Add use_thin_lto switch to start evaluating 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/compiler/compiler.gni")
 import("//build/config/nacl/config.gni")
 import("//build/toolchain/cc_wrapper.gni")
 import("//build/toolchain/toolchain.gni")
@@ skipping 425 matching lines @@
 
   # Add flags for link-time optimization. These flags enable
   # optimizations/transformations that require whole-program visibility at link
   # time, so they need to be applied to all translation units, and we may end up
   # with miscompiles if only part of the program is compiled with LTO flags. For
   # that reason, we cannot allow targets to enable or disable these flags, for
   # example by disabling the optimize configuration.
   # TODO(pcc): Make this conditional on is_official_build rather than on gn
   # flags for specific features.
   if (!is_debug && (allow_posix_link_time_opt || is_cfi) && !is_nacl) {
-    cflags += [ "-flto" ]
-    ldflags += [ "-flto" ]
+    if (use_thin_lto) {
+      cflags += [ "-flto=thin" ]
+      ldflags += [ "-flto=thin" ]
+    } else {
+      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" ]
+      # 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" ]
       }
-    } else if (is_mac) {
-      ldflags += [ "-Wl,-mllvm,-O1" ]
+
+      # Note: ThinLTO does not currently have this feature implemented
+      # For Full LTO, it provides a measurable runtime speedup of Chrome.
+      cflags += [ "-fwhole-program-vtables" ]
+      ldflags += [ "-fwhole-program-vtables" ]

[Dirk Pranke, 2016/09/12 21:15:38]

Can you move these four lines up next to lines 450-451 (or even merge them)? I
think that'd read better.

[krasin1, 2016/09/12 21:35:06]

Done. Good idea.

     }
 
     # Work-around for http://openradar.appspot.com/20356002
     if (is_mac) {
       ldflags += [ "-Wl,-all_load" ]
     }
 
     # Allows the linker to apply 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) {
       ldflags += [ "-Wl,-plugin-opt,-function-sections" ]
     }
-
-    cflags += [ "-fwhole-program-vtables" ]
-    ldflags += [ "-fwhole-program-vtables" ]
   }
 
   # Pass the same C/C++ flags to the objective C/C++ compiler.
   cflags_objc += cflags_c
   cflags_objcc += cflags_cc
 
   # Assign any flags set for the C compiler to asmflags so that they are sent
   # to the assembler. The Windows assembler takes different types of flags
   # so only do so for posix platforms.
   if (is_posix) {
@@ skipping 1116 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
   }
 }