[Bug c++/14024] New: g++ isn't reporting aliasing warnings

[Bug c++/14024] New: g++ isn't reporting aliasing warnings

[tjw at omnigroup dot com]

int test(int input)
{
    float *f = (float *)&input;
    return input;
}


When built with gcc:

$PREFIX/bin/gcc -Wstrict-aliasing -fstrict-aliasing -c alias.c
/tmp/alias.c: In function `test':
/tmp/alias.c:3: warning: dereferencing type-punned pointer will break strict-aliasing rules

But, when the same input is built with g++, no warning is issued.  This is bad since -fstrict-aliasing 
does seem to be taking effect (i.e., my real program produces crazy output with -fstrict-aliasing 
on), but no warnings are generating, meaning that it will be potentially very hard to track down the 
issue.

-- 
           Summary: g++ isn't reporting aliasing warnings
           Product: gcc
           Version: 3.4.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tjw at omnigroup dot com
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: powerpc-apple-darwin7.2.0
  GCC host triplet: powerpc-apple-darwin7.2.0
GCC target triplet: powerpc-apple-darwin7.2.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024

[Bug c++/14024] g++ isn't reporting aliasing warnings

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2004-02-05 15:59 -------
Confirmed. Unfortunate indeed. 
 
W. 

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
           Keywords|                            |diagnostic
      Known to fail|                            |3.3.3 3.4.0 3.5.0
   Last reconfirmed|0000-00-00 00:00:00         |2004-02-05 15:59:47
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024

[Bug c++/14024] g++ isn't reporting aliasing warnings

[pinskia at gcc dot gnu dot org]

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024

[Bug c++/14024] g++ isn't reporting aliasing warnings

[uros at kss-loka dot si]

------- Additional Comments From uros at kss-loka dot si  2004-12-29 07:49 -------
This example is from PR 16111:
#include <stdio.h>

float foo ()
{
  unsigned int MASK = 0x80000000;
  return (float &) MASK;
}

int main() {
  printf ("%f\n", foo());
  return 0;
}

'g++ -Wstrict-aliasing' does not generate a warning about breaking
strict-aliasing rules. When '(float&)MASK' is replaced with equivalent '*(float
*)&MASK', 'gcc -Wstrict-aliasing' produces expected warning.


-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |djp at volny dot cz
  GCC build triplet|powerpc-apple-darwin7.2.0   |*-*-*
   GCC host triplet|powerpc-apple-darwin7.2.0   |*-*-*
 GCC target triplet|powerpc*-*-*                |*-*-*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024

[Bug c++/14024] g++ isn't reporting aliasing warnings

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-09-16 14:15 -------
*** Bug 23915 has been marked as a duplicate of this bug. ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |larschri at pvv dot ntnu dot
                   |                            |no


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024

[Bug c++/14024] g++ isn't reporting aliasing warnings

[rguenth at gcc dot gnu dot org]

------- Comment #4 from rguenth at gcc dot gnu dot org  2005-11-24 10:48 -------
Subject: Bug 14024

Author: rguenth
Date: Thu Nov 24 10:48:15 2005
New Revision: 107459

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=107459
Log:
2005-11-24  Richard Guenther  <rguenther@suse.de>
        Dirk Mueller <dmueller@suse.de>

        PR c++/14024
        * c-common.h (strict_aliasing_warning): Declare.
        * c-common.c (strict_aliasing_warning): New function,
        split out from ...
        * c-typeck.c (build_c_cast): ... here.

        * typeck.c (build_reinterpret_cast_1): Use it.

        * g++.dg/warn/Wstrict-aliasing-1.C: New testcase.
        * g++.dg/warn/Wstrict-aliasing-2.C: Likewise.
        * g++.dg/warn/Wstrict-aliasing-3.C: Likewise.
        * g++.dg/warn/Wstrict-aliasing-4.C: Likewise.
        * g++.dg/warn/Wstrict-aliasing-5.C: Likewise.
        * g++.dg/warn/Wstrict-aliasing-6.C: Likewise.

Added:
    trunk/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-1.C
    trunk/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-2.C
    trunk/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-3.C
    trunk/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-4.C
    trunk/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-5.C
    trunk/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-6.C
Modified:
    trunk/gcc/ChangeLog
    trunk/gcc/c-common.c
    trunk/gcc/c-common.h
    trunk/gcc/c-typeck.c
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/typeck.c
    trunk/gcc/testsuite/ChangeLog


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024

[Bug c++/14024] g++ isn't reporting aliasing warnings

[rguenth at gcc dot gnu dot org]

------- Comment #5 from rguenth at gcc dot gnu dot org  2005-11-24 10:55 -------
Fixed in 4.2.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
      Known to fail|3.3.3 3.4.0 3.4.4 4.0.0     |3.3.3 3.4.0 3.4.4 4.0.0
                   |                            |4.1.0
      Known to work|                            |4.2.0
         Resolution|                            |FIXED
   Target Milestone|---                         |4.2.0


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024

[Bug c++/14024] g++ isn't reporting aliasing warnings

[howarth at nitro dot med dot uc dot edu]

------- Comment #6 from howarth at nitro dot med dot uc dot edu  2005-12-07 16:14 -------
Can we please have the fix for PR 14024 backported into gcc 4.1? It will be
very difficult
to get upstream packages like swig for c++ fixed with regards to the
dereferencing type-punned
pointers breaking strict-aliasing if I have to force them to use an unstable
trunk version of
gcc to do so.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024

[Bug c++/14024] g++ isn't reporting aliasing warnings

[pinskia at gcc dot gnu dot org]

------- Comment #7 from pinskia at gcc dot gnu dot org  2005-12-07 16:19 -------
(In reply to comment #6)
> Can we please have the fix for PR 14024 backported into gcc 4.1? It will be
> very difficult to get upstream packages like swig for c++ fixed with regards to the
> dereferencing type-punned
> pointers breaking strict-aliasing if I have to force them to use an unstable
> trunk version of gcc to do so.

Why?  Most of the places where aliasing violations happen are very obvious (see
the example in the PR).
You just need to go through every source and audit it (which should happen for
overflows and all other undefined behaviors).  The warning is only for helping
and nothing else.  There are a lot of other undefined behaviors which GCC does
not warn for which most people forget to look for.  One is signed overflow
being undefined.  If we warn for that, almost every single line would warn as a
= b+c; could overflow.  There are many more.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024

[Bug c++/14024] g++ isn't reporting aliasing warnings

[mueller at kde dot org]

------- Comment #8 from mueller at kde dot org  2005-12-07 16:33 -------
that comparison isn't quite fair. strict aliasing is an optimisation that
breaks code when compiled with a newer version of gcc, and there is lots of
code to fix because of that. 

Sure, you can fix it by reading through all of the source code and debugging
where a particular failure comes from, but its easier to recompile with Wall
and grep the build log. 


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024

[Bug c++/14024] g++ isn't reporting aliasing warnings

[pinskia at gcc dot gnu dot org]

------- Comment #9 from pinskia at gcc dot gnu dot org  2005-12-07 16:38 -------
(In reply to comment #8)
> that comparison isn't quite fair. strict aliasing is an optimisation that
> breaks code when compiled with a newer version of gcc, and there is lots of
> code to fix because of that. 

Actually it is.  Since GCC now uses the fact that signed types overflow is
undefined more than before.
I know since I was the one who made GCC use that fact more.  (there are even
more going in for 4.2).
Aliasing is not that much different as every one who writes C or C++ should
know the rules about aliasing and signed overflow but most don't.  Though
aliasing violations is the most reported bug, we still get bugs about signed
overflow.  Both have a way to turn off the optimization that uses the facts
that the standard provides.  Though both make the code really invalid C++.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024

[Bug c++/14024] g++ isn't reporting aliasing warnings

[rguenth at gcc dot gnu dot org]

------- Comment #10 from rguenth at gcc dot gnu dot org  2005-12-08 11:24 -------
Subject: Bug 14024

Author: rguenth
Date: Thu Dec  8 11:24:07 2005
New Revision: 108226

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=108226
Log:
2005-12-08  Richard Guenther  <rguenther@suse.de>

        Backport from mainline
        2005-11-28  Richard Guenther  <rguenther@suse.de>

        * c-common.c (strict_aliasing_warning): Handle all
        component-ref like accesses.

        * gcc.dg/alias-9.c: New testcase.
        * g++.dg/warn/Wstrict-aliasing-7.C: Likewise.

        2005-11-24  Richard Guenther  <rguenther@suse.de>
                Dirk Mueller <dmueller@suse.de>

        PR c++/14024
        * c-common.h (strict_aliasing_warning): Declare.
        * c-common.c (strict_aliasing_warning): New function,
        split out from ...
        * c-typeck.c (build_c_cast): ... here.

        * typeck.c (build_reinterpret_cast_1): Use
        strict_aliasing_warning.

        * g++.dg/warn/Wstrict-aliasing-1.C: New testcase.
        * g++.dg/warn/Wstrict-aliasing-2.C: Likewise.
        * g++.dg/warn/Wstrict-aliasing-3.C: Likewise.
        * g++.dg/warn/Wstrict-aliasing-4.C: Likewise.
        * g++.dg/warn/Wstrict-aliasing-5.C: Likewise.
        * g++.dg/warn/Wstrict-aliasing-6.C: Likewise.

        2005-11-23  Paolo Carlini  <pcarlini@suse.de>

        PR libstdc++/24975 (basic_string)
        * include/bits/basic_string.h (_Rep::_S_empty_rep): Avoid
        strict-aliasing warnings.

Added:
    branches/gcc-4_1-branch/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-1.C
    branches/gcc-4_1-branch/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-2.C
    branches/gcc-4_1-branch/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-3.C
    branches/gcc-4_1-branch/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-4.C
    branches/gcc-4_1-branch/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-5.C
    branches/gcc-4_1-branch/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-6.C
    branches/gcc-4_1-branch/gcc/testsuite/g++.dg/warn/Wstrict-aliasing-7.C
    branches/gcc-4_1-branch/gcc/testsuite/gcc.dg/alias-9.c
Modified:
    branches/gcc-4_1-branch/gcc/ChangeLog
    branches/gcc-4_1-branch/gcc/c-common.c
    branches/gcc-4_1-branch/gcc/c-common.h
    branches/gcc-4_1-branch/gcc/c-typeck.c
    branches/gcc-4_1-branch/gcc/cp/ChangeLog
    branches/gcc-4_1-branch/gcc/cp/typeck.c
    branches/gcc-4_1-branch/gcc/testsuite/ChangeLog
    branches/gcc-4_1-branch/libstdc++-v3/ChangeLog
    branches/gcc-4_1-branch/libstdc++-v3/include/bits/basic_string.h


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14024