Clock-Gating Inference: Silicon PPA Impact

Silicon PPA and closure impact

Constraint debt leaks into PD as false confidence and extra ECO loops.

Area drivers

• Buffer growth

• Cell mix shifts

• Retiming duplication

Power drivers

• Leakage from LVT

• Clock power from gating/skew choices

Timing impact

• Critical path depth changes

• Hold/setup coupling

PD consequences

• Congestion ripple

• Handoff predictability

Verification burden

• Equivalence burden

• Regression matrix depth

PPA SNAPSHOT — Clock-Gating Inference
area / dynamic / leakage / WNS / runtime

PPA takeaways

• QoR wins must survive signoff checks

• Correlation beats one-off gains

QoR snapshot

BEFORE / AFTER QOR — Clock-Gating Inference

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

Validate timing + area + power, not one number.

Synthesis deep dive

Constraint hygiene determines whether synthesis QoR is trustworthy.

Concept diagram

CONSTRAINT FLOW

RTL intent -> synthesis SDC -> compile checks -> QoR dashboard -> review gate

Metric graph

QOR HEALTH

constraint lint pass   ███████████████
unconstrained count    ███
policy violations      ██

Reports and artifacts

• compile QoR

• unconstrained endpoints

• constraint lint

• policy audit log

Mini case study

Nightly WNS improved but unconstrained paths jumped; strict SDC lint restored true QoR picture.

Debug branches

• Diff SDC first

• Audit dont_touch scope

• Track unconstrained trend weekly

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

Inference quality depends on RTL style and constraints; wrong enable semantics or exclusion policy causes either missed savings or unsafe gating.

Metric: clock-gating insertion rate, gating check violations, clock power delta