[M3devel] returning record by value vs. by ref?

[Tony Hosking]

Correct!  Anytime you create an l-value pointer.

On Dec 1, 2007, at 2:31 PM, Rodney M. Bates wrote:

> Tony Hosking wrote:
>> I am assuming 's' here is an open array (REF ARRAY OF item) in  
>> which  case it is allocated in the GC'd heap.  There is certainly  
>> no way of  safely getting an interior pointer to items in the  
>> stack in Modula-3  -- at least not one that you can upward expose  
>> (to callers) via  return from a procedure.  The difficulty in  
>> doing this is that the GC  moves objects around and would need to  
>> know where your manufactured  interior pointer is being held and  
>> to which *object* (ie, the open  array in this case) it refers so  
>> that it can 'fix' the pointer when  the array object moves.   
>> Modula-3 provides a small concession to  obtaining downward  
>> exposed interior pointers using the VAR parameter  mode.  For  
>> example you can pass 's[i]' as an actual parameter to a  VAR mode  
>> formal, effectively passing a pointer to the callee.  GC can  cope  
>> with this in one of two possible ways: 1) "Pin" the array so  that  
>> it cannot be moved while the interior pointer is held on the   
>> stack or registers of any thread (this is the approach that CM3's   
>> conservative collector uses for now); or 2) track the creation of   
>> such interior pointers and how they are derived from base object   
>> references for use during GC.  2) requires much more co-operation   
>> from the compiler than the current gcc-based backend (with all of  
>> its  lovely optimizations and register allocation) is capable of  
>> doing.   1) is very cheap and does not impede optimizations and  
>> register  allocation.
>
> Presumably, this all also applies WITH-bound identifiers, when they  
> are
> designators of interior components of heap objects?  Are there any  
> other
> cases?
>
> -- 
> -------------------------------------------------------------
> 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