[Bug debug/54971] New: SRA pessimizes debug info by not creating debug stmts for fields without replacements

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=54971

             Bug #: 54971
           Summary: SRA pessimizes debug info by not creating debug stmts
                    for fields without replacements
    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: jamborm@gcc.gnu.org


Consider the testcase in PR54970, or the following one.

int
main ()
{
  int a[] = { 1, 2, 3 };
  int *p = a + 2;
  int *q = a + 1;
  asm volatile ("nop");
  *p += 10;
  asm volatile ("nop");
  *q += 10;
  asm volatile ("nop");
  __builtin_memcpy (&a, (int [3]) { 4, 5, 6 }, sizeof (a));
  asm volatile ("nop");
  *p += 10;
  asm volatile ("nop");
  *q += 10;
  asm volatile ("nop");
  return 0;
}

For a[1] and a[2] replacements are created and we eventually get corret debug
info, tracking the value changes of those array elements.
But for a[0] SRA decides not to create a replacement, as it seems to be only
assigned and never read (in PR54970 testcase just once, in the above testcase
twice).
Would it be possible for MAY_HAVE_DEBUG_STMTS to create also replacements for
those fields/elements that are only ever assigned, and at the places they used
to be assigned instead of setting the replacement to the value it should have
create a debug bind stmt?  In the above testcase that would be at the place
of former a[0] = 1; add DEBUG a$0 => 1 stmt (where a$0's DECL_DEBUG_EXPR would
be a[0]), and at the point of the MEM[&a, 0] = MEM[something, 0] assignment
DEBUG a$0 => 4 ?