What is Common Path Pessimism, and how does CPPR remove it?

Short Answer

Common Path Pessimism Removal (CPPR), also called CRPR, credits back STA slack lost when launch and capture clock paths share a segment forced into both an early and a late derate under on-chip variation. The tool finds that shared point, computes the delay difference between the two derated values, and adds it back as a credit on both setup and hold checks. For example, a 0.40 ns common-path segment derated to 0.42 ns late and 0.38 ns early would otherwise cost 0.04 ns of slack that never really existed -- CPPR credits that 0.04 ns back.

Technical Explanation

• A well-built clock tree branches near the leaf cells, not near the source, so under OCV the delay differences along each path stay local to that path.
• STA under OCV applies worst-case delays to the launch path and best-case delays to the capture path for setup checks (the combination flips for hold). Where launch and capture share a common clock segment before diverging, that segment gets analyzed under both assumptions simultaneously -- physically impossible on the same wire.
• That impossible double-counting is Common Path Pessimism (CPP) -- reported slack looks worse than the design actually is, on setup checks and hold checks alike.
• CPPR / CRPR identifies the real common clock path and removes that artificial pessimism from the analysis, crediting it back regardless of which check type is being evaluated.
• A later branch point means a longer common path, which means more pessimism for CPPR to remove -- a tree that branches early not only risks more real skew, it also leaves less common path for CPPR to correct.

Illustrative numbers for one CPPR credit on a shared segment:

Healthy, Suspicious & Hard-stop Results

• Expected: report_timing shows a nonzero "clock reconvergence pessimism removed" line on paths with a genuinely shared clock segment, and removing it materially improves reported slack versus the raw graph-based number.
• Investigate: the CRPR line is zero or near-zero on a path you expect to share a long common clock segment -- check whether the clock tree structure actually branches where you think it does.
• Stop: reported slack without CRPR applied is being used as the signoff number -- that's exactly the artificial pessimism this mechanism exists to remove, and using it uncorrected will make real designs look like they fail when they don't.

What The Interviewer Is Testing

Whether you understand CPPR is correcting an analysis artifact (not a real physical effect), and whether you can connect it back to why CTS branch-point discipline matters in the first place.