SSE Comparison Intrinsics - How to get 1 or 0 from a comparison?

问题

I am trying to write the equivalent of an if statement with SSE intrinsics.

I am using __m128 _mm_cmplt_ps(__m128 a, __m128 b) to do the comparison a < b, and this returns 0xffffffff or 0x0 if the comparison was respectively true or false. I would like to convert these values into 1 and 0. In order to do this, is it correct to implement the logical "and" __m128 _mm_and_ps(__m128 c , __m128 d), where c is the result of the conversion and d is, e.g., 0xffffffff?

Thank you for your attention.

回答1:

You're comparing to get a 1 or a 0, then multiplying by another number. That is essentially this:

c = (a < b) * d;

which is the same as this:

c = 0;
if (a < b)
    c = d;

This is also known as a conditional move.

If that's what you want, then you don't need the 0 or the 1. Just AND the result of the compare directly with the number you will multiply with.

__m128 c = _mm_cmplt_ps(a,b);
c = _mm_and_ps(c,d);

This works because the comparison returns either all 0's or all 1's. So ANDing them with the result will either zero it, or keep it entirely.

It was intentionally designed that way. There's no need for a multiplication.

来源：https://stackoverflow.com/questions/13256983/sse-comparison-intrinsics-how-to-get-1-or-0-from-a-comparison