[Bug debug/54970] New: Missing DW_OP_GNU_implicit_pointer in debuginfo

[jakub at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54970

             Bug #: 54970
           Summary: Missing DW_OP_GNU_implicit_pointer in debuginfo
    Classification: Unclassified
           Product: gcc
           Version: 4.7.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
        AssignedTo: jakub@gcc.gnu.org
        ReportedBy: jakub@gcc.gnu.org
                CC: aoliva@gcc.gnu.org


/* { dg-do run } */
/* { dg-options "-g" } */

#include "../nop.h"

int
main ()
{
  int a[] = { 1, 2, 3 };        /* { dg-final { gdb-test 12 "a\[1\]" "2" } } */
  int *p = a + 2;               /* { dg-final { gdb-test 12 "a\[2\]" "3" } } */
  int *q = a + 1;               /* { dg-final { gdb-test 12 "*p" "3" } } */
  asm volatile ("NOP");         /* { dg-final { gdb-test 12 "*q" "2" } } */
  *p += 10;                     /* { dg-final { gdb-test 16 "a\[1\]" "2" } } */
                                /* { dg-final { gdb-test 16 "a\[2\]" "13" } }
*/
                                /* { dg-final { gdb-test 16 "*p" "13" } } */
  asm volatile ("NOP");         /* { dg-final { gdb-test 16 "*q" "2" } } */
  *q += 10;                     /* { dg-final { gdb-test 20 "a\[1\]" "12" } }
*/
                                /* { dg-final { gdb-test 20 "a\[2\]" "13" } }
*/
                                /* { dg-final { gdb-test 20 "*p" "13" } } */
  asm volatile ("NOP");         /* { dg-final { gdb-test 20 "*q" "12" } } */
  return 0;
}

fails, apparently because we have # DEBUG p => &MEM[(void *)&a + 8B]
and we fail to expand that as (plus (debug_implicit_ptr a) (const_int 8)).
The reason for that is that we don't want (mem (debug_implicit_ptr x)) to
appear
in debug insns, that confuses aliasing.  When expanding &a, we expand it as
DEBUG_IMPLICIT_PTR, but when returning to MEM, we return NULL, as that is not
appropriate thing to construct.