MAC units for a 2x2 int8 systolic array on Tiny Tapeout

Overview & Synopsis

We take the multiply-accumulate (MAC) unit of an open-source 2$\times$2 int8 systolic array taped out on Tiny Tapeout (GF180 process) and propose two replacements. The first one, \emph{hybrid}, uses a different arithmetic structure for each row of the array. It reduces the logic area of one MAC unit by 4--6\% and of the whole chip by 0.7--3.2\% after place and route. The second one, \emph{fused\_eq}, never builds the long carry chain of the accumulation. It shortens the critical path by 20\% for one MAC unit and by 14\% for the whole chip. All numbers come from the standard Tiny Tapeout flow (LibreLane) run in GitHub Actions. Correctness is checked by exhaustive simulation of all $2^{24}$ inputs, a formal equivalence proof, and the original test bench.