[Bug tree-optimization/100366] spurious warning - std::vector::clear followed by std::vector::insert(vec.end(), ...) with -O2

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100366

Marc Glisse <glisse at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2021-05-02
     Ever confirmed|0                           |1
          Component|c++                         |tree-optimization
           Keywords|                            |diagnostic,
                   |                            |missed-optimization
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
Assuming the warning happens during the strlen pass, we are still missing a lot
of optimizations at that point

  if (_6 != _7)
    goto <bb 4>; [70.00%]
  else
    goto <bb 3>; [30.00%]

  <bb 3> [local count: 322122544]:
  _158 = _7 - _6;

once VRP2 (2 passes after strlen) replaces _158 with 0 and propagates it, maybe
the code becomes nice enough to avoid confusing this fragile warning (I didn't
check).

Before FRE3, we have

  _6 = vec_2(D)->D.33506._M_impl.D.32819._M_start;
  _7 = vec_2(D)->D.33506._M_impl.D.32819._M_finish;
  if (_6 != _7)
    goto <bb 3>; [70.00%]
  else
    goto <bb 4>; [30.00%]

  <bb 4> [local count: 1073741824]:
  _5 = MEM[(char * const &)vec_2(D) + 8];
  MEM[(struct __normal_iterator *)&D.33862] ={v} {CLOBBER};
  MEM[(struct __normal_iterator *)&D.33862]._M_current = _5;
  __position = D.33862;
  _12 = MEM[(const char * const &)vec_2(D)];
  _13 = MEM[(const char * const &)&__position];
  _14 = _13 - _12;

and after FRE3

  <bb 4> [local count: 1073741824]:
  _5 = MEM[(char * const &)vec_2(D) + 8];
  MEM[(struct __normal_iterator *)&D.33862] ={v} {CLOBBER};
  MEM[(struct __normal_iterator *)&D.33862]._M_current = _5;
  __position = D.33862;
  _14 = _5 - _6;

Only PRE manages to notice that _5 is the same as _7, which is already late.
And it then takes until VRP2 to realize that _7 - _6 must be 0 in the else
branch of _6 != _7.

* I am not sure why FRE manages to optimize _12 and not _5, that seems like the
first thing to check (maybe the +8 means it is obviously "partial")
* I don't know if some other pass than VRP could learn that b-a is 0 if not
a!=b.