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

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

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

--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> 2012-10-18 13:35:05 UTC ---
>From quick skimming of tree-sra.c, I'd say we could add another bool flag like
grp_to_be_replaced (say grp_to_be_debug_replaced), and in the else block of
  if (allow_replacements && scalar && !root->first_child
      && (root->grp_hint
          || ((root->grp_scalar_read || root->grp_assignment_read)
              && (root->grp_scalar_write || root->grp_assignment_write))))
do something like
    if (MAY_HAVE_DEBUG_STMTS && allow_replacements
        && scalar && !root->first_child
        && (root->grp_scalar_write || root->grp_assignment_write))
      root->grp_to_be_debug_replaced = 1;
(in addition to what the else block already does) and then in the actual
modification of stmts for grp_to_be_debug_replaced prepend a debug stmt before
the statement (or in the middle of statements).

For the two stmts that modify such grp_to_be_debug_replaced = 1; access,
  a[0] = 1;
and
  MEM[(char * {ref-all})&a] = MEM[(char * {ref-all})&D.1719];
we would have:
  DEBUG a$0 => 1         // Newly added stmt
  a[0] = 1;
and
  SR.2_14 = 4;
  # DEBUG SR.2 => SR.2_14
  SR.3_13 = 5;
  # DEBUG SR.3 => SR.3_13
  SR.4_12 = 6;
  # DEBUG SR.4 => SR.4_12
  # DEBUG a$0 => SR.2_14           // Newly added stmt
  a$1_9 = SR.3_13;
  # DEBUG a$1 => a$1_9
  a$2_4 = SR.4_12;
  # DEBUG a$2 => a$2_4

Martin, is that possible, does it make sense and where are all the places in
the sra_modify_* etc. code that would need to be tweaked?