Re: Software landmines (was: Why C++ is successful)

[stt@houdini.camb.inmet.com (Tucker Taft)]

dewarr@my-dejanews.com wrote:
: In article <35F3FF42.D1E163E0@elca-matrix.ch>,
:   Mats.Weber@elca-matrix.ch wrote:
: > John G. Volan wrote:
: >
: > > > declare
: > > >    A : Integer renames Get_A (10);
: > > >    B : Float renames Get_B (X);
: > > >    C : Boolean renames Get_C (Y);
: > > > begin
: > > >    Op (A, B, C);
: > > > end;
: > >
: > > I'm curious: Does this have the same effect as the above, i.e., are the
: > > function calls forced into a sequential order?  Or is their evaluation
: > > deferred until the aliases A, B, and C are actually used as parameters
: > > to the Op call, in which case the aliased function calls get executed in
: > > an undefined order?
: >
: > Yes, it has the same effect. Each function call is evaluated during the
: > elaboration of the renames it appears in. You are in fact renaming the object
: > that contains the function result. (See RM 3.3(10)).
: >


: I must say that I find the renaming of function calls
: like this to be a confusing oddity. It seems much clearer
: to me to use ordinary constant declarations. The two
: seem completely equivalent!

They are equivalent unless the result type is return-by-reference.

If the result-type is return-by-reference, only the rename would be legal, 
and simply denotes the same object denoted by the original return statement's
expression.  

If the result-type is the more common return-by-reference,
then the two are equivalent for all intents and purposes, though you
might think that the renaming approach could result in fewer copies
in some (not-too-smart ;-) compilers.

By the way, the original point that a "smart" compiler can parallelize:

   Op(Get_A(..), Get_B(..), Get_C(..))

more easily than the "sequential" form involving named constants is a
bit bogus.  To parallelize, the compiler needs to prove that Get_A, 
Get_B, and Get_C don't interfere with one another.  Given that, there 
is no reason that the "sequential" form couldn't be parallelized as well.

--
-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
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