[tree-ssa-strlen] possible strlen optimization for memcmp/strcmp of arrays

[xiangzhai]

Hi fellows,

As PR82950 mentioned:
Testcase:

$ cat c.c
char a[4];
char b[4];

int f (void)
{
  __builtin_strcpy (a, "abc");
  __builtin_strcpy (b, "abc");
  return __builtin_strcmp (a, b);   // not folded but could be
}

int g (const char *s)
{
  __builtin_strcpy (a, s);
  __builtin_strcpy (b, s);
  return __builtin_strcmp (a, b);   // not folded but could be
}

GCC 5.5 and 8x. failed to folded:

$ /opt/loongson-gnu-5.5/bin/mips64-linux-gnu-gcc -O2 -S -Wall -Wextra -Wpedantic -fdump-tree-optimized=/dev/stdout t.c

;; Function f (f, funcdef_no=0, decl_uid=1486, cgraph_uid=0, symbol_order=0)

f (char * d)
{
  void * _4;
  long unsigned int _6;
  unsigned int _7;

  <bb 2>:
  __builtin_memcpy (d_2(D), "abc", 3);
  _4 = d_2(D) + 3;
  __builtin_memcpy (_4, "def", 4);
  _6 = __builtin_strlen (d_2(D));
  _7 = (unsigned int) _6;
  return _7;

}



;; Function g (g, funcdef_no=1, decl_uid=1489, cgraph_uid=1, symbol_order=1)

g (char * d)
{
  void * _4;
  long unsigned int _6;
  unsigned int _7;

  <bb 2>:
  _4 = __builtin_mempcpy (d_2(D), "abc", 3);
  __builtin_mempcpy (_4, "def", 4);
  _6 = __builtin_strlen (d_2(D));
  _7 = (unsigned int) _6;
  return _7;

}

Regards,
Leslie Zhai