From 5e7b32c59d53f8240dfd7a17e31fc533fa811f75 Mon Sep 17 00:00:00 2001
From: Eli Schwartz <eschwartz93@gmail.com>
Date: Tue, 16 Jan 2024 00:43:58 -0500
Subject: [PATCH] meson.eclass: wire up LTO support directly into the meson
 options

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>
Signed-off-by: Sam James <sam@gentoo.org>
---
 eclass/meson.eclass | 40 +++++++++++++++++++++++++++++++++++-----
 1 file changed, 35 insertions(+), 5 deletions(-)

diff --git a/eclass/meson.eclass b/eclass/meson.eclass
index 629bf3be4f2e8..b24bb40eb0a0f 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
 	${NINJA_DEPEND}
@@ -283,6 +283,38 @@ meson_feature() {
 # variables. Invoke via "${MESONARGS[@]}" in the calling environment.
 # This function is called from meson_src_configure.
 setup_meson_src_configure() {
+	MESONARGS=()
+	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.
+		MESONARGS+=( -Db_lto=true )
+
+		# respect -flto value, e.g. -flto=8, -flto=thin
+		local v=$(get-flag flto)
+		case ${v} in
+			thin)
+				MESONARGS+=( -Db_lto_mode=thin )
+				;;
+			''|*[!0-9]*)
+				;;
+			*)
+				MESONARGS+=( -Db_lto_threads=${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.
+		MESONARGS+=( -Db_lto=false )
+	fi
+
 	local BUILD_CFLAGS=${BUILD_CFLAGS}
 	local BUILD_CPPFLAGS=${BUILD_CPPFLAGS}
 	local BUILD_CXXFLAGS=${BUILD_CXXFLAGS}
@@ -311,7 +343,7 @@ setup_meson_src_configure() {
 		: "${BUILD_PKG_CONFIG_PATH:=${PKG_CONFIG_PATH}}"
 	fi
 
-	MESONARGS=(
+	MESONARGS+=(
 		--libdir "$(get_libdir)"
 		--localstatedir "${EPREFIX}/var/lib"
 		--prefix "${EPREFIX}/usr"
@@ -331,9 +363,7 @@ setup_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