[New-bugs-announce] [issue45542] Using multiple comparison operators can cause performance issues

[Maja]

New submission from Maja <mavioko7 at gmail.com>:

For example:

    def f(x):
        return 1 < x < 3

will be slower than:

    def f(x):
        return 1 < x and x < 3

The first function will generate following bytecode:

              0 LOAD_CONST               1 (1)
              2 LOAD_FAST                0 (x)
              4 DUP_TOP
              6 ROT_THREE
              8 COMPARE_OP               0 (<)
             10 JUMP_IF_FALSE_OR_POP    18
             12 LOAD_CONST               2 (3)
             14 COMPARE_OP               0 (<)
             16 RETURN_VALUE
        >>   18 ROT_TWO
             20 POP_TOP
             22 RETURN_VALUE

Performs unnecessary stack operations: duplicates x, rotates 3 items for no reason, then re-rotates 2 items and pops a value. This is fine if the value in the middle was more complex and needed to be duplicated rather than recalculated, which would be definitely slower, but for simpler values like names or constants, it's actually bad. The bytecode for the first function should look the same as second one which is:

              0 LOAD_CONST               1 (1)
              2 LOAD_FAST                0 (x)
              4 COMPARE_OP               0 (<)
              6 JUMP_IF_TRUE_OR_POP     14
              8 LOAD_FAST                0 (x)
             10 LOAD_CONST               2 (3)
             12 COMPARE_OP               0 (<)
        >>   14 RETURN_VALUE

----------
components: Interpreter Core
messages: 404508
nosy: akuvfx
priority: normal
severity: normal
status: open
title: Using multiple comparison operators can cause performance issues
type: performance
versions: Python 3.11

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45542>
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