From 9a7de50c680861d2c573999fb7d59d3fdeba2316 Mon Sep 17 00:00:00 2001
From: Matt Jolly <kangie@gentoo.org>
Date: Sat, 16 May 2026 12:44:33 +1000
Subject: [PATCH] wip: mold for other toolchains

Signed-off-by: Matt Jolly <kangie@gentoo.org>
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -762,7 +762,7 @@ config("compiler") {
   # [0]: https://pinpoint-dot-chromeperf.appspot.com/job/15efb0313e0000
   # [1]: https://pinpoint-dot-chromeperf.appspot.com/job/157f0b42be0000
   if (!is_debug && use_thin_lto && is_a_target_toolchain) {
-    assert(use_lld, "LTO is only supported with lld")
+    assert(use_lld || use_mold, "LTO is only supported with lld or mold")
 
     cflags += [
       "-flto=thin",
--- a/build/config/compiler/compiler.gni
+++ b/build/config/compiler/compiler.gni
@@ -56,16 +56,27 @@
         (current_os == "android" || current_os == "linux") &&
         (current_cpu == "x64" || current_cpu == "arm64" ||
          current_cpu == "x86" || current_cpu == "arm")
+
+    # Optional path to a linker binary (for example, a local mold binary).
+    # If non-empty, this takes precedence over normal linker selection below.
+    # Leave empty to keep regular ld/lld behavior.
+    linker_path = ""
   }
 }
 
 if (!defined(use_mold)) {
   use_mold = false
+}
+if (!defined(linker_path)) {
   linker_path = ""
-} else if (use_mold) {
-  linker_path = "//buildtools/third_party/mold/cipd/mold"
-} else {
-  linker_path = experimental_linker_path
+}
+
+if (linker_path == "") {
+  if (use_mold) {
+    linker_path = "//buildtools/third_party/mold/cipd/mold"
+  } else {
+    linker_path = experimental_linker_path
+  }
 }
 
 declare_args() {
--- a/build/toolchain/android/BUILD.gn
+++ b/build/toolchain/android/BUILD.gn
@@ -35,8 +35,12 @@ template("android_clang_toolchain") {
     # generate_linker_map=true.
     enable_linker_map = true
 
-    use_mold_separate_debug_file = true
-    strip = rebase_path("$clang_base_path/bin/llvm-strip", root_build_dir)
+    # Mold emits the split debug file directly. Other linkers still rely on a
+    # separate strip step, including explicit linker_path overrides that do not
+    # go through use_lld.
+    if (!use_mold) {
+      strip = rebase_path("$clang_base_path/bin/llvm-strip", root_build_dir)
+    }
 
     use_unstripped_as_runtime_outputs = android_unstripped_runtime_outputs
 
--- a/build/toolchain/gcc_toolchain.gni
+++ b/build/toolchain/gcc_toolchain.gni
@@ -329,9 +329,13 @@ template("single_gcc_toolchain") {
     enable_linker_map = defined(invoker.enable_linker_map) &&
                         invoker.enable_linker_map && generate_linker_map
 
-    use_mold_separate_debug_file =
-        defined(invoker.use_mold_separate_debug_file) &&
-        invoker.use_mold_separate_debug_file && toolchain_args.use_mold
+    # Default to mold's separate-debug-file behavior when use_mold is enabled.
+    # Individual toolchains can still override this explicitly.
+    use_mold_separate_debug_file = use_mold
+    if (defined(invoker.use_mold_separate_debug_file)) {
+      use_mold_separate_debug_file = invoker.use_mold_separate_debug_file
+    }
+
     if (use_mold_separate_debug_file) {
       not_needed(invoker, [ "strip" ])
     }
--- a/build/toolchain/linux/unbundle/README.md
+++ b/build/toolchain/linux/unbundle/README.md
@@ -39,3 +39,43 @@ v8_snapshot_toolchain="//build/toolchain/linux/unbundle:host"
 
 Note: when cross-compiling for a 32-bit target, a matching 32-bit toolchain
 may be needed.
+
+## Linker Selection
+
+The unbundle toolchain uses the C++ compiler to perform linking (via compiler
+flags). The default linker is LLD (for clang builds).
+
+### Using Mold
+
+To use mold as the linker:
+
+```bash
+gn gen out/Default --args='
+custom_toolchain="//build/toolchain/linux/unbundle:default"
+use_mold=true
+'
+```
+
+This uses the mold binary from CIPD. For separate debug symbols (faster
+linking), the build automatically enables separate debug file handling when
+using mold.
+
+### Using a System Mold Binary
+
+To use a locally-installed mold instead of the CIPD-bundled one, set both
+`use_mold=true` and `linker_path` to the mold binary path:
+
+```bash
+gn gen out/Default --args='
+custom_toolchain="//build/toolchain/linux/unbundle:default"
+use_mold=true
+linker_path="/usr/bin/mold"
+'
+```
+
+`use_mold=true` is required: it enables separate-debug-file handling and
+allows ThinLTO. Setting only `linker_path` without `use_mold=true` will
+use the binary as a generic non-mold linker (no mold-specific optimisations).
+
+This also works for cross-compilation builds that use the unbundle host
+toolchain.
-- 
2.54.0