[Bug c/20573] New: unable to use cast to suppress "warning: comparison is always false due to limited range of data type"

[Bug c/20573] New: unable to use cast to suppress "warning: comparison is always false due to limited range of data type"

[gcc-bugzilla at gcc dot gnu dot org]

There doesn't seem to be an easy way to suppress this warning in a macro like
#define MILL(in) ((in) > 1000000)
where the macro may be passed different sizes of ints including shorts.
I would have expected the warning to be suppressed when a cast is present
like:
#define MILL(in) ((int)(in) > 1000000)
but it isn't.

Environment:
System: CYGWIN_NT-5.1 DHX98431 1.5.14s(0.124/4/2) 20050319 16:46:09 i686 unknown unknown Cygwin


	
host: i686-pc-cygwin
build: i686-pc-cygwin
target: i686-pc-cygwin
configured with: /gcc/3.4/gcc-3.4.1-1/configure --verbose --prefix=/usr --exec-prefix=/usr --sysconfdir=/etc --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --enable-languages=c,ada,c++,f77,java,objc --enable-nls --without-included-gettext --enable-libgcj --with-system-zlib --enable-interpreter --enable-threads=posix --enable-java-gc=boehm --enable-sjlj-exceptions --disable-version-specific-runtime-libs --disable-win32-registry

How-To-Repeat:
Given warnme.c:
#define MILL(in) ((in) > 1000000)
int foo(short s) { return MILL(s); }
int bar(short s) { return MILL((int)s); }
int baz(short s) { int i=s; return MILL(i); }

with preprocessor output warnme.i:
# 1 "warnme.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "warnme.c"

int foo(short s) { return ((s) > 1000000); }
int bar(short s) { return (((int)s) > 1000000); }
int baz(short s) { int i=s; return ((i) > 1000000); }

$ gcc -Wall -save-temps -c warnme.c
warnme.c: In function `foo':
warnme.c:2: warning: comparison is always false due to limited range of data type
warnme.c: In function `bar':
warnme.c:3: warning: comparison is always false due to limited range of data type

The warning in foo is expected; the warning in bar is not.  baz is an ugly
workaround.
------- Additional Comments From sthoenna at efn dot org  2005-03-21 00:16 -------
Fix:
An ugly workaround is to assign to a temporary variable of large enough data
type where the macro is called, and pass the temporary variable instead.

-- 
           Summary: unable to use cast to suppress "warning: comparison is
                    always false due to limited range of data type"
           Product: gcc
           Version: 3.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: sthoenna at efn dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-cygwin
  GCC host triplet: i686-pc-cygwin
GCC target triplet: i686-pc-cygwin


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20573

[Bug c/20573] unable to use cast to suppress "warning: comparison is always false due to limited range of data type"

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-21 00:24 -------
A better way is to just to suppress the warning point, see PR 12963.

*** This bug has been marked as a duplicate of 12963 ***

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |DUPLICATE
            Summary|unable to use cast to       |unable to use cast to
                   |suppress "warning:          |suppress "warning:
                   |comparison is always false  |comparison is always false
                   |due to limited range of data|due to limited range of data
                   |type"                       |type"


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20573

[Bug c/20573] unable to use cast to suppress "warning: comparison is always false due to limited range of data type"

[sthoenna at efn dot org]

------- Additional Comments From sthoenna at efn dot org  2005-03-21 03:49 -------
Subject: Re:  unable to use cast to suppress "warning: comparison is always false due to limited range of data type"

On Mon, Mar 21, 2005 at 12:24:32AM -0000, pinskia at gcc dot gnu dot org wrote:
> 
> ------- Additional Comments From pinskia at gcc dot gnu dot org  2005-03-21 00:24 -------
> A better way is to just to suppress the warning point, see PR 12963.

But I don't want to suppress the warning in general; it's a valuable
warning.  I just want to be able to have a macro that takes an
arbitrarily sized int and compares it.
 
> *** This bug has been marked as a duplicate of 12963 ***

Thanks.  I mistakenly searched Comments containing '"comparison is
always"' instead of 'comparison is always' :) or I would have found
that one.

It's interesting to note that the apparent ending point of the discussion
there is my starting point: that a cast should suppress the warning (but
I would hope not the optimization).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20573