Disable --as-needed for libpthread to work around linker bug.

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 1262 matching lines @@
       # Specifically tell the linker to perform optimizations.
       # See http://lwn.net/Articles/192624/ .
       "-Wl,-O1",
       "-Wl,--gc-sections",
     ]
 
     if (!using_sanitizer) {
       # Functions interposed by the sanitizers can make ld think
       # that some libraries aren't needed when they actually are,
       # http://crbug.com/234010. As workaround, disable --as-needed.
+      if (!is_nacl && !is_android) {

[Nico, 2016/06/28 21:56:23]

what about chromeos? should this only happen if is_clang, and/or if lto is on?

[pcc1, 2016/06/28 22:19:34]

This should probably be testing for whether the standard library is glibc, but
I'm not sure that we have a good way to test that.

It's only by chance that we saw this issue with Clang+LTO; the same issue could
happen with GCC depending on how code is inlined, or with any non-PIC object
file (although I don't think our build system can build any of those).

Regarding ChromeOS, according to [1] it uses glibc so I don't think we need to
do anything about it.

[1]
http://unix.stackexchange.com/questions/269487/is-chromium-os-a-gnu-linux-distro

+        # TODO(pcc): Fix linker bug which requires us to link pthread
+        # unconditionally here (crbug.com/623236).
+        common_optimize_on_ldflags += [
+          "-Wl,--no-as-needed",
+          "-lpthread",
+        ]
+      }
       common_optimize_on_ldflags += [ "-Wl,--as-needed" ]
     }
   }
 }
 
 # Default "optimization on" config.
 config("optimize") {
   if (is_win) {
     # TODO(thakis): Remove is_clang here, https://crbug.com/598772
     if (is_official_build && full_wpo_on_official && !is_clang) {
@@ skipping 251 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
   }
 }