Fix various accumulator-saving problems

We need to keep the accumulator alive across function calls. This
requires:

1, Saving the accumulator on the stack if the function might allocate, to
   protect it from the garbage collector. However, we don't need to do
   that if the result of the function is to be saved in the accumulator
   and the function itself doesn't use the accumulator as argument. In
   this case the previous value becomes unaccessible and we might as
   well GC it.
2, In the JIT, restoring the accumulator from the stack after the
   function call if we want to ignore the return value.
3, Therefore, also saving the accumulator on the stack before calling in
   case of 2.

We assume that we don't need to keep the accumulator alive across the
jump to the exception handler. Saving the accumulator more often than
necessary is detrimental for performance. To make sure the assumption
holds, explicitly load the accumulator with undefined _before_ jumping
to any exception handler.

Change-Id: I78cbc42847b8885a0659b23f3b81655b7f1a0bc4
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/qml/jsruntime/qv4vme_moth.cpp b/src/qml/jsruntime/qv4vme_moth.cpp
index c69bb67061..47ad22f4f7 100644
--- a/src/qml/jsruntime/qv4vme_moth.cpp
+++ b/src/qml/jsruntime/qv4vme_moth.cpp
@@ -640,7 +640,6 @@ QV4::ReturnedValue VME::interpret(CppStackFrame *frame, ExecutionEngine *engine,
 
     MOTH_BEGIN_INSTR(LoadSuperProperty)
         STORE_IP();
-        STORE_ACC();
         acc = Runtime::method_loadSuperProperty(engine, STACK_VALUE(property));
         CHECK_EXCEPTION;
     MOTH_END_INSTR(LoadSuperProperty)
@@ -787,12 +786,14 @@ QV4::ReturnedValue VME::interpret(CppStackFrame *frame, ExecutionEngine *engine,
 
     MOTH_BEGIN_INSTR(Construct)
         STORE_IP();
+        STORE_ACC();
         acc = Runtime::method_construct(engine, STACK_VALUE(func), ACC, stack + argv, argc);
         CHECK_EXCEPTION;
     MOTH_END_INSTR(Construct)
 
     MOTH_BEGIN_INSTR(ConstructWithSpread)
         STORE_IP();
+        STORE_ACC();
         acc = Runtime::method_constructWithSpread(engine, STACK_VALUE(func), ACC, stack + argv, argc);
         CHECK_EXCEPTION;
     MOTH_END_INSTR(ConstructWithSpread)
@@ -820,7 +821,6 @@ QV4::ReturnedValue VME::interpret(CppStackFrame *frame, ExecutionEngine *engine,
     MOTH_BEGIN_INSTR(DeadTemporalZoneCheck)
         if (ACC.isEmpty()) {
             STORE_IP();
-            STORE_ACC();
             Runtime::method_throwReferenceError(engine, name);
             goto handleUnwind;
         }
@@ -989,6 +989,7 @@ QV4::ReturnedValue VME::interpret(CppStackFrame *frame, ExecutionEngine *engine,
             if (t->isNullOrUndefined()) {
                 *t = engine->globalObject->asReturnedValue();
             } else {
+                STORE_ACC();
                 *t = t->toObject(engine)->asReturnedValue();
                 CHECK_EXCEPTION;
             }
@@ -1001,6 +1002,7 @@ QV4::ReturnedValue VME::interpret(CppStackFrame *frame, ExecutionEngine *engine,
     MOTH_END_INSTR(LoadSuperConstructor)
 
     MOTH_BEGIN_INSTR(ToObject)
+        STORE_ACC();
         acc = ACC.toObject(engine)->asReturnedValue();
         CHECK_EXCEPTION;
     MOTH_END_INSTR(ToObject)