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

Cython/Compiler/Nodes.py

@@ -3905,20 +3905,25 @@ class DefNodeWrapper(FuncDefNode):
                     first_optional_arg = max_positional_args + i
                 optional_args.append(arg.name)
             if optional_args:
-                # 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
-                code.putln('if (kw_args > 0 && %s(kw_args <= %d)) {' % (
-                    not self.starstar_arg and 'likely' or '',
-                    len(optional_args)))
-                code.putln('Py_ssize_t index;')
-                # 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)))
+                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
+                    code.putln('if (kw_args > 0 && %s(kw_args <= %d)) {' % (
+                        not self.starstar_arg and 'likely' or '',
+                        len(optional_args)))
+                    code.putln('Py_ssize_t index;')
+                    # 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--; }')
-                code.putln('}')
+                if len(optional_args) > 1:
+                    code.putln('}')
                 code.putln('}')
 
         code.putln('if (unlikely(kw_args > 0)) {')