[Bug tree-optimization/83821] New: local aggregate initialization defeats strlen optimization

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83821

            Bug ID: 83821
           Summary: local aggregate initialization defeats strlen
                    optimization
           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: ---

For the test case below, GCC optimizes the strlen call in f() but fails to do
the same in g().  It appears because the maybe_invalidate() function in the
pass considers the initialization of/assignment to b.i as possibly clobbering
the value of b.s, not realizing that there is no way for the two members to
alias.

$ cat z.c && gcc -O2 -S -fdump-tree-optimized=/dev/stdout z.c 
#define STR "0123456789"

struct A
{
  char s[sizeof STR];
};

void f (void)
{
  struct A a = { STR };
  if (__builtin_strlen (a.s) != sizeof STR - 1)   // folded
    __builtin_abort ();                           // eliminated
}

struct B
{
  char s[sizeof STR];
  int i;
};

void g (void)
{
  struct B b = { STR, 123 };
  if (__builtin_strlen (b.s) != sizeof STR - 1)   // not folded
    __builtin_abort ();                           // not eliminated
}


;; Function f (f, funcdef_no=0, decl_uid=1952, cgraph_uid=0, symbol_order=0)

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

}



;; Function g (g, funcdef_no=1, decl_uid=1959, cgraph_uid=1, symbol_order=1)

g ()
{
  struct B b;
  long unsigned int _1;

  <bb 2> [local count: 1073741825]:
  b.s = "0123456789";
  b.i = 123;
  _1 = __builtin_strlen (&b.s);
  if (_1 != 10)
    goto <bb 3>; [0.00%]
  else
    goto <bb 4>; [99.96%]

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

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

}