testing.B.Loop() reports incorrect memory allocation if the code use iterators

[Pierre Durand]

Hello

I noticed a weird behavior when I'm benchmarking with testing.B.Loop() code that uses iterators .

The benchmark shows allocations where I start to iterate the iterator, and where I declare variables (before the loop) that are used inside the iterator loop.

I know that my code is not doing any allocation, so it's strange.

If I change my benchmark to use the old "range b.N", then it doesn't show this strange behavior.

If I check allocations with "testing.AllocsPerRun", I don't see any allocation.

Is that a know issue ? Should I open a bug ?

What should I do ? Use the old benchmarking method ?

Regards

[Pierre Durand]

I can reproduce the issue with this code https://github.com/pierrre/go-libs/blob/31bec3f12a86382924cc0b486c2008db062a14bd/runtimeutil/runtimeutil_test.go#L101-L118
Sorry I can't write a smaller code snippet, because I think it's related to escape analysis, and I don't fully understand it.

As you can see, I'm benchmarking the same function WriteFrames() with the old and the new benchmarking methods.

When I run the benchmark, here is the result

go test -v -run=^$ -bench="^BenchmarkWriteFrames" -benchmem ./runtimeutil
goos: linux
goarch: amd64
pkg: github.com/pierrre/go-libs/runtimeutil
cpu: Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz
BenchmarkWriteFrames
BenchmarkWriteFrames-12           366825              3091 ns/op               0 B/op          0 allocs/op
BenchmarkWriteFramesNew
BenchmarkWriteFramesNew-12        328620              3445 ns/op              80 B/op          4 allocs/op
PASS
ok      github.com/pierrre/go-libs/runtimeutil  2.303s

Le ven. 1 août 2025 à 22:04, Junyang Shao <shaoj...@google.com> a écrit :
>
> Hello Pierre,
>
> Thank you for bringing up this issue.
>
> May you share the code snippet that triggers this behavior? Thanks.

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>
>
>
> --
> Thanks,
> Junyang



--
Pierre Durand

[Pierre Durand]

I think it's related to the fact that b.Loop() disables some optimizations, but I'm not sure.

[Axel Wagner]

`b.Loop` disables inlining, specifically.

I assume in your code, `WriteFrames` gets inlined when you use `for range b.N`. After inlinining `WriteFrames`, the iterator function itself also gets inlined, as it directly `range`s over it. After that inlining step, the iterator function doesn't need to be allocated.

With `for b.Loop()`, `WriteFrames` itself does not get inlined, meaning the function needs to be allocated to get passed as an argument.

To view this discussion visit https://groups.google.com/d/msgid/golang-nuts/f7519a16-a4cd-4d98-8559-bc1cfe5c8fdfn%40googlegroups.com.

[Junyang Shao]

Hello Pierre,

Thank you for bringing up this issue.

May you share the code snippet that triggers this behavior? Thanks.

On Fri, Aug 1, 2025 at 12:50 PM Pierre Durand <pierre...@gmail.com> wrote:

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--

Thanks,

Junyang

[peterGo]

On Friday, August 1, 2025 at 4:39:17 PM UTC-4 Pierre Durand wrote:

I think it's related to the fact that b.Loop() disables some optimizations, but I'm not sure.

Pierre,

The difference between b.N and b.Loop  is a known issue. It is by design,

https://groups.google.com/g/golang-nuts/c/Z017c0f59vY/m/AY1-wT0OBAAJ

Peter

[Pierre Durand]

OK, thank you everyone :)

[Pierre Durand]

For future reference: https://github.com/golang/go/issues/73137