[Bug c++/32716] [4.2/4.3 Regression] Wrong code generation. Alias and C++ virtual bases problem.

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

------- Comment #2 from rguenth at gcc dot gnu dot org  2007-07-10 11:29 -------
This is an aliasing problem (or rather a C++ FE problem):

  # SFT.41_31 = VDEF <SFT.41_30(D)>
  d.D.2508.a = 0;
  x.0_10 = (struct A *) &d;
...
  D.2747_16 = x.0_10 + D.2746_15;
  # VUSE <SFT.44_27>
  D.2748_17 = D.2747_16->a;
  D.2749_18 = D.2748_17 + 1;
  # SFT.44_34 = VDEF <SFT.44_27>
  D.2747_16->a = D.2749_18;
  # VUSE <SFT.41_31>
  D.2707_1 = d.D.2508.a;
  D.2706_2 = D.2707_1 != 1;
  return D.2706_2;

note how we don't identify the contrived access through the virtual base
with the zero-initialization and use in main().  Instead, it seems to
alias with

  iftmp.1_6 = (int (*__vtbl_ptr_type) (void) *) D.2735_5;
  # SFT.44_21 = VDEF <SFT.44_20(D)>
  d.D.2504._vptr.C = iftmp.1_6;
...
  # SFT.44_27 = VDEF <SFT.44_21>
  d.D.2504._vptr.C = &_ZTV1D[3];


Of course the original trees created for the virtual base access is
"somewhat" contrieved:

  (void) ((struct A *) (struct D *) x + (long unsigned int) *(long int *)
(((struct D *) x)->D.2504._vptr.C + 0xffffffffffffffffffffffffffffffe8))->a++ ;

while in main() we manage to do:

    struct D d;
  <<cleanup_point <<< Unknown tree: expr_stmt
  __comp_ctor  (&d) >>>
>>;
  <<cleanup_point <<< Unknown tree: expr_stmt
  (void) (d.D.2508.a = 0) >>>
>>;
  <<cleanup_point <<< Unknown tree: expr_stmt
  h ((struct D &) &d) >>>
>>;
  return <retval> = d.D.2508.a != 1;


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |dberlin at gcc dot gnu dot
                   |                            |org, rguenth at gcc dot gnu
                   |                            |dot org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
 GCC target triplet|arm-none-eabi               |
           Keywords|                            |alias, wrong-code
      Known to fail|                            |4.2.0 4.2.1 4.3.0
      Known to work|                            |4.1.2
   Last reconfirmed|0000-00-00 00:00:00         |2007-07-10 11:29:31
               date|                            |
            Summary|Wrong code generation.      |[4.2/4.3 Regression] Wrong
                   |Inlining problem.           |code generation. Alias and
                   |                            |C++ virtual bases problem.
   Target Milestone|---                         |4.3.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32716