[builtins] Add v8_enable_embedded_builtins gn flag

This flag will be used to toggle things for isolate-independent
builtins during development.

Bug: v8:6666
Change-Id: I8a97f08b3d677a01a2a55a4c6445e71e74471f51
Reviewed-on: https://chromium-review.googlesource.com/924067
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51325}

BUILD.gn

@@ -64,6 +64,9 @@ declare_args() {
   # Enable fast mksnapshot runs.
   v8_enable_fast_mksnapshot = false
 
+  # Enable embedded builtins.
+  v8_enable_embedded_builtins = false
+
   # Enable code-generation-time checking of types in the CodeStubAssembler.
   v8_enable_verify_csa = false
 
@@ -319,6 +322,9 @@ config("features") {
   if (v8_check_microtasks_scopes_consistency) {
     defines += [ "V8_CHECK_MICROTASKS_SCOPES_CONSISTENCY" ]
   }
+  if (v8_enable_embedded_builtins) {
+    defines += [ "V8_EMBEDDED_BUILTINS" ]
+  }
 }
 
 config("toolchain") {

src/flag-definitions.h

@@ -1024,8 +1024,13 @@ DEFINE_INT(fuzzer_random_seed, 0,
 DEFINE_BOOL(trace_rail, false, "trace RAIL mode")
 DEFINE_BOOL(print_all_exceptions, false,
             "print exception object and stack trace on each thrown exception")
+#ifdef V8_EMBEDDED_BUILTINS
 DEFINE_BOOL(stress_off_heap_code, false,
             "Move code objects off-heap for testing.")
+#else
+FLAG_READONLY(BOOL, bool, stress_off_heap_code, false,
+              "Move code objects off-heap for testing.")
+#endif
 
 // runtime.cc
 DEFINE_BOOL(runtime_call_stats, false, "report runtime call counts and times")

src/isolate.cc

@@ -2826,6 +2826,7 @@ void PrintBuiltinSizes(Isolate* isolate) {
   }
 }
 
+#ifdef V8_EMBEDDED_BUILTINS
 #ifdef DEBUG
 bool BuiltinAliasesOffHeapTrampolineRegister(Isolate* isolate,
                                              int builtin_index) {
@@ -2931,6 +2932,7 @@ void MoveBuiltinsOffHeap(Isolate* isolate) {
     isolate->PushOffHeapCode(stream);
   }
 }
+#endif  // V8_EMBEDDED_BUILTINS
 }  // namespace
 
 bool Isolate::Init(StartupDeserializer* des) {
@@ -3079,12 +3081,14 @@ bool Isolate::Init(StartupDeserializer* des) {
 
   if (FLAG_print_builtin_size) PrintBuiltinSizes(this);
 
+#ifdef V8_EMBEDDED_BUILTINS
   if (FLAG_stress_off_heap_code && !serializer_enabled()) {
     // Artificially move code off-heap to help find & verify related code
     // paths. Lazy deserialization should be off to avoid confusion around
     // replacing just the kDeserializeLazy code object.
     MoveBuiltinsOffHeap(this);
   }
+#endif
 
   // Finish initialization of ThreadLocal after deserialization is done.
   clear_pending_exception();