[Bug tree-optimization/86085] New: I/O built-ins considered argument clobbers

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86085

            Bug ID: 86085
           Summary: I/O built-ins considered argument clobbers
           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: ---

The buggy test case in pr67610 should be diagnosed by -Wrestrict but isn't. 
Debugging shows that the strlen pass discards the length information for the
array in g() in response to the alias oracle determining that the printf() call
might clobber the array argument.  Except for printf with the %n directive,
standard output functions don't modify their arguments.

$ cat z.c && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=/dev/stdout z.c
void f (void)
{
  char s[] = "123";
  char d[sizeof s];

  __builtin_sprintf (d, "%s", s);   // eliminated (transformed to strcpy by
sprintf pass)

  if (__builtin_strlen (s) != 3)    // folded
    __builtin_abort ();

  if (__builtin_strlen (d) != 3)    // also folded
    __builtin_abort ();
}

void g (void)
{
  char s[] = "123";

  __builtin_puts (s);

  if (__builtin_strlen (s) != 3)   // not folded
    __builtin_abort ();
}


;; Function f (f, funcdef_no=0, decl_uid=1956, cgraph_uid=0, symbol_order=0)

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

}



;; Function g (g, funcdef_no=1, decl_uid=1961, cgraph_uid=1, symbol_order=1)

g ()
{
  char s[4];
  long unsigned int _1;

  <bb 2> [local count: 1073741825]:
  s = "123";
  __builtin_puts (&s);
  _1 = __builtin_strlen (&s);
  if (_1 != 3)
    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;

}