From a4eae64aa24da57d3f6292768565ed3a4115c99a Mon Sep 17 00:00:00 2001
From: Sam James <sam@gentoo.org>
Date: Mon, 10 Nov 2025 04:04:50 +0000
Subject: [PATCH] meson.eclass: support incremental LTO for GCC

Support -flto-incremental=/path/to/cache being used with GCC. Personally,
I am using this hook in /etc/portage/bashrc:
```
pre_src_configure() {
        if [[ -n ${PORTAGE_LTO_CACHE} ]] ; then
                local lto_path=/var/tmp/portage/lto/${CATEGORY}/${PN}
                mkdir -p "${lto_path}"
                export CFLAGS="${CFLAGS} -flto-incremental=${lto_path} -flto-compression-level=19"
                export CXXFLAGS="${CXXFLAGS} -flto-incremental=${lto_path} -flto-compression-level=19"
                export LDFLAGS="${LDFLAGS} -flto-incremental=${lto_path} -flto-compression-level=19"
        fi
}
```

... and then `PORTAGE_LTO_CACHE=1` in my development environment.

Signed-off-by: Sam James <sam@gentoo.org>
---
 eclass/meson.eclass | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/eclass/meson.eclass b/eclass/meson.eclass
index a2bc5537e4589..06675e02b82be 100644
--- a/eclass/meson.eclass
+++ b/eclass/meson.eclass
@@ -1,4 +1,4 @@
-# Copyright 2017-2024 Gentoo Authors
+# Copyright 2017-2025 Gentoo Authors
 # Distributed under the terms of the GNU General Public License v2
 
 # @ECLASS: meson.eclass
@@ -305,6 +305,18 @@ setup_meson_src_configure() {
 				MESONARGS+=( -Db_lto_threads=${v} )
 				;;
 		esac
+
+		if has_version -b ">=dev-build/meson-1.10.0" ; then
+			local v=$(get-flag -flto-incremental=)
+			v=${v#-flto-incremental=}
+			if [[ ${v} ]] ; then
+				MESONARGS+=(
+					-Db_thinlto_cache=true
+					-Db_thinlto_cache_dir=${v}
+				)
+			fi
+		fi
+
 		# finally, remove it from *FLAGS to avoid passing it:
 		# - twice, with potentially different values
 		# - on excluded targets