This pertains to Tcl 8.5 Say I have a very large dictionary. From performance points of view (memory footprint, etc), assuming that I do not modify the dictionary, should upvar provide a massive performance improvement in terms of memory? I am using an EDA tool, which has TCL shell, but the vendor disabled the TCL memory command. I know that Tcl can share strings under the hood for performance... The same dictionary can be passed several nested procs' calls. Thanks.
As long as you don't modify the dictionary, it won't provide much noticeable performance difference or memory-consumption difference.
Tcl passes values by immutable reference, and copies them when you write an update to them if they're shared, e.g., between a global variable and a local variable (procedure formal parameters are local variables). If you never change anything, you're going to use a shared reference and everything will be quick. If you do need to change something, you should use upvar or global (or one of the more exotic variants) to make a local variable alias to the caller's/global variable and change via that, as that's fastest. But that's only an issue if you're going to change the value.