Bug 1289318 - Part 7: Store the Promise reactions list in the same slot as the result. r=efaust

A Promise can only have either a list of reactions, if it's pending, or a result/reason, if it's resolved.

This gets us down to 3 non-debug-info slots. If we now move the debug info into its own object and only allocate that when the debugger is open, Promise instances only need 4 slots.

MozReview-Commit-ID: FuLAwhmFTBe

js/src/builtin/Promise.cpp

@@ -152,13 +152,12 @@ ResolvePromise(JSContext* cx, Handle<PromiseObject*> promise, HandleValue valueO
     // instead of getting the right list of reactions, we determine the
     // resolution type to retrieve the right information from the
     // reaction records.
-    RootedValue reactionsVal(cx, promise->getFixedSlot(PROMISE_REACTIONS_SLOT));
+    RootedValue reactionsVal(cx, promise->getFixedSlot(PROMISE_REACTIONS_OR_RESULT_SLOT));
 
-    // Step 3.
-    promise->setFixedSlot(PROMISE_RESULT_SLOT, valueOrReason);
-
-    // Steps 4-5.
-    promise->setFixedSlot(PROMISE_REACTIONS_SLOT, UndefinedValue());
+    // Step 3-5.
+    // The same slot is used for the reactions list and the result, so setting
+    // the result also removes the reactions list.
+    promise->setFixedSlot(PROMISE_REACTIONS_OR_RESULT_SLOT, valueOrReason);
 
     // Step 6.
     promise->setFixedSlot(PROMISE_STATE_SLOT, Int32Value(int32_t(state)));
@@ -554,7 +553,10 @@ PromiseObject::getID()
 bool
 PromiseObject::dependentPromises(JSContext* cx, MutableHandle<GCVector<Value>> values)
 {
-    RootedValue reactionsVal(cx, getReservedSlot(PROMISE_REACTIONS_SLOT));
+    if (state() != JS::PromiseState::Pending)
+        return true;
+
+    RootedValue reactionsVal(cx, getReservedSlot(PROMISE_REACTIONS_OR_RESULT_SLOT));
     if (reactionsVal.isNullOrUndefined())
         return true;
     RootedObject reactions(cx, &reactionsVal.toObject());

js/src/builtin/Promise.h

@@ -30,11 +30,11 @@ class PromiseObject : public NativeObject
     }
     Value value()  {
         MOZ_ASSERT(state() == JS::PromiseState::Fulfilled);
-        return getFixedSlot(PROMISE_RESULT_SLOT);
+        return getFixedSlot(PROMISE_REACTIONS_OR_RESULT_SLOT);
     }
     Value reason() {
         MOZ_ASSERT(state() == JS::PromiseState::Rejected);
-        return getFixedSlot(PROMISE_RESULT_SLOT);
+        return getFixedSlot(PROMISE_REACTIONS_OR_RESULT_SLOT);
     }
 
     MOZ_MUST_USE bool resolve(JSContext* cx, HandleValue resolutionValue);

js/src/builtin/Promise.js

@@ -480,19 +480,19 @@ function BlockOnPromise(promise, blockedPromise, onResolve, onReject) {
 function AddDependentPromise(dependentPromise) {
     assert(IsPromise(this), "AddDependentPromise expects an unwrapped Promise as the receiver");
 
+    if (GetPromiseState(this) !== PROMISE_STATE_PENDING)
+        return;
+
     let reaction = new PromiseReactionRecord(dependentPromise, NullFunction, NullFunction,
                                              NullFunction, NullFunction, null);
 
-    let reactions = UnsafeGetReservedSlot(this, PROMISE_REACTIONS_SLOT);
+    let reactions = UnsafeGetReservedSlot(this, PROMISE_REACTIONS_OR_RESULT_SLOT);
 
-    // The reactions list might not have been allocated yet or been reset
-    // because the Promise was resolved.
-    if (!reactions) {
-        if (GetPromiseState(this) === PROMISE_STATE_PENDING)
-            UnsafeSetReservedSlot(promise, PROMISE_REACTIONS_SLOT, [reaction]);
-        return;
-    }
-    _DefineDataProperty(reactions, reactions.length, reaction);
+    // The reactions list might not have been allocated yet.
+    if (!reactions)
+        UnsafeSetReservedSlot(dependentPromise, PROMISE_REACTIONS_OR_RESULT_SLOT, [reaction]);
+    else
+        _DefineDataProperty(reactions, reactions.length, reaction);
 }
 
 // ES6, 25.4.4.4.
@@ -739,9 +739,9 @@ function PerformPromiseThen(promise, onFulfilled, onRejected, resultCapability)
     if (state === PROMISE_STATE_PENDING) {
         // Steps 7.a,b.
         // We only have a single list for fulfill and reject reactions.
-        let reactions = UnsafeGetReservedSlot(promise, PROMISE_REACTIONS_SLOT);
+        let reactions = UnsafeGetReservedSlot(promise, PROMISE_REACTIONS_OR_RESULT_SLOT);
         if (!reactions)
-            UnsafeSetReservedSlot(promise, PROMISE_REACTIONS_SLOT, [reaction]);
+            UnsafeSetReservedSlot(promise, PROMISE_REACTIONS_OR_RESULT_SLOT, [reaction]);
         else
             _DefineDataProperty(reactions, reactions.length, reaction);
     }
@@ -749,7 +749,7 @@ function PerformPromiseThen(promise, onFulfilled, onRejected, resultCapability)
     // Step 8.
     else if (state === PROMISE_STATE_FULFILLED) {
         // Step 8.a.
-        let value = UnsafeGetReservedSlot(promise, PROMISE_RESULT_SLOT);
+        let value = UnsafeGetReservedSlot(promise, PROMISE_REACTIONS_OR_RESULT_SLOT);
 
         // Step 8.b.
         EnqueuePromiseReactionJob(reaction, PROMISE_JOB_TYPE_FULFILL, value);
@@ -761,7 +761,7 @@ function PerformPromiseThen(promise, onFulfilled, onRejected, resultCapability)
         assert(state === PROMISE_STATE_REJECTED, "Invalid Promise state " + state);
 
         // Step 9.b.
-        let reason = UnsafeGetReservedSlot(promise, PROMISE_RESULT_SLOT);
+        let reason = UnsafeGetReservedSlot(promise, PROMISE_REACTIONS_OR_RESULT_SLOT);
 
         // Step 9.c.
         if (UnsafeGetInt32FromReservedSlot(promise, PROMISE_IS_HANDLED_SLOT) !==

js/src/builtin/SelfHostingDefines.h

@@ -57,16 +57,15 @@
 #define ITEM_KIND_VALUE 1
 #define ITEM_KIND_KEY_AND_VALUE 2
 
-#define PROMISE_STATE_SLOT             0
-#define PROMISE_RESULT_SLOT            1
-#define PROMISE_REACTIONS_SLOT         2
-#define PROMISE_RESOLVE_FUNCTION_SLOT  3
-#define PROMISE_ALLOCATION_SITE_SLOT   4
-#define PROMISE_RESOLUTION_SITE_SLOT   5
-#define PROMISE_ALLOCATION_TIME_SLOT   6
-#define PROMISE_RESOLUTION_TIME_SLOT   7
-#define PROMISE_ID_SLOT                8
-#define PROMISE_IS_HANDLED_SLOT        9
+#define PROMISE_STATE_SLOT               0
+#define PROMISE_REACTIONS_OR_RESULT_SLOT 1
+#define PROMISE_RESOLVE_FUNCTION_SLOT    2
+#define PROMISE_ALLOCATION_SITE_SLOT     3
+#define PROMISE_RESOLUTION_SITE_SLOT     4
+#define PROMISE_ALLOCATION_TIME_SLOT     5
+#define PROMISE_RESOLUTION_TIME_SLOT     6
+#define PROMISE_ID_SLOT                  7
+#define PROMISE_IS_HANDLED_SLOT          8
 
 #define PROMISE_STATE_PENDING   0
 #define PROMISE_STATE_FULFILLED 1

js/src/builtin/TestingFunctions.cpp

@@ -1445,7 +1445,7 @@ SettlePromiseNow(JSContext* cx, unsigned argc, Value* vp)
 
     RootedNativeObject promise(cx, &args[0].toObject().as<NativeObject>());
     promise->setReservedSlot(PROMISE_STATE_SLOT, Int32Value(PROMISE_STATE_FULFILLED));
-    promise->setReservedSlot(PROMISE_RESULT_SLOT, UndefinedValue());
+    promise->setReservedSlot(PROMISE_REACTIONS_OR_RESULT_SLOT, UndefinedValue());
 
     JS::dbg::onPromiseSettled(cx, promise);
     return true;