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From: Iain Sandoe <iain@sandoe.co.uk>
To: Richard Biener <richard.guenther@gmail.com>
Cc: GCC Development <gcc@gcc.gnu.org>
Subject: Re: An odd case with structure field alignment.
Date: Mon, 5 Sep 2022 10:06:11 +0100	[thread overview]
Message-ID: <4CE0F7B6-8253-46DF-AF97-98886BAED50E@sandoe.co.uk> (raw)
In-Reply-To: <CAFiYyc3JrtfMLdYtRT=1jo5QCCgUG2-ZyazBso8KD5GCtJGO6g@mail.gmail.com>



> 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 <iain@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


      reply	other threads:[~2022-09-05  9:06 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-09-04 13:32 Iain Sandoe
2022-09-05  8:53 ` Richard Biener
2022-09-05  9:06   ` Iain Sandoe [this message]

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