( ESNUG 316 Item 8 ) ----------------------------------------------- [4/8/99]
Subject: ( ESNUG 313 #13 ) Hidden Synopsys Variables Are Very Dangerous
> Steve Hwang's post about finding hidden variables got me thinking. I went
> into mole mode and burrowed into dc_shell to see what I'd find. The
> result is the following list of undocumented variables and their default
> values, which might be useful. "Undocumented" here means that they exist
> when dc_shell starts up, but have no man page exists. ...
>
> - [ Bit Mole ]
From: [ A Senior Synopsys CAE ]
Hi John,
I am not going to reward the behavior of looking at strings by giving
detailed information about hidden variables. I just know some designers
like examples better than a general message saying hidden variables are
dangerous.
So lets take one of the LESS risky BC variables mentioned.
bc_use_old_group : false
Three years ago BC had a lot of problems with meeting timing after
compilation. After a lot of work and effort by the BC team we turned that
situation around about 1.5 years ago. Well one of the things we did was
fundamentally change how we group logic, internally to BC during timing
analysis, etc. By setting this variable to true you go back to the
previous grouping structure which will give you worse QOR, for most if
not all testcases. We kept this for backward compatibility but do not
recommend people to use it and hence it is hidden.
There are other switchs that may do worse things to your design than just
give you poor QOR.
- [ A Senior Synopsys CAE ]
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