[offthread] Make NextScriptId thread-safe

Make the NextScriptId value incrementing method thread-safe, using
CompareAndSwap primitives. This will allow it to be used in the
future for off-thread compilation.

Bug: chromium:1011762
Change-Id: I5b449e75c7dc63d24fa045ae880b0680b54c2f3d
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2046883Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/master@{#66200}

src/base/atomic-utils.h

@@ -104,6 +104,16 @@ class AsAtomicImpl {
         cast_helper<T>::to_storage_type(new_value)));
   }
 
+  template <typename T>
+  static T Relaxed_CompareAndSwap(
+      T* addr, typename std::remove_reference<T>::type old_value,
+      typename std::remove_reference<T>::type new_value) {
+    STATIC_ASSERT(sizeof(T) <= sizeof(AtomicStorageType));
+    return cast_helper<T>::to_return_type(base::Relaxed_CompareAndSwap(
+        to_storage_addr(addr), cast_helper<T>::to_storage_type(old_value),
+        cast_helper<T>::to_storage_type(new_value)));
+  }
+
   template <typename T>
   static T AcquireRelease_CompareAndSwap(
       T* addr, typename std::remove_reference<T>::type old_value,

src/heap/heap-inl.h

@@ -10,6 +10,7 @@
 // Clients of this interface shouldn't depend on lots of heap internals.
 // Do not include anything from src/heap other than src/heap/heap.h and its
 // write barrier here!
+#include "src/base/atomicops.h"
 #include "src/heap/heap-write-barrier.h"
 #include "src/heap/heap.h"
 #include "src/heap/third-party/heap-api.h"
@@ -575,11 +576,27 @@ Oddball Heap::ToBoolean(bool condition) {
 }
 
 int Heap::NextScriptId() {
-  int last_id = last_script_id().value();
-  if (last_id == Smi::kMaxValue) last_id = v8::UnboundScript::kNoScriptId;
-  last_id++;
-  set_last_script_id(Smi::FromInt(last_id));
-  return last_id;
+  FullObjectSlot last_script_id_slot(&roots_table()[RootIndex::kLastScriptId]);
+  Smi last_id = Smi::cast(last_script_id_slot.Relaxed_Load());
+  Smi new_id, last_id_before_cas;
+  do {
+    if (last_id.value() == Smi::kMaxValue) {
+      STATIC_ASSERT(v8::UnboundScript::kNoScriptId == 0);
+      new_id = Smi::FromInt(1);
+    } else {
+      new_id = Smi::FromInt(last_id.value() + 1);
+    }
+
+    // CAS returns the old value on success, and the current value in the slot
+    // on failure. Therefore, we want to break if the returned value matches the
+    // old value (last_id), and keep looping (with the new last_id value) if it
+    // doesn't.
+    last_id_before_cas = last_id;
+    last_id =
+        Smi::cast(last_script_id_slot.Relaxed_CompareAndSwap(last_id, new_id));
+  } while (last_id != last_id_before_cas);
+
+  return new_id.value();
 }
 
 int Heap::NextDebuggingId() {

src/objects/slots-inl.h

@@ -49,6 +49,12 @@ void FullObjectSlot::Release_Store(Object value) const {
   base::AsAtomicPointer::Release_Store(location(), value.ptr());
 }
 
+Object FullObjectSlot::Relaxed_CompareAndSwap(Object old, Object target) const {
+  Address result = base::AsAtomicPointer::Relaxed_CompareAndSwap(
+      location(), old.ptr(), target.ptr());
+  return Object(result);
+}
+
 Object FullObjectSlot::Release_CompareAndSwap(Object old, Object target) const {
   Address result = base::AsAtomicPointer::Release_CompareAndSwap(
       location(), old.ptr(), target.ptr());

src/objects/slots.h

@@ -116,6 +116,7 @@ class FullObjectSlot : public SlotBase<FullObjectSlot, Address> {
   inline Object Relaxed_Load() const;
   inline void Relaxed_Store(Object value) const;
   inline void Release_Store(Object value) const;
+  inline Object Relaxed_CompareAndSwap(Object old, Object target) const;
   inline Object Release_CompareAndSwap(Object old, Object target) const;
 };