Specifying alignment of pointer targets

[Richard Guenther]

Hi!

I'd like to specify (for vectorization) the alignment of the
target of a pointer.  I.e. I have a vector of floats that I
know is suitable aligned and that get's passed to a function
like

typedef ???? afloatp;

void foo(afloatp __restrict__ a, afloatp __restrict__ b,
	 afloatp __restrict__ c)
{
  int i;
  for (i=0; i<4; ++i)
    a[i] = b[i] + c[i];
}

now, the obvious

typedef float __attribute__((aligned(16))) * afloatp;

doesn't have any effect on (*a)s alignment, and specifying
the alignment in the function argument list like

void foo(float __attribute__((aligned(16))) * __restrict__ a,
         float __attribute__((aligned(16))) * __restrict__ b,
         float __attribute__((aligned(16))) * __restrict__ c)

gets me

simd.c:12: error: alignment may not be specified for 'a'
simd.c:13: error: alignment may not be specified for 'b'
simd.c:14: error: alignment may not be specified for 'c'

which I find confusing.  Specifying alignment of the pointer
itself gets me beyond compiling but of course doesn't buy me
anything (the results are similar to using the typedef).

The only way I was able to convince gcc that the target of
a is aligned is using *ghasp* an aligned struct like

struct v4sf { float v[4]; } __attribute__((aligned(16)));

void foo(struct v4sf * __restrict__ a, struct v4sf * __restrict__ b,
struct v4sf * __restrict__ c)
{
        int i;
        for (i=0; i<4; ++i)
                a->v[i] = b->v[i] + c->v[i];
}

!?

Is this really the only way?  Is it supposed to be the only way?
I remember the thread about alignment specifications for arrays
and agree that

  float __attribute__((aligned(16))) x[4];

is ill-formed, but

  float x[4] __attribute__((aligned(16)));
and
  float __attribute__((aligned(16))) *x;

are not?


Thanks for any suggestions,
Richard.

--
Richard Guenther <richard dot guenther at uni-tuebingen dot de>
WWW: http://www.tat.physik.uni-tuebingen.de/~rguenth/