[Bug debug/54796] New: [4.8 Regression] Non-addressable stack parameter debug quality regression

["jakub at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54796

             Bug #: 54796
           Summary: [4.8 Regression] Non-addressable stack parameter debug
                    quality regression
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: jakub@gcc.gnu.org
                CC: aoliva@gcc.gnu.org
            Target: i686-linux


/* PR debug/NNNNN */
/* { dg-do run } */
/* { dg-options "-g" } */

__attribute__((noinline, noclone)) void
bar (char *a, int b)
{
  __asm volatile ("" : "+r" (a), "+r" (b) : : "memory");
}

__attribute__((noinline, noclone)) void
foo (int a, int b)
{
  int c = a;
  char d[b];/* { dg-final { gdb-test 17 "a" "5" } } */
  bar (d, 2);/* { dg-final { gdb-test 17 "b" "6" } } */
  bar (d, 4);/* { dg-final { gdb-test 17 "c" "5" } } */
}

int
main ()
{
  foo (5, 6);
  return 0;
}

(line 17 is the second call to bar) regressed for -m32 -Os -g on
{i686,x86_64}-linux supposedly with the
http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=188871
change.  I've mentioned this already in
http://gcc.gnu.org/ml/gcc-patches/2012-10/msg00262.html
but filing it now on a testcase that is not dependent on any GCC patches.

The problem is that vt_canon_true_dep/canon_true_dependence doesn't figure out
that the stack stores (after prologue where hard fp is initialized) can't alias
argp.  Without the VLA or other reasons forcing use of frame pointer
var-tracking replaces sp accesses with argp + offset accesses and
canon_true_dependence then disambiguates it using the offsets.