[Bug tree-optimization/120944] New: Incorrect optimization with accessing a volatile structure member

[bmei at broadcom dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120944

            Bug ID: 120944
           Summary: Incorrect optimization with accessing a volatile
                    structure member
           Product: gcc
           Version: 15.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bmei at broadcom dot com
  Target Milestone: ---

For the following snippet, in function foo, v is wrongly optimized to always
true and abort is called unconditionally despite the volatile structure and
pointer.  Function bar is compiled correctly. I checked that GCC has this
behaviour since 10.1.0.  

#include <stdlib.h>
typedef union {
  int u32;
  struct
  {
     int A:1;
     int B:2;
     int C:3;
  };
} u_t;

typedef union {
   volatile int u[3];
   volatile struct {
        u_t a;
        int b;
        int c;
   };
} DATA;

void foo (volatile DATA *d)
{
     d->a.u32 = ~0;
     u_t u = d->a;
     int v = u.A;
     if (v)
        abort();
}

void bar (volatile DATA *d)
{
     d->a.u32 = ~0;
     if (d->a.A)
        abort();
}

in the tree .optimized dump: 
void foo (volatile union DATA * d)
{
  union u_t u;

  <bb 2> [count: 0]:
  d_2(D)->D.3445.a.u32 ={v} -1;
  u ={v} d_2(D)->D.3445.a;
  abort ();

}


;; Function bar (bar, funcdef_no=12, decl_uid=3453, cgraph_uid=13,
symbol_order=12)

void bar (volatile union DATA * d)
{
  <unnamed-signed:1> _1;

  <bb 2> [local count: 1073741824]:
  d_3(D)->D.3445.a.u32 ={v} -1;
  _1 ={v} d_3(D)->D.3445.a.D.3437.A;
  if (_1 != 0)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [100.00%]

  <bb 3> [count: 0]:
  abort ();

  <bb 4> [local count: 1073741824]:
  return;

}