Dont-Touch & Preserve Policy

What this topic teaches

Dont-Touch & Preserve Policy teaches how to turn synthesis intent into stable QoR outcomes. dont_touch and preserve are surgical controls; overuse blocks optimization and underuse breaks intended structures such as CDC wrappers or DFT hooks. Senior practice is proving whether metric movement is real, reproducible, and owned.

The senior-engineer question

When preserved object count, blocked optimization opportunities, ECO rework moves, can you name the run context, mechanism, owner, and minimal fix that holds under regression?

SYNTHESIS FLOW — Dont-Touch & Preserve Policy

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

Primary metric: preserved object count, blocked optimization opportunities, ECO rework

Picture the synthesis flow

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

Preserve policy decision tree

OBJECT MARKED preserve/dont_touch?
        |
  functional or signoff reason?
     /                \
   yes                 no
   |                   |
keep + audit       remove, re-compile

QoR trend shape

QOR TREND — Dont-Touch & Preserve Policy

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

Track: preserved object count, blocked optimization opportunities, ECO rework

Who owns which layer

SYNTHESIS OWNERSHIP LAYERS — Dont-Touch & Preserve Policy

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: preserved object count, blocked optimization opportunities, ECO rework.

• Primary artifact: constraint audit, preserve list, optimization-block report.

• Owners to involve: synthesis lead, RTL owner, DFT owner.

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

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

Ownership map

OWNERSHIP MAP — Dont-Touch & Preserve Policy

artifact              owner
----------------      -----------------
compile owner       synthesis lead
timing/power owner  RTL owner
cross-team review   DFT 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

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.