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

gcc versions have been dodgy about align() on simple typedefs and arrays. Typically to do what you want, you would have to wrap the float in a struct, and have the contained float have the alignment restriction.

With operator overloading you can almost make this painless, but it does assume you can use c++ syntax.

#include 
#include 

#define restrict __restrict__

typedef float oldfloat8 __attribute__ ((aligned(8)));

struct float8
{
    float f __attribute__ ((aligned(8)));

    float8 &operator=(float _f) { f = _f; return *this; }
    float8 &operator=(double _f) { f = _f; return *this; }
    float8 &operator=(int _f) { f = _f; return *this; }

    operator float() { return f; }
};

int Myfunc(float8 * restrict a, float8 * restrict b, float8 * restrict c);

int MyFunc(float8 * restrict a, float8 * restrict b, float8 * restrict c)
{
    return *c = *a* *b;
}

int main(int argc, char **argv)
{
    float8 a, b, c;

    float8 p[4];

    printf("sizeof(oldfloat8) == %d\n", (int)sizeof(oldfloat8));
    printf("sizeof(float8) == %d\n", (int)sizeof(float8));

    printf("addr p[0] == %p\n", &p[0] );
    printf("addr p[1] == %p\n", &p[1] );

    a = 2.0;
    b = 7.0;
    MyFunc( &a, &b, &c );
    return 0;
}