C - How to access elements of vector using GCC SSE vector extension

Question

Usually I work with 3D vectors using following types:

typedef vec3_t float[3];

initializing vectors using smth. like:

vec3_t x_basis = {1.0, 0.0, 0.0};
vec3_t y_basis = {0.0, 1.0, 0.0};
vec3_t z_basis = {0.0, 0.0, 1.0};

and accessing them using smth. like:

x_basis[X] * y_basis[X] + ...

Now I need a vector arithmetics using SSE instructions. I have following code:

typedef float v4sf __attribute__ ((mode(V4SF)))
int main(void)
{
    v4sf   a,b,c;
    a = (v4sf){0.1f,0.2f,0.3f,0.4f};
    b = (v4sf){0.1f,0.2f,0.3f,0.4f};
    c = (v4sf){0.1f,0.2f,0.3f,0.4f};
    a = b + c;
    printf("a=%f \n", a);
    return 0;
}

GCC supports such way. But... First, it gives me 0.00000 as result. Second, I cannot access the elements of such vectors. My question is: how can I access elements of such vectors? I need smth. like a[0] to access X element, a[1] to access Y element, etc.

PS: I compile this code using:

gcc -msse testgcc.c -o testgcc

Answer 1

You are forgetting that you need to reinterpret a as array of floats. Following code works properly:

int main(){
    v4sf a,b,c;
    a = (v4sf){0.1f,0.2f,0.3f,0.4f};
    b = (v4sf){0.1f,0.2f,0.3f,0.4f};
    c = (v4sf){0.1f,0.2f,0.3f,0.4f};
    a = b + c;
    float* pA = (float*) &a;
    printf("a=[%f %f %f %f]\n",pA[0], pA[1], pA[2], pA[3]);
    return 0;
}

P.S.: thanks for this question, I didn't know that gcc has such SSE support.

UPDATE: This solution fails once arrays got unaligned. Solution provided by @drhirsh is free from this problem.

Answer 2

The safe and recommended way to access the elements is with a union, instead of pointer type punning, which fools the aliasing detection mechanisms of the compiler and may lead to unstable code.

union Vec4 {
    v4sf v;
    float e[4];
}

Vec4 vec;
vec.v = (v4sf){0.1f,0.2f,0.3f,0.4f};
printf("%f %f %f %f\n", vec.e[0], vec.e[1], vec.e[2], vec.e[3]);