Is an __m128i variable zero?

Question

How do I test if a __m128i variable has any nonzero value on SSE-2-and-earlier processors?

Answer 1

In SSE2 you can do:

__m128i zero = _mm_setzero_si128();
if(_mm_movemask_epi8(_mm_cmpeq_epi32(x,zero)) == 0xFFFF)
{
    //the code...
}

this will test four int's vs zero then return a mask for each byte, so your bit-offsets of each corresponding int would be at 0, 4, 8 & 12, but the above test will catch if any bit is set, then if you preserve the mask you can work with the finer grained parts directly if need be.

Answer 2

For the sake of completeness, with SSE4 one can use _mm_testz_si128.

const bool isAllZero = _mm_testz_si128(a,a);

Note that this is true when all bits are zero.