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 app-misc/tracker no-lto gnome-base/gvfs no-lto gnome-base/gnome-desktop no-lto gnome-base/gnome-control-center no-lto