AdvancedQuestion 20 of 63

How do you make PrimeTime remember the source file and line number of each exception, and what's the hard restriction?

From PDVerse STA Mentor Guide, part of the pdVerse Mentor Guide

By Tabish Iqbal, who has 9+ years of VLSI Physical Design experience

Short Answer

Set sdc_save_source_file_information (PT) to true - its default is false - and PrimeTime records the file and line of every exception command going forward. The hard restriction is that it can only be changed before any exception has been read in; once one has, an attempt to change it errors out and the setting stays unchanged.

Technical Reference DiagramHow do you make PrimeTime remember the source file and line number of each exception, and what's the hard restriction?

Technical Explanation

  • What has to be turned on. sdc_save_source_file_information (PT) is a variable that defaults to false. Nothing about a timing exception's origin is recorded unless this is explicitly set true.
  • The restriction is about ordering, not syntax. The value can only be changed before any exception command has been input into the session. That means it belongs in a setup script, or in the very first lines of an interactive session, before any set_false_path or set_multicycle_path (SDC) runs.
  • What happens if you try to change it late. Once even a single exception has already run, attempting to set the variable produces a CMD-013 error, and the value is left unchanged - there is no retroactive way to attach source-file information to exceptions already sitting in memory.
  • The only recovery. Restart the session with the variable set true first, before reading in any constraints.
  • What gets tracked once it's on. The file and line number behind every set_false_path, set_multicycle_path, set_max_delay, and set_min_delay (SDC) command, visible through report_exceptions (PT) or inline next to a specific exception as a [ location = multi.tcl:17 ]-style annotation.
  • Why this is worth the setup discipline. On a design whose constraints arrive from several teams across several generated Tcl files, the difference between knowing a path is masked and knowing exactly which line masked it is the difference between a five-minute conversation and an afternoon spent grepping through scripts.

Common Mistake

  • Setting sdc_save_source_file_information true partway through a session, after the SDC has already been read, then being confused when the variable silently stays false or errors out.
  • Assuming the setting can be turned on retroactively "when needed," treating it as an on-demand debug switch rather than a session-start decision.
  • Cost: reaching a point where tracing an exception's origin would genuinely help, and discovering the tracking was never enabled at the one point in the session it could have been.

Follow-up Question & Model Response

A colleague's session hits the CMD-013 error trying to enable this variable mid-session. What went wrong, and what's the only real fix?

Candidate Model Response: The CMD-013 error means at least one timing exception - likely a set_false_path or set_multicycle_path from an earlier-sourced SDC file - has already been read into the session before the attempt to set sdc_save_source_file_information true. The variable's restriction is strict: once any exception exists, it can no longer be changed, and the attempt is rejected outright rather than partially applied. The only real fix is to restart the PrimeTime session and set the variable true as one of the very first commands, before sourcing any SDC file that contains exception commands, so that every exception picks up its file-and-line information from the moment it's read in.

Practical Example

A signoff session sources three team-generated SDC files totaling 40 set_false_path and set_multicycle_path commands. With sdc_save_source_file_information set true before any of the three files are read, report_exceptions -location later lists each exception with its originating file and line, letting a reviewer trace a suspicious false path directly to team_b_constraints.tcl:212 and the engineer who wrote it, instead of grepping three files by hand for a matching -from/-to pattern.

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