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
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
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
app-crypt/gnupg						no-lto
net-libs/glib-networking				no-lto
dev-libs/gobject-introspection				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

<www-client/firefox-33.0				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