What does RESUME opcode actually do?

Question

The documentation is not very informative (at least for me):

opcode:: RESUME (context)

A no-op. Performs internal tracing, debugging and optimization checks.

The context oparand consists of two parts. The lowest two bits indicate where the RESUME occurs:

0 The start of a function, which is neither a generator, coroutine nor an async generator

1 After a yield expression

2 After a yield from expression

3 After an await expression

The next bit is 1 if the RESUME is at except-depth 1, and 0 otherwise.

Example:

>>> import dis
>>> 
>>> def f(): ...
... 
>>> dis.dis(f)
  1           0 RESUME                   0
              2 LOAD_CONST               0 (None)
              4 RETURN_VALUE

Can someone provide an explanation of what this opcode really does?

@AdesojiAlu Thanks for your response, but I couldn't find anything useful related to my question in the link. — Amir reza Riahi
– Amir reza Riahi, Commented Nov 17, 2023 at 16:30
Your question is pretty vague. What exactly do you need to know about it? How are we supposed to know what's "useful" to you? — Barmar
– Barmar, Commented Nov 17, 2023 at 17:33
@Barmar I want to know what does the interpreter do when it reaches RESUME opcode. — Amir reza Riahi
– Amir reza Riahi, Commented Nov 17, 2023 at 18:31
It looks to me like an interpreter equivalent to the ENDBR assembler instruction. — rodrigo
– rodrigo, Commented Jun 11, 2024 at 15:23

AmericanJael · Accepted Answer · 2024-06-11 19:04:47Z

I can't provide an exhaustive answer; but RESUME is a type of NOP opcode, meaning it doesn't actually do anything, but rather simplifies the generation of bytecodes. For example, pass is a NOP in python.

The RESUME NOP is used when the interpreter enters a function. So it is used 1) at the beginning of a python function, and 2) after a yield statement. It is placed after the yield statement since functions with yield statements don't actually run their body code, but merely return a generator object (as opposed to an iterable).

In case it's helpful to anyone, the blog linked below talks in detail about python bytecode and how to view it: https://www.synopsys.com/blogs/software-security/understanding-python-bytecode.html

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