Package: g++-7 Version: 7.3.0-12 Severity: important This is an apparently impossible bug which nevertheless can be reliably observed when using Debian versions of g++-7 and also i686-w64-mingw32-g++. It consists in compiler generating different, and broken, code when compiling the same input with only warning options -- which are, of course, not supposed to affect the code generation at all -- added.
The upstream bug at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85091 contains several test cases. Using the one from https://gcc.gnu.org/bugzilla/attachment.cgi?id=43774 it can be seen that running g++-7 -S -std=c++17 -O2 gcc-7.3-x86_64-linux.cpp -o nowarn.s and g++-7 -S -std=c++17 -O2 gcc-7.3-x86_64-linux.cpp -Wnonnull -Woverloaded-virtual -o warn.s commands produces different assembly. Subsequently, Martin Liška has created a further reduced test case which allows to reproduce the problem using just "-O1 -Woverloaded-virtual", please see https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85091#c31 The bug is really mysterious but quite worrisome, as it results in broken code being silently generated in practice and, worse, the generated code oscillates between correct and broken versions when any, even apparently completely unrelated, changes are made. Another interesting detail is that using -fchecking=2 makes the bug disappear (but -fchecking does not). Finally, please note that the bug doesn't seem to happen with the upstream versions nor with g++ 7.2 from Fedora, so it's highly likely that it's due to one of the Debian-specific patches, even if I really can't see anything that could explain it in any of them. Thanks in advance for any help with this! -- System Information: Debian Release: buster/sid APT prefers unstable APT policy: (990, 'unstable'), (500, 'unstable-debug'), (100, 'experimental') Architecture: amd64 (x86_64) Foreign Architectures: i386 Kernel: Linux 4.6.0-0.bpo.1-amd64 (SMP w/8 CPU cores) Locale: LANG=C.UTF-8, LC_CTYPE=C.UTF-8 (charmap=UTF-8) (ignored: LC_ALL set to C.UTF-8), LANGUAGE=C.UTF-8 (charmap=UTF-8) (ignored: LC_ALL set to C.UTF-8) Shell: /bin/sh linked to /bin/dash Init: unable to detect Versions of packages g++-7 depends on: ii gcc-7 7.3.0-12 ii gcc-7-base 7.3.0-12 ii libc6 2.27-2 ii libgmp10 2:6.1.2+dfsg-3 ii libisl19 0.19-1 ii libmpc3 1.1.0-1 ii libmpfr6 4.0.1-1 ii libstdc++-7-dev 7.3.0-12 ii zlib1g 1:1.2.8.dfsg-5 g++-7 recommends no packages. Versions of packages g++-7 suggests: pn g++-7-multilib <none> pn gcc-7-doc <none> pn libstdc++6-7-dbg <none> -- no debconf information
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