[Bug tree-optimization/86043] New: strlen after memcpy partially overwriting a string not optimized

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86043

            Bug ID: 86043
           Summary: strlen after memcpy partially overwriting a string not
                    optimized
           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 tree-ssa-strlen optimization has code to detect non-nul character stores
into the initial elements of a string of a known length and avoid invalidating
its length information, but it's missing the same optimization for
corresponding stores by memcpy (or similarly, strcpy).

$ cat d.c && gcc -O2 -S -Wall -Wextra -fdump-tree-optimized=/dev/stdout d.c
char a[5];

int f (void)
{
  __builtin_strcpy (a, "123");

  a[0] = '1';
  return __builtin_strlen (a);       // folded, good
}

int g (void)
{
  __builtin_strcpy (a, "123");

  __builtin_memcpy (a + 1, "2", 1);
  return __builtin_strlen (a);       // not folded
}

;; Function f (f, funcdef_no=0, decl_uid=1957, cgraph_uid=0, symbol_order=1)

f ()
{
  <bb 2> [local count: 1073741825]:
  __builtin_memcpy (&a, "123", 4);
  return 3;

}



;; Function g (g, funcdef_no=1, decl_uid=1960, cgraph_uid=1, symbol_order=2)

g ()
{
  long unsigned int _1;
  int _5;

  <bb 2> [local count: 1073741825]:
  __builtin_memcpy (&a, "123", 4);
  __builtin_memcpy (&MEM[(void *)&a + 1B], "2", 1);
  _1 = __builtin_strlen (&a);
  _5 = (int) _1;
  return _5;

}