Answers Database

How does the XILINX_PATHLIMIT environment variable work?


Record #3136

Product Family:  Software

Product Line:  Merged Core

Problem Title:
How does the XILINX_PATHLIMIT environment variable work?


Problem Description:
Keywords: XILINX_PATHLIMIT, path, limit, constraint, environment, variable

Urgency: Standard

General Description:
Sometimes long runtimes are experienced, and could be because there are many
paths to analyze. Users may also get a message during par such as the
following:

WARNING:xiltw:1 - The current connection evaluation limit of 1000 caused the
   truncation of timing analysis for paths through 2.20% of your constrained
   connections, which may limit the accuracy of this analysis.	You may
   specify a larger limit with the XILINX_PATHLIMIT environment variable to

   increase the accuracy of this analysis.

So the user may want to use this XILINX_PATHLIMIT, but are unsure how it is
going to affect the results.


Solution 1:

The XILINX_PATHLIMIT will limit the number of path evaluations per connection
to the specified limit. So the specified number will run a n# depth-first
search passes through each connection (for each constraint), consequently
limiting the total number of overall paths analyzed for the design, and
possibly reducing runtime.

Using this environment variable does NOT guarantee that the worst-case path
is analyzed. The paths it picks is not random, but also it will not guarantee
that the worst case path is analyzed. So using this environment variable could
be risky if expecting to get full timing coverage on the design. So down the
road this could lead to timing errors. So to get a faster evaluation on the
design this environment variable could be used, but later it would be a good
idea to re-run TRCE without the XILINX_PATHLIMIT, to ensure that no timing
errors remain.



End of Record #3136