Synthesis & Logic Optimization · All levels

Timing-Driven Synthesis: Expanded Case Study

Expanded Case Study for Timing-Driven Synthesis.

Extended case study

Closure review: critical path WNS, violating endpoint count, compile effort distribution regressed after a change touching Timing-Driven Synthesis.

Background

Team previously had stable baseline across weekly compile runs and agreed guardbands.

Symptoms observed

critical path WNS, violating endpoint count, compile effort distribution moved beyond threshold
Mismatch between local and CI runs
Owner disagreement on first fix

Investigation timeline

Hour 0: freeze manifests and compare run metadata
Hour 1: isolate section/topic-specific failure pattern
Hour 2: verify constraints and library view consistency
Hour 3: inspect transform and mapping deltas
Hour 4: choose one reversible fix
Hour 5: execute full regression matrix
Hour 6: publish closure memo and next actions

Root cause

Root cause centered on Timing-Driven Synthesis: Timing-driven compile focuses optimization where slack is most constrained, balancing global QoR instead of over-optimizing non-critical logic.

Fix and validation

Targeted synthesis tweak with audit trail
Re-run critical range report, path-group WNS report, effort histogram
Update dashboard and handoff notes

Lessons learned

Compare like-for-like runs only
Assign clear owner before ECO
Keep rollback ready

diagram

CASE STUDY — Timing-Driven Synthesis
baseline / regression / recovery QoR snapshot

Sequence under stress

diagram

SYNTHESIS FLOW — Timing-Driven Synthesis

RTL + constraints
      |
      v
elaboration + checks
      |
      v
mapping + optimization
      |
      v
QoR reports (timing/area/power)
      |
      v
incremental ECO + regression

Primary metric: critical path WNS, violating endpoint count, compile effort distribution

Synthesis deep dive

Mapping converts logic intent into real PPA outcomes.

Concept diagram

diagram

MAPPING LOOP

boolean net -> library mapping -> optimization -> report and iterate

Metric graph

diagram

MAPPING IMPACT

depth reduction   ███████
fanout cleanup    █████
runtime overhead  ███

Reports and artifacts

mapping summary
critical path cell list
fanout/slew report
library coverage

Mini case study

Cell-heavy critical path improved after restructuring, while blanket buffering had worsened power.

Debug branches

Depth issue -> structure
Fanout issue -> buffers
Library mismatch -> view audit

Senior review question

Ask: what evidence proves this QoR move is real and stable?

Key takeaways

State exact run context (RTL, SDC, libs, switches) with every QoR claim.
Re-run timing, area, and power regressions after each synthesis ECO.

Common pitfalls

Comparing runs with mismatched constraints or library views.
Timing-only fixes that violate power or area budgets.
Skipping equivalence checks after structural changes.

Principal synthesis review addendum

Timing-driven compile focuses optimization where slack is most constrained, balancing global QoR instead of over-optimizing non-critical logic.

Metric: critical path WNS, violating endpoint count, compile effort distribution

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