Re: Storage for uninitialized objects (PR 24626)

["John David Anglin" <dave at hiauly1 dot hia dot nrc dot ca>]

> > This seems pretty clear. C99 requires that storage be allocated
> >
> >for uninitialized objects, that an indeterminate value be stored
> >in the object when the declarator for the object is reached in the
> >block, that the last-stored value be retained for the duration of
> >the block.
> >  
> >
> I think that is an incorrect interpretation. Remember the standard is
> always "as-if" when it gives an implementation approach. Please
> show a correct C program that can tell that gcc is not following
> the above scheme.

#include <stdio.h>
unsigned char
T (unsigned char x)
{
  static int first = 1;
  static unsigned char firstx;

  if (first)
    {
      first = 0;
      firstx = x;
      return ~x;
    }

  if (x == firstx)
    printf ("Behavior is pre GCC 4.0\n");
  else
    printf ("Behavior is GCC 4.0 and later\n");
  return 0;
}

*** cut ***

extern int T (unsigned char);
int
foo (void)
{
  int result;
  unsigned char x;

  result = T (x);
  T (x);
  return result;
}
int
main ()
{
  return foo ();
}

Compile as follows:

gcc-3.4 -O2 -c T.c
gcc-3.4 -o und -O2 und.c T.o
./und
Behavior is pre GCC 4.0
gcc-4.0 -o und -O2 und.c T.o
Behavior is GCC 4.0 and later

That's what I see on hppa-unknown-linux-gnu.  The 4.0 behavior is
caused by GCC using result for the second call to T, whereas 3.3
and 3.4 use the same undefined value for both calls to T.

Dave
-- 
J. David Anglin                                  dave.anglin@nrc-cnrc.gc.ca
National Research Council of Canada              (613) 990-0752 (FAX: 952-6602)