[GN]: Precompiled header support for GCC.

build/config/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/allocator.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/crypto.gni")
 import("//build/config/features.gni")
 import("//build/config/sanitizers/sanitizers.gni")
 import("//build/config/ui.gni")
@@ skipping 399 matching lines @@
       "CoreText.framework",
       "Foundation.framework",
     ]
   } else if (is_linux) {
     libs = [ "dl" ]
   }
 }
 
 # Add this config to your target to enable precompiled headers.
 #
-# On Windows, precompiled headers are done on a per-target basis. If you have
-# just a couple of files, the time it takes to precompile (~2 seconds) can
-# actually be longer than the time saved. On a Z620, a 100 file target compiles
-# about 2 seconds faster with precompiled headers, with greater savings for
-# larger targets.
+# Precompiled headers are done on a per-target basis. If you have just a couple
+# of files, the time it takes to precompile (~2 seconds) can actually be longer
+# than the time saved. On a Z620, a 100 file target compiles about 2 seconds
+# faster with precompiled headers, with greater savings for larger targets.
 #
 # Recommend precompiled headers for targets with more than 50 .cc files.
 config("precompiled_headers") {
   if (is_win && !is_official_build && !use_goma) {
     # This is a string rather than a file GN knows about. It has to match
     # exactly what's in the /FI flag below, and what might appear in the source
     # code in quotes for an #include directive.
     precompiled_header = "build/precompile.h"
 
     # This is a file that GN will compile with the above header. It will be
     # implicitly added to the sources (potentially multiple times, with one
     # variant for each language used in the target).
     precompiled_source = "//build/precompile.cc"
 
     # Force include the header.
     cflags = [ "/FI$precompiled_header" ]
 
     # Disable warning for "this file was empty after preprocessing". This
     # error is generated only in C mode for ANSI compatibility. It conflicts
     # with precompiled headers since the source file that's "compiled" for
     # making the precompiled header is empty.
     #
     # This error doesn't happen every time. In VS2013, it seems if the .pch
     # file doesn't exist, no error will be generated (probably MS tested this
     # case but forgot the other one?). To reproduce this error, do a build,
     # then delete the precompile.c.obj file, then build again.
     cflags_c = [ "/wd4206" ]
+  } else if (is_mac && !is_official_build && !use_goma) {
+    # TODO:(andybons): enable this when GCC PCH support in the binary has been
+    # rolled.
+    #precompiled_header = "build/precompile.h"
+    #precompiled_source = "//build/precompile.h"
   }
 }