[Bug lto/41058] FAIL: ext/pb_ds/regression/hash_data_map_rand.cc

[rguenth at gcc dot gnu dot org]

------- Comment #9 from rguenth at gcc dot gnu dot org  2009-08-27 15:43 -------
In fact the code generated by the C++ frontend (or the library) looks bogus.

  other.1157_15 = (const struct hash_eq_fn *) D.107435_2;
  this.1158_16 = (struct hash_eq_fn *) D.107436_4;
  other.1161_17 = (struct equal_to *) other.1157_15;
  this.1162_18 = (struct equal_to *) this.1158_16;
  __tmp = *this.1162_18;
  *this.1162_18 = *other.1161_17;
  *other.1161_17 = __tmp;

this swaps the first byte(!) of the struct which contains the vtable pointer.
In fact gp_hash_table::swap(gp_hash_table&) seems to cause this.  Forcing
to not inline that function fixes the issue as well.

(so the dead store really is not dead - but only because of that bogus swap).

Thus, things like

      void
      _M_swap(_Hash_code_base& __x)
      {
 std::swap(_M_extract, __x._M_extract);
 std::swap(_M_eq, __x._M_eq);
 std::swap(_M_h1, __x._M_h1);
 std::swap(_M_h2, __x._M_h2);
      }

    protected:
      _ExtractKey _M_extract;
      _Equal _M_eq;
      _H1 _M_h1;
      _H2 _M_h2;
    };

seem to corrupt the vtable pointer as the empty function classes are
tail-packed into that field.  OTOH std::swap should do nothing for
empty classes, but instead it does

;; Function void std::swap(_Tp&, _Tp&) [with _Tp =
std::equal_to<__gnu_pbds::test::basic_type>] (null)
;; enabled by -tree-original

{
  struct equal_to __tmp;

    struct equal_to __tmp;
  <<cleanup_point <<< Unknown tree: expr_stmt
  (void) (__tmp = TARGET_EXPR <D.100064, *(const struct equal_to &) (const
struct equal_to *) __a>) >>>
>>;
  <<cleanup_point <<< Unknown tree: expr_stmt
  (void) (*(struct equal_to *) __a = *(const struct equal_to &) (const struct
equal_to *) __b) >>>
>>;
  <<cleanup_point <<< Unknown tree: expr_stmt
  (void) (*(struct equal_to *) __b = *(const struct equal_to &) (const struct
equal_to *) &__tmp) >>>
>>;
}

bah.  Paolo, who is at fault here?  The library or the frontend?  The
issue must be latent on trunk.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |paolo dot carlini at oracle
                   |                            |dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41058