[Bug c/108298] Wrong optimization of volatile access from gcc 11 and beyond

[segher at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108298

--- Comment #5 from Segher Boessenkool <segher at gcc dot gnu.org> ---
This is not x86-specific.  Like on powerpc64 we get

        addi 3,3,3       # 11   [c=4 l=4]  *addsi3/1
        extsw 3,3        # 17   [c=4 l=4]  extendsidi2/1
        blr              # 25   [c=4 l=4]  simple_return

Before RTL all is fine still:

int foo (int a, int b, int c)
{
  int _1;
  int _2;

;;   basic block 2, loop depth 0
;;    pred:       ENTRY
  _1 = a_3(D) + 1;
  b = _1;
  _2 ={v} MEM[(volatile int *)&b];
  a_6 = _2 + 2;
  return a_6;
;;    succ:       EXIT

}

But it is expanded to something that is not going through memory:

;; _2 ={v} MEM[(volatile int *)&b];

(insn 10 9 0 (set (reg:SI 118 [ _2 ])
        (subreg/s/u:SI (reg/v:DI 121 [ b+-4 ]) 4)) "108298.c":10:9 -1
     (nil))