The old vec_dot decoded every element with its own pair of multiplications
(256 inlined get_trit per block, each qs byte re-read 5 times). Decode each
byte once instead, iterating q -> (q*3) & 0xFF two bytes at a time in 16-bit
lanes, and accumulate dp4a over base-3 digits in {0, 1, 2}; one extra dp4a
with 0x01010101 per q8_1 int, shared by both planes, turns the digit sums
back into trit sums in exact integer arithmetic, so the result stays
bit-identical to the per-trit decode. The qh bytes keep their own 4-digit
path so the padding digit is never decoded.
Millaguie
10e1fe3d3c