Win/Clang: turn on LTO for Official builds that use LLD

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 1227 matching lines @@
       # This optimization can't be used when ASan is enabled because
       # it is not compatible with the ASan ODR checker.
       "/Gw",
     ]
   }
   common_optimize_on_ldflags = []
   if (!is_component_build) {
     common_optimize_on_ldflags += [ "/OPT:ICF" ]  # Redundant COMDAT folding.
   }
   if (is_official_build) {
-    common_optimize_on_ldflags += [
-      "/OPT:REF",  # Remove unreferenced data.
-      "/LTCG",  # Link-time code generation.
+    common_optimize_on_ldflags += [ "/OPT:REF" ]  # Remove unreferenced data.
+    if (!use_lld) {
+      common_optimize_on_ldflags += [
+        "/LTCG",  # Link-time code generation.
 
-      # Set the number of LTCG code-gen threads to eight. The default is four.
-      # This gives a 5-10% link speedup.
-      "/cgthreads:8",
-    ]
+        # Set the number of LTCG code-gen threads to eight. The default is four.
+        # This gives a 5-10% link speedup.
+        "/cgthreads:8",
+      ]
+    }
     if (full_wpo_on_official) {
       arflags = [ "/LTCG" ]
     }
   }
 } else {
   common_optimize_on_cflags = []
   common_optimize_on_ldflags = []
 
   if (is_android) {
     # TODO(jdduke) Re-enable on mips after resolving linking
@@ skipping 131 matching lines @@
     # So this config has to have the selection logic just like
     # "default_optimization", below.
     configs = [ "//build/config/nacl:irt_optimize" ]
   } else {
     ldflags = common_optimize_on_ldflags
     if (is_win) {
       # Favor speed over size, /O2 must be before the common flags. The GYP
       # build also specifies /Ot, /Oi, and /GF, but these are implied by /O2.
       cflags = [ "/O2" ] + common_optimize_on_cflags
 
-      # TODO(thakis): Remove is_clang here, https://crbug.com/598772
-      if (is_official_build && !is_clang) {
-        cflags += [
-          "/GL",  # Whole program optimization.
+      if (is_official_build) {
+        if (!is_clang) {
+          cflags += [
+            "/GL",  # Whole program optimization.

[Reid Kleckner, 2016/10/04 22:25:40]

Should we alias /GL to -flto in clang, or do you think that would be confusing
because it doesn't actually interoperate with link.exe /LTCG?

[hans, 2016/10/04 22:28:52]

Yes, I think that might be confusing. We discussed this a little already in
https://bugs.chromium.org/p/chromium/issues/detail?id=598772#c2

 
-          # Disable Warning 4702 ("Unreachable code") for the WPO/PGO builds.
-          # Probably anything that this would catch that wouldn't be caught in a
-          # normal build isn't going to actually be a bug, so the incremental
-          # value of C4702 for PGO builds is likely very small.
-          "/wd4702",
-        ]
+            # Disable Warning 4702 ("Unreachable code") for the WPO/PGO builds.
+            # Probably anything that this would catch that wouldn't be caught
+            # in a normal build isn't going to actually be a bug, so the
+            # incremental value of C4702 for PGO builds is likely very small.
+            "/wd4702",
+          ]
+        } else if (is_clang && use_lld) {
+          cflags += [ "-flto" ]  # Link-time optimization (whole program optimization).
+        }
       }
     } else {
       cflags = [ "-O2" ] + common_optimize_on_cflags
     }
   }
 }
 
 # This config can be used to override the default settings for per-component
 # and whole-program optimization, optimizing the particular target for speed
 # instead of code size. This config is exactly the same as "optimize_max"
@@ skipping 162 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
   }
 }