 +60,30 @@ func (v *Int) Set(value int64) {
 
 // Float is a 64-bit float variable that satisfies the Var interface.
 type Float struct {
-	mu sync.RWMutex
-	f  float64
+	f uint64
 }
 
 func (v *Float) String() string {
-	v.mu.RLock()
-	defer v.mu.RUnlock()
-	return strconv.FormatFloat(v.f, 'g', -1, 64)
+	return strconv.FormatFloat(
+		math.Float64frombits(atomic.LoadUint64(&v.f)), 'g', -1, 64)
 }
 
 // Add adds delta to v.
 func (v *Float) Add(delta float64) {
-	v.mu.Lock()
-	defer v.mu.Unlock()
-	v.f += delta
+	for {
+		cur := atomic.LoadUint64(&v.f)
+		curVal := math.Float64frombits(cur)
+		nxtVal := curVal + delta
+		nxt := math.Float64bits(nxtVal)
+		if atomic.CompareAndSwapUint64(&v.f, cur, nxt) {
+			return
+		}
+	}
 }
 
 // Set sets v to value.
 func (v *Float) Set(value float64) {
-	v.mu.Lock()
-	defer v.mu.Unlock()
-	v.f = value
+	atomic.StoreUint64(&v.f, math.Float64bits(value))
 }
 
 // Map is a string-to-Var map variable that satisfies the Var interface.

src/expvar/expvar_test.go

@@ -7,11 +7,13 @@ package expvar
 import (
 	"bytes"
 	"encoding/json"
+	"math"
 	"net"
 	"net/http/httptest"
 	"runtime"
 	"strconv"
 	"sync"
+	"sync/atomic"
 	"testing"
 )
 
@@ -70,6 +72,10 @@ func BenchmarkIntSet(b *testing.B) {
 	})
 }
 
+func (v *Float) val() float64 {
+	return math.Float64frombits(atomic.LoadUint64(&v.f))
+}
+
 func TestFloat(t *testing.T) {
 	RemoveAll()
 	reqs := NewFloat("requests-float")
@@ -82,8 +88,8 @@ func TestFloat(t *testing.T) {
 
 	reqs.Add(1.5)
 	reqs.Add(1.25)
-	if reqs.f != 2.75 {
-		t.Errorf("reqs.f = %v, want 2.75", reqs.f)
+	if v := reqs.val(); v != 2.75 {
+		t.Errorf("reqs.val() = %v, want 2.75", v)
 	}
 
 	if s := reqs.String(); s != "2.75" {
@@ -91,8 +97,8 @@ func TestFloat(t *testing.T) {
 	}
 
 	reqs.Add(-2)
-	if reqs.f != 0.75 {
-		t.Errorf("reqs.f = %v, want 0.75", reqs.f)
+	if v := reqs.val(); v != 0.75 {
+		t.Errorf("reqs.val() = %v, want 0.75", v)
 	}
 }
 
@@ -157,8 +163,8 @@ func TestMapCounter(t *testing.T) {
 	if x := colors.m["blue"].(*Int).i; x != 4 {
 		t.Errorf("colors.m[\"blue\"] = %v, want 4", x)
 	}
-	if x := colors.m[`green "midori"`].(*Float).f; x != 4.125 {
-		t.Errorf("colors.m[`green \"midori\"] = %v, want 3.14", x)
+	if x := colors.m[`green "midori"`].(*Float).val(); x != 4.125 {
+		t.Errorf("colors.m[`green \"midori\"] = %v, want 4.125", x)
 	}
 
 	// colors.String() should be '{"red":3, "blue":4}',