> On Aug 21, 2019, at 10:28 AM, Alexander Monakov <amona...@ispras.ru> wrote:
>
> On Tue, 20 Aug 2019, "Markus Fröschle" wrote:
>
>> Thank you (and others) for your answers. Now I'm just as smart as before,
>> however.
>>
>> Is it a supported, documented, 'long term' feature we can rely on or not?
>>
>> If yes, I would expect it to be properly documented. If not, never mind.
>
> I think it's properly documented in gcc-9:
>
> https://gcc.gnu.org/onlinedocs/gcc-9.2.0/gcc/Common-Type-Attributes.html
>
> (the "old" behavior where the compiler would neither honor reduced alignment
> nor issue a warning seems questionable, the new documentation promises a more
> sensible approach)
I agree, but if the new approach generates a warning for code that was written
to the old rules, that would be unfortunate.
> In portable code one can also use memcpy to move unaligned data, the compiler
> should translate it like an unaligned load/store when size is a suitable
> constant:
>
> int val;
> memcpy(&val, ptr, sizeof val);
>
> (or __builtin_memcpy when -ffreestanding is in effect)
Yes. But last I tried, optimizing that for > 1 alignment is problematic
because that information often doesn't make it down to the target code even
though it is documented to do so.
paul
