Re: ARG process (was += in ada)

["Robert I. Eachus" <rieachus@comcast.net>]

Randy Brukardt wrote:

> I can't say in general, but since my watch began (1998), the record is
> AI95-00147. It was approved at meetings 4 times; the first three, new issues
> surfaced afterwards, causing it to go back. It took six years to finish.

I can't say for sure, but in Ada 83 it was probably AI-2,7,50 or AI-315. 
  Of course, AI-315 probably holds a different record.  I don't think we 
EVER got as far as an approved AI.  AI-315 was about exceptions and 
optimization.  The horrible example case was:

X,Y: Integer;

declare
   Junk: Integer;
begin
   Get(X);
   Get(Y);
   Junk := X * Y;
exception
   when Constraint_Error => Do_Something;
end;

The goal of course, is to determine if X*Y > Integer'Last.  But 11.6 
allowed the multiplication and the (potential) exception to be removed.

The problem is fixed in Ada 95.  The "effect" of a program is defined, 
rather than allowing a compiler to decide to eliminate code because a 
particular value is not used.  Of course, in Ada 83 there was no real 
need to "fix" the language as long as no compiler tried to get too 
agressive in eliminating dead code.  Since all compiler vendors were 
aware of the problem, they tended to avoid that particular optimization.

Yes, dead code elimination is important in a good optimizer.  But the 
optimizer and code generator can check if a variable is a compiler 
generated temporary or a user declared variable.  Eliminating user 
declared variables entirely (and the constraint checks on its value) 
was the potentially upsetting case, eliminating compiler generated 
temporaries was not a problem.

-- 
                                            Robert I. Eachus

"Quality is the Buddha. Quality is scientific reality. Quality is the 
goal of Art. It remains to work these concepts into a practical, 
down-to-earth context, and for this there is nothing more practical or 
down-to-earth than what I have been talking about all along...the repair 
of an old motorcycle."  -- from Zen and the Art of Motorcycle 
Maintenance by Robert Pirsig