How to tell GCC that a pointer argument is always double-word-aligned?

Question

In my program I have a function that does a simple vector addition c[0:15] = a[0:15] + b[0:15]. The function prototype is:

void vecadd(float * r         


        

Answer 1

Alignment specifications usually only work for alignments that are smaller than the base type of a pointer, not larger.

I think easiest is to declare your whole array with an alignment specification, something like

typedef float myvector[16];
typedef myvector alignedVector __attribute__((aligned (8));

(The syntax might not be correct, I always have difficulties to know where to put these __attribute__s)

And use that type throughout your code. For your function definition I'd try

void vecadd(alignedVector * restrict a, alignedVector * restrict b, alignedVector * restrict c);

This gives you an additional indirection but this is only syntax. Something like *a is just a noop and only reinterprets the pointer as a pointer to the first element.