How to use atomic.Int64?

[Benz]

Since go `1.19`, the [`atomic.Int64`](https://github.com/golang/go/blob/go1.19/src/sync/atomic/type.go#L82) was added

```go
type Int64 struct {
    _ noCopy
    _ align64
    v int64
}
```

There is one additional [`align64`](https://github.com/golang/go/blob/go1.19/src/sync/atomic/type.go#L191) in `atomic.Int64`

```go
// align64 may be added to structs that must be 64-bit aligned.
// This struct is recognized by a special case in the compiler
// and will not work if copied to any other package.
type align64 struct{}
```

With the test codes under go 1.19

```go
    var i64 int64
    var a64 atomic.Int64
    fmt.Println(unsafe.Alignof(i64))
    fmt.Println(unsafe.Alignof(a64))
```

The result is

```
8
8
```

The test environment is MacOS with CPU info

```
hw.ncpu: 12
hw.activecpu: 12
hw.perflevel0.cpusperl2: 2
hw.perflevel0.cpusperl3: 12
hw.perflevel0.logicalcpu: 12
hw.perflevel0.logicalcpu_max: 12
hw.perflevel0.physicalcpu: 6
hw.perflevel0.physicalcpu_max: 6
hw.cpu64bit_capable: 1
```

It seems both `int64` and `atomic.Int64` have the same align size `8`. What does `align64 may be added to structs that must be 64-bit aligned` means? What situations can `atomic.Int64` be used？

[Axel Wagner]

I'm not sure under what situations you get misaligned values. But note that atomic.Int64 has other benefits besides alignment guarantees. Most importantly, it prevents you from accidentally mixing atomic and non-atomic accesses to the same address - every access of an atomic.Int64 is always atomic.

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[Keith Randall]

There's a difference only on platforms that wouldn't normally 8-byte align an int64. Those are the 32-bit platforms, e.g. linux/386. On those platforms your program prints 4 8.