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The-OpenROAD-Project / OpenSTA

OpenSTA engine
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Unmatched results when using `disconnect_pin` and `connect_pin` command. #85

Closed KKA92 closed 2 years ago

KKA92 commented 2 years ago

When I used your OpenSTA library with the standard TCL commands , and I ran into a weird problem. My task involved using fan-in swap a particular cell with another type and calculating the slack improvement. I tried to use the function disconnect_pin and connect_pin from the STA API to do that. However, when I tried to revert back to the original fan-in for further processing, the slack values before and after were not matched.

In order to reproduce the problem, please refer to the following steps:

  1. unzip gcdfile.zip in your STA examples folder
  2. move from examples/gcdfile/sta.tcl to /examples/sta.tcl
  3. modify directory in examples/gcdfile/gcd/runs/test2/config.tcl file GCD_ROOT at line 1
  4. run with your sta file use command : ../app/sta sta.tcl

report_checks result in the original design. image

report_checks result after reverting image

this is a link to download my design and sky130 lib files, because this file is too big so I uploaded file to google drive https://drive.google.com/file/d/16TFSIY1JPvaup6r6TIbA9eYz4SSLaT6C/view?usp=sharing

thanks.

rovinski commented 2 years ago

This just seems like floating point error, likely from changing the order of operations. The net is still within 2E-7 accuracy. If your time unit is nanoseconds, the difference is less than 1 femtosecond. Plus, the resulting path slack is exactly the same down to the last digit.

I'll leave this to @jjcherry56 to decide if this is buggy, but my bet is that this is expected floating point error, and it's also why we never go more than 3 or so decimal places in the timing report. If you are doing some kind of comparison based on slacks, you need to use rounded comparisons rather than exact comparisons (e.g. see here)

KKA92 commented 2 years ago

Thank you for your answer, @rovinski

I need one more of your confirmation about comparing delay numbers. In OpenSTA/util/Fuzzy.cc, there are some functions for comparison between two delay values (such as delayEqual or fuzzyEqual). I wonder can I use these functions as an alternative way for this tutorial: https://floating-point-gui.de/errors/comparison/.

rovinski commented 2 years ago

@KKA92 that's probably a good idea if that's what you need.