[M3devel] Static chains

[Rodney M. Bates]

See embedded corrections

I (Rodney M. Bates)  wrote:
> Think of the debugger as an interpreter for a substantial subset of the
> language.  When the program is stopped at some point, m3gdb can, in effect,
> inject code to evaluate an arbitrary expression, typed by the debugger
> user, or execute an arbitrary assignment or call statement.  Although
> there are a number of gaps yet in the implementation, m3gdb is approaching
> fitting this description.
> 
> The user-typed code can both create and use static link values that do
> not occur in the original code that the compiler saws.  The simplest
                                                   ^saw
> example is, when stopped in a nested procedure, the user types
> 'print NonLocalVariable'.  This works fine right now, unless the compiler
> has not stored the SL in the current proc's activation record, which a
> compiler might well do if the nested proc, as coded, has no references
> to any nonlocal variables.  (Or keeps the SL only in a register, etc.)
> 
> You could say this is only a convenience, as the user could work around
> it by just typing 'up' the exact number of times to get to the static
> parent's frame (which is, of course, also a dynamic ancestor).  But that
> can slow down debugging a great deal, and I often find it a very important
> convenience.  Anyway, you could say that to a degree about much of what a
> debugger does.
> 
> A more complex example might be 'print Nested1(Nested2,NonlocalVar)',
> where Nested1 and Nested2 are nested procedures.  Here, the debugger
> has create and pass the appropriate SL value in order to call Nested1,
> which, unless Nested1 is one scope deeper than the current stop point,
> also entails m3gdb's fetching the SL that was given to some procedure
> on the current dynamic call chain.  The same things are required of
                 ^static chain.
Although it is true that the proc is also on the dynamic chain, that is
not what I meant and is confusing at best.
> m3gdb to construct the closure for Nested2, which it must pass to Nested1.
> And if NonlocalVar is in an outer scope where Nested1 and/or Nested2
> aren't visible, there would be no workaround possible at all.
> 
> Tony, I haven't been able to try out your latest change anyway, as
> I seem to have once again regressed to not being able to get CM3 to
> link to a working version of libpthread on Mandriva.  Also, I am about
> to travel for a week, so may be out of this discussion for a while.
> 
> 
> Tony Hosking wrote:
> 
>> Rodney,
>>
>> I just realised that the static chains variables are *not* always  
>> being set up for nested function internal use.  i.e., callers will  
>> pass them, but nested functions that don't explicitly use them will  
>> not have a static chain.  Is this something essential for you?  Why  
>> would you need a static chain from within a nested function that  
>> doesn't use any outer scope variables?  Since it doesn't use any  
>> outer scope variables they cannot affect its execution, so debugging  
>> the value of those variables seems unnecessary.  This is the default  
>> behavior of gcc for static chains.
>>
>> -- Tony
>>
>>
> 

-- 
-------------------------------------------------------------
Rodney M. Bates, retired assistant professor
Dept. of Computer Science, Wichita State University
Wichita, KS 67260-0083
316-978-3922
rodney.bates at wichita.edu