Re: A note on GNAT 3.05 performance

[dewar@cs.nyu.edu (Robert Dewar)]

Dale said

When I first began looking at Ada, I wondered if if was 'pass by value'
or 'pass by reference'. The answer was, apparently, 'neither and both'.
It seemed to me that Ada was attempting to 'idealize' a concept by
using copy-in/copy-out that it shouldn't be.

   Ada was not doing anything new here. Fortran has also always had an
   implementation defined choice between pass by reference and pass by
   copy for exactly the same reason (efficiency).

Now maintaining copy-in and copy-out will apparently have a performance
penalty, yet changing that aspect to pass-by-reference will not be
consistant with regular 'in' parameter passing.

   Regular in parameter passing allows records to be passed by copy or
   reference, so there is no inconsistency here. The issue was being
   consistent with C by default, there is no problem at the Ada
   semantic level.

Are Ada pragmas frozen in concrete the way the rest of the language
is? Is it feasible to make 'pass by copy' and 'pass by reference'
for record parameters adjustable on a pragma basis? Is it desirable?

   New pragmas can be added (see the features file in the GNAT distribution
   for many interesting ones added to GNAT). We will be implementing all the
   Dec pragmas in the near future, which among other things allow this level
   of control.

   Of course the use of pragma Convention (or Import or Export) will in
   practice control the choice of record passing semantics.