pub const DECISION_CHAIN_BUDGET: f64 = 9.6;Expand description
The stage budget a decision is cut to on a fabric with carry chains, in the units of op_delay: one unlowered
add or subtract (a carry chain, 9) and an AND. The XORs that finish a compare’s sign start the next stage, so the
mask a select fans out to its 64 bits leaves a register, never the end of a carry chain. MEASURED (2026-10-17, K1’s
decide, Artix-7 fabric map, yosys 0.33 synth_xilinx + nextpnr-xilinx, seeds 1-3, median): 12 (the compare and
its select in one stage) = 852 LUTs at 119.2 MHz, 10 = 848 at 147.3, 9.6 = 784 at 145.4, 6 = 848 at 147.8; at 8
and 9 one seed in three does not finish routing within 300 s.