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>

1 files changed, 47 insertions, 58 deletions

diff --git a/src/qml/jit/qv4assembler.cpp b/src/qml/jit/qv4assembler.cpp
index a32039dc96..5702774cd7 100644
--- a/src/qml/jit/qv4assembler.cpp
+++ b/src/qml/jit/qv4assembler.cpp
@@ -712,6 +712,11 @@ struct PlatformAssembler64 : PlatformAssemblerCommon
         patches.push_back({ jump, offset });
     }
 
+    Jump jumpEmpty()
+    {
+        return branch64(Equal, AccumulatorRegister, TrustedImm64(Primitive::emptyValue().asReturnedValue()));
+    }
+
     void toBoolean(std::function<void(RegisterID)> continuation)
     {
         urshift64(AccumulatorRegister, TrustedImm32(Value::IsIntegerConvertible_Shift), ScratchRegister);
@@ -815,26 +820,6 @@ struct PlatformAssembler64 : PlatformAssemblerCommon
         return branch32(Equal, TrustedImm32(3), ScratchRegister);
     }
 
-    void jumpStrictEqualStackSlotInt(int lhs, int rhs, int offset)
-    {
-        Address lhsAddr(JSStackFrameRegister, lhs * int(sizeof(Value)));
-        load64(lhsAddr, ScratchRegister);
-        Jump isUndef = branch64(Equal, ScratchRegister, TrustedImm64(0));
-        Jump equal = branch32(Equal, TrustedImm32(rhs), ScratchRegister);
-        patches.push_back({ equal, offset });
-        isUndef.link(this);
-    }
-
-    void jumpStrictNotEqualStackSlotInt(int lhs, int rhs, int offset)
-    {
-        Address lhsAddr(JSStackFrameRegister, lhs * int(sizeof(Value)));
-        load64(lhsAddr, ScratchRegister);
-        Jump isUndef = branch64(Equal, ScratchRegister, TrustedImm64(0));
-        patches.push_back({ isUndef, offset });
-        Jump notEqual = branch32(NotEqual, TrustedImm32(rhs), ScratchRegister);
-        patches.push_back({ notEqual, offset });
-    }
-
     void setAccumulatorTag(QV4::Value::ValueTypeInternal tag, RegisterID sourceReg = NoRegister)
     {
         if (sourceReg == NoRegister)
@@ -1163,6 +1148,11 @@ struct PlatformAssembler32 : PlatformAssemblerCommon
         patches.push_back({ jump, offset });
     }
 
+    Jump jumpEmpty()
+    {
+        return branch32(Equal, AccumulatorRegisterTag, TrustedImm32(Primitive::emptyValue().asReturnedValue() >> 32));
+    }
+
     void toBoolean(std::function<void(RegisterID)> continuation)
     {
         urshift32(AccumulatorRegisterTag, TrustedImm32(Value::IsIntegerConvertible_Shift - 32),
@@ -1202,34 +1192,6 @@ struct PlatformAssembler32 : PlatformAssemblerCommon
         done.link(this);
     }
 
-    void jumpStrictEqualStackSlotInt(int lhs, int rhs, int offset)
-    {
-        Address lhsAddr(JSStackFrameRegister, lhs * int(sizeof(Value)));
-        load32(lhsAddr, ScratchRegister);
-        Jump notEqInt = branch32(NotEqual, ScratchRegister, TrustedImm32(rhs));
-        Jump notEqUndefVal = branch32(NotEqual, ScratchRegister, TrustedImm32(0));
-        patches.push_back({ notEqUndefVal, offset });
-        lhsAddr.offset += 4;
-        load32(lhsAddr, ScratchRegister);
-        Jump notEqUndefTag = branch32(NotEqual, ScratchRegister, TrustedImm32(0));
-        patches.push_back({ notEqUndefTag, offset });
-        notEqInt.link(this);
-    }
-
-    void jumpStrictNotEqualStackSlotInt(int lhs, int rhs, int offset)
-    {
-        Address lhsAddr(JSStackFrameRegister, lhs * int(sizeof(Value)));
-        load32(lhsAddr, ScratchRegister);
-        Jump notEqual = branch32(NotEqual, TrustedImm32(rhs), ScratchRegister);
-        patches.push_back({ notEqual, offset });
-        Jump notUndefValue = branch32(NotEqual, TrustedImm32(0), ScratchRegister);
-        lhsAddr.offset += 4;
-        load32(lhsAddr, ScratchRegister);
-        Jump equalUndef = branch32(Equal, TrustedImm32(0), ScratchRegister);
-        patches.push_back({ equalUndef, offset });
-        notUndefValue.link(this);
-    }
-
     void setAccumulatorTag(QV4::Value::ValueTypeInternal tag, RegisterID sourceReg = NoRegister)
     {
         if (sourceReg != NoRegister)
@@ -2009,6 +1971,16 @@ void Assembler::jumpFalse(int offset)
     });
 }
 
+void Assembler::jumpNoException(int offset)
+{
+    auto jump = pasm()->branch32(
+        PlatformAssembler::Equal,
+        PlatformAssembler::Address(PlatformAssembler::EngineRegister,
+                                   offsetof(EngineBase, hasException)),
+        TrustedImm32(0));
+    pasm()->patches.push_back({ jump, offset });
+}
+
 void Assembler::jumpNotUndefined(int offset)
 {
     pasm()->jumpNotUndefined(offset);
@@ -2019,16 +1991,6 @@ void JIT::Assembler::jumpEmpty(int offset)
     pasm()->jumpEmpty(offset);
 }
 
-void Assembler::jumpStrictEqualStackSlotInt(int lhs, int rhs, int offset)
-{
-    pasm()->jumpStrictEqualStackSlotInt(lhs, rhs, offset);
-}
-
-void Assembler::jumpStrictNotEqualStackSlotInt(int lhs, int rhs, int offset)
-{
-    pasm()->jumpStrictNotEqualStackSlotInt(lhs, rhs, offset);
-}
-
 void Assembler::prepareCallWithArgCount(int argc)
 {
 #ifndef QT_NO_DEBUG
@@ -2220,12 +2182,14 @@ void Assembler::getException()
 
 void Assembler::setException()
 {
+    auto noException = pasm()->jumpEmpty();
     Address addr(PlatformAssembler::EngineRegister, offsetof(EngineBase, exceptionValue));
     pasm()->loadPtr(addr, PlatformAssembler::ScratchRegister);
     pasm()->storeAccumulator(Address(PlatformAssembler::ScratchRegister));
     addr.offset = offsetof(EngineBase, hasException);
     Q_STATIC_ASSERT(sizeof(QV4::EngineBase::hasException) == 1);
     pasm()->store8(TrustedImm32(1), addr);
+    noException.link(pasm());
 }
 
 void Assembler::setUnwindHandler(int offset)
@@ -2240,6 +2204,31 @@ void Assembler::clearUnwindHandler()
     pasm()->storePtr(TrustedImmPtr(nullptr), pasm()->exceptionHandlerAddress());
 }
 
+void JIT::Assembler::unwindDispatch()
+{
+    checkException();
+    pasm()->load32(Address(PlatformAssembler::CppStackFrameRegister, offsetof(CppStackFrame, unwindLevel)), PlatformAssembler::ScratchRegister);
+    auto noUnwind = pasm()->branch32(PlatformAssembler::Equal, PlatformAssembler::ScratchRegister, TrustedImm32(0));
+    pasm()->sub32(TrustedImm32(1), PlatformAssembler::ScratchRegister);
+    pasm()->store32(PlatformAssembler::ScratchRegister, Address(PlatformAssembler::CppStackFrameRegister, offsetof(CppStackFrame, unwindLevel)));
+    auto jump = pasm()->branch32(PlatformAssembler::Equal, PlatformAssembler::ScratchRegister, TrustedImm32(0));
+    gotoCatchException();
+    jump.link(pasm());
+
+    pasm()->loadPtr(Address(PlatformAssembler::CppStackFrameRegister, offsetof(CppStackFrame, unwindLabel)), PlatformAssembler::ScratchRegister);
+    pasm()->jump(PlatformAssembler::ScratchRegister);
+
+    noUnwind.link(pasm());
+}
+
+void JIT::Assembler::unwindToLabel(int level, int offset)
+{
+    auto l = pasm()->storePtrWithPatch(TrustedImmPtr(nullptr), Address(PlatformAssembler::CppStackFrameRegister, offsetof(CppStackFrame, unwindLabel)));
+    pasm()->ehTargets.push_back({ l, offset });
+    pasm()->store32(TrustedImm32(level), Address(PlatformAssembler::CppStackFrameRegister, offsetof(CppStackFrame, unwindLevel)));
+    gotoCatchException();
+}
+
 void Assembler::pushCatchContext(int index, int name)
 {
     prepareCallWithArgCount(3);