What is path grouping, and how does it help organize timing reports and optimization?

Short Answer

group_path (PT) organizes timing paths into named groups — by clock, by endpoint type, or by a custom set the designer defines — so reporting and optimization tools can treat different classes of paths distinctly instead of lumping every path into one undifferentiated list. Common groupings by startpoint/endpoint type include reg2reg, in2reg, reg2out, and in2out.

Technical Explanation

• The problem path grouping solves. A design typically has thousands of timing paths of very different character — some register-to-register, some from a chip input straight into a register, some from a register out to a chip output. Reporting or optimizing all of them as one undifferentiated pool makes it hard to see which category actually needs attention.
• What group_path does. group_path (PT) assigns a name to a defined set of paths — selected by -from, -to, or a clock — so those paths can be reported and optimized as their own group, separately from every other group.
• The four common startpoint/endpoint categories. reg2reg (register to register, the most common case), in2reg (a chip input directly into a register with no intermediate clock), reg2out (a register directly to a chip output), and in2out (a purely combinational input-to-output path with no register at all) — each behaves differently enough in typical designs that grouping them separately gives a much clearer signal.
• Why optimization benefits, not just reporting. Synthesis and place-and-route optimization tools can spend their effort budget unevenly across groups — for instance treating in2reg paths, which often have looser, externally-defined input delay budgets, differently from reg2reg paths, which are usually the tightest and most numerous.
• What a grouped report actually shows. Instead of one design-wide WNS and TNS number, a grouped report_timing or summary shows a separate worst slack and total negative slack per group, letting an engineer immediately tell whether the design's worst violation lives in the reg2reg domain or is isolated to, say, a handful of in2out paths.

Follow-up Question & Model Response

If two path groups both show negative WNS, how would you decide which one to prioritize fixing first?

Candidate Model Response: Worst slack (WNS) alone only tells you the single worst path in each group; total negative slack (TNS) tells you how widespread the problem is across that whole group, so comparing both numbers together is more informative than either alone. A group with a deeply negative WNS but a small TNS likely has one isolated bad path, while a group with a moderate WNS but a large TNS suggests a systemic issue affecting many paths, which usually needs a structural fix — like retiming or a floorplan change — rather than a one-off path fix. Prioritizing the group with the larger TNS, or the group whose failure mode looks systemic rather than isolated, typically produces a better return on the same amount of engineering effort.