SSE intrinsics: masking a float and using bitwise and?

Question

Basically the problem is related to x86 assembler where you have a number that you want to set to either zero or the number itself using an and. If you and that number with negative one you get back the number itself but if you and it with zero you get zero.

Now the problem I'm having with SSE instrinsics is that floats aren't the same in binary as doubles (or maybe I'm mistaken). Anyways here's the code, I've tried using all kinds of floats to mask the second and third numbers (127.0f and 99.0f respectively) but no luck.

#include <xmmintrin.h>
#include <stdio.h>

void print_4_bit_num(const char * label, __m128 var)
{
    float *val = (float *) &var;
    printf("%s: %f %f %f %f\n",
       label, val[3], val[2], val[1], val[0]);
}
int main()
{
    __m128 v1 = _mm_set_ps(1.0f, 127.0f,  99.0f, 1.0f);
    __m128 v2 = _mm_set_ps(1.0f, 65535.0f, 127.0f, 0.0f);
    __m128 v = _mm_and_ps(v1, v2);

    print_4_bit_num("v1", v1);
    print_4_bit_num("v2", v2);
    print_4_bit_num("v ", v);

    return 0;
}

Answer 1

You need to use a bitwise (integer) mask when you AND, so to e.g. clear alternate values in a vector you might do something like this:

__m128 v1 = _mm_set_ps(1.0f, 127.0f,  99.0f, 1.0f);
__m128 v2 = _mm_castsi128_ps(_mm_set_epi32(0, -1, 0, -1));
__m128 v = _mm_and_ps(v1, v2); // => v = { 0.0f, 127.0f, 0.0f, 1.0f }