SIMD (SSE) implementation of the infamous Fast Inverse Square Root algorithm from Quake III Arena.
Why not.
This video explains it well.
Here is the results of running benchmark.c compiled with -O2 on my hardware:
Q_rsqrt took(x) 110.617000ms
1.0f/sqrtf(x) took 343.441000ms
Q_rsqrt_sse(x) took 31.145000ms
_mm_div_ps(_mm_set1_ps(1.0f), _mm_sqrt_ps(x)) took 85.942000ms
_mm_rsqrt_ps(x) took 13.855000ms
We can clearly see that Q_rsqrt_sse significantly faster (about 3.5x, the theoretical maximum being 4x) than the scalar version, with the fastest being SSE's native inverse square root function.
If for some god forsaken reason you want to use this just include the simd_fastinvsqrt.h header in your program, define INCLUDE_ORIGINAL before including to bring in the original Q_rsqrt as well.