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From: stolfi (Jorge Stolfi)
Newsgroups: comp.lang.modula3
Subject: Re: Two Oberon questions
Message-ID: <9104190753.AA03720@jumbo.pa.dec.com>
Date: 19 Apr 91 07:53:10 GMT
Lines: 42
In-Reply-To: Message of Thu, 18 Apr 1991 19:31:34 GMT
    from vsanchez@casbah.acns.nwu.edu (Victor Sanchez)
    <1991Apr18.193134.28552@casbah.acns.nwu.edu>
To: m3
X-Folder-Carbon: lang-des


    
    [sakkinen] Can function procedures in Oberon return structured values?
    
    [vsanchez] ...  if you have automatic garbage collection you can
    use the pointers to structures ...  perhaps one pays a price for
    that, but returning structures is not very efficient anyway.
    
Garbage collection and pointers can replace structured return values
in some uses, but not all.

It should cost only two or three instructions to return a two-word
record as a procedure result.  If you account for the cost of running
the garbage collector, allocating that same record from the heap may
well cost thousands of instructions---two or three orders of magnitude
more.

So, the pointer/heap solution is acceptable only in some cases: for
low-intensity uses, for large structures, for values that will get
copied many times, etc..  For many other uses (complex numbers, points,
dates, etc.) structured return values will be a lot cheaper.

Moreover, in languages like Modula-2 and Pascal, the use
of heap records requires extra code: NEW calls, "^" operators,
temporary variables.  Structured return values are more natural
and easier to use.

Finally, allowing structured return values does not mean making the
language more complex; on the contrary, it means removing a rather
arbitrary rule...

  Jorge Stolfi
  DEC Systems Research Center, Palo Alto
  stolfi@src.dec.com

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