On 01/08/2014 11:01 PM, Chen Gang wrote:
> On 01/06/2014 06:31 PM, James Hogan wrote:
>> I suspect this is due to bad assumptions in the code. The metag ABI is
>> unusual in padding the size of structs to a 32bit boundary even if all
>> members are <32bit. This is actually permitted by the C standard but
>> it's a bit of a pain. e.g.
>>
>> struct s {
>> short x
>> struct {
>> short x[3];
>> } y;
>> short z;
>> };
>>
>> on x86
>> alignof(s::y) == 2
>> s::y at offset 2
>> sizeof(s::y) == 6
>> s::z at offset 6+2 = 8
>> sizeof(struct s) == 10
>>
>> but on metag
>> alignof(s::y) == 4
>> s::y at offset 4
>> sizeof(s::y) == 8 (padding, this is what catches people out)
>> s::z at offset 4+8 = 12
>> sizeof(struct s) == 16 (and here too)
>>
>> Adding packed attribute on outer struct reduces sizeof(struct s) to 12
>> on metag:
>> alignof(s::y) == 4
>> s::y at offset 2 (packed)
>> sizeof(s::y) == 8 (still padded)
>
> In my memory, when packed(2), it breaks the C standard (although I am
> not quit sure).
>
> And I guess, all C programmers will assume it will be 6 when within
> pack(2) or pack(1).
>
>> s::z at offset 2+8 = 10
>> sizeof(struct s) == 12 (packed)
>>
>> Also reduced to 12 if only inner struct is marked packed:
>> alignof(s::y) == 2
>> s::y at offset 2
>> sizeof(s::y) == 6 (packed)
>> s::z at offset 2+6 = 8
>> sizeof(struct s) == 12 (still padded)
>>
>> Adding packed attribute on both outer and inner struct reduces
>> sizeof(struct s) to 10 to match x86.
>>
>> Unfortunately it's years too late to change this ABI, so we're stuck
>> with it.
>>
>
> Unfortunately too, most using cases are related with API (the related
> structure definition must be the same in binary data).
>
> I am sure there are still another ways to bypass this issue, but that
> will make the code looks very strange (especially they are API).
>
> :-(
>
I guess most C programmers will use this way to describe protocol/data
format, and keep compatible for it (since it is API).
So even if it really does not break C standard, I still recommend our
compiler to improve itself to support this features.
Thanks.
--
Chen Gang
Open, share and attitude like air, water and life which God blessed
--
To unsubscribe from this list: send the line "unsubscribe linux-metag" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
