Alexandre Ferrieux wrote:
>  Hello Mo,
>   
>  On 5/28/07, *Mo DeJong* <mo@[...].com <mailto:mo@[...].com>> wrote:
> 
>       You can write a compiler that assumes that a
>      proc does not deal with linked variables,
>      but it is trivial to create a test case that breaks this
>      optimization. 
> 
>   
>  Yes, if we do nothing actively to deal with upvars.
>  But I was thinking about intercepting those accesses at runtime (see 
>  my answer to Donal's).
Yes, but then you end up with a least common denominator approach. You 
end up having to check
for traces and other changes before each variable access as well as 
changes to command epochs before
each command invocation. All these checks to deal with the most unlikely 
cases waste much CPU time.

If you ask me, the right way to create an optimized Tcl compiler is to 
provide a way to declare procs
that don't "do anything funky". This includes upvars, uplevels, traces 
vars, and so on. Basically, a
proc would declare that it will not mess with variables in calling 
scopes and that other procs invoked
by the command will not mess with variables in the current scope. The 
runtime would also need to
ensure that crazy edge cases like the "set" command being redefined 
while the proc is running don't happen.

With the runtime enforcing these rules, a command that does no do 
anything crazy could invoke another
command that does nothing crazy without having to check for every 
possible edge case. Simple procs
like the following could be significantly optimized:

proc_non_funky { arg } {
    set sum 0
    for {set i 0} {$i < 1000} {incr i} {
        incr sum $i
    }
    return $sum
}

In this command, 3000 checks on the variable i could be avoided, along 
with 1000 checks on the variable sum.
It really adds up.

cheers
Mo DeJong


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