[Python-Dev] CALL_FUNCTION_EX arg and stack_effect

[Matthieu Dartiailh]

Hi,

I have a question about the use of CALL_FUNCTION_EX in 
https://github.com/python/cpython/blob/master/Python/compile.c#L3624. 
Looking at the code it appears that the argument will be either 1 or 0 
depending on whether or not the function is taking keywords arguments 
(which means that CALL_FUNCTION_EX cannot be used on function taking no 
argument).
Executing that opcode will remove from the stack the function code, the 
positional arguments (packed in a tuple) and the potential keyword 
arguments packed in a dict and push the return value. So the stack 
effect will be either -1 or -2 (could be 0 if the possibility to pass 0 
arguments existed).
Looking at the stack effect computation 
(https://github.com/python/cpython/blob/master/Python/compile.c#L1047), 
it appears that the stack effect will be 0 if the argument is 0, -1 for 
either 1 or 2, and -2 for 3. Which means that the code generated at 
https://github.com/python/cpython/blob/master/Python/compile.c#L3624 can 
never allow to compute the right stack effect using the stack_effect 
function (as it will return either 0 or -1 instead of -1 and -2)

I would say that this is a bug and that the oparg should be 1 + 2 if 
keywords arguments are present at line 3624.

I am not sure what consequence this can have on CPython but it means the 
bytecode becomes weird as a the stack looks like it can grow during a 
list comprehension (calling a function f using * syntax) :
      BUILD_LIST 0 1
      LOAD_FAST .0 1
      FOR_ITER 22 1     ---> after each jump it looks like the stack is 
higher by one
      STORE_FAST i -1
      LOAD_GLOBAL f 1
      LOAD_DEREF a 1
      LOAD_FAST i 1
      BINARY_SUBSCR None -1
      CALL_FUNCTION_EX 0 0
      LIST_APPEND 2 -1
      JUMP_ABSOLUTE 4 0
      RETURN_VALUE None -1

What do you think ? Should I open an issue on https://bugs.python.org/ ?

Best regards

Matthieu