Add an `auto_profile_path` flag to GN to control AFDO.

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 62 matching lines @@
   # If true, optimize for size. Does not affect windows builds.
   # Linux & Mac favor speed over size.
   # TODO(brettw) it's weird that Mac and desktop Linux are different. We should
   # explore favoring size over speed in this case as well.
   optimize_for_size = is_android || is_ios
 
   # Enable fatal linker warnings. Building Chromium with certain versions
   # of binutils can cause linker warning.
   # See: https://bugs.chromium.org/p/chromium/issues/detail?id=457359
   fatal_linker_warnings = true
+
+  # AFDO (Automatic Feedback Directed Optimizer) is a form of profile-guided
+  # optimization that GCC supports. It used by ChromeOS in their official
+  # builds. To use it, set auto_profile_path to the path to a file containing
+  # the needed gcov profiling data.
+  auto_profile_path = ""
 }
 
 if (is_clang && !is_nacl) {
   update_args = [ "--print-revision" ]
   if (llvm_force_head_revision) {
     update_args += [ "--llvm-force-head-revision" ]
   }
   clang_revision =
       exec_script("//tools/clang/scripts/update.py", update_args, "trim string")
 }
@@ skipping 1379 matching lines @@
     # The NaCl IRT is a special case and always wants its own config.
     # It gets optimized the same way regardless of the type of build.
     configs = [ "//build/config/nacl:irt_optimize" ]
   } else if (is_debug) {
     configs = [ ":no_optimize" ]
   } else {
     configs = [ ":optimize" ]
   }
 }
 
+# GCC supports a form of profile-guided optimization called AFDO, which
+# is used by ChromeOS in their official builds. However,
+# //base/allocator:tcmalloc currently doesn't work correctly with AFDO
+# so we provide separate config so that the flag can be disabled per-target.
+# TODO(crbug.com/633719): Remove this config once tcmalloc works with AFDO
+# or we remove tcmalloc or we stop using AFDO.
+config("afdo") {
+  if (auto_profile_path) {
+    cflags = [ "-fauto-profile=${auto_profile_path}" ]
+  }
+}
+
 # Symbols ----------------------------------------------------------------------
 
 # The BUILDCONFIG file sets the "default_symbols" config on targets by
 # default. It will be equivalent to one the three specific symbol levels.
 #
 # You can override the symbol level on a per-target basis by removing the
 # default config and then adding the named one you want:
 #
 #   configs -= [ "//build/config/compiler:default_symbols" ]
 #   configs += [ "//build/config/compiler:symbols" ]
@@ skipping 98 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
   }
 }