testing: Benchmark shows zero allocations

[rogpeppe]

testing.Benchmark does not seem to produce valid memory stats any more:

The following program:

package main

import (
	"fmt"
	"testing"
)

var global *int

func main() {
	r := testing.Benchmark(func(b *testing.B) {
		for i := 0; i < b.N; i++ {
			global = new(int)
		}
	})
	fmt.Printf("creating filter: %v\n", r.MemString())
}

always prints:

       0 B/op	       0 allocs/op

I'd expect it to print

       8 B/op	       1 allocs/op

like previous Go versions.

go version devel +eab99a8 Mon Jun 26 21:12:22 2017 +0000 linux/amd64

[rogpeppe]

It turns out that it prints memory statistics when b.ReportAllocs is called.
As this is a behaviour change, perhaps this should be documented in
the Go 1.9 release notes.

[ALTree]

This is already documented

MemAllocs and MemBytes may be zero if memory benchmarking is not requested using B.ReportAllocs or the -benchmem command line flag.

http://tip.golang.org/pkg/testing/#BenchmarkResult

but if it was changed in go1.9 then I agree it's worth mentioning in release notes, too.

[ALTree]

The change was introduced in https://go-review.googlesource.com/c/36791/

[gopherbot]

CL https://golang.org/cl/47331 mentions this issue.

[bradfitz]

@ALTree I would rather revert CL 36791 now that ReadMemStats is fast (see https://tip.golang.org/doc/go1.9#gc).

I'll send a CL.

[bradfitz]

/cc @josharian @aclements

[aclements]

I thought this was fixed a few days ago by https://golang.org/cl/46612?

[bradfitz]

Yeah, I was just about to point that out. Looks like that only reverted 1 of the 3 spots.

[gopherbot]

CL https://golang.org/cl/47350 mentions this issue.

[ALTree]

Ah, thanks for noticing. I didn't notice that that change was partly reverted.