http://bugs.python.org/issue4715 This patch by Antoine Pitrou optimizes the bytecode for conditional branches by merging the following "POP_TOP" instruction into the conditional jump. For example, the list comprehension "[x for x in l if not x]" produced the following bytecode: 1 0 BUILD_LIST 0 3 LOAD_FAST 0 (.0) >> 6 FOR_ITER 23 (to 32) 9 STORE_FAST 1 (x) 12 LOAD_FAST 1 (x) 15 JUMP_IF_TRUE 10 (to 28) 18 POP_TOP 19 LOAD_FAST 1 (x) 22 LIST_APPEND 2 25 JUMP_ABSOLUTE 6 >> 28 POP_TOP 29 JUMP_ABSOLUTE 6 >> 32 RETURN_VALUE but after the patch it produces the following bytecode: 1 0 BUILD_LIST 0 3 LOAD_FAST 0 (.0) >> 6 FOR_ITER 18 (to 27) 9 STORE_FAST 1 (x) 12 LOAD_FAST 1 (x) 15 POP_JUMP_IF_TRUE 6 18 LOAD_FAST 1 (x) 21 LIST_APPEND 2 24 JUMP_ABSOLUTE 6 >> 27 RETURN_VALUE Notice that not only the code is shorter, but the conditional jump (POP_JUMP_IF_TRUE) jumps right to the start of the loop instead of going through the JUMP_ABSOLUTE at the end. "continue" statements are helped similarly. Furthermore, the old jump opcodes (JUMP_IF_FALSE, JUMP_IF_TRUE) have been replaced by two new opcodes: - JUMP_IF_TRUE_OR_POP, which jumps if true and pops otherwise - JUMP_IF_FALSE_OR_POP, which jumps if false and pops otherwise

#! /usr/bin/env python
"""Generate C code for the jump table of the threaded code interpreter
(for compilers supporting computed gotos or "labels-as-values", such as gcc).
"""

# This code should stay compatible with Python 2.3, at least while
# some of the buildbots have Python 2.3 as their system Python.

import imp
import os


def find_module(modname):
    """Finds and returns a module in the local dist/checkout.
    """
    modpath = os.path.join(
        os.path.dirname(os.path.dirname(__file__)), "Lib")
    return imp.load_module(modname, *imp.find_module(modname, [modpath]))

def write_contents(f):
    """Write C code contents to the target file object.
    """
    opcode = find_module("opcode")
    targets = ['_unknown_opcode'] * 256
    for opname, op in opcode.opmap.items():
        if opname == "STOP_CODE":
            # XXX opcode not implemented
            continue
        targets[op] = "TARGET_%s" % opname
    f.write("static void *opcode_targets[256] = {\n")
    f.write(",\n".join(["\t&&%s" % s for s in targets]))
    f.write("\n};\n")


if __name__ == "__main__":
    import sys
    assert len(sys.argv) < 3, "Too many arguments"
    if len(sys.argv) == 2:
        target = sys.argv[1]
    else:
        target = "Python/opcode_targets.h"
    f = open(target, "w")
    try:
        write_contents(f)
    finally:
        f.close()