Re: Garbage Collection

[sommar@enea.se (Erland Sommarskog)]

Bill Wolfe (wtwolfe@hubcap.UUCP) first said:
>>>     The deallocation of every object in the local environment is
>>>     performed as an automatic service when a procedure, function,
>>>     or local block is exited.  This is not garbage collection,
>>>     because the programmer has implicitly directed that the 
>>>     destruction be performed.  

I tried to understand:
>> I read this as "on block exit all memory allocated to variables
>> declared in that block should be deallocated". 
>>   Isn't this very dangerous? What if programmer copied the object to
>> a variable declared in a outer block? Or stored it in a table of some
>> sort in a subprogram call made in block? 

And Bill explains:
>   Not at all.  If a programmer assigns the value stored in some local
>   object to some nonlocal object, then we simply have a new value for
>   a nonlocal object, assuming we have not engaged in the foul practice
>   of structural sharing.  Those who engage in structural sharing will
>   get the run-time errors they deserve for engaging in such space-hacking. 


And I still don't understand. Let's say we have a tree handler
   Generic 
      Data_type is limited private;
      Procedure -- Assign, "<" and ">" 
   Package Tree_handler
      Tree_type is private;
Assume further that we instantiate this package with Some_access_type, 
it could be an imported limited type, it could be a locally defined.
Then we have the procedure:

    Procedure Insert_data_to_tree(Tree : in out Tree_type;
                                  Data_of_some_sort : in Any_type); 
    Reference : Some_access_type;
    Begin
       Reference := new Some_type;  -- Or subprogram call for external type
       Prepare(Reference, Data_of_some_sort);
       Insert(Tree, Reference);
    End Insert_data_to_tree;
    
According to Bill the data allocated to Reference should be freed when 
we exit this procedure, but if Insert does what we think this is of
course a severe error. Where did I go wrong? Did I plead guilty to
structure sharing just because I inserted a pointer to a tree? 
  Or does Bill mean that when Reference is inserted to the tree that 
Reference.all should be inserted to the tree, not the pointer itself?
But what if I also insert the reference into another tree (or some other 
structure) and then modify the referred data (some counter, for instance) 
and want this change to appear in both places?
  Note also that in this example, there are no side effects. We are only
dealing with local variables and paramerters. And still deallocation
on block exit is completely wrong. (And there is no way the compiler
could deduce that Insert really saves Reference somewhere.)

Several times in this discussion Bill Wolfe has claimed that 
garbage collection is a trouble-maker for maintainers. In what 
way? I can only see one: If the system due to a heavy use of memory
is spending a lot of time on garbage collecttion, something must be 
done. But in this case the problem is often quite easy to spot.
  Much worse for maintainence is when the system dies with "System 
access violation" or "segmentation fault" from time to time and you 
don't know why. The reason may be that an allocated area was erroneously 
released and then reused and the original data, that someone appearently 
is relying in, has been over-written. Or because an already deallocated 
was deallocated a second time.
  Bill Wolfe is trying to outline some alternative, but I
think he has to think more about, until it is equally safe.
(And really, I don't think he should. He is just reinventing
the wheel.)
  
Bill Wolfe said earlier that garbage collection encourages  
sloppy programming. Whether it is sloppy or not, I don't care,
but obviously garbage collection is taking a lot of the burden
of the programmer's shoulder and the maintainer's too. Why
should a programmer waste time bothering about something that
the computer could do much safer for him? After all, the  
programmer does not work for free.

So to conclude: I think Ada should require garbage collection
that could be disabled with a pragma for critical applications.
As it is now a portable application cannot rely on it, with
all the troubles explicit deallocation implies.
-- 
Erland Sommarskog
ENEA Data, Stockholm              This signature is not to be quoted.
sommar@enea.se