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

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 4bbc34f4e7d0cc77a69d13856bbd843c116a0dab..24e29d24518870695577ea35c09633ade59ef7be 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -1245,14 +1245,16 @@ if (is_win) {
     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" ]
     }
@@ -1404,17 +1406,20 @@ config("optimize_max") {
       # 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