strconv: make malloc tests more reliable

Fixes #3495.

I adapted fmt.TestCountMallocs to fix the
existing tests. As the resulting tests did not
appear to belong to either itoa or ftoa I moved
them into their own file.

R=bradfitz, fullung
CC=golang-dev
https://golang.org/cl/5985072

src/pkg/strconv/ftoa_test.go

@@ -173,23 +173,6 @@ func TestFtoaRandom(t *testing.T) {
 	}
 }
 
-func TestAppendFloatDoesntAllocate(t *testing.T) {
-	n := numAllocations(func() {
-		var buf [64]byte
-		AppendFloat(buf[:0], 1.23, 'g', 5, 64)
-	})
-	want := 1 // TODO(bradfitz): this might be 0, once escape analysis is better
-	if n != want {
-		t.Errorf("with local buffer, did %d allocations, want %d", n, want)
-	}
-	n = numAllocations(func() {
-		AppendFloat(globalBuf[:0], 1.23, 'g', 5, 64)
-	})
-	if n != 0 {
-		t.Errorf("with reused buffer, did %d allocations, want 0", n)
-	}
-}
-
 func BenchmarkFormatFloatDecimal(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		FormatFloat(33909, 'g', -1, 64)

src/pkg/strconv/itoa_test.go

@@ -5,7 +5,6 @@
 package strconv_test
 
 import (
-	"runtime"
 	. "strconv"
 	"testing"
 )
@@ -126,35 +125,6 @@ func TestUitoa(t *testing.T) {
 	}
 }
 
-func numAllocations(f func()) int {
-	runtime.GC()
-	memstats := new(runtime.MemStats)
-	runtime.ReadMemStats(memstats)
-	n0 := memstats.Mallocs
-	f()
-	runtime.ReadMemStats(memstats)
-	return int(memstats.Mallocs - n0)
-}
-
-var globalBuf [64]byte
-
-func TestAppendUintDoesntAllocate(t *testing.T) {
-	n := numAllocations(func() {
-		var buf [64]byte
-		AppendInt(buf[:0], 123, 10)
-	})
-	want := 1 // TODO(bradfitz): this might be 0, once escape analysis is better
-	if n != want {
-		t.Errorf("with local buffer, did %d allocations, want %d", n, want)
-	}
-	n = numAllocations(func() {
-		AppendInt(globalBuf[:0], 123, 10)
-	})
-	if n != 0 {
-		t.Errorf("with reused buffer, did %d allocations, want 0", n)
-	}
-}
-
 func BenchmarkFormatInt(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		for _, test := range itob64tests {

src/pkg/strconv/strconv_test.go

@@ -0,0 +1,51 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strconv_test
+
+import (
+	"runtime"
+	. "strconv"
+	"testing"
+)
+
+var (
+	globalBuf [64]byte
+
+	mallocTest = []struct {
+		count int
+		desc  string
+		fn    func()
+	}{
+		// TODO(bradfitz): this might be 0, once escape analysis is better
+		{1, `AppendInt(localBuf[:0], 123, 10)`, func() {
+			var localBuf [64]byte
+			AppendInt(localBuf[:0], 123, 10)
+		}},
+		{0, `AppendInt(globalBuf[:0], 123, 10)`, func() { AppendInt(globalBuf[:0], 123, 10) }},
+		// TODO(bradfitz): this might be 0, once escape analysis is better
+		{1, `AppendFloat(localBuf[:0], 1.23, 'g', 5, 64)`, func() {
+			var localBuf [64]byte
+			AppendFloat(localBuf[:0], 1.23, 'g', 5, 64)
+		}},
+		{0, `AppendFloat(globalBuf[:0], 1.23, 'g', 5, 64)`, func() { AppendFloat(globalBuf[:0], 1.23, 'g', 5, 64) }},
+	}
+)
+
+func TestCountMallocs(t *testing.T) {
+	for _, mt := range mallocTest {
+		const N = 100
+		memstats := new(runtime.MemStats)
+		runtime.ReadMemStats(memstats)
+		mallocs := 0 - memstats.Mallocs
+		for i := 0; i < N; i++ {
+			mt.fn()
+		}
+		runtime.ReadMemStats(memstats)
+		mallocs += memstats.Mallocs
+		if mallocs/N > uint64(mt.count) {
+			t.Errorf("%s: expected %d mallocs, got %d", mt.desc, mt.count, mallocs/N)
+		}
+	}
+}