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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/112653] We should optimize memmove to memcpy using
 alias oracle
Date: Wed, 22 Nov 2023 10:46:04 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112653

--- Comment #4 from Richard Biener <rguenth at gcc dot gnu.org> ---
We do use the alias oracle in folding memmove:

          /* If the destination and source do not alias optimize into
             memcpy as well.  */
          if ((is_gimple_min_invariant (dest)
               || TREE_CODE (dest) =3D=3D SSA_NAME)
              && (is_gimple_min_invariant (src)
                  || TREE_CODE (src) =3D=3D SSA_NAME))
            {
              ao_ref destr, srcr;
              ao_ref_init_from_ptr_and_size (&destr, dest, len);
              ao_ref_init_from_ptr_and_size (&srcr, src, len);
              if (!refs_may_alias_p_1 (&destr, &srcr, false))
                {
                  tree fn;
                  fn =3D builtin_decl_implicit (BUILT_IN_MEMCPY);
                  if (!fn)
                    return false;

but the issue is that test2 escapes which makes this conflict:

  # PT =3D null { D.2775 } (escaped, escaped heap)
  # ALIGN =3D 8, MISALIGN =3D 0
  # USE =3D nonlocal escaped
  # CLB =3D nonlocal escaped
  test2_4 =3D __builtin_malloc (1000);
  # PT =3D nonlocal escaped null
  test.0_1 =3D test;
  __builtin_memmove (test2_4, test.0_1, 1000);

it works for

char *test, *test3;
void
copy_test ()
{
        char *test2 =3D __builtin_malloc (1000);
        __builtin_memmove (test2, test, 1000);
        __builtin_memmove (test3, test2, 1000);
  __builtin_free (test2);
}

where both memmove calls become memcpy.  So this isn't asking for better
folding but for better pointer analysis I guess.=