Re: Fortran's Equivalence

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

Keith argues that somehow unchecked conversion is more guaranteed by the
RM than address overlay. I find all these arguments bogus. First they
are rather useless pedantry, if you are in the business of trying to 
find out what the RM absolutely guarantees will work in chapter 13,
even with annex B implemented, you will likely be disappointed. Second
in this particular case, there simply are no guarantees on either side.

Keith try this excercise. Consider a case in which we are trying to model
a Fortran EQUIVALENCE statement that maps a big 2D array into a big 1D
array.

Now write the EXACT code using UC and address overlays for this problem,
and we will look at what you come up with. I am willing to bet that if
you write reasonable code in both cases, we will find that

(a) neither is guaranteed to work portably by the RM
(b) in practice both will work fine with any reasonable implementation

It pays to get out of pedantic mode when you are dealing with chapter
13 in real life. In porting millions of lines of code to GNAT, we have
found that real code is full of stuff which is definitely NOT guaranteed
to work by the RM, but we still work to have GNAT accept it properly and
process it as expected. The minimum guarantees of the RM are so thin as
to be pretty uninteresting except for the purpose of writing ACVC tests.