[Python-Dev] Expose stack effects to Python?

[Victor Stinner]

Hi,

For my registervm project (fork of CPython using register-based
bytecode, instead of stack-based bytecode), I implemented a
Instruction.use_stack() method which just checks if the stack is
"used": read the stack, exchange values in the stack (like "ROT"
instruction), push or pop a value.

Instruction.use_stack():
http://hg.python.org/sandbox/registervm/file/ff24dfecc27d/Lib/registervm.py#l546

The method uses a dummy heuristic, just because it was quick to
implement it, and it's enough for my use case.

To answer your question: yes, I would like a opcode_stack_effect() function.

registervm has its own disassembler which creates objects, rather than
just generating a text representation of the bytecode. I'm using
objects because I rewrite most instructions and I need more
information and functions:

* disassembler (raw bytes => list of instructions)
* assembler (list of instructions => raw bytes)
* format an instruction (human readable assembler)
* is_terminal(): last instruction of a block
* is_cond_jump(): the instruction is a conditional jump? hesitate to
move this disassembler to CPython directly. I'm not sure that it would
be useful, its API is maybe too specific to my registervm project.
* is_reg_used(), is_reg_replaced(), is_reg_modified(), etc.: checks on registers
* etc.

Would it be useful to have such high-level API in Python?

Victor

2013/7/3 Larry Hastings <larry at hastings.org>:
>
>
> I wrote my own assembler for Python bytecode called "Maynard".  I had to
> statically compute the stack effects for each bytecode instruction by hand;
> what I did was copied and pasted opcode_stack_effect() (which is static) out
> of Python/compile.c and into my own driver program, then I probed it with
> test values to produce a table.  I then coded up a function using that
> table, but hand-calculating the value sometimes as there are some opcodes
> whose stack effect varies based on the oparg.
>
> It sure would be nice if this information was simply available to the Python
> interpreter; theoretically it can change between point releases.  Would
> anybody mind if I added it somewhere?  I'd probably just expose
> opcode_stack_effect to Python, then add all this other junk to the dis
> module and make it available there.
>
>
> /arry
>
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