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From: Robert Dewar <robert_dewar@my-deja.com>
Subject: Re: Ada OO Mechanism
Date: 1999/06/09
Message-ID: <7jlfnv$g3a$1@nnrp1.deja.com>#1/1
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References: <7i05aq$rgl$1@news.orbitworld.net> <7i17gj$1u1k@news2.newsguy.com>
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 <t7zp2sr6yf.fsf@calumny.jyacc.c <t7emjmmx8w.fsf@calumny.jyacc.com>
 <1999Jun8.151014.1@eisner> <t7r9nmz8ou.fsf@calumny.jyacc.com>
 <375D9A3D.E1CCCC63@averstar.com>
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Date: 1999-06-09T00:00:00+00:00
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In article <375D9A3D.E1CCCC63@averstar.com>,
  Tucker Taft <stt@averstar.com> wrote:
> For functions that return objects of size unknown to the
caller, the
> function creates the object on (or copies the object to) the
secondary
> stack, and then returns from the function *without* releasing
the
> secondary stack.  The caller takes care of releasing the
secondary stack
> once they are through with using the returned object.
>
> There are other compilers that use "local" heaps, or other
heap-based
> mechanisms.  They all still typically use some kind of
mark/release,
> though it may be a mark/release on a linked-list of objects,
rather
> than on a secondary stack.

By far the most efficient method of handling variable length
returned values is to return with the stack pointer depressed
so that the newly created value is still available. However
this cannot be done without wandering a bit outside the standard
ABI, and is thus infeasible in GNAT, which always uses the
standard ABI calling sequence.

Actually there is one routine alloca, which works the same way,
and indeed alloca has to be built in on many compilers,
precisely because it does not fit the standard ABI calling
sequence.


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