Merge commit '98dfb53b' into refcounting

Conflicts:
	from_cpython/Python/traceback.c
	src/codegen/unwinding.cpp
	src/runtime/builtin_modules/sys.cpp
	src/runtime/capi.cpp
	src/runtime/cxx_unwind.cpp
	src/runtime/exceptions.cpp

from_cpython/Python/traceback.c

@@ -124,14 +124,9 @@ PyTraceBack_Here(PyFrameObject *frame)
 int
 PyTraceBack_Here_Tb(PyFrameObject *frame, PyTracebackObject** tb)
 {
-    PyObject* prev_tb = (PyObject*)*tb;
-    if (prev_tb == Py_None) {
-        Py_DECREF(prev_tb);
-        *tb = NULL;
-        prev_tb = NULL;
-    }
-    *tb = newtracebackobject(*tb, frame);
-    Py_XDECREF(prev_tb);
+    PyTracebackObject* oldtb = *tb;
+    *tb = newtracebackobject(oldtb, frame);
+    Py_XDECREF(oldtb);
     if (*tb == NULL)
         return -1;
     return 0;

src/codegen/unwinding.cpp

@@ -703,7 +703,7 @@ ExcInfo* getFrameExcInfo() {
 
     if (!copy_from_exc->type) {
         // No exceptions found:
-        *copy_from_exc = ExcInfo(None, None, None);
+        *copy_from_exc = ExcInfo(None, None, NULL);
     }
 
     for (auto* ex : to_update) {

src/runtime/builtin_modules/sys.cpp

@@ -48,25 +48,24 @@ Box* sysExcInfo() {
     ExcInfo* exc = getFrameExcInfo();
     assert(exc->type);
     assert(exc->value);
-    assert(exc->traceback);
-    return BoxedTuple::create({ exc->type, exc->value, exc->traceback });
+    Box* tb = exc->traceback ? exc->traceback : None;
+    return BoxedTuple::create({ exc->type, exc->value, tb });
 }
 
 Box* sysExcClear() {
     ExcInfo* exc = getFrameExcInfo();
     assert(exc->type);
     assert(exc->value);
-    assert(exc->traceback);
 
     Box* old_type = exc->type;
     Box* old_value = exc->value;
     Box* old_traceback = exc->traceback;
     exc->type = incref(None);
     exc->value = incref(None);
-    exc->traceback = incref(None);
+    exc->traceback = NULL;
     Py_DECREF(old_type);
     Py_DECREF(old_value);
-    Py_DECREF(old_traceback);
+    Py_XDECREF(old_traceback);
 
     Py_RETURN_NONE;
 }

src/runtime/capi.cpp

@@ -717,9 +717,6 @@ void checkAndThrowCAPIException() {
         if (!value)
             value = incref(None);
 
-        if (!tb)
-            tb = incref(None);
-
         // This is similar to PyErr_NormalizeException:
         if (!isSubclass(value->cls, type)) {
             if (value->cls == tuple_cls) {
@@ -733,10 +730,10 @@ void checkAndThrowCAPIException() {
 
         RELEASE_ASSERT(value->cls == type, "unsupported");
 
-        if (tb != None)
+        if (tb)
             throw ExcInfo(value->cls, value, tb);
         Py_DECREF(type);
-        Py_DECREF(tb);
+        Py_XDECREF(tb);
         raiseExc(value);
     }
 }
@@ -761,7 +758,7 @@ extern "C" void PyErr_SetExcInfo(PyObject* type, PyObject* value, PyObject* trac
     ExcInfo* exc = getFrameExcInfo();
     exc->type = type ? type : None;
     exc->value = value ? value : None;
-    exc->traceback = traceback ? traceback : None;
+    exc->traceback = traceback;
 }
 
 extern "C" const char* PyExceptionClass_Name(PyObject* o) noexcept {

src/runtime/cxx_unwind.cpp

@@ -73,7 +73,7 @@ namespace pyston {
 
 void checkExcInfo(const ExcInfo* exc) {
     assert(exc);
-    assert(exc->type && exc->value && exc->traceback);
+    assert(exc->type && exc->value);
 }
 
 static StatCounter us_unwind_loop("us_unwind_loop");

src/runtime/exceptions.cpp

@@ -27,7 +27,7 @@ namespace pyston {
 
 void raiseExc(STOLEN(Box*) exc_obj) {
     assert(!PyErr_Occurred());
-    throw ExcInfo(incref(exc_obj->cls), exc_obj, incref(None));
+    throw ExcInfo(incref(exc_obj->cls), exc_obj, NULL);
 }
 
 // Have a special helper function for syntax errors, since we want to include the location
@@ -47,7 +47,7 @@ void raiseSyntaxError(const char* msg, int lineno, int col_offset, llvm::StringR
         auto args = BoxedTuple::create({ autoDecref(boxString(file)), autoDecref(boxInt(lineno)), None, loc });
         Box* exc = runtimeCall(SyntaxError, ArgPassSpec(2), autoDecref(boxString(msg)), args, NULL, NULL, NULL);
         assert(!PyErr_Occurred());
-        throw ExcInfo(incref(exc->cls), exc, incref(None));
+        throw ExcInfo(incref(exc->cls), exc, NULL);
     } else {
         PyErr_SetString(SyntaxError, msg);
         PyErr_SyntaxLocation(file.str().c_str(), lineno);
@@ -154,10 +154,6 @@ ExcInfo excInfoForRaise(STOLEN(Box*) type, STOLEN(Box*) value, STOLEN(Box*) tb)
 
     assert(PyExceptionClass_Check(type));
 
-    if (tb == NULL) {
-        tb = incref(None);
-    }
-
     return ExcInfo(type, value, tb);
 }
 
@@ -186,7 +182,7 @@ extern "C" void raise0_capi(ExcInfo* frame_exc_info) noexcept {
         frame_exc_info->type = TypeError;
         frame_exc_info->value
             = boxString("exceptions must be old-style classes or derived from BaseException, not NoneType");
-        frame_exc_info->traceback = None;
+        frame_exc_info->traceback = NULL;
         PyErr_NormalizeException(&frame_exc_info->type, &frame_exc_info->value, &frame_exc_info->traceback);
     }