Topographical Synthesis: Worked Example

Worked example

Worked Example for Topographical Synthesis focuses on topo-vs-logic WNS delta, estimated wirelength, congestion hotspots. The goal is to connect observed QoR movement to mechanism, ownership, and regression risk.

A weekly compile reports topo-vs-logic WNS delta, estimated wirelength, congestion hotspots regression. Instead of random knob tuning, start by proving whether input context changed and which mechanism dominates.

Sequence under inspection

SYNTHESIS FLOW — Topographical Synthesis

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

Primary metric: topo-vs-logic WNS delta, estimated wirelength, congestion hotspots

Topographical estimate loop

logic compile
  + estimated placement/RC
  -> topo QoR
  -> PD correlation check

1. Capture baseline and regressed artifact pair.

2. Confirm manifest parity (RTL, SDC, libs, switches).

3. Pin first metric drift and mechanism hypothesis.

4. Reproduce with topo compile report, estimated placement map, QoR delta log.

5. Apply one reversible fix and run matrix regression.

Did the fix hold?

BEFORE / AFTER QOR — Topographical Synthesis

QoR score
  0 |                     --- target zone
 -1 |        ● regression
 -2 |             ● baseline
 -0.5|                  ● after fix
    +----------------------------------> iteration

Validate timing + area + power, not one number.

Synthesis deep dive

Physical-aware synthesis reduces logic-to-route surprise when correlation is monitored.

Concept diagram

PHYSICAL-AWARE LOOP

topo compile -> congestion hints -> handoff -> PD correlation

Metric graph

CORRELATION DELTA

logic view WNS      ███████
topo view WNS       █████
route trial WNS     ████

Reports and artifacts

• topo QoR

• congestion heatmap

• handoff manifest

• delta compare vs PD

Mini case study

Topo compile looked clean, but hotspot congestion predicted route failure and prevented late ECO churn.

Debug branches

• Check hotspot ownership

• Bound correlation drift

• Escalate if handoff assumptions stale

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

Topographical mode estimates physical effects during synthesis to reduce handoff surprises between logic and implementation views.

Metric: topo-vs-logic WNS delta, estimated wirelength, congestion hotspots