I recently noticed that
_m128 m = _mm_set_ps(0,1,2,3);
puts the 4 floats into reverse order when cast to a float array:
(float*) p = (float*)(&m);
// p[0] == 3
// p[1] == 2
// p[2] == 1
// p[3] == 0
The same happens with a union { _m128 m; float[4] a; } also.
Why do SSE operations use this ordering? It's not a big deal but slightly confusing.
And a follow-up question:
When accessing elements in the array by index, should one access in the order 0..3 or the order 3..0 ?
It's just a convention; they had to pick some order, and it really doesn't matter what the order is as long as everyone follows it. Intel happens to like little-endianness.
As far as accessing by index goes... the best thing is to try to avoid doing it. Nothing kills vector performance like element-wise accesses. If you must, try set things up so that the indexing matches the hardware vector lanes; that's what most vector programmers (in my experience) will expect.