Does writing the shared a + b sum of a carry-select adder explicitly change what Yosys and ABC synthesise?
research/chip-cse-style
Writing the shared a + b sum of a carry-select adder by hand (explicitly, or as an incrementer) gives the same netlist Yosys already finds; attaching the constant to b first defeats the sharing and is 17-22% smaller under ABC's optimising scripts (stylised, Yosys 0.33).
What it found
Six equivalent 16-bit adder styles (each proved against a + b with a Yosys miter) are scored with three ABC scripts. Writing the shared sum by hand (p = a + b, y = p + 1) or as an incrementer (y = x + 1) gives exactly the netlist opt_merge finds for the plain a + b + 0 / a + b + 1 text (cells×depth 2090 / 1232 / 1665), so a hand common-subexpression rewrite is a no-op. Attaching the constant to b first, a + (b + k), defeats the sharing: 2268 / 1024 / 1300, 17–22% smaller under the two optimising scripts and 9% larger under yosys_default. One design, one tool version (0.33), generic gates. Stylised.
Paper, code and results
More in LLM chip design loops
- Does a rewritten adder or multiplier still beat the reference once the ABC script is allowed to vary?Re-scores the
chip-ppa-looprewrites and sweep under five ABC scripts with every mapped netlist proved equivalent: the adder rewrite gain survives (0.80–0.86× the reference per script) but the multiplier's whole gain is the script (stylised, generic-gate). - Does an equivalence-gated rewrite loop beat a parameter sweep on popcount, leading-zero count and absolute difference?Equivalence-gated rewrites of 16-bit popcount, 16-bit leading-zero count and 8-bit |a−b| against a no-model sweep: popcount 0.98×, |a−b| 0.86×, but on leading-zero count the sweep (0.56×) beats the session-written rewrites (0.80×) in 168 of 200 equal-budget draws (stylised, gate counts).
- Does the rewrite-versus-sweep verdict for a 16-bit leading-zero count survive five ABC scripts?Re-scores
chip-ppa-logic's popcount, leading-zero-count and |a−b| candidates under five ABC scripts with every mapped netlist proved equivalent: the clz16 sweep-versus-rewrite verdict flips with the script (best 0.35× the reference); stylised, generic gates. - Does fusing the accumulate into the partial-product tree beat multiply-then-add once the ABC script varies?A fused 4×4 multiply-accumulate: no written rewrite beats
a*b+cunder the default or three of five ABC scripts, but carry-save forms win by up to 9% cells×depth underdchandresyn2(stylised, generic-gate). - Do the carry-save multiply-accumulate rewrites still win once mapped to real SKY130 cells, and does the generic-gate score predict that?The
chip-ppa-maccarry-save MAC rewrites mapped to SKY130 HD cells: the plaina*b+cunder its best script is the overall best, the generic-gate score ranks candidates only weakly like cell areaxdelay (Spearman 0.20; stylised, ABC delay estimate). - How rugged is the landscape of SKY130 area×delay over ABC pass sequences for a multiply-accumulate, and do simple searches beat random at 10–40 evaluations?An exhaustive 800-point table of SKY130 areaxdelay over ABC pass sequences for the MAC (optimum 11% below the best earlier script; needle-like landscape), with random search, hill climbing and session-written proposals compared at equal budget (stylised, preliminary).
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Produced with AI assistance (Claude). Results are from the code in this repository and are simulated or stylised unless the abstract says otherwise; nothing here implies affiliation with or endorsement by any lab, researcher or company cited. MIT licensed.