Python has a peephole optimizer which does not remove dead code that it
just created.
Victor
Le 18 juin 2016 23:14, "Obiesie ike-nwosu via Python-Dev" <
python-dev@python.org> a écrit :
> Hi,
>
> Could some one give a hand with explaining to me why we have a
> JUMP_ABSOLUTE followed by a JUMP_FORWARD op code when this function is
> disassembled.
>
> >>> def f1():
> ... a, b = 10, 11
> ... if a >= 10:
> ... if b >= 11:
> ... print("hello world")
> …
>
> The disassembled function is shown below.
> >>> dis(f1)
> 2 0 LOAD_CONST 4 ((10, 11))
> 3 UNPACK_SEQUENCE 2
> 6 STORE_FAST 0 (a)
> 9 STORE_FAST 1 (b)
>
> 3 12 LOAD_FAST 0 (a)
> 15 LOAD_CONST 1 (10)
> 18 COMPARE_OP 5 (>=)
> 21 POP_JUMP_IF_FALSE 47
>
> 4 24 LOAD_FAST 1 (b)
> 27 LOAD_CONST 2 (11)
> 30 COMPARE_OP 5 (>=)
> 33 POP_JUMP_IF_FALSE 47
>
> 5 36 LOAD_CONST 3 ('hello world')
> 39 PRINT_ITEM
> 40 PRINT_NEWLINE
> 41 JUMP_ABSOLUTE 47
> 44 JUMP_FORWARD 0 (to 47)
> >> 47 LOAD_CONST 0 (None)
> 50 RETURN_VALUE
>
> From my understanding, once JUMP_ABSOLUTE is executed, then JUMP_FORWARD
> is never gotten to so must be dead code so why is it being generated?
> Furthermore why is JUMP_ABSOLUTE rather than JUMP_FORWARD used in this
> particular case of nested if statements? I have tried other types of nested
> if statements and it has always been JUMP_FORWARD that
> is generated.
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