fmt: fix Malloc test

We need to avoid allocating an extra word for the interface value
passing the floating-point value as an interface{}. It's easy.

Fixes #2722.

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/5553044

src/pkg/fmt/fmt_test.go

@@ -509,7 +509,7 @@ func BenchmarkSprintfFloat(b *testing.B) {
 var mallocBuf bytes.Buffer
 
 var mallocTest = []struct {
-	max  int
+	count int
 	desc  string
 	fn    func()
 }{
@@ -518,7 +518,9 @@ var mallocTest = []struct {
 	{1, `Sprintf("%x")`, func() { Sprintf("%x", 7) }},
 	{2, `Sprintf("%s")`, func() { Sprintf("%s", "hello") }},
 	{1, `Sprintf("%x %x")`, func() { Sprintf("%x %x", 7, 112) }},
-	{2, `Sprintf("%g")`, func() { Sprintf("%g", 3.14159) }}, // TODO: should be 1. See Issue 2722.
+	// For %g we use a float32, not float64, to guarantee passing the argument
+	// does not need to allocate memory to store the result in a pointer-sized word.
+	{2, `Sprintf("%g")`, func() { Sprintf("%g", float32(3.14159)) }},
 	{0, `Fprintf(buf, "%x %x %x")`, func() { mallocBuf.Reset(); Fprintf(&mallocBuf, "%x %x %x", 7, 8, 9) }},
 	{1, `Fprintf(buf, "%s")`, func() { mallocBuf.Reset(); Fprintf(&mallocBuf, "%s", "hello") }},
 }
@@ -535,8 +537,8 @@ func TestCountMallocs(t *testing.T) {
 		}
 		runtime.UpdateMemStats()
 		mallocs += runtime.MemStats.Mallocs
-		if mallocs/N > uint64(mt.max) {
-			t.Errorf("%s: expected at most %d mallocs, got %d", mt.desc, mt.max, mallocs/N)
+		if mallocs/N > uint64(mt.count) {
+			t.Errorf("%s: expected %d mallocs, got %d", mt.desc, mt.count, mallocs/N)
 		}
 	}
 }