[Bug c++/84151] [6/7/8 Regression] g++ generates two identical loads in a volatile-qualified member function.

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84151

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2018-02-01
   Target Milestone|---                         |6.5
            Summary|[4.9/5/6/7 Regression] g++  |[6/7/8 Regression] g++
                   |generates two identical     |generates two identical
                   |loads in a                  |loads in a
                   |volatile-qualified member   |volatile-qualified member
                   |function.                   |function.
     Ever confirmed|0                           |1

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
The C++ FE generates

;; Function int A::foo() volatile (null)
;; enabled by -tree-original


{
  int v;

    int v;
  <<cleanup_point <<< Unknown tree: expr_stmt
  (void) (v = (int) ((volatile struct A *) this)->c) >>>>>;
  <<cleanup_point return <retval> = *(volatile struct A *) this;, *A::bar ((int
&) &v);>>;
}

which results in

int A::foo() volatile (volatile struct A * const this)
{
  int v;
  struct A vol.0;
  int _1;
  int & _2;
  int _8;

  <bb 2> [0.00%]:
  _1 ={v} this_4(D)->c;
  v = _1;
  vol.0 ={v} *this_4(D);
  _2 = A::bar (&v);
  _8 = *_2;
  v ={v} {CLOBBER};
  return _8;


I'm not sure how the behavior of a volatile this is specified but this
unused copy of *this looks unwarranted.