[gentoo-dev] [PATCH v2 1/5] meson.eclass: wire up LTO support directly into the meson options

[Eli Schwartz <eschwartz93@gmail.com>]

meson's builtin LTO support allows meson to introspect whether LTO is
enabled and do some fancy things, such as forcing LTO off for a single
target that is known to be special(ly bad) and not support LTO.

Signed-off-by: Eli Schwartz <eschwartz93@gmail.com>
---

no change

 eclass/meson.eclass | 36 ++++++++++++++++++++++++++++++++----
 1 file changed, 32 insertions(+), 4 deletions(-)

diff --git a/eclass/meson.eclass b/eclass/meson.eclass
index d8bd93082ea5..c2ae289019e9 100644
--- a/eclass/meson.eclass
+++ b/eclass/meson.eclass
@@ -41,7 +41,7 @@ esac
 if [[ -z ${_MESON_ECLASS} ]]; then
 _MESON_ECLASS=1
 
-inherit multiprocessing ninja-utils python-utils-r1 toolchain-funcs
+inherit flag-o-matic multiprocessing ninja-utils python-utils-r1 toolchain-funcs
 
 BDEPEND=">=dev-build/meson-1.2.1
 	${NINJA_DEPEND}
@@ -286,6 +286,36 @@ meson_src_configure() {
 
 	[[ -n "${NINJA_DEPEND}" ]] || ewarn "Unknown value '${NINJA}' for \${NINJA}"
 
+	local ltoflags=()
+	if tc-is-lto; then
+		# We want to connect -flto in *FLAGS to the dedicated meson option,
+		# to ensure that meson has visibility into what the user set. Although
+		# it is unlikely projects will check `get_option('b_lto')` and change
+		# their behavior, individual targets which are broken with LTO can
+		# disable it per target. Injecting via *FLAGS means that meson cannot
+		# strip -flto from that target.
+		ltoflags+=( -Db_lto=true )
+
+		# respect -flto value, e.g. -flto=8, -flto=thin
+		local v=$(get-flag flto)
+		case ${v} in
+			thin)
+				ltoflags+=( -Db_lto_mode=thin )
+				;;
+			''|*[!0-9]*)
+				;;
+			*) ltoflags+=( -Db_lto_threads=${v} )
+		esac
+		# finally, remove it from *FLAGS to avoid passing it:
+		# - twice, with potentially different values
+		# - on excluded targets
+		filter-lto
+	else
+		# Prevent projects from enabling LTO by default.  In Gentoo, LTO is
+		# enabled via setting *FLAGS appropriately.
+		ltoflags+=( -Db_lto=false )
+	fi
+
 	local BUILD_CFLAGS=${BUILD_CFLAGS}
 	local BUILD_CPPFLAGS=${BUILD_CPPFLAGS}
 	local BUILD_CXXFLAGS=${BUILD_CXXFLAGS}
@@ -335,9 +365,7 @@ meson_src_configure() {
 		# an upstream development matter. bug #754279.
 		-Dwerror=false
 
-		# Prevent projects from enabling LTO by default.  In Gentoo, LTO is
-		# enabled via setting *FLAGS appropriately.
-		-Db_lto=false
+		"${ltoflags[@]}"
 	)
 
 	if [[ -n ${EMESON_BUILDTYPE} ]]; then
-- 
2.43.0