Use custom static scopes to improve QML/JavaScript performance

This commit introduces a new internal JS object type,
QScriptStaticScopeObject, that enables the JS compiler to make
more aggressive optimizations of scoped property access.

QScriptStaticScopeObject registers all its properties in a
symbol table that the JS compiler has access to. If the compiler
finds the property in the symbol table, it will generate the
fast index-based op_{get,put}_scoped_var bytecodes, rather than
the dynamic (slow) op_resolve and friends.

If the compiler _doesn't_ find the property in the symbol table,
it infers that it's safe to skip the scope object when later
resolving the property, which will also improve performance
(see op_resolve_skip bytecode).

QScriptStaticScopeObject is only safe to use when all relevant
properties are known at JS compile time; that is, when a
function that has the static scope object in its scope chain is
compiled.
It's up to the user of the class (e.g. QtDeclarative) to ensure
that this constraint is not violated.

The API for constructing QScriptStaticScopeObject instances is
not public; it lives in QScriptDeclarativeClass for now, an
internal class exported for the purpose of QML. The instance is
returned as a QScriptValue and can be manipulated like any
other JS object (e.g. by QScriptValue::setProperty()).

The other part of this commit utilizes QScriptStaticScopeObject
in QtDeclarative in the two major places where it's currently
possible:

1) QML disallows adding properties to the Global Object.
Furthermore, it's not possible for QML IDs and properties to
"shadow" global variables. Hence, a QScriptStaticScopeObject
can be used to hold all the standard ECMA properties, and this
scope object can come _before_ the QML component in the scope
chain. This enables binding expressions and scripts to have
optimized (direct) access to e.g. Math.sin.

2) Imported scripts can have their properties (resulting from
variable declarations ("var" statements) and function
declarations) added to a static scope object. This enables
functions in the script to have optimized (direct) access to
the script's own properties, as well as to global properties
such as Math.

With this change, it's no longer possible to delete properties
of the Global Object, nor delete properties of an imported
script. It's a compromise we make in order to make the
optimization safe.

Task-number: QTBUG-8576
Reviewed-by: Aaron Kennedy
Reviewed-by: Olivier Goffart
Reviewed-by: Jedrzej Nowacki

2 files changed, 18 insertions, 9 deletions

diff --git a/src/script/api/qscriptengine.cpp b/src/script/api/qscriptengine.cpp
index e2999c1..655026c 100644
--- a/src/script/api/qscriptengine.cpp
+++ b/src/script/api/qscriptengine.cpp
@@ -64,6 +64,7 @@
 #include "bridge/qscriptqobject_p.h"
 #include "bridge/qscriptglobalobject_p.h"
 #include "bridge/qscriptactivationobject_p.h"
+#include "bridge/qscriptstaticscopeobject_p.h"
 
 #ifndef QT_NO_QOBJECT
 #include <QtCore/qcoreapplication.h>
@@ -905,6 +906,7 @@ QScriptEnginePrivate::QScriptEnginePrivate()
     JSC::ExecState* exec = globalObject->globalExec();
 
     scriptObjectStructure = QScriptObject::createStructure(globalObject->objectPrototype());
+    staticScopeObjectStructure = QScriptStaticScopeObject::createStructure(JSC::jsNull());
 
     qobjectPrototype = new (exec) QScript::QObjectPrototype(exec, QScript::QObjectPrototype::createStructure(globalObject->objectPrototype()), globalObject->prototypeFunctionStructure());
     qobjectWrapperObjectStructure = QScriptObject::createStructure(qobjectPrototype);
@@ -1770,15 +1772,7 @@ void QScriptEnginePrivate::setProperty(JSC::ExecState *exec, JSC::JSValue object
         } else if (flags != QScriptValue::KeepExistingFlags) {
             if (thisObject->hasOwnProperty(exec, id))
                 thisObject->deleteProperty(exec, id); // ### hmmm - can't we just update the attributes?
-            unsigned attribs = 0;
-            if (flags & QScriptValue::ReadOnly)
-                attribs |= JSC::ReadOnly;
-            if (flags & QScriptValue::SkipInEnumeration)
-                attribs |= JSC::DontEnum;
-            if (flags & QScriptValue::Undeletable)
-                attribs |= JSC::DontDelete;
-            attribs |= flags & QScriptValue::UserRange;
-            thisObject->putWithAttributes(exec, id, value, attribs);
+            thisObject->putWithAttributes(exec, id, value, propertyFlagsToJSCAttributes(flags));
         } else {
             JSC::PutPropertySlot slot;
             thisObject->put(exec, id, value, slot);
diff --git a/src/script/api/qscriptengine_p.h b/src/script/api/qscriptengine_p.h
index 56366e2..1b35704 100644
--- a/src/script/api/qscriptengine_p.h
+++ b/src/script/api/qscriptengine_p.h
@@ -205,6 +205,7 @@ public:
 
     inline QScriptValue scriptValueFromJSCValue(JSC::JSValue value);
     inline JSC::JSValue scriptValueToJSCValue(const QScriptValue &value);
+    static inline unsigned propertyFlagsToJSCAttributes(const QScriptValue::PropertyFlags &flags);
 
     static inline JSC::JSValue jscValueFromVariant(JSC::ExecState*, const QVariant &value);
     static QVariant jscValueToVariant(JSC::ExecState*, JSC::JSValue value, int targetType);
@@ -346,6 +347,7 @@ public:
     JSC::ExecState *currentFrame;
 
     WTF::RefPtr<JSC::Structure> scriptObjectStructure;
+    WTF::RefPtr<JSC::Structure> staticScopeObjectStructure;
 
     QScript::QObjectPrototype *qobjectPrototype;
     WTF::RefPtr<JSC::Structure> qobjectWrapperObjectStructure;
@@ -639,6 +641,19 @@ inline JSC::JSValue QScriptEnginePrivate::scriptValueToJSCValue(const QScriptVal
     return vv->jscValue;
 }
 
+inline unsigned QScriptEnginePrivate::propertyFlagsToJSCAttributes(const QScriptValue::PropertyFlags &flags)
+{
+    unsigned attribs = 0;
+    if (flags & QScriptValue::ReadOnly)
+        attribs |= JSC::ReadOnly;
+    if (flags & QScriptValue::SkipInEnumeration)
+        attribs |= JSC::DontEnum;
+    if (flags & QScriptValue::Undeletable)
+        attribs |= JSC::DontDelete;
+    attribs |= flags & QScriptValue::UserRange;
+    return attribs;
+}
+
 inline QScriptValuePrivate::~QScriptValuePrivate()
 {
     if (engine)