[Bug tree-optimization/84794] New: memcpy between a struct and its first member not eliminated

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84794

            Bug ID: 84794
           Summary: memcpy between a struct and its first member not
                    eliminated
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
  Target Milestone: ---

GCC eliminates most calls to memcpy where the first two arguments are
guaranteed to be the same.  However, it fails to eliminate such calls when one
of the arguments points to a struct object and the other to the first member of
the same struct object.

$ cat z.c && gcc -O2 -S -Wall -Wextra -Wpedantic  -Wrestrict
-fdump-tree-optimized=/dev/stdout z.c
extern void* memcpy (void* restrict, const void* restrict, __SIZE_TYPE__);

struct S {
  union {
    char a[4], b[4];
  };
} s;

void f (struct S *p)
{
  if ((void*)p != p->a || (void*)p != p->b)   // folded to false (good)
    __builtin_abort ();                       // eliminated
}

void g (struct S *p, unsigned n)
{
  memcpy (p->a, p->b, n);   // undefined and eliminated (good)
}

void h (struct S *p, unsigned n)
{
  memcpy (p, p->b, n);   // also undefined but not eliminated
}



;; Function f (f, funcdef_no=0, decl_uid=1968, cgraph_uid=0, symbol_order=1)

f (struct S * p)
{
  <bb 2> [local count: 1073741825]:
  return;

}


z.c: In function ‘g’:
z.c:17:3: warning: ‘memcpy’ source argument is the same as destination
[-Wrestrict]
   memcpy (p->a, p->b, n);   // -Wrestrict (good)
   ^~~~~~~~~~~~~~~~~~~~~~

;; Function g (g, funcdef_no=1, decl_uid=1972, cgraph_uid=1, symbol_order=2)

g (struct S * p, unsigned int n)
{
  <bb 2> [local count: 1073741825]:
  return;

}



;; Function h (h, funcdef_no=2, decl_uid=1976, cgraph_uid=2, symbol_order=3)

h (struct S * p, unsigned int n)
{
  long unsigned int _1;
  char[4] * _2;

  <bb 2> [local count: 1073741825]:
  _1 = (long unsigned int) n_3(D);
  _2 = &p_4(D)->D.1965.b;
  memcpy (p_4(D), _2, _1); [tail call]
  return;

}