Slack is required time minus arrival time on one path. The path with the smallest (worst) slack is the critical path.
When all slacks are positive, no path violates, but a critical path still exists.
A synthesis tool recomputes these ranks as the netlist changes. Each round of effort targets the current worst path.
EXAMPLE parameters: required time 6.0 delay units
path arrival = 4.0 + 3.0 + 3.0 = 10.0 delay units
slack = required - arrival = 6.0 - 10.0 = -4.0 delay units
negative slack means the path misses its target by 4.0 delay unitsTiming-driven synthesis applies its transforms to the critical path first. Restructuring, sizing, and duplication each aim at the same ranked list.
Fixing a non-critical path changes no WNS; it can still improve TNS. Its value shows in TNS until that path becomes the worst one.
Intermediate runs explore structures, so their slack can move either way between runs. A worse middle run does not prove the approach is wrong.
Final quality is read at the real target after effort converges. Judge a strategy by its trend across runs, not by one middle number.
Path A. It is the critical path, and only the worst slack sets the reported worst total.
No. Intermediate structures are exploration. The trend across runs and the final result at the target decide.