https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91043
--- Comment #24 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #23)
> (In reply to Hanoch Haim from comment #22)
> >
> > "Of course it does, because without aligning the container you cannot have
> > aligned members. Maximum alignment always propagates outwards."
> >
> > Sorry, your answer is still not clear, so let give a short example
> > In this case there is a discrepancy betwean two gcc modules
> >
> > 1. The module that generates the code think that it is aligned
> > (CCPortLatency)
> > 2. However the linker puts it in a none aligned location
> >
> >
> > "
> > class CTimeHistogram {
> >
> > } __rte_cache_aligned;
> >
> > class CCPortLatency {
> > public:
> > CTimeHistogram m_hist;
> > };
> > class Root {
> >
> > CCPortLatency port;
> >
> > } __rte_cache_aligned;
> >
> > static Root root;
> > "
> >
> > In this case can I expect root.port to be aligned because its child (m_hist)
> > was defined as aligned and it propogate? Or should I explicitly ask both to
> > be aligned?
>
> Yes, for
>
> class CTimeHistogram {
> } __attribute__((aligned(64)));
> class CCPortLatency {
> public:
> CTimeHistogram m_hist;
> };
> class Root {
> CCPortLatency port;
> };
> static Root root;
>
> 'root' will be aligned to 64 bytes. This is also what you can easily
> observe when inspecting the ELF object:
>
> Section Headers:
> [Nr] Name Type Address Offset
> Size EntSize Flags Link Info Align
> ...
> [ 3] .bss NOBITS 0000000000000000 00000040
> 0000000000000040 0000000000000000 WA 0 0 64
>
> Symbol table '.symtab' contains 8 entries:
> Num: Value Size Type Bind Vis Ndx Name
> ...
> 5: 0000000000000000 64 OBJECT LOCAL DEFAULT 3 _ZL4root
>
> compiled without optimization since root is unused and will otherwise
> be eliminated.
You can also check with
static_assert (__alignof__(root.port.m_hist) == 64, "oops");
(need to make port public for this, eh)
