Zheaoli / recursion

Playing with recursion, trampolines etc.

Recursion.

This project explores a variety of ways to optimize recursion in Python. So far, the following approaches have been explored: Trampolines.

TCO using Python standard means. Taking a tail-recursive function that returns a thunk in place of a recursive call, the trampoline function keeps calling the thunked result until a non-callable value is reached.

Associated files:
trampolines.py - all source code for trampolines
trampolines_in_Python.ipynb

AST trampolines.

TCO by manipulating AST of the function (with Paul Tagliamonte). The ast of the tail-recursive function is altered so that all returns are "thunked", a trampolining decorator is injected into the tree.

Associated files:
ast_trampoline.py

Tailbytes.

TCO using direct bytecode manipulation (with Allison Kaptur). By swapping the bytecode representation of a tail-recursive function, the recursive call is substituted by resetting the variables and jumping to the beginning of the function. Problems we've faced and resolved so far:

• Problem: Deleting and inserting bytes into the bytecode changes the location of the original bytes. Solution: Absolute jumps are updated after bytecode alternation has been performed.
• Problem: Our initial algorithm was removing all calls to any function. This caused a problem if the function involve non-recursive function calls. Solution: Remove a CALL_FUNCTION instruction only if it is a part of the recursive call (to determine that, we keep track of the stack size). We target the CALL_FUNCTION instructions which return the stack to the same size that it had when the recursive function was loaded onto it.

Associated files:
tailbytes.py
stack_heights.py
See also presentation slides

Transform regular recursion into tail recursion.

Interpreter that reads an ast representation of a regular recursion and mutates it to a tree that compiles to the same function using tail recursion.

Associated files:
translator.py

Further goals:

• Transform binary recursion (i.e. fibonacci-like functions) into tail-recursion.
• Find a way to optimize mutual recursion.

Tools:

• ast_pretty_printer.py
  print_ast takes an ast tree and prettyprints it, working off of ast.dump