Incremental Compile

What this topic teaches

Incremental Compile teaches how to turn synthesis intent into stable QoR outcomes. Incremental compile reuses stable optimization regions while targeting changed cones, reducing churn and iteration cost. Senior practice is proving whether metric movement is real, reproducible, and owned.

The senior-engineer question

When reuse ratio, localized QoR delta, compile turnaround time moves, can you name the run context, mechanism, owner, and minimal fix that holds under regression?

SYNTHESIS FLOW — Incremental Compile

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

Primary metric: reuse ratio, localized QoR delta, compile turnaround time

Picture the synthesis flow

Draw the behavior before diving into tool commands. These diagrams are the whiteboard models to memorize for reviews and interviews.

Incremental compile zones

stable cones: reuse
changed cones: re-optimize
global checks: timing/power consistency

QoR trend shape

QOR TREND — Incremental Compile

metric quality
  ^
  |                      target band
  |                o  o  o
  |            o
  |      o  regression
  +----------------------------------> synthesis iteration
   baseline    tuning     signoff-ready

Track: reuse ratio, localized QoR delta, compile turnaround time

Who owns which layer

SYNTHESIS OWNERSHIP LAYERS — Incremental Compile

layer               owns                          failure mode
----------------    ---------------------------   -------------------------
constraints         clocks/exceptions/policy      fake QoR optimism
mapping             cell choices/structure        depth/fanout regressions
optimization        timing/power tradeoffs        one-metric overfitting
physical-aware      topo/congestion estimates     handoff delta surprises
closure             ECO order/regression          fixes break other corners

Evidence to collect

• Primary metric: reuse ratio, localized QoR delta, compile turnaround time.

• Primary artifact: incremental reuse report, changed-cone map, QoR diff.

• Owners to involve: synthesis owner, CAD owner.

• One baseline run and one regressed run with matching manifests.

• A rollback-safe change proposal with full regression scope.

Ownership map

OWNERSHIP MAP — Incremental Compile

artifact              owner
----------------      -----------------
compile owner       synthesis owner
timing/power owner  CAD owner

Every QoR movement needs a named owner before ECO.

Subpages in this topic

Each topic is taught through mechanism, interfaces, reports, debug, worked example, pitfalls, interview, checklist, theory, design space, expanded case study, walkthrough, comparison matrix, software view, and silicon PPA impact.

Key takeaways

• Always present QoR with run context and artifact.

• Prefer reversible fixes with explicit rollback criteria.

• Re-run timing, area, and power checks before closing.

Common pitfalls

• Comparing unlike compile contexts.

• Timing-only wins that worsen power or area.

• Skipping equivalence checks after structural changes.

Synthesis deep dive

Incremental and ECO synthesis protect schedule only with strict regression discipline.

Concept diagram

INCREMENTAL ECO LOOP

identify delta -> localized compile -> patch -> regression -> release

Metric graph

ECO CYCLE TIME

full compile      ███████████
incremental       █████

Reports and artifacts

• incremental reuse

• ECO patch list

• LEC status

• regression matrix

Mini case study

Localized compile cut turnaround 40%, but skipped LEC caused rollback.

Debug branches

• Reuse ratio is not enough

• Patch locality check

• Always include equivalence hooks

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.