Re: GCC 3.3.1, new warnings with <limits>

From: Craig Rodrigues (rodrigc_at_crodrigues.org)
Date: 07/13/03

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    Date: Sat, 12 Jul 2003 23:13:12 -0400
To: David Leimbach <leimy2k@mac.com>

    On Sat, Jul 12, 2003 at 09:50:21PM -0500, David Leimbach wrote:
    > Heh that's because the offending macro __glibcpp_digits calls
    > __glibcpp_signed (T)
    > on an unsigned type which does a < compareison.
    >
    > std::numeric_limits <signed long>::digits on a 32bit FBSD will yield 31
    > because its
    > got 31 bits for magnitude.
    >
    > Unfortunately the way it seems to go about calculating that stuff at
    > compile time
    > seems to be invalid due to the fact that it does < 0 compares on
    > unsigned types.
    >
    > Is this a gcc issue or a FBSD issue? [is this the original gcc c++
    > header file or has
    > it been tweaked?]

    Hi,

    I think that this is a FreeBSD issue. I compiled
    the same file under Linux, with a GCC 3.3.1 checked out on 7/11
    and did not encounter this warning.

    I looked in the source code of gcc and found this:
    /usr/src/contrib/gcc/c-common.c

       2597 case LT_EXPR:
       2598 if (extra_warnings && !in_system_header
       2599 && ! (TREE_CODE (primop0) == INTEGER_CST
       2600 && ! TREE_OVERFLOW (convert (c_common_signed_typ
    e (type),
       2601 primop0))))
       2602 warning ("comparison of unsigned expression < 0 is alway
    s false");
       2603 value = boolean_false_node;
       2604 break;

    I am guessing that the C preprocessor does not think that it is
    in a system header, and thus prints out the warning.

    If I take the following preprocessed source (test.ii) and compile it
    under FreeBSD with g++ -W -c test.ii:

    ===============================================================================
    # 1 "test.cc"
    # 1 "<built-in>"
    # 1 "<command line>"
    # 1 "test.cc"
    # 1 "/usr/include/c++/3.3/iostream" 1 3
    # 43 "/usr/include/c++/3.3/iostream" 3

    static const int digits = (sizeof(unsigned int) * 8 - ((unsigned int)(-1) < 0));

    ===============================================================================

    I get:

    In file included from test.cc:1:
    /usr/include/c++/3.3/iostream:44: warning: comparison of unsigned expression <
       0 is always false

    If I compile the same file on my Linux box, with a gcc checked out
    from the FSF CVS repository (gcc version 3.3.1 20030711 (prerelease)),
    I do not get the warning.

    I am not an expert on the GNU C preprocessor format, but I changed
    two of the lines in the above file to:

    # 1 "/usr/include/c++/3.3/iostream" 1
    # 43 "/usr/include/c++/3.3/iostream"

    and when I recompiled it under Linux, I also got the warning:

    In file included from test.cc:1:
    /usr/include/c++/3.3/iostream:44: warning: comparison of unsigned expression <
       0 is always false

    Any GCC/FreeBSD expert care to comment? ;) 

    -- 
Craig Rodrigues        
http://crodrigues.org
rodrigc@crodrigues.org
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