Synthesis & Logic Optimization · All levels

Clock-Gating Inference: Design Space

Design Space for Clock-Gating Inference.

Design space exploration

For Clock-Gating Inference, options trade QoR gain, risk, and schedule.

Option A — conservative

Conservative constraints: helps predictability
Risk: slower PPA gains
Validate with: first-silicon safety

Option B — balanced

Balanced QoR tuning: helps stable closure
Risk: requires discipline
Validate with: typical product mode

Option C — aggressive compile

Aggressive compile push: helps fast timing recovery
Risk: power/hold regressions
Validate with: late schedule

Option D — structural change

Structural RTL change: helps long-term fix
Risk: schedule impact
Validate with: recurring bottleneck

diagram

DESIGN SPACE — Clock-Gating Inference
QoR gain <-> regression risk <-> schedule pressure

Design pitfalls

Single-metric optimization
No rollback path
Weak run comparability

Tradeoff curve

diagram

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

diagram

CONSTRAINT FLOW

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

Metric graph

diagram

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

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