[Bug c/55009] New: compile-time assertions no longer usable: error: variably modified âari_sign_32_bit_and_wrapâ at file scope

["tg at mirbsd dot org" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55009

             Bug #: 55009
           Summary: compile-time assertions no longer usable: error:
                    variably modified âari_sign_32_bit_and_wrapâ at file
                    scope
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: tg@mirbsd.org


Hi everyone,

Iâve been using compile-time assertions, that is, compiling .c files with
arrays that are either 1 or -1 bytes long, depending on some assertion, for a
while now, and one of these now no longer works: the check whether a certain
integer type is 32-bit with usable wraparound, as asked for by -fwrapv. (I need
to check this because the program in question is a shell that guarantees this
behaviour to scripts run in it, even for signed integers, and recent GCC
versions are known to optimise the wraparound away, as itâs UD, strictly
spoken, but can be made to work.)

This affects recent gcc-4.6 and gcc-4.7 uploads into Debian and Ubuntu, at
least, as well as gcc trunk:

tg@zigo:~ $ /usr/lib/gcc-snapshot/bin/gcc -v                                    
Using built-in specs.
COLLECT_GCC=/usr/lib/gcc-snapshot/bin/gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc-snapshot/libexec/gcc/x86_64-linux-gnu/4.8.0/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 20121008-1'
--with-bugurl=file:///usr/share/doc/gcc-snapshot/README.Bugs
--enable-languages=c,ada,c++,java,go,fortran,objc,obj-c++
--prefix=/usr/lib/gcc-snapshot --enable-shared --enable-linker-build-id
--with-system-zlib --disable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object
--enable-plugin --disable-browser-plugin --enable-java-awt=gtk
--enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.5.0-gcj-4.8-snap/jre
--enable-java-home --with-jvm-root-dir=/usr/lib/jvm/java-1.5.0-gcj-4.8-snap
--with-jvm-jar-dir=/usr/lib/jvm-exports/java-1.5.0-gcj-4.8-snap
--with-arch-directory=amd64 --with-ecj-jar=/usr/share/java/eclipse-ecj.jar
--enable-objc-gc --with-arch-32=i586 --with-tune=generic --disable-werror
--enable-checking=yes --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.8.0 20121008 (experimental) [trunk revision 192192] (Debian
20121008-1) 
tg@zigo:~ $ cat t.c
#include <stdint.h>
typedef int32_t mksh_ari_t;
char ari_sign_32_bit_and_wrap[(

        (mksh_ari_t)(((((mksh_ari_t)1 << 15) << 15) - 1) * 2 + 1) >
        (mksh_ari_t)(((((mksh_ari_t)1 << 15) << 15) - 1) * 2 + 2)

    ) ? 1 : -1];

int main(void) { return (sizeof(ari_sign_32_bit_and_wrap)); }
tg@zigo:~ $ if /usr/lib/gcc-snapshot/bin/gcc -O t.c; then ./a.out; echo ok $?;
else echo error $?; fi
t.c:6:62: warning: integer overflow in expression [-Woverflow]
         (mksh_ari_t)(((((mksh_ari_t)1 << 15) << 15) - 1) * 2 + 2)
                                                              ^
t.c:3:6: error: variably modified 'ari_sign_32_bit_and_wrap' at file scope
 char ari_sign_32_bit_and_wrap[(
      ^
error 1
tg@zigo:~ $ gcc-4.7 -v
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc-4.7.real
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.7/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.7.2-2'
--with-bugurl=file:///usr/share/doc/gcc-4.7/README.Bugs
--enable-languages=c,c++,go,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.7 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.7
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object
--enable-plugin --enable-objc-gc --with-arch-32=i586 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.7.2 (Debian 4.7.2-2) 
tg@zigo:~ $ if gcc-4.7 -O t.c; then ./a.out; echo ok $?; else echo error $?; fi
t.c:6:62: warning: integer overflow in expression [-Woverflow]
t.c:3:6: error: variably modified âari_sign_32_bit_and_wrapâ at file scope
error 1
tg@zigo:~ $ gcc-4.6 -v
Using built-in specs.
COLLECT_GCC=/usr/bin/gcc-4.6.real
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/4.6/lto-wrapper
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 4.6.3-10'
--with-bugurl=file:///usr/share/doc/gcc-4.6/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr
--program-suffix=-4.6 --enable-shared --enable-linker-build-id
--with-system-zlib --libexecdir=/usr/lib --without-included-gettext
--enable-threads=posix --with-gxx-include-dir=/usr/include/c++/4.6
--libdir=/usr/lib --enable-nls --with-sysroot=/ --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes --enable-gnu-unique-object
--enable-plugin --enable-objc-gc --with-arch-32=i586 --with-tune=generic
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu
Thread model: posix
gcc version 4.6.3 (Debian 4.6.3-10) 
tg@zigo:~ $ if gcc-4.6 -O t.c; then ./a.out; echo ok $?; else echo error $?; fi
t.c:6:62: warning: integer overflow in expression [-Woverflow]
t.c:3:6: error: variably modified âari_sign_32_bit_and_wrapâ at file scope
error 1

This used to be no problem (though gcc did warn about the overflow despite
passing -fwrapv). Swapping out the cc1 binary with a slightly older version
confirms that this is a recent development localised to that.

Iâm reporting this here by request of the Debian GCC Maintainers to decide
whether this is, indeed, a bug in GCC, or whether applications cannot use this
kind of compile-time check any more, in which case I would be _really_ thankful
if someone can point me out a way to check for the integer size and wraparound.
(Note that the unsigned version of the check passes with no issue.)