Date: Sat, 12 Jul 2003 23:13:12 -0400 From: Craig Rodrigues <rodrigc@crodrigues.org> To: David Leimbach <leimy2k@mac.com> Cc: freebsd-current@freebsd.org Subject: Re: GCC 3.3.1, new warnings with <limits> Message-ID: <20030713031312.GA89014@crodrigues.org> In-Reply-To: <BEDC8C48-B4DC-11D7-BE3B-0003937E39E0@mac.com> References: <20030712155333.GA79322@crodrigues.org> <BEDC8C48-B4DC-11D7-BE3B-0003937E39E0@mac.com>
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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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