What version of Go are you using (go version)?

$ go version
go version devel +cfe3cd903f Tue Jan 14 19:54:20 2020 +0000 linux/amd64

Does this issue reproduce with the latest release?

yes, it is reproducible in go1.13 and go tip.

What did you do?

Run the following benchmark:

func BenchmarkAtomicAddUint64(b *testing.B) {
        b.ReportAllocs()
        for i := 0; i < b.N; i++ {
                n := uint64(123)
                sink += atomic.AddUint64(&n, 1)
        }
}

var sink uint64

What did you expect to see?

Zero memory allocations in the benchmark loop.

What did you see instead?

A memory allocation. The following is the output from go tool pprof --alloc_objects:

  25919883   25919883 (flat, cum)   100% of Total
         .          .     10:)
         .          .     11:
         .          .     12:func BenchmarkAtomicAddUint64(b *testing.B) {
         .          .     13:	b.ReportAllocs()
         .          .     14:	for i := 0; i < b.N; i++ {
  25919883   25919883     15:		n := uint64(123)
         .          .     16:		sink += atomic.AddUint64(&n, 1)
         .          .     17:	}
         .          .     18:}
         .          .     19:
         .          .     20:var sink uint64

It looks like Go compiler thinks that args passed to sync/atomic functions may escape.

Possible solution is to add //go:noescape to these functions.