Enable whole-program virtual function optimization in LTO mode.

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 898b25ff2e07982597603e6b9390211c718ca404..f0c8cd947c34b50c277adc79e56e091c6ba13ead 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -7,6 +7,7 @@ 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")
 
 if (current_cpu == "arm") {
   import("//build/config/arm.gni")
@@ -370,6 +371,52 @@ config("compiler") {
     }
   }
 
+  # 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" ]
+
+    # Apply a lower LTO optimization level as the default is too slow.
+    if (is_linux) {
+      ldflags += [ "-Wl,-plugin-opt,O1" ]
+    } else if (is_mac) {
+      ldflags += [ "-Wl,-mllvm,-O1" ]
+    }
+
+    # 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

[brettw, 2016/04/06 06:25:36]

80 cols.

[pcc1, 2016/04/06 21:42:01]

Done.

+    # 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" ]
+    }
+
+    # TODO(pcc): Make these flags work correctly with CFI.
+    if (!is_cfi) {
+      cflags += [
+        "-fwhole-program-vtables",
+
+        # TODO(pcc): Remove this flag once the upstream interface change
+        # (http://reviews.llvm.org/D18635) lands.
+        "-fwhole-program-vtables-blacklist=" +
+            rebase_path("//tools/cfi/blacklist.txt", root_build_dir),
+      ]
+      ldflags += [ "-fwhole-program-vtables" ]
+    }
+  }
+
   # Pass the same C/C++ flags to the objective C/C++ compiler.
   cflags_objc += cflags_c
   cflags_objcc += cflags_cc