{sys-devel/gcc,dev-lang/gnat-gpl}: add a USE for building GCC with LTO

Bug: https://bugs.gentoo.org/685634

Signed-off-by: Shane Peelar <lookatyouhacker@gmail.com>
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

eclass/toolchain.eclass

@@ -174,6 +174,7 @@ if [[ ${PN} != "kgcc64" && ${PN} != gcc-* ]] ; then
 	tc_version_is_at_least 8.0 &&
 		IUSE+=" systemtap" TC_FEATURES+=(systemtap)
 	tc_version_is_at_least 9.0 && IUSE+=" d"
+	tc_version_is_at_least 9.1 && IUSE+=" lto"
 fi
 
 SLOT="${GCC_CONFIG_VER}"
@@ -993,6 +994,11 @@ toolchain_src_configure() {
 		confgcc+=( --enable-libstdcxx-time )
 	fi
 
+	# Build compiler using LTO
+	if tc_version_is_at_least 9.1 && use_if_iuse lto ; then
+		confgcc+=( --with-build-config=bootstrap-lto )
+	fi
+
 	# Support to disable pch when building libstdcxx
 	if tc_version_is_at_least 6.0 && ! use_if_iuse pch ; then
 		confgcc+=( --disable-libstdcxx-pch )

sys-devel/gcc/metadata.xml

@@ -21,6 +21,7 @@
       This will slow down the compiler a bit as it forces all of the toolchain to be shared libs.</flag>
     <flag name="libssp">Build SSP support into a dedicated library rather than use the
       code in the C library (DO NOT ENABLE THIS IF YOU DON'T KNOW WHAT IT DOES)</flag>
+    <flag name="lto">Build using Link Time Optimizations (LTO)</flag>
     <flag name="mpx">Enable support for Intel Memory Protection Extensions (MPX)</flag>
     <flag name="mudflap">Add support for mudflap, a pointer use checking library</flag>
     <flag name="nopie">Disable PIE support (NOT FOR GENERAL USE)</flag>