[Bug c++/92559] Returning std∷map breaks tail-recursion optimization

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92559

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2019-11-19
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We cannot currently deal with the intermediate stmts in bb 10 and maybe also
not with the EH.  I'm not sure whether in the middle-end we really will
ever be able to elide the ctor/dtor of m around the recursive call.  Using
by-reference passing might be a way around this (I don't think the return
of std::map is really the issue).

  *_20(D) = foo (&D.34875); [return slot optimization]
;;    succ:       14 (EH,EXECUTABLE)
;;                10 (FALLTHRU,EXECUTABLE)

  <bb 10> :
  _40 = MEM[(struct _Rb_tree *)&D.34875]._M_impl.D.33073._M_header._M_parent;
  std::_Rb_tree<unsigned int, std::pair<const unsigned int, unsigned int>,
std::_Select1st<std::pair<const unsigned int, unsigned int> >,
std::less<unsigned int>, std::allocator<std::pair<const unsigned int, unsigned
int> > >::_M_erase (&D.34875._M_t, _40);
  D.34875 ={v} {CLOBBER};
  D.34875 ={v} {CLOBBER};
;;    succ:       11 (FALLTHRU,EXECUTABLE)

  <bb 11> :
  return _20(D);