Correct way to make a 16-byte aligned double* for SSE vectorization?

[Jie Zhang]

Hi,

On 12/31/2009 03:30 AM, Benjamin Redelings I wrote:
> Hi,
>
> I am trying to figure out how to make a double* that is 16-byte aligned
> in the way that SSE instructions want. Hopefully this would allow GCC to
> auto-vectorize loops in a better way. The problem that I am having is
> that I want a pointer to an aligned double, not an aligned pointer to a
> double.
>
> I am compiling with these options:
> % gcc -c test.C -O3 -ftree-vectorizer-verbose=3 -ffast-math
>
> According to the output of the vectorizer, none of the three ways
> (below) of declaring an aligned pointer actually work. They are treated
> as unaligned accesses, so presumably the location of the pointer itself
> is being aligned, but it does not point to an aligned location. In
> contrast, if I define an aligned double, and then define a pointer to
> it, this works. Is this recommended?
>
Below is just taken from the GCC Manual:
[quote]
As another example,

      char *__attribute__((aligned(8))) *f;

specifies the type “pointer to 8-byte-aligned pointer to char”. Note 
again that this does not work with most attributes; for example, the 
usage of `aligned' and `noreturn' attributes given above is not yet 
supported.
[/quote]

If it had been supported, you could use

 > //typedef const real __attribute__((aligned(16))) *SSE_PTR;

But since it is not yet supported now, you have to use

> typedef double real;
>
> // these two lines work (together)
> typedef real aligned_real __attribute__((aligned(16)));
> typedef const aligned_real* SSE_PTR;
>

Jie