avoid formatting call overhead for simple Unicode value case

Cython/Compiler/ExprNodes.py

@@ -3024,8 +3024,15 @@ class FormattedValueNode(ExprNode):
             ))
             code.put_gotref(conversion_result)
 
-        code.put("%s = PyObject_Format(%s, %s); " % (
+        if isinstance(self.format_spec, UnicodeNode) and not self.format_spec.value:
+            # common case: no format spec => expect Unicode pass-through
+            format_func = '__Pyx_PyObject_FormatSimple'
+        else:
+            format_func = 'PyObject_Format'
+
+        code.put("%s = %s(%s, %s); " % (
             self.result(),
+            format_func,
             conversion_result,
             self.format_spec.py_result()))
 

Cython/Utility/ModuleSetupCode.c

@@ -123,6 +123,7 @@
   #define PyObject_Realloc(p)  PyMem_Realloc(p)
 #endif
 
+#define __Pyx_PyObject_FormatSimple(s, f) (likely(PyUnicode_CheckExact(s)) ? (Py_INCREF(s), s) : PyObject_Format(s, f))
 #define __Pyx_PyString_FormatSafe(a, b)   ((unlikely((a) == Py_None)) ? PyNumber_Remainder(a, b) : __Pyx_PyString_Format(a, b))
 #define __Pyx_PyUnicode_FormatSafe(a, b)  ((unlikely((a) == Py_None)) ? PyNumber_Remainder(a, b) : PyUnicode_Format(a, b))