testing: add AllocsPerRun

This CL also replaces similar loops in other stdlib
package tests with calls to AllocsPerRun.

Fixes #4461.

R=minux.ma, rsc
CC=golang-dev
https://golang.org/cl/7002055

src/pkg/testing/allocs.go

@@ -0,0 +1,41 @@
+// Copyright 2013 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 testing
+
+import (
+	"runtime"
+)
+
+// AllocsPerRun returns the average number of allocations during calls to f.
+//
+// To compute the number of allocations, the function will first be run once as
+// a warm-up.  The average number of allocations over the specified number of
+// runs will then be measured and returned.
+//
+// AllocsPerRun sets GOMAXPROCS to 1 during its measurement and will restore
+// it before returning.
+func AllocsPerRun(runs int, f func()) (avg float64) {
+	defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(1))
+
+	// Warm up the function
+	f()
+
+	// Measure the starting statistics
+	var memstats runtime.MemStats
+	runtime.ReadMemStats(&memstats)
+	mallocs := 0 - memstats.Mallocs
+
+	// Run the function the specified number of times
+	for i := 0; i < runs; i++ {
+		f()
+	}
+
+	// Read the final statistics
+	runtime.ReadMemStats(&memstats)
+	mallocs += memstats.Mallocs
+
+	// Average the mallocs over the runs (not counting the warm-up)
+	return float64(mallocs) / float64(runs)
+}