On Sun, 27 Oct 2024, Thorsten Glaser wrote:
Finn Thain dixit:
That would mean __alignof__(foo.b) == sizeof(foo.b) but that's not the
case on my Linux/i686 system. 4 != 8:
struct baa {
int a;
long long b;
} foo;
That struct is just 12 bytes for you then?
Right. i686 and m68k agree on that.
That’s possible on i386 but almost nowhere else, and has multiple
penalties on i386 even (cacheline split, loss of atomicity, merging
of accesses, possible cross-page split, …).
Well, the GCC manual says the Power architecture also has ABI variants
with differing alignment rules.
https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/RS_002f6000-and-PowerPC-Options.html#index-malign-natural
https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/x86-Options.html#index-malign-data-1
https://gcc.gnu.org/onlinedocs/gcc-14.2.0/gcc/x86-Options.html#index-malign-double
Of course, in this case, the software author ought to have made the
padding explicit or, better, sorted the struct members. But if all did
that we wouldn’t have this problem.
Right. And I don't think the problem is going to go away. One solution
that is sometimes raised is better tooling. I'm not sure how that would
work. Perhaps a better solution would be language changes to allow a
struct definition to be qualified with an explicit data model.
More importantly, and relevant for Qt, struct baa is also 8-byte
aligned...
If the struct itself is not naturally aligned, it too will eventually
break someone's assumption of natural alignment.
No, structs are not naturally aligned but aligned to the max() of the
alignment of its members. Natural alignment is only applied to primitive
types.
Makes sense.
