New submission from Saul Shanabrook <s.shanabr...@gmail.com>:
I noticed that in Python 3.10, and also in main, a certain control flow
construct produces some very odd bytecode (showing on main but same on python
3.10 tags):
```
./python.exe -c 'import dis; dis.dis("while not (a < b < c): pass")'
0 RESUME 0
1 2 LOAD_NAME 0 (a)
4 LOAD_NAME 1 (b)
6 SWAP 2
8 COPY 2
10 COMPARE_OP 0 (<)
12 POP_JUMP_IF_FALSE 11 (to 22)
14 LOAD_NAME 2 (c)
16 COMPARE_OP 0 (<)
18 POP_JUMP_IF_TRUE 28 (to 56)
20 JUMP_FORWARD 1 (to 24)
>> 22 POP_TOP
>> 24 LOAD_NAME 0 (a)
26 LOAD_NAME 1 (b)
28 SWAP 2
30 COPY 2
32 COMPARE_OP 0 (<)
34 POP_JUMP_IF_FALSE 23 (to 46)
36 LOAD_NAME 2 (c)
38 COMPARE_OP 0 (<)
40 POP_JUMP_IF_FALSE 12 (to 24)
42 LOAD_CONST 0 (None)
44 RETURN_VALUE
>> 46 POP_TOP
48 EXTENDED_ARG 255
50 EXTENDED_ARG 65535
52 EXTENDED_ARG 16777215
54 JUMP_FORWARD 4294967280 (to 8589934616)
>> 56 LOAD_CONST 0 (None)
58 RETURN_VALUE
```
The last JUMP_FORWARD has a rather larger argument! This was the minimal
example I could find to replicate this.
However, this is an example of some runnable code that also encounters it:
```
a = b = c = 1
while not (a < b < c):
if c == 1:
c = 3
else:
b = 2
print(a, b, c)
```
This actually executes fine, but I notice that when it's executing it does
execute that very large arg, but that the `oparg` to JUMP_FORWARD ends up being
negative! By adding some tracing, I was able to see that the `oparg` variable
in the `TARGET(JUMP_FORWARD)` case is `-32`.
I am not sure if this is a bug or intended behavior. It does seem a bit odd to
have this unnecessarily large argument that ends up turning into a negative
jump! But the behavior seems fine.
At the least, maybe `dis` should be modified so that it properly sees this as a
negative jump and debugs it properly? I am happy to submit a PR to modify `dis`
to handle this case, but I also wanted to flag that maybe it's a bug to being
with.
----------
components: Interpreter Core
messages: 413088
nosy: saulshanabrook
priority: normal
severity: normal
status: open
title: Odd Bytecode Generation in 3.10
versions: Python 3.10, Python 3.11
_______________________________________
Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue46724>
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