Debugging QoR · synthesis

Part 8 · Flows and Debugging · Intermediate

Poor QoR answers to a fixed method. Reproduce the result, isolate the worst path, classify the cause, and change one thing at a time.

Term	Meaning
QoR	Quality of results: area, timing slack, and cell counts for one run.
Worst path	The path with the smallest slack. It sets the timing verdict.
Slack	Required time minus arrival time. Negative slack misses the ask.
Cause class	Constraint, RTL structure, library, or tool behaviour. Each has its own fix.

The QoR debug loop.

Reproduce, isolate, classify, then fix one thing.

Lesson	What it covers
The QoR debug method	Reproduce a result, isolate the worst path, and fix one thing.
Reading reports to localise	Area, fanout, and worst-path tables that point at the cause.
Constraint causes	Missing constraints and over-constraint, with illustrative SDC.
RTL structure causes	Latches, deep priority logic, and unshared arithmetic.
Fanout, black boxes and classifying paths	Huge fanout, black boxes, and a runnable path classifier.

Check yourself Timing fails after an RTL edit. What is the first debug step?

Reproduce the run with pinned revisions, constraints, and scripts. Only then isolate the worst path and classify it.