[compiler] Put shared function marking behind a flag

This lets us investigate regressions caused by this marking while
letting others continue their work without being impacted.

BUG=v8:5512

Review-Url: https://codereview.chromium.org/2446673002
Cr-Commit-Position: refs/heads/master@{#40563}

src/compiler.cc

@@ -978,6 +978,8 @@ MaybeHandle<Code> GetLazyCode(Handle<JSFunction> function) {
   }
 
   if (function->shared()->was_marked_for_optimization()) {
+    DCHECK(FLAG_optimize_shared_functions);
+
     function->shared()->set_was_marked_for_optimization(false);
 
     if (FLAG_trace_opt) {

src/flag-definitions.h

@@ -275,6 +275,8 @@ DEFINE_BOOL(track_field_types, true, "track field types")
 DEFINE_IMPLICATION(track_field_types, track_fields)
 DEFINE_IMPLICATION(track_field_types, track_heap_object_fields)
 DEFINE_BOOL(smi_binop, true, "support smi representation in binary operations")
+DEFINE_BOOL(optimize_shared_functions, false,
+            "mark shared functions for optimization")
 
 // Flags for optimization types.
 DEFINE_BOOL(optimize_for_size, false,

src/objects.cc

@@ -12212,9 +12212,11 @@ void JSFunction::MarkForOptimization() {
 
 
 void JSFunction::AttemptConcurrentOptimization() {
-  // Mark the shared function for optimization regardless of whether the
-  // optimization is concurrent or not.
-  shared()->set_was_marked_for_optimization(true);
+  if (FLAG_optimize_shared_functions) {
+    // Mark the shared function for optimization regardless of whether the
+    // optimization is concurrent or not.
+    shared()->set_was_marked_for_optimization(true);
+  }
 
   Isolate* isolate = GetIsolate();
   if (!isolate->concurrent_recompilation_enabled() ||