V4: Pass scope around as parameters inside the runtime.

The implementation of many (or all) runtime functions consist of first
creating a QV4::Scope, which saves and restores the JS stack pointer.
It also prevents tail-calls because of that restoring behavior. In many
cases it suffices to do that at the entry-point of the runtime.

The return value of a JS function call is now also stored in the scope.
Previously, all return values were stored in a ScopedValue, got loaded
on return, and immediately stored in another ScopedValue in the caller.
This resulted in a lot of stores, where now there is only one store
needed, and no extra ScopedValue for every function.

Change-Id: I13d80fc0ce72c5702ef1536d41d12f710c5914fa
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 27 insertions, 22 deletions

diff --git a/src/qml/jsruntime/qv4scopedvalue_p.h b/src/qml/jsruntime/qv4scopedvalue_p.h
index a6f7f8e0e7..aa811ddba4 100644
--- a/src/qml/jsruntime/qv4scopedvalue_p.h
+++ b/src/qml/jsruntime/qv4scopedvalue_p.h
@@ -71,14 +71,18 @@ struct ScopedValue;
 struct Scope {
     inline Scope(ExecutionContext *ctx)
         : engine(ctx->d()->engine)
+        , mark(engine->jsStackTop)
+        , result(*engine->jsAlloca(1))
     {
-        mark = engine->jsStackTop;
+        result = Encode::undefined();
     }
 
     explicit Scope(ExecutionEngine *e)
         : engine(e)
+        , mark(engine->jsStackTop)
+        , result(*engine->jsAlloca(1))
     {
-        mark = engine->jsStackTop;
+        result = Encode::undefined();
     }
 
     ~Scope() {
@@ -93,7 +97,7 @@ struct Scope {
         engine->jsStackTop = mark;
     }
 
-    Value *alloc(int nValues) {
+    Value *alloc(int nValues) const {
         return engine->jsAlloca(nValues);
     }
 
@@ -103,6 +107,7 @@ struct Scope {
 
     ExecutionEngine *engine;
     Value *mark;
+    Value &result;
 
 private:
     Q_DISABLE_COPY(Scope)
@@ -190,59 +195,59 @@ struct Scoped
 
     Scoped(const Scope &scope)
     {
-        ptr = scope.engine->jsStackTop++;
-        ptr->setM(0);
+        ptr = scope.engine->jsAlloca(1);
     }
 
     Scoped(const Scope &scope, const Value &v)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.engine->jsAlloca(1);
         setPointer(v.as<T>());
     }
     Scoped(const Scope &scope, Heap::Base *o)
     {
         Value v;
         v = o;
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.engine->jsAlloca(1);
         setPointer(v.as<T>());
     }
     Scoped(const Scope &scope, const ScopedValue &v)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.engine->jsAlloca(1);
         setPointer(v.ptr->as<T>());
     }
 
     Scoped(const Scope &scope, const Value &v, ConvertType)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.engine->jsAlloca(1);
         ptr->setRawValue(value_convert<T>(scope.engine, v));
     }
 
     Scoped(const Scope &scope, const Value *v)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.engine->jsAlloca(1);
         setPointer(v ? v->as<T>() : 0);
     }
 
     Scoped(const Scope &scope, T *t)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.engine->jsAlloca(1);
         setPointer(t);
     }
     Scoped(const Scope &scope, typename T::Data *t)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.engine->jsAlloca(1);
         *ptr = t;
     }
 
     Scoped(const Scope &scope, const ReturnedValue &v)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.engine->jsAlloca(1);
         setPointer(QV4::Value::fromReturnedValue(v).as<T>());
     }
+
     Scoped(const Scope &scope, const ReturnedValue &v, ConvertType)
     {
-        ptr = scope.engine->jsStackTop++;
+        ptr = scope.engine->jsAlloca(1);
         ptr->setRawValue(value_convert<T>(scope.engine, QV4::Value::fromReturnedValue(v)));
     }
 
@@ -311,7 +316,7 @@ struct Scoped
 };
 
 struct ScopedCallData {
-    ScopedCallData(Scope &scope, int argc = 0)
+    ScopedCallData(const Scope &scope, int argc = 0)
     {
         int size = qMax(argc, (int)QV4::Global::ReservedArgumentCount) + qOffsetOf(QV4::CallData, args)/sizeof(QV4::Value);
         ptr = reinterpret_cast<CallData *>(scope.alloc(size));
@@ -362,19 +367,19 @@ struct ScopedProperty
 
 struct ExecutionContextSaver
 {
-    ExecutionEngine *engine;
+    Scope scope; // this makes sure that a reference to context on the JS stack goes out of scope as soon as the context is not used anymore.
     ExecutionContext *savedContext;
 
-    ExecutionContextSaver(Scope &scope)
-        : engine(scope.engine)
+    ExecutionContextSaver(const Scope &scope)
+        : scope(scope.engine)
     {
-        savedContext = engine->currentContext;
+        savedContext = scope.engine->currentContext;
     }
     ~ExecutionContextSaver()
     {
-        Q_ASSERT(engine->jsStackTop > engine->currentContext);
-        engine->currentContext = savedContext;
-        engine->current = savedContext->d();
+        Q_ASSERT(scope.engine->jsStackTop > scope.engine->currentContext);
+        scope.engine->currentContext = savedContext;
+        scope.engine->current = savedContext->d();
     }
 };