yield: make it steal the passed argument

the idea is that stealing the value allows us to immediately free the variable in more case when it's unused
otherwise we would always have to continue the generator in order to decrease the refcount

src/codegen/ast_interpreter.cpp

@@ -993,7 +993,6 @@ Value ASTInterpreter::visit_langPrimitive(AST_LangPrimitive* node) {
 
 Value ASTInterpreter::visit_yield(AST_Yield* node) {
     Value value = node->value ? visit_expr(node->value) : getNone();
-    AUTO_DECREF(value.o);
     assert(generator && generator->cls == generator_cls);
 
     return Value(yield(generator, value.o), jit ? jit->emitYield(value) : NULL);

src/codegen/baseline_jit.cpp

@@ -537,7 +537,9 @@ std::vector<RewriterVar*> JitFragmentWriter::emitUnpackIntoArray(RewriterVar* v,
 
 RewriterVar* JitFragmentWriter::emitYield(RewriterVar* v) {
     RewriterVar* generator = getInterp()->getAttr(ASTInterpreterJitInterface::getGeneratorOffset());
-    return call(false, (void*)yield, generator, v)->setType(RefType::OWNED);
+    auto rtn = call(false, (void*)yield, generator, v)->setType(RefType::OWNED);
+    v->refConsumed();
+    return rtn;
 }
 
 void JitFragmentWriter::emitDelAttr(RewriterVar* target, BoxedString* attr) {

src/codegen/irgen/irgenerator.cpp

@@ -1544,8 +1544,9 @@ private:
         CompilerVariable* value = node->value ? evalExpr(node->value, unw_info) : emitter.getNone();
         ConcreteCompilerVariable* convertedValue = value->makeConverted(emitter, value->getBoxType());
 
-        llvm::Value* rtn
+        llvm::Instruction* rtn
             = emitter.createCall2(unw_info, g.funcs.yield, convertedGenerator->getValue(), convertedValue->getValue());
+        emitter.refConsumed(convertedValue->getValue(), rtn);
         emitter.setType(rtn, RefType::OWNED);
 
         return new ConcreteCompilerVariable(UNKNOWN, rtn);

src/runtime/generator.cpp

@@ -332,13 +332,13 @@ Box* generatorHasnext(Box* s) {
 }
 
 
-extern "C" Box* yield(BoxedGenerator* obj, Box* value) {
+extern "C" Box* yield(BoxedGenerator* obj, STOLEN(Box*) value) {
     STAT_TIMER(t0, "us_timer_generator_switching", 0);
 
     assert(obj->cls == generator_cls);
     BoxedGenerator* self = static_cast<BoxedGenerator*>(obj);
     assert(!self->returnValue);
-    self->returnValue = incref(value);
+    self->returnValue = value;
 
     threading::popGenerator();
 
@@ -371,6 +371,7 @@ extern "C" Box* yield(BoxedGenerator* obj, Box* value) {
     if (self->exception.type) {
         ExcInfo e = self->exception;
         self->exception = ExcInfo(NULL, NULL, NULL);
+        Py_CLEAR(self->returnValue);
         throw e;
     }
 

src/runtime/generator.h

@@ -29,7 +29,7 @@ void setupGenerator();
 void generatorEntry(BoxedGenerator* g);
 Context* getReturnContextForGeneratorFrame(void* frame_addr);
 
-extern "C" Box* yield(BoxedGenerator* obj, Box* value);
+extern "C" Box* yield(BoxedGenerator* obj, STOLEN(Box*) value);
 extern "C" BoxedGenerator* createGenerator(BoxedFunctionBase* function, Box* arg1, Box* arg2, Box* arg3, Box** args);
 }