diff --git a/BUILD.gn b/BUILD.gn
index 381dd1e769..4551e1eb9e 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -76,6 +76,9 @@ declare_args() {
   # Sets -dV8_TRACE_IGNITION.
   v8_enable_trace_ignition = false
 
+  # Sets -dV8_CONCURRENT_MARKING
+  v8_enable_concurrent_marking = false
+
   # With post mortem support enabled, metadata is embedded into libv8 that
   # describes various parameters of the VM for use by debuggers. See
   # tools/gen-postmortem-metadata.py for details.
@@ -257,6 +260,9 @@ config("features") {
   if (v8_use_external_startup_data) {
     defines += [ "V8_USE_EXTERNAL_STARTUP_DATA" ]
   }
+  if (v8_enable_concurrent_marking) {
+    defines += [ "V8_CONCURRENT_MARKING" ]
+  }
 }
 
 config("toolchain") {
diff --git a/src/flag-definitions.h b/src/flag-definitions.h
index 30a294ad12..887b89eab7 100644
--- a/src/flag-definitions.h
+++ b/src/flag-definitions.h
@@ -668,7 +668,13 @@ DEFINE_BOOL(age_code, true,
 DEFINE_BOOL(incremental_marking, true, "use incremental marking")
 DEFINE_BOOL(incremental_marking_wrappers, true,
             "use incremental marking for marking wrappers")
-DEFINE_BOOL(concurrent_marking, V8_CONCURRENT_MARKING, "use concurrent marking")
+#ifdef V8_CONCURRENT_MARKING
+#define V8_CONCURRENT_MARKING_BOOL true
+#else
+#define V8_CONCURRENT_MARKING_BOOL false
+#endif
+DEFINE_BOOL(concurrent_marking, V8_CONCURRENT_MARKING_BOOL,
+            "use concurrent marking")
 DEFINE_BOOL(trace_concurrent_marking, false, "trace concurrent marking")
 DEFINE_BOOL(minor_mc_parallel_marking, true,
             "use parallel marking for the young generation")
diff --git a/src/globals.h b/src/globals.h
index 6d34ab9f0a..1194472d29 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -118,8 +118,6 @@ namespace internal {
 #define V8_DOUBLE_FIELDS_UNBOXING 0
 #endif
 
-#define V8_CONCURRENT_MARKING 0
-
 // Some types of tracing require the SFI to store a unique ID.
 #if defined(V8_TRACE_MAPS) || defined(V8_TRACE_IGNITION)
 #define V8_SFI_HAS_UNIQUE_ID 1
diff --git a/src/heap/concurrent-marking.cc b/src/heap/concurrent-marking.cc
index 32afac63f3..1c1bf1b2df 100644
--- a/src/heap/concurrent-marking.cc
+++ b/src/heap/concurrent-marking.cc
@@ -236,10 +236,10 @@ ConcurrentMarking::ConcurrentMarking(Heap* heap, ConcurrentMarkingDeque* deque)
       deque_(deque),
       visitor_(new ConcurrentMarkingVisitor(deque_)),
       is_task_pending_(false) {
-  // Concurrent marking does not work with double unboxing.
-  STATIC_ASSERT(!(V8_CONCURRENT_MARKING && V8_DOUBLE_FIELDS_UNBOXING));
   // The runtime flag should be set only if the compile time flag was set.
-  CHECK(!FLAG_concurrent_marking || V8_CONCURRENT_MARKING);
+#ifndef V8_CONCURRENT_MARKING
+  CHECK(!FLAG_concurrent_marking);
+#endif
 }
 
 ConcurrentMarking::~ConcurrentMarking() { delete visitor_; }
diff --git a/src/heap/heap.cc b/src/heap/heap.cc
index 209373ac14..df1df79af3 100644
--- a/src/heap/heap.cc
+++ b/src/heap/heap.cc
@@ -5651,7 +5651,7 @@ bool Heap::SetUp() {
   mark_compact_collector_ = new MarkCompactCollector(this);
   incremental_marking_->set_marking_deque(
       mark_compact_collector_->marking_deque());
-#if V8_CONCURRENT_MARKING
+#ifdef V8_CONCURRENT_MARKING
   concurrent_marking_ =
       new ConcurrentMarking(this, mark_compact_collector_->marking_deque());
 #else
diff --git a/src/heap/incremental-marking.cc b/src/heap/incremental-marking.cc
index cb9f46782d..eb97176e95 100644
--- a/src/heap/incremental-marking.cc
+++ b/src/heap/incremental-marking.cc
@@ -141,7 +141,7 @@ void IncrementalMarking::MarkBlackAndPush(HeapObject* obj) {
   // Color the object black and push it into the bailout deque.
   ObjectMarking::WhiteToGrey<kAtomicity>(obj, marking_state(obj));
   if (ObjectMarking::GreyToBlack<kAtomicity>(obj, marking_state(obj))) {
-#if V8_CONCURRENT_MARKING
+#ifdef V8_CONCURRENT_MARKING
     marking_deque()->Push(obj, MarkingThread::kMain, TargetDeque::kBailout);
 #else
     if (!marking_deque()->Push(obj)) {
diff --git a/src/heap/incremental-marking.h b/src/heap/incremental-marking.h
index 998d0b279e..4a88ab3fae 100644
--- a/src/heap/incremental-marking.h
+++ b/src/heap/incremental-marking.h
@@ -182,7 +182,7 @@ class V8_EXPORT_PRIVATE IncrementalMarking {
   static const intptr_t kActivationThreshold = 0;
 #endif
 
-#if V8_CONCURRENT_MARKING
+#ifdef V8_CONCURRENT_MARKING
   static const MarkBit::AccessMode kAtomicity = MarkBit::AccessMode::ATOMIC;
 #else
   static const MarkBit::AccessMode kAtomicity = MarkBit::AccessMode::NON_ATOMIC;
diff --git a/src/heap/mark-compact.h b/src/heap/mark-compact.h
index fa57bfb872..29c871d406 100644
--- a/src/heap/mark-compact.h
+++ b/src/heap/mark-compact.h
@@ -33,7 +33,7 @@ class RecordMigratedSlotVisitor;
 class ThreadLocalTop;
 class YoungGenerationMarkingVisitor;
 
-#if V8_CONCURRENT_MARKING
+#ifdef V8_CONCURRENT_MARKING
 using MarkingDeque = ConcurrentMarkingDeque;
 #else
 using MarkingDeque = SequentialMarkingDeque;
diff --git a/src/objects-inl.h b/src/objects-inl.h
index 11162bf7b0..00441babef 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -1284,7 +1284,7 @@ bool JSObject::PrototypeHasNoElements(Isolate* isolate, JSObject* object) {
   reinterpret_cast<Object*>(base::NoBarrier_Load( \
       reinterpret_cast<const base::AtomicWord*>(FIELD_ADDR_CONST(p, offset))))
 
-#if V8_CONCURRENT_MARKING
+#ifdef V8_CONCURRENT_MARKING
 #define WRITE_FIELD(p, offset, value)                             \
   base::NoBarrier_Store(                                          \
       reinterpret_cast<base::AtomicWord*>(FIELD_ADDR(p, offset)), \