bpo-39320: Handle unpacking of **values in compiler (GH-18141)

* Add DICT_UPDATE and DICT_MERGE bytecodes. Use them for ** unpacking.

* Remove BUILD_MAP_UNPACK and BUILD_MAP_UNPACK_WITH_CALL, as they are now unused.

* Update magic number for ** unpacking opcodes.

* Update dis.rst to incorporate new bytecodes.

* Add blurb entry.

1 files changed, 19 insertions, 36 deletions

diff --git a/Python/ceval.c b/Python/ceval.c
index 528ad7f..673b401 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -2801,49 +2801,32 @@ main_loop:
             DISPATCH();
         }
 
-        case TARGET(BUILD_MAP_UNPACK): {
-            Py_ssize_t i;
-            PyObject *sum = PyDict_New();
-            if (sum == NULL)
-                goto error;
-
-            for (i = oparg; i > 0; i--) {
-                PyObject *arg = PEEK(i);
-                if (PyDict_Update(sum, arg) < 0) {
-                    if (_PyErr_ExceptionMatches(tstate, PyExc_AttributeError)) {
-                        _PyErr_Format(tstate, PyExc_TypeError,
-                                      "'%.200s' object is not a mapping",
-                                      arg->ob_type->tp_name);
-                    }
-                    Py_DECREF(sum);
-                    goto error;
+        case TARGET(DICT_UPDATE): {
+            PyObject *update = POP();
+            PyObject *dict = PEEK(oparg);
+            if (PyDict_Update(dict, update) < 0) {
+                if (_PyErr_ExceptionMatches(tstate, PyExc_AttributeError)) {
+                    _PyErr_Format(tstate, PyExc_TypeError,
+                                    "'%.200s' object is not a mapping",
+                                    update->ob_type->tp_name);
                 }
+                Py_DECREF(update);
+                goto error;
             }
-
-            while (oparg--)
-                Py_DECREF(POP());
-            PUSH(sum);
+            Py_DECREF(update);
             DISPATCH();
         }
 
-        case TARGET(BUILD_MAP_UNPACK_WITH_CALL): {
-            Py_ssize_t i;
-            PyObject *sum = PyDict_New();
-            if (sum == NULL)
-                goto error;
+        case TARGET(DICT_MERGE): {
+            PyObject *update = POP();
+            PyObject *dict = PEEK(oparg);
 
-            for (i = oparg; i > 0; i--) {
-                PyObject *arg = PEEK(i);
-                if (_PyDict_MergeEx(sum, arg, 2) < 0) {
-                    Py_DECREF(sum);
-                    format_kwargs_error(tstate, PEEK(2 + oparg), arg);
-                    goto error;
-                }
+            if (_PyDict_MergeEx(dict, update, 2) < 0) {
+                format_kwargs_error(tstate, PEEK(2 + oparg), update);
+                Py_DECREF(update);
+                goto error;
             }
-
-            while (oparg--)
-                Py_DECREF(POP());
-            PUSH(sum);
+            Py_DECREF(update);
             PREDICT(CALL_FUNCTION_EX);
             DISPATCH();
         }