Bug 1666853 - Part 4: Remove use of ApplyGCThingTyped from Value barriers as this doesn't generate good code r=sfink

Differential Revision: https://phabricator.services.mozilla.com/D91311

js/public/Id.h

@@ -92,8 +92,13 @@ struct PropertyKey {
     return reinterpret_cast<JS::Symbol*>(asBits ^ JSID_TYPE_SYMBOL);
   }
 
+  js::gc::Cell* toGCThing() const {
+    MOZ_ASSERT(isGCThing());
+    return reinterpret_cast<js::gc::Cell*>(asBits & ~(size_t)JSID_TYPE_MASK);
+  }
+
   GCCellPtr toGCCellPtr() const {
-    void* thing = (void*)(asBits & ~(size_t)JSID_TYPE_MASK);
+    js::gc::Cell* thing = toGCThing();
     if (isString()) {
       return JS::GCCellPtr(thing, JS::TraceKind::String);
     }

js/src/gc/Barrier.cpp

@@ -103,20 +103,103 @@ AutoTouchingGrayThings::~AutoTouchingGrayThings() {
 
 #endif  // DEBUG
 
+// Tagged pointer barriers
+//
+// It's tempting to use ApplyGCThingTyped to dispatch to the typed barrier
+// functions (e.g. gc::ReadBarrier(JSObject*)) but this does not compile well
+// (clang generates 1580 bytes on x64 versus 296 bytes for this implementation
+// of ValueReadBarrier).
+//
+// Instead, check known special cases and call the generic barrier functions.
+
+static MOZ_ALWAYS_INLINE bool ValueIsPermanent(const Value& value) {
+  gc::Cell* cell = value.toGCThing();
+
+  if (value.isString()) {
+    return cell->as<JSString>()->isPermanentAndMayBeShared();
+  }
+
+  if (value.isSymbol()) {
+    return cell->as<JS::Symbol>()->isPermanentAndMayBeShared();
+  }
+
+#ifdef DEBUG
+  // Using mozilla::DebugOnly here still generated code in opt builds.
+  bool isPermanent = MapGCThingTyped(value, [](auto t) {
+    return t->isPermanentAndMayBeShared();
+  }).value();
+  MOZ_ASSERT(!isPermanent);
+#endif
+
+  return false;
+}
+
 void gc::ValueReadBarrier(const Value& v) {
-  ApplyGCThingTyped(v, [](auto t) { gc::ReadBarrier(t); });
+  MOZ_ASSERT(v.isGCThing());
+
+  if (!ValueIsPermanent(v)) {
+    ReadBarrierImpl(v.toGCThing());
+  }
 }
 
 void gc::ValuePreWriteBarrier(const Value& v) {
-  ApplyGCThingTyped(v, [](auto t) { gc::PreWriteBarrier(t); });
+  MOZ_ASSERT(v.isGCThing());
+
+  if (!ValueIsPermanent(v)) {
+    PreWriteBarrierImpl(v.toGCThing());
+  }
+}
+
+static MOZ_ALWAYS_INLINE bool IdIsPermanent(jsid id) {
+  gc::Cell* cell = id.toGCThing();
+
+  if (id.isString()) {
+    return cell->as<JSString>()->isPermanentAndMayBeShared();
+  }
+
+  if (id.isSymbol()) {
+    return cell->as<JS::Symbol>()->isPermanentAndMayBeShared();
+  }
+
+#ifdef DEBUG
+  bool isPermanent = MapGCThingTyped(id, [](auto t) {
+    return t->isPermanentAndMayBeShared();
+  }).value();
+  MOZ_ASSERT(!isPermanent);
+#endif
+
+  return false;
 }
 
 void gc::IdPreWriteBarrier(jsid id) {
-  ApplyGCThingTyped(id, [](auto t) { gc::PreWriteBarrier(t); });
+  MOZ_ASSERT(id.isGCThing());
+
+  if (!IdIsPermanent(id)) {
+    PreWriteBarrierImpl(&id.toGCThing()->asTenured());
+  }
+}
+
+static MOZ_ALWAYS_INLINE bool CellPtrIsPermanent(JS::GCCellPtr thing) {
+  if (thing.mayBeOwnedByOtherRuntime()) {
+    return true;
+  }
+
+#ifdef DEBUG
+  bool isPermanent = MapGCThingTyped(thing, [](auto t) {
+    return t->isPermanentAndMayBeShared();
+  });
+  MOZ_ASSERT(!isPermanent);
+#endif
+
+  return false;
 }
 
 void gc::CellPtrPreWriteBarrier(JS::GCCellPtr thing) {
-  ApplyGCThingTyped(thing, [](auto t) { gc::PreWriteBarrier(t); });
+  MOZ_ASSERT(thing);
+
+  if (!CellPtrIsPermanent(thing)) {
+    PreWriteBarrierImpl(thing.asCell());
+  }
 }
 
 template <typename T>