Clock-Gating Inference: Mechanism

Mechanism to understand

Mechanism for Clock-Gating Inference focuses on clock-gating insertion rate, gating check violations, clock power delta. The goal is to connect observed QoR movement to mechanism, ownership, and regression risk.

Inference quality depends on RTL style and constraints; wrong enable semantics or exclusion policy causes either missed savings or unsafe gating. Treat synthesis as a contract: constraints + transforms + reports must agree.

• Identify which lane moved first: timing, area, power, or runtime.

• Identify whether drift is input, transform, or correlation related.

• Identify owner before applying any ECO.

Flow model

SYNTHESIS FLOW — Clock-Gating Inference

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

Primary metric: clock-gating insertion rate, gating check violations, clock power delta

Clock-gating insertion loop

RTL enable intent
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gating inference rules
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insert ICG cells + checks
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power delta + gating-safe verification

Layer responsibilities

SYNTHESIS OWNERSHIP LAYERS — Clock-Gating Inference

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

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.

Mechanism deep dive

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