I have a small piece of C code that does not do what is expected. Reproducible: Always Steps to Reproduce: run "make" Actual Results: Output: ERROR Expected Results: Output: Ok The output should be "Ok" but it is "ERROR". Runs fine on a Redhat GCC 4.4.7.
Portage 2.2.18 (python 2.7.9-final-0, default/linux/amd64/13.0/desktop/kde, gcc-4.8.4, glibc-2.20-r2, 3.14.24 x86_64) ================================================================= System uname: Linux-3.14.24-x86_64-Intel-R-_Core-TM-_i5_CPU_M_450_@_2.40GHz-with-gentoo-2.2 KiB Mem: 3842344 total, 533628 free KiB Swap: 12582908 total, 12274604 free Timestamp of repository gentoo: Sun, 14 Jun 2015 14:30:01 +0000 sh bash 4.3_p33-r2 ld GNU ld (Gentoo 2.24 p1.4) 2.24 app-shells/bash: 4.3_p33-r2::gentoo dev-java/java-config: 2.2.0::gentoo dev-lang/perl: 5.20.2::gentoo dev-lang/python: 2.7.9-r1::gentoo, 3.3.5-r1::gentoo, 3.4.1::gentoo dev-util/cmake: 3.2.2::gentoo dev-util/pkgconfig: 0.28-r2::gentoo sys-apps/baselayout: 2.2::gentoo sys-apps/openrc: 0.16.4::gentoo sys-apps/sandbox: 2.6-r1::gentoo sys-devel/autoconf: 2.13::gentoo, 2.69::gentoo sys-devel/automake: 1.11.6-r1::gentoo, 1.14.1::gentoo sys-devel/binutils: 2.24-r3::gentoo sys-devel/gcc: 4.8.4::gentoo sys-devel/gcc-config: 1.7.3::gentoo sys-devel/libtool: 2.4.6::gentoo sys-devel/make: 4.1-r1::gentoo sys-kernel/linux-headers: 3.14::gentoo (virtual/os-headers) sys-libs/glibc: 2.20-r2::gentoo Repositories: gentoo location: /usr/portage sync-type: rsync sync-uri: rsync://rsync.gentoo.org/gentoo-portage priority: -1000 Ole location: /usr/local/portage/overlay/local masters: gentoo priority: 0 sunrise location: /var/lib/layman/sunrise masters: gentoo priority: 1 vmware location: /var/lib/layman/vmware masters: gentoo priority: 2 ACCEPT_KEYWORDS="amd64" ACCEPT_LICENSE="* -@EULA AdobeFlash-11.x skype-4.0.0.7-copyright Broadcom google-chrome TeamViewer PUEL" CBUILD="x86_64-pc-linux-gnu" CFLAGS="-pipe -O2 -march=core2 -fomit-frame-pointer" CHOST="x86_64-pc-linux-gnu" CONFIG_PROTECT="/etc /usr/share/config /usr/share/gnupg/qualified.txt /usr/share/themes/oxygen-gtk/gtk-2.0" CONFIG_PROTECT_MASK="/etc/ca-certificates.conf /etc/dconf /etc/env.d /etc/fonts/fonts.conf /etc/gconf /etc/gentoo-release /etc/revdep-rebuild /etc/sandbox.d /etc/terminfo" CXXFLAGS="-pipe -O2 -march=core2 -fomit-frame-pointer" DISTDIR="/usr/portage/distfiles" EMERGE_DEFAULT_OPTS="--jobs 5 --load-average 4 --keep-going" FCFLAGS="-O2 -pipe" FEATURES="assume-digests binpkg-logs config-protect-if-modified distlocks ebuild-locks fixlafiles merge-sync news parallel-fetch protect-owned sandbox sfperms strict unknown-features-warn unmerge-logs unmerge-orphans userfetch userpriv usersandbox usersync" FFLAGS="-O2 -pipe" GENTOO_MIRRORS="http://de-mirror.org/distro/gentoo/ http://ftp.uni-erlangen.de/pub/mirrors/gentoo http://mirror.ovh.net/gentoo-distfiles/" LANG="de_DE.utf8" LDFLAGS="-Wl,-O1 -Wl,--as-needed" MAKEOPTS="-j 5 -l 4" PKGDIR="/home/pkgdir/march-core2" PORTAGE_CONFIGROOT="/" PORTAGE_RSYNC_OPTS="--recursive --links --safe-links --perms --times --omit-dir-times --compress --force --whole-file --delete --stats --human-readable --timeout=180 --exclude=/distfiles --exclude=/local --exclude=/packages" PORTAGE_TMPDIR="/var/tmp" USE="X a52 aac acpi alsa amd64 bash-completion berkdb bluetooth branding bzip2 cairo cdda cddb cdparanoia cdr cli consolekit cracklib crypt css cups cxx dbus declarative dga dri dts dvd dvdr emacs emboss encode exif faac fam ffmpeg firefox flac gdbm gif gimp glamor gpm gstreamer gtk iconv icu id3tag ipod java joystick jpeg kde kipi lame lcms ldap libnotify libsamplerate lm_sensors mad matroska mmx mmxext mng modules mp3 mp4 mpeg mplayer multilib ncurses nls nptl nsplugin ogg opengl openmp pam pango pcre pdf phonon plasma png policykit popcnt ppds pulseaudio qt3support qt4 rdesktop readline samba scanner sdl semantic-desktop session smp spell sse sse2 sse3 sse4_1 sse4_2 ssl ssse3 startup-notification svg tcpd theora threads tiff truetype twolame udev udisks unicode upnp upower usb v4l vcd vorbis wmf wxwidgets x264 xcb xcomposite xft xine xinerama xml xpm xscreensaver xv xvid zlib" ABI_X86="64" ALSA_CARDS="ali5451 als4000 atiixp atiixp-modem bt87x ca0106 cmipci emu10k1x ens1370 ens1371 es1938 es1968 fm801 hda-intel intel8x0 intel8x0m maestro3 trident usb-audio via82xx via82xx-modem ymfpci" APACHE2_MODULES="authn_core authz_core socache_shmcb unixd actions alias auth_basic authn_alias authn_anon authn_dbm authn_default authn_file authz_dbm authz_default authz_groupfile authz_host authz_owner authz_user autoindex cache cgi cgid dav dav_fs dav_lock deflate dir disk_cache env expires ext_filter file_cache filter headers include info log_config logio mem_cache mime mime_magic negotiation rewrite setenvif speling status unique_id userdir usertrack vhost_alias" CALLIGRA_FEATURES="kexi words flow plan sheets stage tables krita karbon braindump author" CAMERAS="ptp2" COLLECTD_PLUGINS="df interface irq load memory rrdtool swap syslog" CPU_FLAGS_X86="mmx mmxext popcnt sse sse2 sse3 ssse3 sse4_1 sse4_2" DRACUT_MODULES="lvm" ELIBC="glibc" GPSD_PROTOCOLS="ashtech aivdm earthmate evermore fv18 garmin garmintxt gpsclock itrax mtk3301 nmea ntrip navcom oceanserver oldstyle oncore rtcm104v2 rtcm104v3 sirf superstar2 timing tsip tripmate tnt ublox ubx" INPUT_DEVICES="keyboard mouse evdev synaptics" KERNEL="linux" LCD_DEVICES="bayrad cfontz cfontz633 glk hd44780 lb216 lcdm001 mtxorb ncurses text" LIBREOFFICE_EXTENSIONS="presenter-console presenter-minimizer" LINGUAS="de" OFFICE_IMPLEMENTATION="libreoffice" PHP_TARGETS="php5-5" PYTHON_SINGLE_TARGET="python2_7" PYTHON_TARGETS="python2_7 python3_3" RUBY_TARGETS="ruby19 ruby20" USERLAND="GNU" VIDEO_CARDS="intel i965" XTABLES_ADDONS="quota2 psd pknock lscan length2 ipv4options ipset ipp2p iface geoip fuzzy condition tee tarpit sysrq steal rawnat logmark ipmark dhcpmac delude chaos account" Unset: CPPFLAGS, CTARGET, INSTALL_MASK, LC_ALL, PORTAGE_BUNZIP2_COMMAND, PORTAGE_COMPRESS, PORTAGE_COMPRESS_FLAGS, PORTAGE_RSYNC_EXTRA_OPTS, USE_PYTHON
Created attachment 405410 [details] C snippet that reproduces the problem
Created attachment 405412 [details] Makefile to compile and run the C code
Forgot to mention that the code runs fine with -O0 or -O1. -O2 or -O3 is required to trigger the issue.
It's not a bug, your code is breaking strict aliasing rules and so the behavior is undefined. To make it work with higher optimization levels use -fno-strict-aliasing. You can find out more about this here https://stackoverflow.com/questions/2958633/gcc-strict-aliasing-and-horror-stories
...adding '-fno-strict-aliasing' makes things work, therefore it's yet another case of Revenge of The Compiler. Sure, gcc doesn't display the expected warning - not the first case it fails to. As I've been told in an upstream bug way back, these are sometimes quite a bitch to detect correctly and not get smothered by false positives.
Yes, I assumed something like this. The thing is, the actual issue occurs with a program generated by Csmith https://embed.cs.utah.edu/csmith/ (not writing such crazy stuff myself). And it seems, I stripped down the test case a bit too much. What remained was not the cause of the original problem. But it turned out, the actual issue is an aliasing as well. So, finally I found it and have reported it to the Csmith bug mailing list. Thanks for your patience guys!
