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Re: x86_64: Should the -mavx* options affected __alignof__ (max_align_t)?


On 04/02/2015 01:13 PM, H.J. Lu wrote:
> On Thu, Apr 2, 2015 at 4:08 AM, Florian Weimer <fweimer@redhat.com> wrote:
>> On 04/02/2015 01:06 PM, H.J. Lu wrote:
>>> On Thu, Apr 2, 2015 at 1:46 AM, Florian Weimer <fweimer@redhat.com> wrote:
>>>> On 04/02/2015 10:40 AM, Andrew Haley wrote:
>>>>
>>>>> So, max_align_t is an object type, and therefore malloc returns a
>>>>> pointer suitable for max_align_t.
>>>>
>>>> Then the GCC definition of max_align_t is incorrect, it should be 8 on
>>>> x86_64 GNU/Linux, because traditionally, that's what mallocs implement
>>>> for this architecture.  (dlmalloc in glibc is an exception.)
>>>>
>>>
>>> x86-64 psABI specifies that a memory >= 16 bytes is 16-byte aligned.
>>> If malloc doesn't do it, it is a broken.
>>
>> My concern is different.  I think _Alignof (max_align_t) == 16 (as it is
>> in GCC now) implies that malloc return values for sizes less than 16
>> bytes are 16-byte-aligned, too, which is not required by the x86-64 psABI.
>>
> 
> If you take this way, malloc of 1 byte can return 1-byte aligned memory.

If this is the case, is this code valid?

  struct S {
    char a __attribute__ ((aligned (__alignof__ (max_align_t))));
  };

  struct S *p = malloc (sizeof (struct S));

-- 
Florian Weimer / Red Hat Product Security


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