Re: AQ&S Guidance on pragma Elaborate_Body

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

Robert Duff said

<<If you're willing to live with some restrictions, and some complicated
(slow?) processing at link time, you could dispense with all
elaboration-control pragmas, and have the link phase determine an
elaboration order that's guaranteed to not raise Program_Error.  But
note that some of these pragmas (Pure, especially) have purposes other
than just elaboration order.>>

No, that's quite wrong. There are many things that can place restrictions
on the order of elaboration.

Suppose we have a package Glorp that has several procedures including
Thunk, Boggle, and Glub. In the comments of package Glorp, we find the
comment:

-- WARNING: It is essential to call Thunk before calling Boggle. If you
-- fail to do this, all subsequent calls to Glub will raise the Glob-Glob
-- exception or otherwise malfunction. Clients of Glorp are responsible for
-- obeying this rule, since no check is made (a check would introduce too
-- much overhead).

Now, suppose we have two clients of Glorp, Jabber and Wocky. The elaboration
code of Jabber calls Thunk, and the elaboration code of Wocky calls Boggle.

Well now it is essential that Jabber is elaborated before Wocky, and no
static analysis of the program code can determine this, and a pragma
Elaborate is required.

And if you think this is just theoretical, think again, there are
instantiations of the names above in the SGI GNAT runtime library, and
when we installed some new clever elaboration stuff (a bit like Bob
proposes, but more effective, and easier to understand), the binder
chose a much better elaboration order, much more clearly guaranteed
to prevent Program_Error, but unfortunately wrong (of course this is
a bug in the runtime, there is a missing pragma Elaborate).

It is quite true that you can guarantee absence from Program_Error for a wide
variety of programs automatically, and version 3.10 of GNAT does that. BUT
it is far from true that this means that you could "dispense with all
elaboration-control pragmas".