[Bug tree-optimization/82950] New: possible strlen optimization for memcmp/strcmp of arrays

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82950

            Bug ID: 82950
           Summary: possible strlen optimization for memcmp/strcmp of
                    arrays
           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: ---

While testing some unrelated work I noticed that GCC doesn't eliminate the call
to strcmp in the program below even though it could based on the knowledge that
the two arrays a and b contain a copy of the same string.  A nsimilar
optimization is possible for memcpy (regardless of the size specified by the
third argument).  It seems that a good place to add this optimization might be
the tree-ssa-strlen.c pass.

$ cat c.c && gcc -O2 -S -Wall -fdump-tree-optimized=/dev/stdout 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
}

;; Function f (f, funcdef_no=0, decl_uid=1893, cgraph_uid=0, symbol_order=2)

f ()
{
  int _4;

  <bb 2> [local count: 10000]:
  MEM[(char * {ref-all})&a] = "abc";
  MEM[(char * {ref-all})&b] = "abc";
  _4 = __builtin_strcmp (&a, &b); [tail call]
  return _4;

}



;; Function g (g, funcdef_no=1, decl_uid=1896, cgraph_uid=1, symbol_order=3)

g (const char * s)
{
  int _5;

  <bb 2> [local count: 10000]:
  __builtin_strcpy (&a, s_2(D));
  __builtin_strcpy (&b, s_2(D));
  _5 = __builtin_strcmp (&a, &b); [tail call]
  return _5;

}