[Bug tree-optimization/86318] New: const local aggregates can be assumed not to be modified even when escaped

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86318

            Bug ID: 86318
           Summary: const local aggregates can be assumed not to be
                    modified even when escaped
           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: ---

Because the local object in all three function is defined const it can be
assumed not to change as a result of the call to f()  (otherwise the behavior
of a program that changed it would be undefined).  GCC takes advantage of this
constraint when the object is of a basic/fundamental type like in g0() but not
when the object is an aggregate like in g1() or g2(), unless the object has a
static storage duration.

$ cat c.c && gcc -S -O2 -Wall -Wextra -Wpedantic -Wstrict-prototypes
-fdump-tree-optimized=/dev/stdout c.c
struct S { int i; };

void f (const void*);

void g0 (void)
{
  const int i = 1;

  f (&i);

  if (i != 1)             // folded into false
    __builtin_abort ();
}

void g1 (void)
{
  const int a[1] = { 1 };

  f (a);

  if (*a != 1)             // not folded but could be
    __builtin_abort ();
}

void g2 (void)
{
  const struct { int i; } s = { 1 };

  f (&s.i);

  if (s.i != 1)             // not folded but could be
    __builtin_abort ();
}

;; Function g0 (g0, funcdef_no=0, decl_uid=1902, cgraph_uid=1, symbol_order=0)

g0 ()
{
  const int i;

  <bb 2> [local count: 1073741825]:
  i = 1;
  f (&i);
  i ={v} {CLOBBER};
  return;

}



;; Function g1 (g1, funcdef_no=1, decl_uid=1906, cgraph_uid=2, symbol_order=1)

g1 ()
{
  const int a[1];
  int _1;

  <bb 2> [local count: 1073741825]:
  a[0] = 1;
  f (&a);
  _1 = a[0];
  if (_1 != 1)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [99.96%]

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

  <bb 4> [local count: 1073312327]:
  a ={v} {CLOBBER};
  return;

}



;; Function g2 (g2, funcdef_no=2, decl_uid=1910, cgraph_uid=3, symbol_order=2)

g2 ()
{
  const struct 
  {
    int i;
  } s;
  int _1;

  <bb 2> [local count: 1073741825]:
  s.i = 1;
  f (&s.i);
  _1 = s.i;
  if (_1 != 1)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [99.96%]

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

  <bb 4> [local count: 1073312327]:
  s ={v} {CLOBBER};
  return;

}