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？
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