Use -fno-delete-null-pointer-checks with gcc builds

R=bmeurer@chromium.org, jochen@chromium.org, machenbach@chromium.org
BUG=v8:3782

Review-Url: https://codereview.chromium.org/2310513002
Cr-Commit-Position: refs/heads/master@{#39286}

BUILD.gn

@@ -322,6 +322,13 @@ config("toolchain") {
   if (is_android && v8_android_log_stdout) {
     defines += [ "V8_ANDROID_LOG_STDOUT" ]
   }
+  if (!is_win && !is_clang) {
+    # GCC 6+ can optimize away pointer comparisons to null. This is
+    # problematic as V8 encodes Values through tagged pointers and comparisons
+    # with 0 are actually necessary in many cases. As a temporary Workaround
+    # we disable this optimization. See: https://crbug.com/v8/3782
+    cflags += [ "-fno-delete-null-pointer-checks" ]
+  }
 
   # TODO(jochen): Support v8_enable_prof on Windows.
   # TODO(jochen): Add support for compiling with simulators.

gypfiles/toolchain.gypi

@@ -1098,6 +1098,13 @@
           }],
         ],  # conditions
       }],
+      # GCC 6+ can optimize away pointer comparisons to null. This is
+      # problematic as V8 encodes Values through tagged pointers and comparisons
+      # with 0 are actually necessary in many cases. As a temporary Workaround
+      # we disable this optimization. See: https://crbug.com/v8/3782
+      ['OS!="win" and clang==0', {
+        'cflags': [ '-fno-delete-null-pointer-checks' ]
+      }],
       ['OS=="solaris"', {
         'defines': [ '__C99FEATURES__=1' ],  # isinf() etc.
       }],