[fpc-devel] State of SSE/AVX intrinsics

J. Gareth Moreton gareth at moreton-family.com
Tue Apr 21 20:40:05 CEST 2020

I made a bit of a big post with "Future development plans" where I 
mentioned 'aligned allocation', especially since C11 has the feature and 
Microsoft have their own non-portable set of functions.  Hopefully it 
will spawn some discussion.

Sorry to keep going back to the dynamic array issue.  The comparison 
with static arrays is a good one, because this is where bugs often appear.

Take the common Move function for some array V (assume it's an array of 
Integers for the sake of this example) and a pointer to a block of memory:

Move(V, DestPtr^, Length(V) * SizeOf(Integer)); { V and DestPtr can be 
swapped around if you like }

If V is a static array, this will work as expected, since V points to 
the first element in the array.  However, if V is a dynamic array, then 
this call will likely raise an access violation, or if it doesn't, 
DestPtr^ will contain garbage - if you're writing to V, you'll end up 
giving it an invalid pointer to its data, while the real pointer will 
end up dangling.  You have to specify V[0] rather than V for it to work 
as expected (which works for static arrays too).

Gareth aka. Kit

On 21/04/2020 16:18, Sven Barth wrote:
> denisgolovan <denisgolovan at yandex.ru <mailto:denisgolovan at yandex.ru>> 
> schrieb am Di., 21. Apr. 2020, 14:58:
>     > I think Gareth meant the address of V instead (Pointer(@V)).
>     >
>     > Regards,
>     > Sven
>     Hm. I have no idea why anyone would take pointer to pointer :)
> Because they might have forgotten that dynamic arrays behave different 
> from static ones?
>     BTW, Sven, what's your opinion on dynamic arrays alignment problem?
> I have none, cause I'm not dealing with such problems.
> Regards,
> Sven

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