Bug 1618995 - Stop storing the Empty Global Scope in the GCThingList r=mgaudet

Differential Revision: https://phabricator.services.mozilla.com/D67101

--HG--
extra : moz-landing-system : lando

js/src/frontend/BytecodeSection.cpp

@@ -13,8 +13,9 @@
 #include "frontend/SharedContext.h"  // FunctionBox
 #include "frontend/Stencil.h"        // ScopeCreationData
 #include "vm/BytecodeUtil.h"         // INDEX_LIMIT, StackUses, StackDefs
-#include "vm/JSContext.h"            // JSContext
-#include "vm/RegExpObject.h"         // RegexpObject
+#include "vm/GlobalObject.h"
+#include "vm/JSContext.h"     // JSContext
+#include "vm/RegExpObject.h"  // RegexpObject
 
 using namespace js;
 using namespace js::frontend;
@@ -43,6 +44,16 @@ void GCThingList::finishInnerFunctions() {
   }
 }
 
+AbstractScopePtr GCThingList::getScope(size_t index) const {
+  auto& elem = vector[index].get();
+  if (elem.is<JS::GCCellPtr>()) {
+    return AbstractScopePtr(&elem.as<JS::GCCellPtr>().as<Scope>());
+  } else if (elem.is<EmptyGlobalScopeType>()) {
+    return AbstractScopePtr(&compilationInfo.cx->global()->emptyGlobalScope());
+  }
+  return AbstractScopePtr(compilationInfo, elem.as<ScopeIndex>());
+}
+
 bool js::frontend::EmitScriptThingsVector(JSContext* cx,
                                           CompilationInfo& compilationInfo,
                                           const ScriptThingsVector& objects,
@@ -112,6 +123,12 @@ bool js::frontend::EmitScriptThingsVector(JSContext* cx,
       output[i] = JS::GCCellPtr(data.as<JSFunction*>());
       return true;
     }
+
+    bool operator()(const EmptyGlobalScopeType& emptyGlobalScope) {
+      Scope* scope = &cx->global()->emptyGlobalScope();
+      output[i] = JS::GCCellPtr(scope);
+      return true;
+    }
   };
 
   for (uint32_t i = 0; i < objects.length(); i++) {

js/src/frontend/BytecodeSection.h

@@ -67,16 +67,6 @@ struct MOZ_STACK_CLASS GCThingList {
     }
     return true;
   }
-  MOZ_MUST_USE bool append(Scope* scope, uint32_t* index) {
-    *index = vector.length();
-    if (!vector.append(mozilla::AsVariant(JS::GCCellPtr(scope)))) {
-      return false;
-    }
-    if (!firstScopeIndex) {
-      firstScopeIndex.emplace(*index);
-    }
-    return true;
-  }
   MOZ_MUST_USE bool append(BigIntLiteral* literal, uint32_t* index) {
     *index = vector.length();
     return vector.append(mozilla::AsVariant(literal->index()));
@@ -91,19 +81,25 @@ struct MOZ_STACK_CLASS GCThingList {
   }
   MOZ_MUST_USE bool append(FunctionBox* funbox, uint32_t* index);
 
+  MOZ_MUST_USE bool appendEmptyGlobalScope(uint32_t* index) {
+    *index = vector.length();
+    EmptyGlobalScopeType emptyGlobalScope;
+    if (!vector.append(mozilla::AsVariant(emptyGlobalScope))) {
+      return false;
+    }
+    if (!firstScopeIndex) {
+      firstScopeIndex.emplace(*index);
+    }
+    return true;
+  }
+
   uint32_t length() const { return vector.length(); }
 
   const ScriptThingsVector& objects() { return vector; }
 
   void finishInnerFunctions();
 
-  AbstractScopePtr getScope(size_t index) const {
-    auto& elem = vector[index].get();
-    if (elem.is<JS::GCCellPtr>()) {
-      return AbstractScopePtr(&elem.as<JS::GCCellPtr>().as<Scope>());
-    }
-    return AbstractScopePtr(compilationInfo, elem.as<ScopeIndex>());
-  }
+  AbstractScopePtr getScope(size_t index) const;
 
   AbstractScopePtr firstScope() const {
     MOZ_ASSERT(firstScopeIndex.isSome());

js/src/frontend/EmitterScope.cpp

@@ -338,7 +338,8 @@ bool EmitterScope::internEmptyGlobalScopeAsBody(BytecodeEmitter* bce) {
   hasEnvironment_ = scope->hasEnvironment();
 
   bce->bodyScopeIndex = bce->perScriptData().gcThingList().length();
-  return bce->perScriptData().gcThingList().append(scope, &scopeIndex_);
+  return bce->perScriptData().gcThingList().appendEmptyGlobalScope(
+      &scopeIndex_);
 }
 
 template <typename ScopeCreator>

js/src/frontend/Stencil.h

@@ -441,12 +441,15 @@ class ScopeCreationData {
   }
 };
 
+class EmptyGlobalScopeType {};
+
 // These types all end up being baked into GC things as part of stencil
 // instantiation. Currently, GCCellPtr is part of this list while we complete
 // Stencil, but eventually will be removed.
 using ScriptThingVariant =
     mozilla::Variant<JS::GCCellPtr, BigIntIndex, ObjLiteralCreationData,
-                     RegExpIndex, ScopeIndex, FunctionIndex>;
+                     RegExpIndex, ScopeIndex, FunctionIndex,
+                     EmptyGlobalScopeType>;
 
 // A vector of things destined to be converted to GC things.
 using ScriptThingsVector = GCVector<ScriptThingVariant>;
@@ -518,5 +521,9 @@ struct GCPolicy<js::frontend::TypedIndex<T>>
 template <>
 struct GCPolicy<js::frontend::FunctionIndex>
     : JS::IgnoreGCPolicy<js::frontend::FunctionIndex> {};
+
+template <>
+struct GCPolicy<js::frontend::EmptyGlobalScopeType>
+    : JS::IgnoreGCPolicy<js::frontend::EmptyGlobalScopeType> {};
 }  // namespace JS
 #endif /* frontend_Stencil_h */