Is _mm_broadcast_ss faster than _mm_set1_ps?

Question

Is this code

float a = ...;
__m256 b = _mm_broadcast_ss(&a)

always faster than this code

float a = ...;
_mm_set1_ps(a)         


        

Answer 1

mm_broadcast_ss is likely to be faster than mm_set1_ps. The former translates into a single instruction (VBROADCASTSS), while the latter is emulated using multiple instructions (probably a MOVSS followed by a shuffle). However, mm_broadcast_ss requires the AVX instruction set, while only SSE is required for mm_set1_ps.