[Bug c++/47504] New: some constexpr calculations erroneously overflow when using negative numbers

[Bug c++/47504] New: some constexpr calculations erroneously overflow when using negative numbers

[yacwroy at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47504

           Summary: some constexpr calculations erroneously overflow when
                    using negative numbers
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: yacwroy@gmail.com
            Target: x86_64-unknown-linux-gnu


Created attachment 23148
  --> http://gcc.gnu.org/bugzilla/attachment.cgi?id=23148
overflow.cpp

Easiest to describe via code:

overflow.cpp
=================================================
char constexpr sub(char arg)
  { return char(arg - char(1)); }

int main()
  { static char constexpr m = sub(-1); }
=================================================
g++ overflow.cpp --std=c++0x
=================================================
overflow.cpp: In function ‘int main()’:
overflow.cpp:5:36: error: overflow in constant expression [-fpermissive]

This is simply attempting to subtract 1 from -1, using "char"s.
I cannot see any reason why this should trigger an overflow error.
I have attempted to cast every intermediate state to char to avoid any
unintended casts.
The code is as simple as I could get it while still triggering the seemingly
erroneous overflow.

Compiles OK using int instead of char. (int can still erroneously overflow in
other circumstances, though. Would examples be useful?).
Compiles OK using 1 instead of -1.
Compiles OK using a literal -1 instead of a parameter.
Compiles OK if main()::m isn't constexpr.

Attached source code, but it's probably easier to just copy/paste the above.


SPECS:
gcc: version 4.6.0 2010-12-30 (experimental) (svn = 168358)
 - manually patched by
(http://gcc.gnu.org/ml/gcc-patches/2007-04/msg00620.html)
 - patch shouldn't have any effect here.
ubuntu: 10.10 (64 bit)
intel: core2 duo

Are any other specs relevant here, such as GMP. (Note: my GMP is the standard
one).


I searched other constexpr bugs here - AFAIK none appear to be about overflow.

[Bug c++/47504] some constexpr calculations erroneously overflow when using negative numbers

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47504

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |rejects-valid
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2011.01.28 10:59:58
     Ever Confirmed|0                           |1

--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-01-28 10:59:58 UTC ---
Confirmed (with -fsigned-char, -funsigned-char works ok).

[Bug c++/47504] some constexpr calculations erroneously overflow when using negative numbers

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47504

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jason at gcc dot gnu.org,
                   |                            |kazu at gcc dot gnu.org

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> 2011-03-16 23:34:25 UTC ---
This is happening because convert_to_integer is doing what seems to me like an
invalid transformation: it forces the arithmetic to be done in unsigned char,
so we end up with char((unsigned char)-2), or char(254), which has
implementation-defined behavior according to the C and C++ standards.

This seems to have been introduced with the fix for PR 25125.

[Bug c++/47504] some constexpr calculations erroneously overflow when using negative numbers

[pinskia at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47504

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> 2011-03-16 23:43:38 UTC ---
>which has implementation-defined behavior according to the C and C++ standards.

But that does not mean it has an overflow though, right?

[Bug c++/47504] some constexpr calculations erroneously overflow when using negative numbers

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47504

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> 2011-03-17 03:14:22 UTC ---
True, the language standards seem to distinguish between this and the overflow
you get from saying INT_MAX+1.  But GCC internals do not make this distinction;
in either case, we end up setting TREE_OVERFLOW in force_fit_type_double. 
Given that, I would prefer to avoid this transformation.

[Bug c++/47504] some constexpr calculations erroneously overflow when using negative numbers

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47504

--- Comment #5 from Jason Merrill <jason at gcc dot gnu.org> 2011-03-17 22:00:51 UTC ---
Author: jason
Date: Thu Mar 17 22:00:47 2011
New Revision: 171116

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=171116
Log:
    PR c++/47504
    * semantics.c (cxx_eval_constant_expression) [NOP_EXPR]: Don't let
    the conversion set TREE_OVERFLOW.

Added:
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-overflow2.C
Modified:
    trunk/gcc/cp/ChangeLog
    trunk/gcc/cp/semantics.c
    trunk/gcc/testsuite/ChangeLog
    trunk/gcc/testsuite/g++.dg/cpp0x/constexpr-data2.C

[Bug c++/47504] some constexpr calculations erroneously overflow when using negative numbers

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47504

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
         AssignedTo|unassigned at gcc dot       |jason at gcc dot gnu.org
                   |gnu.org                     |

--- Comment #6 from Jason Merrill <jason at gcc dot gnu.org> 2011-03-18 02:49:36 UTC ---
Fixed for 4.7, planning to fix in 4.6.1.

[Bug c++/47504] some constexpr calculations erroneously overflow when using negative numbers

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47504

--- Comment #7 from Jason Merrill <jason at gcc dot gnu.org> 2011-03-29 14:24:24 UTC ---
Author: jason
Date: Tue Mar 29 14:24:19 2011
New Revision: 171664

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=171664
Log:
    PR c++/47504
    * semantics.c (cxx_eval_constant_expression) [NOP_EXPR]: Don't let
    the conversion set TREE_OVERFLOW.

Added:
    branches/gcc-4_6-branch/gcc/testsuite/g++.dg/cpp0x/constexpr-overflow2.C
Modified:
    branches/gcc-4_6-branch/gcc/cp/ChangeLog
    branches/gcc-4_6-branch/gcc/cp/semantics.c
    branches/gcc-4_6-branch/gcc/testsuite/ChangeLog
    branches/gcc-4_6-branch/gcc/testsuite/g++.dg/cpp0x/constexpr-data2.C

[Bug c++/47504] some constexpr calculations erroneously overflow when using negative numbers

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47504

Jason Merrill <jason at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.6.1

--- Comment #8 from Jason Merrill <jason at gcc dot gnu.org> 2011-03-29 14:37:13 UTC ---
Fixed for 4.6.1.