4 ms·
Adding the length means another shift+or operation at minimum. I already think this is slower than the technique presented in the article, and this would make
by RaisingSpear 3y ago
Adding the length means another shift+or operation at minimum. I already think this is slower than the technique presented in the article, and this would make it worse.
It's an interesting idea, but I don't see it being practical, even if the size of the table wasn't an issue.
- wholesomepotato 3y agoInstruction level parallelism will make extra shift free. Other than this it needs to be benched and might depend on cpu/arch. I don't care enough to bench and optimize further.
- RaisingSpear 3y agoIt's most definitely not free. It'd consume fetch bandwidth, decode/rename/scheduler slots, an execution port etc. The comparison here is: ((v ^ 0x303030) * 0x640a0100) >> (len << 3) against: table[(((v >> 12) | v) & 0xfff) | (len << 12)] The former is 4 ops, the latter is 6 ops, so throughput wise, the former wins. Latency wise, it also wins, considering that L1 cache lookups are generally 3-5 cycles, whilst integer multiply is typically 3-4.