Re: [PATCH ARM]: PR67745: Fix function alignment after __attribute__ 2/2

[Bernd Schmidt <bschmidt@redhat.com>]

On 10/07/2015 12:45 PM, Christian Bruel wrote:
>
>
> On 10/07/2015 12:18 PM, Bernd Schmidt wrote:
>> On 10/07/2015 09:04 AM, Christian Bruel wrote:
>>> +      /* Similarly, relayout function's alignment if not forced.  */
>>> +      if (!DECL_USER_ALIGN (fndecl)
>>> +          && (TREE_CODE (fntype) != METHOD_TYPE
>>> +          || TARGET_PTRMEMFUNC_VBIT_LOCATION !=
>>> ptrmemfunc_vbit_in_pfn))
>>> +        DECL_ALIGN (fndecl) = FUNCTION_BOUNDARY;
>>>        }
>>
>> That's a very odd-looking condition. Why the vbit location test?
>
> This is for member functions to make sure that the lsb address is
> reserved for the virtual function bit.
>
> see cp/decl.c:
>
>    /* If pointers to member functions use the least significant bit to
>       indicate whether a function is virtual, ensure a pointer
>       to this function will have that bit clear.  */
>    if (TARGET_PTRMEMFUNC_VBIT_LOCATION == ptrmemfunc_vbit_in_pfn
>        && TREE_CODE (type) == METHOD_TYPE
>        && DECL_ALIGN (decl) < 2 * BITS_PER_UNIT)
>      DECL_ALIGN (decl) = 2 * BITS_PER_UNIT;
>
> This happens for instance on i386 that aligns member functions on 2
> bytes, bigger than the one byte required boundary. So we cannot
> re-layout bellow that.

Hmm, at least that would need to be spelled out in a comment, but I 
think this would better be abstracted as

#define MINIMUM_FUNCTION_BOUNDARY(FN) \
  ....
in a header file, and used in both places.


Bernd