https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91043
--- Comment #23 from Richard Biener <rguenth at gcc dot gnu.org> ---
(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.
