[Bug c/20318] RFE: add attribute to specify that a function never returns NULL

["dnovillo at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From dnovillo at gcc dot gnu dot org  2005-03-04 18:01 -------

Adding some of the early analysis that went on in private mail.


> So is there an __attribute__ that we can use to mark that function to
> always return non-NULL, so the compiler knows that path is not possible?
>
No.  There is __attribute__ ((noreturn)), but the compiler has no way of knowing
that svn_fs_fs__id_parse is guaranteed to return non-NULL.

This is what the compiler sees in the IL:

  root_id_41 = svn_fs_fs__id_parse (node_id_str_39, D.10885_40, pool_5);
  if (root_id_41 == 0B) goto <L13>; else goto <L17>;

<L13>:;
  svn_error__locate (...);
  D.10887_45 = dgettext (...);
  svn_err__temp_46 = svn_error_create (160004, 0B, D.10887_45);

  # root_id_257 = PHI <root_id_28(10), root_id_41(8)>;
<L17>:;
  [ ... ]
<L19>:;
  *root_id_p_30 = root_id_257;


At L19, it sees two possible values for root_id (root_id_28 and root_id_41). 
root_id_28 is the uninitialized value.

We don't have a "can't return NULL" attribute.  Though it would be useful.  In
this case, you're better off initializing *root_id_p at the start of the function.


Diego.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20318