build: Enable -fwhole-program-vtables under ThinLTO.

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/chromecast_build.gni")
 import("//build/config/compiler/compiler.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 469 matching lines @@
       if (use_lld) {
         ldflags += [
           "-Wl,--thinlto-jobs=8",
           "-Wl,--thinlto-cache-dir=" +
               rebase_path("$root_out_dir/thinlto-cache", root_build_dir),
         ]
       } else {
         ldflags += [ "-Wl,-plugin-opt,jobs=8" ]
       }
     } else {
-      # Note: ThinLTO does not currently have this feature implemented
-      # For Full LTO, it provides a measurable runtime speedup of Chrome.
-      cflags += [
-        "-flto",
-        "-fwhole-program-vtables",
-      ]
-      ldflags += [
-        "-flto",
-        "-fwhole-program-vtables",
-      ]
+      cflags += [ "-flto" ]
+      ldflags += [ "-flto" ]
 
       # Apply a lower LTO optimization level as the default is too slow.
       if (is_linux) {
         if (use_lld) {
           ldflags += [ "-Wl,--lto-O1" ]
         } else {
           ldflags += [ "-Wl,-plugin-opt,O1" ]
         }
       } else if (is_mac) {
         ldflags += [ "-Wl,-mllvm,-O1" ]
       }
     }
 
+    cflags += [ "-fwhole-program-vtables" ]
+    ldflags += [ "-fwhole-program-vtables" ]
+
     # Work-around for http://openradar.appspot.com/20356002
     if (is_mac) {

[Nico, 2017/05/16 19:51:19]

Huh, why do we have mac stuff in here? We don't use LTO on mac, do we?

[pcc1, 2017/05/16 19:57:33]

I think this is from some early work I did to try to support LTO on Mac
(crbug.com/471146). I'd be fine with removing it, we can always bring it back
later.

       ldflags += [ "-Wl,-all_load" ]
     }
 
     # Allows the linker to apply --gc-sections and ICF to the LTO object file.
     # Also, when targeting ARM, without this flag, LTO produces a .text section
     # that is larger than the maximum call displacement, preventing the linker
     # from relocating calls (http://llvm.org/PR22999).
     if (is_linux) {
       if (use_lld) {
         ldflags += [
@@ skipping 1227 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
   }
 }