hdl-tools/digital-chip-design-agents/logic-synthesis

Skill: Logic Synthesis

Invocation

• If invoked by a user presenting a synthesis task: immediately spawn the

digital-chip-design-agents:synthesis-orchestrator agent and pass the full user request and any available context. Do not execute stages directly.

• If invoked by the `synthesis-orchestrator` mid-flow: do not spawn a new

agent. Treat this file as read-only — return the requested stage rules, sign-off criteria, or loop-back guidance to the calling orchestrator.

Spawning the orchestrator from within an active orchestrator run causes recursive delegation and must never happen.

Pre-run Context

<MEM> below is the resolved memory root, never the literal memory/ directory: an explicit --memory-root, else $CHIP_DESIGN_MEMORY_ROOT, else the central default, else the in-repo seed (python3 plugins/infrastructure/skills/memory-keeper/memory_root.py prints it; the orchestrator's ## Memory section is authoritative).

Before executing or advising on any stage, read the following files if they exist:

1. <MEM>/synthesis/knowledge.md — known failure patterns, successful tool flags, PDK/tool quirks.

Incorporate its guidance into every stage decision. If absent, proceed without it.

1. <MEM>/synthesis/run_state.md — current run identity (run_id, design_name, tool,

last_stage). Use this to resume correctly after interruption. If absent, a new run is starting; the orchestrator will create this file before the first stage.

This pre-run read applies whether this skill is loaded by a user or called by the orchestrator mid-flow. It ensures the fix database is consulted before any diagnosis step.

Purpose

Produce a timing-clean, area-efficient, LEC-verified gate-level netlist from RTL. Covers constraint setup, synthesis compilation strategy, and quality checks before PD handoff.

Supported EDA Tools

Open-Source

• Yosys (yosys) — open-source synthesis suite; runs as a sequential pass pipeline (see sequential flow note below)
• Surelog (surelog) — SystemVerilog front-end for Yosys
• ABC — logic optimisation and technology mapping (invoked automatically by Yosys)

Proprietary

• Synopsys Design Compiler (dc_shell, dialect synopsys) — industry-standard logic synthesis
• Cadence Genus (genus, dialect cadence) — RTL-to-netlist with concurrent optimisation
• Synopsys Fusion Compiler (fc_shell, dialect synopsys) — combined synthesis and physical guidance

Wrappers: wrap-dc.sh and wrap-genus.sh (Fusion Compiler through wrap-icc2.sh with WRAP_ICC2_TOOL=fc_shell). Prove a netlist with LEC (wrap-fm.sh / wrap-lec.sh) before handing it to PD.

Tool Notes: commercial synthesis (outline — measured notes pending)

Standard command names only; no option recipe here has been verified on this suite's reference host yet.

• Design Compiler, topographical — dc_shell -topographical_mode -f <script.tcl>.

Topographical mode needs physical data: an NDM reference library (or a Milkyway library on older releases) and the technology file / tech LEF, plus the TLU+ or extraction tech for wire RC. Without them it is wire-load-model synthesis and the timing is not comparable to a physical flow. Compile with compile_ultra (add -spg to pass placement guidance to ICC2), then report_qor, report_area, report_timing, write -format verilog, and keep the SVF for Formality. On an open PDK build the .db/NDM views first (physical-design skill, ## PDK Enablement)

• Genus — genus -batch -files <script.tcl>: read_mmmc (library sets, RC corners,

constraint mode with the SDC, analysis views), read_physical -lef (tech LEF then cell and macro LEFs) for physically aware synthesis, read_hdl, elaborate, then syn_generic, syn_map, syn_opt (each optionally -physical, which also needs a floorplan DEF and the extraction tech), then report_qor, report_timing, report_area, write_hdl, write_sdc. compile_ultra is a Design Compiler command, not a Genus one

• Fusion Compiler — fc_shell: synthesis is compile_fusion (stepwise with -to/-from),

in the same NDM design library the back end continues from; compile_ultra is not the FC command

Sequential Flow Log Review (Yosys)

Yosys runs its synthesis script (yosys -c synth.ys or yosys -p "synth_*") as a sequential pass pipeline. Each pass (read_verilog → synth → opt → techmap → abc → write_verilog) executes in order; errors or warnings in early passes propagate forward.

After a Yosys run the agent must:

1. Read the Yosys log (stdout or redirected yosys.log) for:

• Warning: / Error: lines per pass
• Final statistics block: number of cells, wires, and logic depth
• Unmapped cells (search for $-prefixed cell names in the output netlist)

1. Verify that the output netlist (synth_netlist.v) exists and is non-empty
2. Parse report_area / report_timing output if ABC timing mode (abc -constr) was used

When used inside OpenROAD Flow Scripts (ORFS) or LibreLane, the Yosys log appears at: logs/<platform>/<design>/1_1_yosys.log

Stage: constraint_setup

Domain Rules

1. create_clock: all primary clocks with period, waveform, source pin, name
2. create_generated_clock: all derived/divided clocks with correct source
3. set_clock_uncertainty: setup = skew + jitter — use design_state.constraints.timing.clk_uncertainty_ps if set; otherwise 200–500 ps pre-CTS (default: see constraints.timing.clk_uncertainty_ps, schema default null → use 200–500 ps rule-of-thumb)
4. set_input_delay / set_output_delay: all primary IOs constrained
5. set_false_path: multi-clock crossings, test modes, async resets
6. set_multicycle_path: both setup (-setup N) and hold (-hold 1) must be set
7. set_dont_touch: IPs, memory macros, hand-crafted cells
8. set_max_fanout: from design_state.constraints.timing.fanout_max (default: 32)
9. set_max_transition: per technology DRC rule
10. Operating conditions: explicitly set (never rely on tool defaults)

Common SDC Mistakes

QoR Metrics to Evaluate

• All clocks defined (verify with report_clocks)
• All IOs constrained (verify with report_port -verbose)
• No unconstrained paths (report_timing -unconstrained)

Output Required

• Validated SDC file
• Clock summary
• Constraint QA report

Stage: compile_explore

Domain Rules

1. Run at worst-case timing corner (SS, low voltage, high temperature)
2. Compile explore: faster run to find best logic structure
3. Try multiple architectures: retiming on/off, datapath options
4. Identify critical paths for human review before final compile
5. Check area estimate vs microarch estimate

Optimisation Strategy by Priority

Output Required

• Exploration report (timing, area, power summary)
• Critical path list for architect review
• Recommended compile strategy for final compile

Stage: compile_final

Domain Rules

1. Run multi-scenario if available (setup + hold simultaneously)
2. Enable clock gating synthesis for sequential power reduction
3. Preserve hierarchy for blocks with existing placement intent
4. Ungroup small modules for better cross-boundary optimisation
5. Review critical paths manually — restructure RTL if path cannot close
6. Run incremental compile after initial compile to address remaining violations

QoR Metrics to Evaluate

• WNS: ≥ design_state.constraints.timing.wns_ns_target at worst-case corner for sign-off (default: 0)
• TNS: = design_state.constraints.timing.tns_ns_target for clean sign-off (default: 0)
• Area: within design_state.constraints.area.area_um2 budget (comparison run: recorded, not judged)
• Power: within design_state.constraints.power.power_mw budget (comparison run: recorded, not judged)
• No unmapped cells

Output Required

• Gate-level netlist (.v)
• Timing report (setup and hold, all path groups)
• Area report
• Power report
• Synthesis run log

Stage: netlist_qc

Checks Required

1. No black boxes (undefined modules) in netlist. A black box is an input-set defect, not a

compile result: if it resolves to first-party RTL, that file is missing from the synthesis source list (sv2v and yosys black-box it without an error); otherwise a library, macro or IP view is missing. Either way re-running the compile cannot fix it — name the module, the file or view, and the list, and escalate as input_setup

1. No combinational loops (report_loop)
2. Scan chains intact (if DFT-enabled compile)
3. Power/ground connections correct (tie cells, well ties)
4. Formal equivalence check (RTL vs netlist): PASS required

LEC Requirements

• Golden: RTL (post-lint, post-CDC-clean)
• Revised: gate-level netlist
• Result: all points EQUIVALENT
• Any UNMATCHED point: must be resolved before PD

QoR Metrics to Evaluate

• LEC: 100% EQUIVALENT
• No black boxes
• No combinational loops
• Scan chain integrity: verified

Output Required

• LEC report (pass/fail)
• Netlist QC checklist
• Final gate netlist (ready for PD)
• Back-annotated SDC for PD

Stage: synthesis_signoff

Sign-off Checklist

• [ ] WNS ≥ design_state.constraints.timing.wns_ns_target at all required corners (default: 0)
• [ ] TNS = design_state.constraints.timing.tns_ns_target (default: 0)
• [ ] Area within design_state.constraints.area.area_um2 budget
• [ ] Power within design_state.constraints.power.power_mw budget
• [ ] LEC: EQUIVALENT
• [ ] No black boxes
• [ ] No combinational loops
• [ ] Scan chains verified (if DFT)

Output Required

• PD handoff package: netlist, SDC, timing reports, area/power reports

Constraint Validation

See plugins/meta/skills/pipeline-orchestration/SKILL.md §Constraints Schema for the authoritative schema and stage-entry validation rule.

Required at entry (`constraint_setup`) — hard-fail if missing:

• constraints.clock.clk_mhz — target clock frequency (drives SDC create_clock)
• constraints.area.area_um2 — area budget
• constraints.power.power_mw — power budget

Comparison / benchmark runs (pipeline_config.comparison_run: true, or a user request that is a tool-vs-tool comparison rather than a product build): area.area_um2 and power.power_mw may be absent. Record "comparison run, no target" for each in the stage reason, report the measured area and power, and do not escalate. clock.clk_mhz stays required — every compared synthesis must use the same clock.

Optional (schema defaults apply when absent):

• constraints.timing.wns_ns_target (default: 0) — WNS sign-off threshold
• constraints.timing.tns_ns_target (default: 0) — TNS sign-off threshold
• constraints.timing.fanout_max (default: 32) — set_max_fanout value
• constraints.timing.clk_uncertainty_ps (default: null → use 200–500 ps rule-of-thumb)

Memory