Rework unwind handling

The old code was rather convoluted and expanded to quite
a bit of bytecode. It was also very hard to fix some
of the remaining issues with unwinding in there.

The new code handles unwinding a bit differently. Basically,
we now have three instructions to do what the spec requires.

SetUnwindHandler is the same as the old SetExceptionHandler
instruction. It basically tells the runtime where to jump to
to handle any abrupt completion (ie. throw/break/continue/return)
that requires unwinding.

UnwindToLabel is a new instruction that is used for unwinding
break/continue/return statements. It takes two arguments, one
telling the runtime how many levels to unwind and the second
a target label to jump to when unwinding is done.

UnwindDispatch is the third instruction and is invoked at
the end of each unwind block to dispatch the the parent
unwind handler if required and thus implement the support
for the levelled unwinding.

Change-Id: I079a39d0d897b3ecc2f0dc631ca29b25eae05250
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

2 files changed, 45 insertions, 27 deletions

diff --git a/src/qml/jsruntime/qv4engine_p.h b/src/qml/jsruntime/qv4engine_p.h
index ecc110a16f..b007e65c4b 100644
--- a/src/qml/jsruntime/qv4engine_p.h
+++ b/src/qml/jsruntime/qv4engine_p.h
@@ -97,7 +97,9 @@ struct Q_QML_EXPORT CppStackFrame {
     int originalArgumentsCount;
     int instructionPointer;
     const char *yield;
-    const char *exceptionHandler;
+    const char *unwindHandler;
+    const char *unwindLabel;
+    int unwindLevel;
 
     QString source() const;
     QString function() const;
diff --git a/src/qml/jsruntime/qv4vme_moth.cpp b/src/qml/jsruntime/qv4vme_moth.cpp
index e180298789..165902ed36 100644
--- a/src/qml/jsruntime/qv4vme_moth.cpp
+++ b/src/qml/jsruntime/qv4vme_moth.cpp
@@ -344,7 +344,7 @@ static struct InstrCount {
 #endif
 #define CHECK_EXCEPTION \
     if (engine->hasException) \
-        goto catchException
+        goto handleUnwind
 
 static inline Heap::CallContext *getScope(QV4::Value *stack, int level)
 {
@@ -510,7 +510,10 @@ QV4::ReturnedValue VME::exec(const FunctionObject *fo, const QV4::Value *thisObj
     frame.originalArguments = argv;
     frame.originalArgumentsCount = argc;
     frame.yield = nullptr;
-    frame.exceptionHandler = nullptr;
+    frame.unwindHandler = nullptr;
+    frame.unwindLabel = nullptr;
+    frame.unwindLevel = 0;
+
     Function *function;
 
     {
@@ -810,7 +813,7 @@ QV4::ReturnedValue VME::interpret(CppStackFrame &frame, const char *code)
         if (engine->hasException) {
             // an empty value indicates that the generator was called with return()
             if (engine->exceptionValue->asReturnedValue() != Primitive::emptyValue().asReturnedValue())
-                goto catchException;
+                goto handleUnwind;
             engine->hasException = false;
             *engine->exceptionValue = Primitive::undefinedValue();
     } else {
@@ -823,7 +826,7 @@ QV4::ReturnedValue VME::interpret(CppStackFrame &frame, const char *code)
         Value func = STACK_VALUE(name);
         if (Q_UNLIKELY(!func.isFunctionObject())) {
             acc = engine->throwTypeError(QStringLiteral("%1 is not a function").arg(func.toQStringNoThrow()));
-            goto catchException;
+            goto handleUnwind;
         }
         acc = static_cast<const FunctionObject &>(func).call(nullptr, stack + argv, argc);
         CHECK_EXCEPTION;
@@ -843,7 +846,7 @@ QV4::ReturnedValue VME::interpret(CppStackFrame &frame, const char *code)
 
         if (Q_UNLIKELY(!f.isFunctionObject())) {
             acc = engine->throwTypeError();
-            goto catchException;
+            goto handleUnwind;
         }
 
         acc = static_cast<FunctionObject &>(f).call(stack + base, stack + argv, argc);
@@ -887,14 +890,30 @@ QV4::ReturnedValue VME::interpret(CppStackFrame &frame, const char *code)
     MOTH_END_INSTR(CallContextObjectProperty)
 
     MOTH_BEGIN_INSTR(SetUnwindHandler)
-        frame.exceptionHandler = offset ? code + offset : nullptr;
+        frame.unwindHandler = offset ? code + offset : nullptr;
     MOTH_END_INSTR(SetUnwindHandler)
 
+    MOTH_BEGIN_INSTR(UnwindDispatch)
+        CHECK_EXCEPTION;
+        if (frame.unwindLevel) {
+            --frame.unwindLevel;
+            if (frame.unwindLevel)
+                goto handleUnwind;
+            code = frame.unwindLabel;
+        }
+    MOTH_END_INSTR(UnwindDispatch)
+
+    MOTH_BEGIN_INSTR(UnwindToLabel)
+        frame.unwindLevel = level;
+        frame.unwindLabel = code + offset;
+        goto handleUnwind;
+    MOTH_END_INSTR(UnwindToLabel)
+
     MOTH_BEGIN_INSTR(ThrowException)
         STORE_IP();
         STORE_ACC();
         Runtime::method_throwException(engine, accumulator);
-        goto catchException;
+        goto handleUnwind;
     MOTH_END_INSTR(ThrowException)
 
     MOTH_BEGIN_INSTR(GetException)
@@ -904,8 +923,10 @@ QV4::ReturnedValue VME::interpret(CppStackFrame &frame, const char *code)
     MOTH_END_INSTR(HasException)
 
     MOTH_BEGIN_INSTR(SetException)
-        *engine->exceptionValue = acc;
-        engine->hasException = true;
+        if (acc != Primitive::emptyValue().asReturnedValue()) {
+            *engine->exceptionValue = acc;
+            engine->hasException = true;
+        }
     MOTH_END_INSTR(SetException)
 
     MOTH_BEGIN_INSTR(PushCatchContext)
@@ -979,7 +1000,7 @@ QV4::ReturnedValue VME::interpret(CppStackFrame &frame, const char *code)
             if (function->isStrict()) {
                 STORE_IP();
                 engine->throwTypeError();
-                goto catchException;
+                goto handleUnwind;
             }
             acc = Encode(false);
         } else {
@@ -992,7 +1013,7 @@ QV4::ReturnedValue VME::interpret(CppStackFrame &frame, const char *code)
             if (function->isStrict()) {
                 STORE_IP();
                 engine->throwTypeError();
-                goto catchException;
+                goto handleUnwind;
             }
             acc = Encode(false);
         } else {
@@ -1006,7 +1027,7 @@ QV4::ReturnedValue VME::interpret(CppStackFrame &frame, const char *code)
                 STORE_IP();
                 QString n = function->compilationUnit->runtimeStrings[name]->toQString();
                 engine->throwSyntaxError(QStringLiteral("Can't delete property %1").arg(n));
-                goto catchException;
+                goto handleUnwind;
             }
             acc = Encode(false);
         } else {
@@ -1096,6 +1117,11 @@ QV4::ReturnedValue VME::interpret(CppStackFrame &frame, const char *code)
         }
     MOTH_END_INSTR(JumpFalse)
 
+    MOTH_BEGIN_INSTR(JumpNoException)
+        if (!engine->hasException)
+            code += offset;
+    MOTH_END_INSTR(JumpNoException)
+
     MOTH_BEGIN_INSTR(JumpNotUndefined)
         if (Q_LIKELY(acc != QV4::Encode::undefined()))
             code += offset;
@@ -1242,16 +1268,6 @@ QV4::ReturnedValue VME::interpret(CppStackFrame &frame, const char *code)
         CHECK_EXCEPTION;
     MOTH_END_INSTR(CmpInstanceOf)
 
-    MOTH_BEGIN_INSTR(JumpStrictNotEqualStackSlotInt)
-        if (STACK_VALUE(lhs).int_32() != rhs || STACK_VALUE(lhs).isUndefined())
-            code += offset;
-    MOTH_END_INSTR(JumpStrictNotEqualStackSlotInt)
-
-    MOTH_BEGIN_INSTR(JumpStrictEqualStackSlotInt)
-        if (STACK_VALUE(lhs).int_32() == rhs && !STACK_VALUE(lhs).isUndefined())
-            code += offset;
-    MOTH_END_INSTR(JumpStrictNotEqualStackSlotInt)
-
     MOTH_BEGIN_INSTR(UNot)
         if (ACC.integerCompatible()) {
             acc = Encode(!static_cast<bool>(ACC.int_32()));
@@ -1456,12 +1472,12 @@ QV4::ReturnedValue VME::interpret(CppStackFrame &frame, const char *code)
         STACK_VALUE(result) = Runtime::method_loadQmlImportedScripts(static_cast<QV4::NoThrowEngine*>(engine));
     MOTH_END_INSTR(LoadQmlImportedScripts)
 
-    catchException:
-        Q_ASSERT(engine->hasException);
-        if (!frame.exceptionHandler) {
+    handleUnwind:
+        Q_ASSERT(engine->hasException || frame.unwindLevel);
+        if (!frame.unwindHandler) {
             acc = Encode::undefined();
             return acc;
         }
-        code = frame.exceptionHandler;
+        code = frame.unwindHandler;
     }
 }