avoid generating loop code if there is only one kw-only argument to fetch

Cython/Compiler/Nodes.py

@@ -3905,6 +3905,7 @@ class DefNodeWrapper(FuncDefNode):
                     first_optional_arg = max_positional_args + i
                 optional_args.append(arg.name)
             if optional_args:
+                if len(optional_args) > 1:
                     # if we receive more than the named kwargs, we either have **kwargs
                     # (in which case we must iterate anyway) or it's an error (which we
                     # also handle during iteration) => skip this part if there are more
@@ -3915,9 +3916,13 @@ class DefNodeWrapper(FuncDefNode):
                     # not unrolling the loop here reduces the C code overhead
                     code.putln('for (index = %d; index < %d && kw_args > 0; index++) {' % (
                         first_optional_arg, first_optional_arg + len(optional_args)))
+                else:
+                    code.putln('if (kw_args == 1) {')
+                    code.putln('const Py_ssize_t index = %d;' % first_optional_arg)
                 code.putln('PyObject* value = PyDict_GetItem(%s, *%s[index]);' % (
                     Naming.kwds_cname, Naming.pykwdlist_cname))
                 code.putln('if (value) { values[index] = value; kw_args--; }')
+                if len(optional_args) > 1:
                     code.putln('}')
                 code.putln('}')