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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 media-libs/mesa no-lto-O2.conf x11-base/xorg-server no-lto.conf sys-apps/grep no-lto.conf sys-fs/e2fsprogs no-lto.conf sys-devel/gdb no-lto.conf net-misc/wget no-lto.conf dev-libs/cyrus-sasl no-lto.conf mail-mta/postfix no-lto.conf app-office/akonadi-server no-lto.conf sys-apps/util-linux no-lto.conf sys-fs/udisks no-lto.conf x11-libs/pango no-lto.conf gnome-base/libglade no-lto.conf media-libs/gst-plugins-base no-lto.conf sys-apps/systemd no-lto.conf sys-apps/dbus no-lto.conf media-sound/pulseaudio no-lto.conf dev-qt/qtcore no-lto.conf dev-qt/qtgui no-lto.conf media-video/libav no-lto.conf media-libs/libpostproc no-lto.conf dev-db/mariadb no-lto.conf #maybe net-irc/telepathy-idle no-lto.conf net-misc/openssh no-lto.conf app-admin/sudo no-lto.conf dev-libs/libtasn1 no-lto.conf net-libs/gnutls no-lto.conf net-libs/glib-networking no-lto.conf dev-libs/gobject-introspection no-lto.conf www-servers/apache no-lto.conf net-dns/libidn no-lto.conf dev-lang/vala no-lto.conf net-im/telepathy-mission-control no-lto.conf x11-libs/gdk-pixbuf no-lto.conf net-voip/telepathy-gabble no-lto.conf dev-vcs/git no-lto.conf sys-power/upower no-lto.conf kde-base/kdelibs no-lto.conf