Introduce cfi_vptr build flag.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index f67bf4f0f44327ee634c257636e0bef6484bcb98..b4066adaf0bbdb7f3de9c0fb8e1f22b2b8bc6863 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -646,6 +646,10 @@
       # Libxkbcommon usage.
       'use_xkbcommon%': 0,
 
+      # Control Flow Integrity for virtual calls.
+      # See http://clang.llvm.org/docs/ControlFlowIntegrity.html
+      'cfi_vptr%': 0,
+
       'conditions': [
         # A flag for POSIX platforms
         ['OS=="win"', {
@@ -1220,6 +1224,7 @@
     'video_hole%': '<(video_hole)',
     'support_pre_M6_history_database%': '<(support_pre_M6_history_database)',
     'v8_use_external_startup_data%': '<(v8_use_external_startup_data)',
+    'cfi_vptr%': '<(cfi_vptr)',
 
     # Use system protobuf instead of bundled one.
     'use_system_protobuf%': 0,
@@ -2995,7 +3000,7 @@
       ['v8_use_external_startup_data==1', {
        'defines': ['V8_USE_EXTERNAL_STARTUP_DATA'],
       }],
-      ['use_lto==1 and (target_arch=="ia32" or target_arch=="x64")', {
+      ['(use_lto==1 or cfi_vptr==1) and (target_arch=="ia32" or target_arch=="x64")', {

[Nico, 2015/03/06 21:13:25]

alternatively use_lto could be set to 1 automatically if cfi_vptr is set (like
asan=1 used to set clang=1)

         # Required for third_party/zlib/crc_folding.c and various other code
         # that uses SSE. TODO(pcc): Remove this once we properly support
         # subtarget specific code generation in LLVM.
@@ -5903,7 +5908,7 @@
         ],
       },
     }],
-    ['use_lto==1 and clang==1', {
+    ['(use_lto==1 or cfi_vptr==1) and clang==1', {
       'target_defaults': {
         'target_conditions': [
           ['_toolset=="target"', {
@@ -5936,6 +5941,20 @@
         ],
       },
     }],
+    ['cfi_vptr==1', {
+      'target_defaults': {
+        'target_conditions': [
+          ['_toolset=="target"', {
+            'cflags': [
+              '-fsanitize=cfi-vptr',
+            ],
+            'ldflags': [
+              '-fsanitize=cfi-vptr',
+            ],
+          }],
+        ],
+      },
+    }],
   ],
   'xcode_settings': {
     # DON'T ADD ANYTHING NEW TO THIS BLOCK UNLESS YOU REALLY REALLY NEED IT!