RE: [PATCH v6 1/6] wifi: mac80211: Use struct instead of macro for PREQ frame

[Ping-Ke Shih <pkshih@realtek.com>]

Johannes Berg <johannes@sipsolutions.net> wrote:
> > In my tests with arm-gcc compiler, I did a special case:
> >
> > struct foo {
> >       int a;
> >       char b;
> > } __packed;
> >
> > int bar(struct foo *foo)
> > {
> >       return foo->a;
> > }
> >
> > It is obviously aligned (offset = 0), so I guessed arm-gcc can generate
> > a single load instruction, but actually it doesn't (even with -O3).
> 
> No, it cannot, because in addition it has to assume the pointer 'foo'
> itself has no alignment. We use this a lot too, since a struct like your
> 'struct foo' could be in a frame element where we don't know what came
> before it while parsing the elements, so we don't know that 'foo' has
> any alignment.
> 
> The reasons are related to what happens when you have a
> 
>         struct foo array[N];
> 
> I think, because then __packed means there's no padding at the end of
> 'struct foo' either, and array[1] is a completely unaligned pointer.
> 
> So no, the compiler couldn't assume alignment even for fields that look
> aligned within 'struct foo'.

I learned more. Super thank you. :)

Ping-Ke