Enable gold threaded link

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/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 19 matching lines @@
   treat_warnings_as_errors = true
 
   # Normally, Android builds are lightly optimized, even for debug builds, to
   # keep binary size down. Setting this flag to true disables such optimization
   android_full_debug = false
 
   # Whether to use the binary binutils checked into third_party/binutils.
   # These are not multi-arch so cannot be used except on x86 and x86-64 (the
   # only two architectures that are currently checked in). Turn this off when
   # you are using a custom toolchain and need to control -B in cflags.
   linux_use_bundled_binutils =

[Lei Zhang, 2016/08/29 17:55:43]

I'm happy this CL landed, but now I'm trying to roll DEPS for PDFium, and I'm
stuck because this evaluates to true, even though $binutils_path below does not
exist. Any suggestions how if I should add some kind of a check for
$binutils_path, or just add binutils to the PDFium build on Linux?

[Nico, 2016/08/29 17:56:50]

I'd ask on gn-dev what best practice is supposed to be.

[Dirk Pranke, 2016/08/29 18:11:05]

There's a number of different possible options, but in this particular case 
I'd be inclined to change this so that linux_use_bundled_binutils wasn't 
true if that directory didn't exist. 

There's a //build/dir_exists.py script that can be used for that purpose.

Of course, we'd probably want to do a pass over things and figure out
if there are other places where we assume that //third_party/binutils exists;
hopefully they're all gated on this flag.

       is_linux && (current_cpu == "x64" || current_cpu == "x86")
   binutils_path = rebase_path("//third_party/binutils/Linux_x64/Release/bin",
                               root_build_dir)
 
   # Compile in such a way as to make it possible for the profiler to unwind full
   # stack frames. Setting this flag has a large effect on the performance of the
   # generated code than just setting profiling, but gives the profiler more
   # information to analyze.
   # Requires profiling to be set to true.
   enable_full_stack_frames_for_profiling = false
@@ skipping 150 matching lines @@
 
       # The x86 toolchain currently has problems with stack-protector.
       if (is_android && current_cpu == "x86") {
         cflags += [ "-fno-stack-protector" ]
       } else {
         cflags += [ "-fstack-protector" ]
       }
     }
 
     # Linker warnings.
-    if ( fatal_linker_warnings &&
-        !(is_chromeos && current_cpu == "arm") &&
+    if (fatal_linker_warnings && !(is_chromeos && current_cpu == "arm") &&
         !(is_android && use_order_profiling) && !is_mac && !is_ios) {
       # TODO(jochen): Enable this on chromeos on arm. http://crbug.com/356580
       # TODO(lizeb,pasko): Fix link errors when linking with order_profiling=1
       # crbug.com/485542
       ldflags += [ "-Wl,--fatal-warnings" ]
     }
   }
 
   if (is_clang && is_debug) {
     # Allow comparing the address of references and 'this' against 0
@@ skipping 88 matching lines @@
         # Let clang find the ld.gold in the NDK.
         ldflags += [ "--gcc-toolchain=$_rebased_android_toolchain_root" ]
       }
 
       # Use -mstackrealign due to a bug on ia32 Jelly Bean.
       # See crbug.com/521527
       if (current_cpu == "x86") {
         cflags += [ "-mstackrealign" ]
       }
     } else {
-      ldflags += [
-        "-B$gold_path",
-        # Experimentation found that using four linking threads
-        # saved ~20% of link time.
-        # https://groups.google.com/a/chromium.org/group/chromium-dev/browse_thread/thread/281527606915bb36
-        # Only apply this to the target linker, since the host
-        # linker might not be gold, but isn't used much anyway.
-        # TODO(raymes): Disable threading because gold is frequently
-        # crashing on the bots: crbug.com/161942.
-        #"-Wl,--threads",
-        #"-Wl,--thread-count=4",
-      ]
+      ldflags += [ "-B$gold_path" ]
+
+      if (linux_use_bundled_binutils) {
+        ldflags += [
+          # Experimentation found that using four linking threads
+          # saved ~20% of link time.
+          # https://groups.google.com/a/chromium.org/group/chromium-dev/browse_thread/thread/281527606915bb36
+          # Only apply this to the target linker, since the host
+          # linker might not be gold, but isn't used much anyway.
+          "-Wl,--threads",
+          "-Wl,--thread-count=4",
+        ]
+      }
     }
 
     if (gdb_index) {
       ldflags += [ "-Wl,--gdb-index" ]
     }
 
     # TODO(thestig): Make this flag work with GN.
     #if (!is_official_build && !is_chromeos && !(is_asan || is_lsan || is_tsan || is_msan)) {
     #  ldflags += [
     #    "-Wl,--detect-odr-violations",
@@ skipping 1223 matching lines @@
 
 if (is_ios || is_mac) {
   # On Mac and iOS, this enables support for ARC (automatic ref-counting).
   # See http://clang.llvm.org/docs/AutomaticReferenceCounting.html.
   config("enable_arc") {
     common_flags = [ "-fobjc-arc" ]
     cflags_objc = common_flags
     cflags_objcc = common_flags
   }
 }