Control Flow Graph (CFG)

To analyze or optimize existing code, bytecode provides a ControlFlowGraph class which is a control flow graph (CFG).

The control flow graph is used to perform the stack depth analysis when converting to code. Because it is better at identifying dead code than CPython it can lead to reduced stack size.

Example

Dump the control flow graph of the conditional jump example:

from bytecode import Label, Instr, Bytecode, ControlFlowGraph, dump_bytecode

label_else = Label()
label_print = Label()
bytecode = Bytecode([Instr('LOAD_NAME', 'print'),
                     Instr('LOAD_NAME', 'test'),
                     Instr('POP_JUMP_IF_FALSE', label_else),
                         Instr('LOAD_CONST', 'yes'),
                         Instr('JUMP_FORWARD', label_print),
                     label_else,
                         Instr('LOAD_CONST', 'no'),
                     label_print,
                         Instr('CALL_FUNCTION', 1),
                     Instr('LOAD_CONST', None),
                     Instr('RETURN_VALUE')])

blocks = ControlFlowGraph.from_bytecode(bytecode)
dump_bytecode(blocks)

Output:

block1:
    LOAD_NAME 'print'
    LOAD_NAME 'test'
    POP_JUMP_IF_FALSE <block3>
    -> block2

block2:
    LOAD_CONST 'yes'
    JUMP_FORWARD <block4>

block3:
    LOAD_CONST 'no'
    -> block4

block4:
    CALL_FUNCTION 1
    LOAD_CONST None
    RETURN_VALUE

We get 4 blocks:

  • block #1 is the start block and ends with POP_JUMP_IF_FALSE conditional jump and is followed by the block #2
  • block #2 ends with JUMP_FORWARD uncondition jump
  • block #3 does not contain jump and is followed by the block #4
  • block #4 is the final block

The start block is always the first block.

Analyze the control flow graph

The bytecode module provides two ways to iterate on blocks:

  • iterate on the basic block as a sequential list
  • browse the graph by following jumps and links to next blocks

Iterate on basic blocks

Iterating on basic blocks is a simple as this loop:

for block in blocks:
    ...

Example of a display_blocks() function:

from bytecode import UNSET, Label, Instr, Bytecode, BasicBlock, ControlFlowGraph

def display_blocks(blocks):
    for block in blocks:
        print("Block #%s" % (1 + blocks.get_block_index(block)))
        for instr in block:
            if isinstance(instr.arg, BasicBlock):
                arg = "<block #%s>" % (1 + blocks.get_block_index(instr.arg))
            elif instr.arg is not UNSET:
                arg = repr(instr.arg)
            else:
                arg = ''
            print("    %s %s" % (instr.name, arg))

        if block.next_block is not None:
            print("    => <block #%s>"
                  % (1 + blocks.get_block_index(block.next_block)))

        print()

label_else = Label()
label_print = Label()
bytecode = Bytecode([Instr('LOAD_NAME', 'print'),
                     Instr('LOAD_NAME', 'test'),
                     Instr('POP_JUMP_IF_FALSE', label_else),
                         Instr('LOAD_CONST', 'yes'),
                         Instr('JUMP_FORWARD', label_print),
                     label_else,
                         Instr('LOAD_CONST', 'no'),
                     label_print,
                         Instr('CALL_FUNCTION', 1),
                     Instr('LOAD_CONST', None),
                     Instr('RETURN_VALUE')])

blocks = ControlFlowGraph.from_bytecode(bytecode)
display_blocks(blocks)

Output:

Block #1
    LOAD_NAME 'print'
    LOAD_NAME 'test'
    POP_JUMP_IF_FALSE <block #3>
    => <block #2>

Block #2
    LOAD_CONST 'yes'
    JUMP_FORWARD <block #4>

Block #3
    LOAD_CONST 'no'
    => <block #4>

Block #4
    CALL_FUNCTION 1
    LOAD_CONST None
    RETURN_VALUE

Note

SetLineno is not handled in the example to keep it simple.

Browse the graph

Recursive function is a simple solution to browse the control flow graph.

Example to a recursive display_block() function:

from bytecode import UNSET, Label, Instr, Bytecode, BasicBlock, ControlFlowGraph

def display_block(blocks, block, seen=None):
    # avoid loop: remember which blocks were already seen
    if seen is None:
        seen = set()
    if id(block) in seen:
        return
    seen.add(id(block))

    # display instructions of the block
    print("Block #%s" % (1 + blocks.get_block_index(block)))
    for instr in block:
        if isinstance(instr.arg, BasicBlock):
            arg = "<block #%s>" % (1 + blocks.get_block_index(instr.arg))
        elif instr.arg is not UNSET:
            arg = repr(instr.arg)
        else:
            arg = ''
        print("    %s %s" % (instr.name, arg))

    # is the block followed directly by another block?
    if block.next_block is not None:
        print("    => <block #%s>"
              % (1 + blocks.get_block_index(block.next_block)))

    print()

    # display the next block
    if block.next_block is not None:
        display_block(blocks, block.next_block, seen)

    # display the block linked by jump (if any)
    target_block = block.get_jump()
    if target_block is not None:
        display_block(blocks, target_block, seen)

label_else = Label()
label_print = Label()
bytecode = Bytecode([Instr('LOAD_NAME', 'print'),
                     Instr('LOAD_NAME', 'test'),
                     Instr('POP_JUMP_IF_FALSE', label_else),
                         Instr('LOAD_CONST', 'yes'),
                         Instr('JUMP_FORWARD', label_print),
                     label_else,
                         Instr('LOAD_CONST', 'no'),
                     label_print,
                         Instr('CALL_FUNCTION', 1),
                     Instr('LOAD_CONST', None),
                     Instr('RETURN_VALUE')])

blocks = ControlFlowGraph.from_bytecode(bytecode)
display_block(blocks, blocks[0])

Output:

Block #1
    LOAD_NAME 'print'
    LOAD_NAME 'test'
    POP_JUMP_IF_FALSE <block #3>
    => <block #2>

Block #2
    LOAD_CONST 'yes'
    JUMP_FORWARD <block #4>

Block #4
    CALL_FUNCTION 1
    LOAD_CONST None
    RETURN_VALUE

Block #3
    LOAD_CONST 'no'
    => <block #4>

Block numbers are no displayed in the sequential order: block #4 is displayed before block #3.

Note

Dead code (unreachable blocks) is not displayed by display_block.