x86 - How to compare __m128 types?

Question

__m128 a;
__m128 b;

How to code a != b ?

what to use: _mm_cmpneq_ps or _mm_cmpneq_ss ?

How to process the result ?

Can't find adequate docs.

Answer 1

You should probably use _mm_cmpneq_ps. However the interpretation of comparisons is a little different with SIMD code than with scalar code. Do you want to test for any corresponding element not being equal ? Or all corresponding elements not being equal ?

To test the results of the 4 comparisons from _mm_cmpneq_ps you can use _mm_movemask_epi8.

Note that comparing floating point values for equality or inequality is usually a bad idea, except in very specific cases.

__m128i vcmp = (__m128i)_mm_cmpneq_ps(a, b); // compare a, b for inequality
uint16_t test = _mm_movemask_epi8(vcmp); // extract results of comparison
if (test == 0xffff)
    // *all* elements not equal
else if (test != 0)
    // *some* elements not equal
else
    // no elements not equal, i.e. all elements equal

For documentation you want these two volumes from Intel:

Intel? 64 and IA-32 Architectures Software Developer’s Manual Volume 2A: Instruction Set Reference, A-M

Intel? 64 and IA-32 Architectures Software Developer’s Manual Volume 2B: Instruction Set Reference, N-Z