2 ms·

What is SWAR?

• ncruces 2y ago
  SIMD within a register: https://en.wikipedia.org/wiki/SWAR https://en.wikipedia.org/wiki/SWAR It's generally used for techniques that apply SIMD principles within general-purpose registers and instructions. Assume you've loaded a 64-bit register (a uint64_t) with 8 bytes (unsigned char) of data. Can you answer the question “is any of these 8 bytes zero (the NUL terminator)?” If you find a cheap way to do it, you can make strlen go faster by consuming 8 bytes at a time. Et voilà: #define ONES ((uint64_t)-1/UCHAR_MAX) #define HIGHS (ONES \* (UCHAR_MAX/2+1)) #define HASZERO(x) ((x)-ONES & ~(x) & HIGHS)
  • forrestthewoods 2y ago
    TIL, thanks!