Message-ID: <52D01DBE.6010205@gmail.com>
Date:	Sat, 11 Jan 2014 00:20:14 +0800
From:	Chen Gang <gang.chen.5i5j@...il.com>
To:	James Hogan <james.hogan@...tec.com>
CC:	linux-metag@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [Suggest] arch: metag: compiler: Are they compiler's issues?

On 01/11/2014 12:02 AM, James Hogan wrote:
> On 10/01/14 15:57, Chen Gang wrote:
>> 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.
> 
> The compiler cannot change this without breaking the ABI.
> 
> If the structure describes a set-in-stone data layout (which it sounds
> like it does since it asserts the size of it) then the correct fix is to
> pack the structures in such a way as to guarantee the correct offsets
> and sizes on all compliant compilers. Otherwise if it's just an internal
> programming API it shouldn't be using compile time asserts to enforce
> things that vary between ABIs.
> 

OK, thanks, I guess your meaning is:

	struct s {
		short x;
		struct {
			short x[3];
		} y __attribute__ ((packed));
		short z;
	} __attribute__ ((packed));

That will satisfy all of compilers (include metag), is it correct?


Thanks.
-- 
Chen Gang

Open, share and attitude like air, water and life which God blessed
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