Dont-Touch & Preserve Policy: Mechanism

Mechanism to understand

Mechanism for Dont-Touch & Preserve Policy focuses on preserved object count, blocked optimization opportunities, ECO rework. The goal is to connect observed QoR movement to mechanism, ownership, and regression risk.

dont_touch and preserve are surgical controls; overuse blocks optimization and underuse breaks intended structures such as CDC wrappers or DFT hooks. 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 — 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

Preserve policy decision tree

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

Layer responsibilities

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

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

dont_touch and preserve are surgical controls; overuse blocks optimization and underuse breaks intended structures such as CDC wrappers or DFT hooks.