From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 48) id F17C3385783D; Tue, 13 Apr 2021 10:15:33 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org F17C3385783D From: "t.josefowitz at gmail dot com" To: gcc-bugs@gcc.gnu.org Subject: [Bug c/100061] New: Comparison guarding undefined behaviour disappears Date: Tue, 13 Apr 2021 10:15:33 +0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: new X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: gcc X-Bugzilla-Component: c X-Bugzilla-Version: 10.3.0 X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: t.josefowitz at gmail dot com X-Bugzilla-Status: UNCONFIRMED X-Bugzilla-Resolution: X-Bugzilla-Priority: P3 X-Bugzilla-Assigned-To: unassigned at gcc dot gnu.org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: bug_id short_desc product version bug_status bug_severity priority component assigned_to reporter target_milestone attachments.created Message-ID: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Bugzilla-URL: http://gcc.gnu.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-BeenThere: gcc-bugs@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-bugs mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 13 Apr 2021 10:15:34 -0000 https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100061 Bug ID: 100061 Summary: Comparison guarding undefined behaviour disappears Product: gcc Version: 10.3.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: t.josefowitz at gmail dot com Target Milestone: --- Created attachment 50580 --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D50580&action=3Dedit test case with int a =3D 0, b =3D -1, c =3D MAX_INT, d =3D 0, if ((a > d) && (a > d + 1)) { puts("YES"); ret =3D 1; } else if ((b > c) && (b > c + 1)) { puts("NO NOT GREAT"); ret =3D 0; } else { puts("OK"); ret =3D 2; } and optimization enabled, "else if" gets selected. While b > c + 1 is undef= ined for c =3D=3D MAX_INT, b > c should guard against it. The generated amd64 ma= chine code simply lacks the (b > c) check in the generated code, while curiously = the (a > d) from the first if is present. with -fwrapv the (b > c + 1) becomes "more defined" and thus the bug then d= oes not trigger anymore (at least not in the same way), but I think e.g. optimi= zing away (b > c) is not actually valid here as it seems to be a valid guard for= the undefined behaviour. I reproduced this on GCC 10.2.0 and 10.3.0. The attached cmp.c can be compiled: gcc -O2 cmp.c -Wall -Wextra and then executed: ./a.out 0 -1 2147483647 0 to trigger the issue. Notably, if compiled with -fsanitize=3Dundefined, no undefined behaviour is reported but the issue goes away. # gcc -v Using built-in specs. COLLECT_GCC=3Dgcc COLLECT_LTO_WRAPPER=3D/usr/lib/gcc/x86_64-linux-gnu/10/lto-wrapper OFFLOAD_TARGET_NAMES=3Dnvptx-none:amdgcn-amdhsa:hsa OFFLOAD_TARGET_DEFAULT=3D1 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion=3D'Ubuntu 10.2.0-13ubuntu1' --with-bugurl=3Dfile:///usr/share/doc/gcc-10/README.Bugs --enable-languages=3Dc,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix= =3D/usr --with-gcc-major-version-only --program-suffix=3D-10 --program-prefix=3Dx86_64-linux-gnu- --enable-shared --enable-linker-build-= id --libexecdir=3D/usr/lib --without-included-gettext --enable-threads=3Dposix --libdir=3D/usr/lib --enable-nls --enable-clocale=3Dgnu --enable-libstdcxx-= debug --enable-libstdcxx-time=3Dyes --with-default-libstdcxx-abi=3Dnew --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=3Drelea= se --with-target-system-zlib=3Dauto --enable-objc-gc=3Dauto --enable-multiarch --disable-werror --with-arch-32=3Di686 --with-abi=3Dm64 --with-multilib-list=3Dm32,m64,mx32 --enable-multilib --with-tune=3Dgeneric --enable-offload-targets=3Dnvptx-none=3D/build/gcc-10-JvwpWM/gcc-10-10.2.0/= debian/tmp-nvptx/usr,amdgcn-amdhsa=3D/build/gcc-10-JvwpWM/gcc-10-10.2.0/deb= ian/tmp-gcn/usr,hsa --without-cuda-driver --enable-checking=3Drelease --build=3Dx86_64-linux-gnu --host=3Dx86_64-linux-gnu --target=3Dx86_64-linux-gnu Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 10.2.0 (Ubuntu 10.2.0-13ubuntu1)=