Why isn't this faster than the "standard" loop based version? The disclaimer at the start says it's not, but I would have thought that 4 64bit math operations would be much faster than a loop with 8 steps and comparisons and so on.
That being said. The 64bit multiplication is faster, and also constant in time. I've measured this using the timestamp counter which doesn't necessarily count actual CPU clock cycles, but possibly an integer multiple of it (seems to output always numbers divisable by 8 for me).
(...)
100 loop:01100100(152) 64bit:01100100(48)
101 loop:01100101(128) 64bit:01100101(48)
102 loop:01100110(160) 64bit:01100110(48)
103 loop:01100111(128) 64bit:01100111(48)
104 loop:01101000(192) 64bit:01101000(48)
105 loop:01101001(88) 64bit:01101001(48)
106 loop:01101010(152) 64bit:01101010(48)
107 loop:01101011(128) 64bit:01101011(48)
108 loop:01101100(160) 64bit:01101100(48)
(...)
From your github source:
/* anyone have a big-endian machine at hand to test this? */
#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
#define MULT 0x8040201008040201ULL
#else
#define MULT 0x0102040810204080ULL
#endif
Actually, not the carry bits, the "content" bits. The 8-bit content must appear on more places there for the magic to work.
Intel Core i5-3210M @ 2.50GHz
Then I continued '16 elements', oh, OK then, next time I'll try to read the whole sentence first.