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SubjectRe: [xfs-masters] [PATCH] fs: Add new pre-allocation ioctls to vfs for compatibility with legacy xfs ioctls
Geert Uytterhoeven wrote:
> On Mon, 2 Feb 2009, Boaz Harrosh wrote:
>> Arnd Bergmann wrote:
>>> No, the compiler is correct, it has to generate more complex code
>>> if it cannot assume that data is naturally aligned and the architecture
>>> does not support unaligned loads. If you don't understand this, please
>>> at least read the list archives about the last five times this came up
>>> before claiming that the compiler is broken.
>>>
>> Wrong!! Sorry, you guys don't listen.
>> I'm talking of the case where the structures are EXACTLY the same anyway
>> you look at them. sizeof(foo) == sizeof(foo_packed) and
>> offsetof(foo_memmber) == offsetof(foo_packed_member) for every member of
>> the structure. foo && foo_packed are declared exactly the same but with
>> __attribute__((packed(1))) applied to the later.
>>
>> THEN in ia64 case the compiler is brain dead, because it relates
>> "unalignment" to packed(1) which are two different things.
>
> The natural alignment of a structure is max(alignment(member)), for all
> members. With __attribute__((packed)), the natural alignment of the structure
> is 1, so the compiler cannot assume anything.
>

No the natural alignment is what it is, after the application of
__attribute__((packed(1))). In a well defined structure that is a no-opt.
But yes in ai64 the gcc programmers got lazy and did not make that analysis
after laying out the structure.

> While the ints in the structure may still be at offsets 0, 4, 8, and so on,
> this doesn't say anything about their actual memory addresses, as the struct
> base address itself may be unaligned.
>

The base address can be unaligned even if the structure is aligned. In that
case you need the __atrubute__((aligned)) thingy. It is true that if the sizeof(foo_packed)
is though unaligned, the compiler will have to assume unalignment in array operations.
but if the sizeof(foo_packed) is naturally aligned at the output then the compiler
has all the needed information to know that even if I declared __packed, it calculated
and knows that it is well aligned at the end.

> Gr{oetje,eeting}s,
>
> Geert
>

Please note that I gave up on the compiler and understand that the use of __packed
is dangerous in some cases, sigh. My standing point is to make sure there are no guesses
left, and a BUILD_BUG_ON to make sure of that.

Boaz


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