> On 5 Sep 2022, at 09:53, Richard Biener via Gcc <gcc@gcc.gnu.org> wrote:
>
> On Sun, Sep 4, 2022 at 3:33 PM Iain Sandoe <i...@sandoe.co.uk> wrote:
>>
>> Hi,
>>
>> I am clearly missing something here … can someone point out where it is?
>>
>> https://gcc.gnu.org/onlinedocs/gcc-3.3/gcc/Variable-Attributes.html#Variable%20Attributes
>> in the discussion of applying this to structure fields:
>>
>> "The aligned attribute can only increase the alignment; but you can decrease
>> it by specifying packed as well."
>>
>> Consider:
>>
>> struct odd {
>> int * __attribute__((aligned(2))) a;
>
> I think this applies the attribute to the type.
That was what I wondered - but it does not seem to apply the under-alignment to
a non-pointer type ...
> For non-aggregate
> types 'packed' is ignored. So the above
> is equivalent to
>
> typedef int *A __attribute__((aligned(2)));
>
> struct odd {
> A a;
> char c;
> };
Which (for the record) works as expected on both compilers.
>
>> char c;
>> };
>>
>> I would expect, given reading of the information on the aligned attribute,
>> that the under-alignment of a would be ignored (since there is no packed
>> attribute on either the field or the struct).
>>
>> However, on x86_64, powerpc64 linux and x86_64, powerpc Darwin, I see that
>> the size of the struct is sizeof(pointer) + 2 and the alignment is 2.
>>
>> OTOH:
>>
>> struct OK {
>> int __attribute__((aligned(2))) a;
>> char c;
>> };
However, this does _not_ treat the same sequence as “typedef int A
__attribute__((aligned(2)))”
>> behaves as expected (the under-alignment is ignored, silently).
>>
>> as does this…
>>
>> struct maybe {
>> int *a __attribute__((aligned(2)));
>> char c;
>> };
>
> Where for both of these cases the attribute applies to the FIELD_DECL.
> The documentation refers to
> alignment of fields, not the alignment of types.
sure, but I can’t at the moment see a consistent rule to file a bug about.
> At least that's my understanding of this issue.
>
> IIRC clang has issues when matching GCC attribute parsing rules, esp.
> when applied to pointer types.
probably; when I looked at the decls produced there seemed to be no way to
to tell the position of the attribute in the decl (so to decide if it’s a type
attr or a
field attr). … possibly that means poking at the parser too…
attributes in aggregates are fun, for sure ..
Iain
>
> Richard.
>
>> * the type of the pointer does not seem to be relevant (i.e. AFAICT the
>> behaviour is the same for char * etc.)
>>
>> Is there some special rule about pointers that I have not found ?
>>
>> [it’s making an ABI mismatch with clang, which treats the int * as expected
>> from the documentation quoted above]
>>
>> cheers
>> Iain
