will __attribute__((aligned(CACHE_LINE_SIZE))) break struct alignment

[Florian Weimer]

On 11/06/2017 06:04 AM, Yubin Ruan wrote:
> I am using something like this:
> 
>    struct S1 {
>      int index __attribute__((aligned(CACHE_LINE_SIZE)));
>      struct S2 some_other;
>    };
> 
> I am not using anything like `#pragma pack`.
> 
> I am wondering whether adding that
> `__attribute__((aligned(CACHE_LINE_SIZE)))` will break GCC's automatic
> alignment for S1 and S2. Can anyone point me to any document which
> specify this behavior?

It will change the alignment of struct S1, so code which allocates 
objects of struct S1 will have to be modified, and the offset in structs 
of a member of type struct S1 will likely change (and so will the 
offsets of members after it).

So in general, this is not an ABI-compatible change.

Thanks,
Florian