This is an old revision of the document!
Link Time Optimization (LTO) gives GCC the capability of dumping its internal representation (GIMPLE) to disk, so that all the different compilation units that make up a single executable can be optimized as a single module. This expands the scope of inter-procedural optimizations to encompass the whole program (or, rather, everything that is visible at link time). https://gcc.gnu.org/wiki/LinkTimeOptimization
All you need is add in make.conf append to your CFLAGS -flto.
$ cat /etc/portage/env/no-lto.conf
CFLAGS="${CFLAGS} -fno-lto -fno-use-linker-plugin"
CXXFLAGS="${CXXFLAGS} -fno-lto -fno-use-linker-plugin"
LDFLAGS="${LDFLAGS} -fno-lto -fno-use-linker-plugin"
This is list of packages, which doesn't build with CFLAGS=“-flto”, please update if you encounter some broken.
$ cat /etc/portage/package.env/no-lto sys-libs/glibc no-lto # glibc take care self about it sys-devel/llvm no-lto sys-apps/coreutils no-lto dev-lang/ruby no-lto dev-lang/perl no-lto dev-lang/yasm no-lto sys-devel/flex no-lto net-misc/dhcp no-lto app-shells/bash no-lto sys-apps/findutils no-lto media-libs/mesa no-lto x11-apps/mesa-progs no-lto app-benchmarks/glmark2 no-lto app-emulation/wine no-lto media-libs/alsa-lib no-lto x11-base/xorg-server no-lto sys-apps/grep no-lto sys-fs/e2fsprogs no-lto sys-devel/gdb no-lto sys-devel/gettext no-lto net-misc/wget no-lto net-analyzer/traceroute no-lto dev-libs/cyrus-sasl no-lto mail-mta/postfix no-lto sys-apps/util-linux no-lto sys-fs/udisks no-lto x11-libs/pango no-lto gnome-base/libglade no-lto media-libs/gstreamer no-lto media-libs/gst-plugins-base no-lto media-libs/gst-plugins-bad no-lto app-misc/mc no-lto media-libs/quvi no-lto # OLD GST, remove later media-plugins/gst-plugins-a52dec no-lto media-plugins/gst-plugins-vorbis no-lto media-plugins/gst-plugins-ogg no-lto media-plugins/gst-plugins-xvideo no-lto media-plugins/gst-plugins-theora no-lto media-plugins/gst-plugins-x no-lto media-plugins/gst-plugins-mad no-lto media-plugins/gst-plugins-dvdread no-lto media-plugins/gst-plugins-mpeg2dec no-lto media-plugins/gst-plugins-x264 no-lto media-plugins/gst-plugins-resindvd no-lto media-plugins/gst-plugins-dts no-lto media-plugins/gst-plugins-faad no-lto media-plugins/gst-plugins-flac no-lto media-plugins/gst-plugins-assrender no-lto media-plugins/gst-plugins-pulse no-lto media-plugins/gst-plugins-libav no-lto sys-apps/systemd no-lto sys-apps/kmod no-lto sys-apps/dbus no-lto media-sound/pulseaudio no-lto dev-qt/qtcore no-lto dev-qt/qtgui no-lto dev-qt/qtquick1 no-lto dev-qt/qtwidgets no-lto dev-qt/qtdeclarative no-lto dev-qt/qtprintsupport no-lto dev-qt/qtscript no-lto dev-qt/qtxmlpatterns no-lto dev-qt/qtquickcontrols no-lto dev-qt/qtwebkit no-lto dev-qt/designer no-lto media-video/libav no-lto media-libs/libpostproc no-lto dev-db/mariadb no-lto #maybe net-misc/openssh no-lto app-admin/sudo no-lto sys-libs/pam no-lto sys-apps/shadow no-lto dev-libs/libtasn1 no-lto net-libs/gnutls no-lto net-libs/webkit-gtk no-lto app-crypt/gnupg no-lto net-libs/glib-networking no-lto net-misc/modemmanager no-lto dev-libs/gobject-introspection no-lto dev-libs/gjs no-lto www-servers/apache no-lto net-dns/libidn no-lto dev-lang/vala no-lto x11-libs/gdk-pixbuf no-lto dev-vcs/git no-lto sys-power/upower no-lto dev-libs/xapian no-lto app-office/akonadi-server no-lto net-voip/telepathy-gabble no-lto net-irc/telepathy-idle no-lto net-im/telepathy-mission-control no-lto # KDE kde-base/kdelibs no-lto kde-base/ksmserver no-lto kde-base/pykde4 no-lto kde-base/drkonqi no-lto kde-base/kdepim-runtime no-lto app-office/calligra no-lto net-im/ktp-call-ui no-lto app-office/kmymoney no-lto # KDE 5 dev-util/cmake no-lto kde-base/baloo no-lto kde-base/plasma-workspace no-lto kde-base/kate no-lto kde-base/marble no-lto kde-frameworks/kauth no-lto kde-frameworks/kdoctools no-lto kde-frameworks/kio no-lto dev-util/kdevelop no-lto dev-util/kdevelop-clang no-lto media-sound/mpg123 no-lto media-libs/openal no-lto kde-base/ksysguard no-lto x11-themes/qtcurve no-lto net-print/cups no-lto media-gfx/imagemagick no-lto media-gfx/inkscape no-lto dev-lang/spidermonkey no-lto sys-auth/polkit no-lto dev-lang/lua no-lto net-analyzer/wireshark no-lto net-libs/gupnp-igd no-lto net-libs/libnice no-lto gnome-base/gconf no-lto media-video/vlc no-lto net-libs/libproxy no-lto net-fs/nfs-utils no-lto gnome-base/librsvg no-lto sys-devel/libtool no-lto #phonon fails net-irc/quassel no-lto media-libs/libquvi no-lto media-sound/mpd no-lto app-misc/strigi no-lto media-gfx/nvidia-texture-tools no-lto games-emulation/dosbox no-lto app-arch/tar no-lto sys-apps/accountsservice no-lto net-wireless/aircrack-ng no-lto media-libs/libdvdnav no-lto sys-apps/kbd no-lto dev-libs/elfutils no-lto media-libs/libdvdread no-lto sys-apps/iproute2 no-lto media-libs/flac no-lto sys-block/parted no-lto app-crypt/pinentry no-lto sys-apps/net-tools no-lto app-text/hunspell no-lto app-i18n/ibus no-lto # GNOME gnome-base/gnome-desktop no-lto gnome-base/gnome-control-center no-lto