[Bug tree-optimization/83444] New: [5/6/7/8 Regression] missing strlen optimization on a member array of a local struct

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83444

            Bug ID: 83444
           Summary: [5/6/7/8 Regression] missing strlen optimization on a
                    member array of a local struct
           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 4.9 is the last release to transform both of the functions below into a
no-op.  Starting with 5.1, only the first function is so optimized while the
second is not.

$ cat z.c && gcc -O2 -S -Warray-bounds=2 -fdump-tree-optimized=/dev/stdout z.c
void f (void)
{
  char a[5] = "012";

  __builtin_strcpy (a, "");        // folded into *a = 0

  if (__builtin_strlen (a) != 0)   // folded into false
    __builtin_abort ();            // eliminated
}

void g (void)
{
  char a[5] = "012";
  char b[7] = "";

  __builtin_strcpy (a, b);         // folded into *a = 0

  if (__builtin_strlen (a) != 0)   // not folded
    __builtin_abort ();            // not eliminated
}

;; Function f (f, funcdef_no=0, decl_uid=1892, cgraph_uid=0, symbol_order=0)

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

}



;; Function g (g, funcdef_no=1, decl_uid=1896, cgraph_uid=1, symbol_order=1)

g (int i)
{
  char b[7];
  char a[5];
  char _1;
  unsigned char _9;

  <bb 2> [local count: 1073741825]:
  a = "012";
  b = "";
  _9 = MEM[(char * {ref-all})&b];
  MEM[(char * {ref-all})&a] = _9;
  _1 = MEM[(const char *)&a];
  if (_1 != 0)
    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};
  b ={v} {CLOBBER};
  return;

}