Add _PyObject_FastCallKeywords()

Issue #27830: Similar to _PyObject_FastCallDict(), but keyword arguments are
also passed in the same C array than positional arguments, rather than being
passed as a Python dict.

Objects/abstract.c

@@ -2309,6 +2309,85 @@ exit:
     return result;
 }
 
+static PyObject *
+_PyStack_AsDict(PyObject **stack, Py_ssize_t nkwargs, PyObject *func)
+{
+    PyObject *kwdict;
+
+    kwdict = PyDict_New();
+    if (kwdict == NULL) {
+        return NULL;
+    }
+
+    while (--nkwargs >= 0) {
+        int err;
+        PyObject *key = *stack++;
+        PyObject *value = *stack++;
+        if (PyDict_GetItem(kwdict, key) != NULL) {
+            PyErr_Format(PyExc_TypeError,
+                         "%.200s%s got multiple values "
+                         "for keyword argument '%U'",
+                         PyEval_GetFuncName(func),
+                         PyEval_GetFuncDesc(func),
+                         key);
+            Py_DECREF(kwdict);
+            return NULL;
+        }
+
+        err = PyDict_SetItem(kwdict, key, value);
+        if (err) {
+            Py_DECREF(kwdict);
+            return NULL;
+        }
+    }
+    return kwdict;
+}
+
+PyObject *
+_PyObject_FastCallKeywords(PyObject *func, PyObject **stack, Py_ssize_t nargs,
+                           Py_ssize_t nkwargs)
+{
+    PyObject *args, *kwdict, *result;
+
+    /* _PyObject_FastCallKeywords() must not be called with an exception set,
+       because it may clear it (directly or indirectly) and so the
+       caller loses its exception */
+    assert(!PyErr_Occurred());
+
+    assert(func != NULL);
+    assert(nargs >= 0);
+    assert(nkwargs >= 0);
+    assert((nargs == 0 && nkwargs == 0) || stack != NULL);
+
+    if (PyFunction_Check(func)) {
+        /* Fast-path: avoid temporary tuple or dict */
+        return _PyFunction_FastCallKeywords(func, stack, nargs, nkwargs);
+    }
+
+    if (PyCFunction_Check(func) && nkwargs == 0) {
+        return _PyCFunction_FastCallDict(func, args, nargs, NULL);
+    }
+
+    /* Slow-path: build temporary tuple and/or dict */
+    args = _PyStack_AsTuple(stack, nargs);
+
+    if (nkwargs > 0) {
+        kwdict = _PyStack_AsDict(stack + nargs, nkwargs, func);
+        if (kwdict == NULL) {
+            Py_DECREF(args);
+            return NULL;
+        }
+    }
+    else {
+        kwdict = NULL;
+    }
+
+    result = PyObject_Call(func, args, kwdict);
+    Py_DECREF(args);
+    Py_XDECREF(kwdict);
+    return result;
+}
+
 static PyObject*
 call_function_tail(PyObject *callable, PyObject *args)
 {