Re: Fix misaligned access of ItemPointerData on ARM

On 05/21/2015 04:08 PM, Tom Lane wrote:
> I wrote:
>> But BlockIdData is laid out and accessed as two 16-bit fields, so there
>> should be no problem. On what platform exactly do you see a failure?
> Ah, after reading the gcc manual a bit more closely, I get the point.
> For some reason I think we assumed that "packed" would not result in
> misaligning the struct overall, but it clearly could do so, with possible
> efficiency consequences on hardware that doesn't like misaligned accesses.
>
> If the compiler accepts __attribute__((aligned)) then what you've done is
> clearly better. It's not clear to me whether all compilers that accept
> "packed" also accept "aligned", but there are enough ARM machines in the
> buildfarm that we could hope that we'll find out if this isn't portable.
>
> I wonder whether we should drop the ARM assumption and instead write
>
> #if defined(pg_attribute_packed) && defined(pg_attribute_aligned)
> pg_attribute_packed()
> pg_attribute_aligned(2)
> #endif
>
> so that the annotations are applied on every compiler that accepts them.
>
>

Sounds reasonable.

cheers

andrew