[Bug middle-end/21305] flag_delete_null_pointer_checks is target specific

[Bug middle-end/21305] flag_delete_null_pointer_checks is target specific

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21305

Richard Guenther <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-11 13:13:29 UTC ---
6.5.3.2/4 says that the behavior unary * operator is undefined if the
pointer is a null pointer.  For QOI reasons several targets / frontends have
-fdelete-null-pointer-checks disabled.

[Bug middle-end/21305] flag_delete_null_pointer_checks is target specific

[schlie at comcast dot net]

------- Additional Comments From schlie at comcast dot net  2005-05-19 03:55 -------
(In reply to comment #0)
> The current -fdelete_null_pointer_checks implementation makes assumptions that
> the target processor will trap on reads or writes to address zero.  Unfortunately,
> these assumptions are target (often operating system) specific.
> ...
> I propose we should add target macros, TARGET_NULL_POINTER_READABLE and
> TARGET_NULL_POINTER_WRITABLE and control the effects of the
> -fdelete_null_pointer_checks with that.  The current situation is that
> these transformations are enabled automatically at -O2 without regard to
> the target environment.

Correct, and as coppied from PR21479:

> >    "a null reference is undefined" => "may trap" => "will trap"
> >    is simply wrong, and is not justifyable; such an optimization
> >    is target specific, as it depends on "will trap" target semantics.
> 
> Right. However, the logic here is simply "a null pointer dereference is
> undefined" => "if you still do it, your code may behave however gcc feels
> like", which is backed by the C standard. So this is invalid.

No, only the "null pointer dereference" itself is undefined. which means
that upon a null pointer reference any or no value may be returned.

Is says, implies, and grants no rights what so ever to an implementation,
to define that an arbitrary behavior will occure which may be subsequenlty
relied upon to occure unless the implementation inforces that behavior.

More specifically, unless GCC can warrent that a "null pointer dereference"
will trap will terminate program execution, it must preserve the semantics
of the remaining programs execution as defined by the standard, which
includes but not limited to preserving null-pointer comparision semantics,
as defined by the standard; as not to do so would be in violation of the same.

("If -O2 or greater, enable -fdelete-null-pointer-checks" can not be
 unconditionally enabled without violating the semantics of the language.)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21305

[Bug middle-end/21305] flag_delete_null_pointer_checks is target specific

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
OtherBugsDependingO|                            |21479
              nThis|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21305

[Bug middle-end/21305] flag_delete_null_pointer_checks is target specific

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-04-30 21:19 -------
Note the flag is old, it comes from:
Thu Sep 23 13:55:21 1999  Jeffrey A Law  (law@cygnus.com)
        
        * invoke.texi: Document -fdelete-null-pointer-checks
        * toplev.c (flag_delete_null_pointer_checks): New.
        (f_options): Add entry for -fdelete-null-pointer-checks.
        (rest_of_compilation): Conditionalize null pointer check
        elimination on flag_delete_null_pointer_checks.
        (main): If -O2 or greater, enable -fdelete-null-pointer-checks
        

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21305

[Bug middle-end/21305] flag_delete_null_pointer_checks is target specific

[roger at eyesopen dot com]

------- Additional Comments From roger at eyesopen dot com  2005-04-30 21:18 -------
Apparently the behaviour of this code is undefined in the C/C++ language
standards, though this transformation is performed in front-end independent
code.  Perhaps its only a quality of implementation issue.


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21305

[Bug middle-end/21305] flag_delete_null_pointer_checks is target specific

[pinskia at gcc dot gnu dot org]

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
          Component|tree-optimization           |middle-end


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21305