GN: Refactor Linux Clang toolchain definitions, plumb use_gold for toolchain_args()

build/config/compiler/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/android/config.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/toolchain/ccache.gni")
 
 if (current_cpu == "arm") {
@@ skipping 32 matching lines @@
   # information to analyze.
   # Requires profiling to be set to true.
   enable_full_stack_frames_for_profiling = false
 
   # TODO(GYP): We should be using 64-bit gold for linking on both 64-bit Linux
   # and 32-bit linux; 32-bit Gold runs out of address-space on 32-bit builds.
   # However, something isn't quite working right on the 32-bit builds.
   use_gold = is_linux && current_cpu == "x64"
 
   # When we are going to use gold we need to find it.
-  if (use_gold) {
-    gold_path = rebase_path("//third_party/binutils/Linux_x64/Release/bin",
-                            root_build_dir)
-  } else {
-    gold_path = ""
-  }
+  # This is initialized below, after use_gold might have been overridden.
+  gold_path = false
 
   # use_debug_fission: whether to use split DWARF debug info
   # files. This can reduce link time significantly, but is incompatible
   # with some utilities such as icecc and ccache. Requires gold and
   # gcc >= 4.8 or clang.
   # http://gcc.gnu.org/wiki/DebugFission
-  use_debug_fission = is_debug && !is_win && use_gold &&
-                      linux_use_bundled_binutils && !use_ccache
+  #
+  # This is a placeholder value indicating that the code below should set
+  # the default.  This is necessary to delay the evaluation of the default
+  # value expression until after its input values such as use_gold have
+  # been set, e.g. by a toolchain_args() block.
+  use_debug_fission = "default"
 
   if (is_win) {
     # Whether the VS xtree header has been patched to disable warning 4702. If
     # it has, then we don't need to disable 4702 (unreachable code warning).
     # The patch is preapplied to the internal toolchain and hence all bots.
     msvs_xtree_patched = false
   }
 }
 
+# Apply the default logic for these values if they were not set explicitly.
+if (gold_path == false) {
+  if (use_gold) {
+    gold_path = rebase_path("//third_party/binutils/Linux_x64/Release/bin",
+                            root_build_dir)
+  } else {
+    gold_path = ""
+  }
+}
+
+if (use_debug_fission == "default") {
+  use_debug_fission = is_debug && !is_win && use_gold &&
+                      linux_use_bundled_binutils && !use_ccache
+}
+
 # default_include_dirs ---------------------------------------------------------
 #
 # This is a separate config so that third_party code (which would not use the
 # source root and might have conflicting versions of some headers) can remove
 # this and specify their own include paths.
 config("default_include_dirs") {
   include_dirs = [
     "//",
     root_gen_dir,
   ]
@@ skipping 1029 matching lines @@
   if (symbol_level == 0) {
     configs = [ ":no_symbols" ]
   } else if (symbol_level == 1) {
     configs = [ ":minimal_symbols" ]
   } else if (symbol_level == 2) {
     configs = [ ":symbols" ]
   } else {
     assert(false)
   }
 }