Add the possibility to build with PGO when using Clang

chrome/BUILD.gn

Index: chrome/BUILD.gn
diff --git a/chrome/BUILD.gn b/chrome/BUILD.gn
index ec75dcdb41c4531fca2bb3beef6c097211fbf650..1bc98e42a45f3cc1b63c44008c6a6c1b26ff158d 100644
--- a/chrome/BUILD.gn
+++ b/chrome/BUILD.gn
@@ -5,6 +5,7 @@
 import("//build/buildflag_header.gni")
 import("//build/config/chrome_build.gni")
 import("//build/config/compiler/compiler.gni")
+import("//build/config/compiler/pgo/pgo.gni")
 import("//build/config/features.gni")
 import("//build/config/locales.gni")
 import("//build/config/sanitizers/sanitizers.gni")
@@ -116,6 +117,16 @@ if (!is_android && !is_mac) {
       output_name = "chrome"
     }
 
+    if (!is_win || is_clang) {
+      # Normally, we need to pass specific flags to the linker to
+      # create an executable that gathers profile data. However, when
+      # using MSVC, we need to make sure we *don't* pass /GENPROFILE
+      # when linking without generating any code, or else the linker
+      # will give us fatal error LNK1264. So we add the PGO flags
+      # on all configurations, execpt MSVC on Windows.
+      configs += [ "//build/config/compiler/pgo:default_pgo_flags" ]

[Sébastien Marchand, 2016/11/24 15:57:07]

This config will be applied to chrome.exe, do you also want to apply it to the
'main_dll' target (for the Clang build only).

Again, I really think that this should be behind a "if (chrome_pgo_phase > 0)"
condition. Debugging an error caused by someone adding some default flag to the
PGO config will probably be tricky, so it's probably better to add some more
safety checks.

+    }
+
     # Because the sources list varies so significantly per-platform, generally
     # each platform lists its own files rather than relying on filtering or
     # removing unused files.
@@ -425,28 +436,7 @@ if (is_win) {
         configs -= [ "//build/config/win:default_incremental_linking" ]
         configs += [ "//build/config/win:no_incremental_linking" ]
       }
-      if (chrome_pgo_phase == 1) {
-        ldflags += [
-          "/LTCG",
-
-          # Make sure that enough memory gets allocated for the PGO profiling
-          # buffers and also cap this memory. Usually a PGI instrumented build
-          # of chrome_child.dll requires ~55MB of memory for storing its counter
-          # etc, normally the linker should automatically choose an appropriate
-          # amount of memory but it doesn't always do a good estimate and
-          # sometime allocates too little or too much (and so the instrumented
-          # image fails to start). Making sure that the buffer has a size in the
-          # [128 MB, 512 MB] range should prevent this from happening.
-          "/GENPROFILE:MEMMIN=134217728",
-          "/GENPROFILE:MEMMAX=536870912",
-          "/PogoSafeMode",
-        ]
-      } else if (chrome_pgo_phase == 2) {
-        ldflags += [
-          "/LTCG",
-          "/USEPROFILE",
-        ]
-      }
+      configs += [ "//build/config/compiler/pgo:default_pgo_flags" ]
 
       if (enable_plugins && enable_pdf) {
         deps += [ "//pdf" ]