[Bug middle-end/115606] return slot opt prevents tail calls

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115606

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note CALL_EXPR_RETURN_SLOT_OPT is set in the C++ front-end:
  else if (style == arg)
    {
      /* Just mark it addressable here, and leave the rest to
         expand_call{,_inline}.  */
      cxx_mark_addressable (slot);
      CALL_EXPR_RETURN_SLOT_OPT (call_expr) = true;
      call_expr = cp_build_init_expr (slot, call_expr);
    }

Which comes from:
  enum style_t { ctor, arg, pcc } style;

  if (AGGR_INIT_VIA_CTOR_P (aggr_init_expr))
    style = ctor;
#ifdef PCC_STATIC_STRUCT_RETURN
  else if (1)
    style = pcc;
#endif
  else
    {
      gcc_assert (TREE_ADDRESSABLE (type));
      style = arg;
    }

>- why return slot opt at the gimple level is target specific?

It is not from gimple level, it is from the front-end which comes from target
specific.