Re: Elaboration_check For Instantiations

[kst@king.cts.com (Keith Thompson)]

Charles H. Sampson (csampson@cod.nosc.mil) wrote:
[...]
>      You are right, the GNAT implementation is entirely correct.  I 
> must have mislead you when I said "technically correct".  Technically 
> correct is correct, no doubt about it.  All I meant was that GNAT is not 
> following the programmer's wishes.  If Suppress(Elaboration_check) is 
> written, then the programmer doesn't want the check made; if the check 
> is not made then Program_error can't be raised.  The programmer's wisdom 
> in making such a request is another discussion.
> 
>  				Charlie

As is the programmer's wisdom in assuming that pragma Suppress means
the check won't be made.

The intent of pragma Suppress is to improve the efficiency of generated
code by inhibiting checks that the programmer knows will not fail.
Suppressing a check that would fail if it were not suppressed, as in
the following:

    X : Some_Integer_Subtype := Some_Integer_Subtype'Last;

    declare
	pragma Suppress(Range_Check);
    begin
	X := X + 1;
    end;

is a misuse of the pragma, and results in erroneous execution (and
arbitrarily nasty results).  For example, the compiler is entitled to
assume that X is in the range of its subtype after the assignment, and
to perform optimizations based on that assumption.  The more clever the
compiler, and the more aggressively it optimizes, the more likely it is
to mess things up.

References:

RM95-11.5(26):
    If a given check has been suppressed, and the corresponding error
    situation occurs, the execution of the program is erroneous.

RM95-11.5(29) (a note):
    There is no guarantee that a suppressed check is actually removed;
    hence a pragma Suppress should be used only for efficience reasons.

-- 
Keith Thompson (The_Other_Keith) kst@cts.com <*>
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