Strange disassembly

[Terry Reedy]

On 6/18/2021 8:13 PM, Chris Angelico wrote:
> On Sat, Jun 19, 2021 at 9:50 AM Terry Reedy <tjreedy at udel.edu> wrote:
>>> Why are there two separate bytecode blocks for the "raise Exception"?
>>
>> Because one block must POP_TOP and other must not.
>>
>>> I'd have thought that the double condition would still be evaluated as
>>> one thing, or at least that the jump destinations for both the
>>> early-abort and the main evaluation should be the same.
>>
>> To reuse the exception block with POP_TOP, could jump over it after the
>> 2nd compare.
> 
> Hmm, fair enough I guess. The compiler figured that it'd be faster to
> duplicate the executable code rather than have the jump.

I would not assume that any alternative was considered.

> It made for a
> somewhat confusing disassembly, but I presume it's faster to run.
> 
>> For the simplest and fasted bytecode, write normal logic and let x be
>> reloaded instead of duplicated and rotated.
>>
>>   >>> import dis
>>   >>> def f(x):
>> ...     if x <= 0 or 10 <= x: raise Exception
>> ...
>> ...
>>   >>> dis.dis(f)
>>     2           0 LOAD_FAST                0 (x)
>>                 2 LOAD_CONST               1 (0)
>>                 4 COMPARE_OP               1 (<=)
>>                 6 POP_JUMP_IF_TRUE         8 (to 16)
>>                 8 LOAD_CONST               2 (10)
>>                10 LOAD_FAST                0 (x)
>>                12 COMPARE_OP               1 (<=)
>>                14 POP_JUMP_IF_FALSE       10 (to 20)
>>           >>   16 LOAD_GLOBAL              0 (Exception)
>>                18 RAISE_VARARGS            1
>>           >>   20 LOAD_CONST               0 (None)
>>                22 RETURN_VALUE
>>   >>>
>>
> 
> Makes sense. I'm not sure if this would actually run faster, but I
> can't really justify warping my code around the disassembly :)
> 
> Thanks for the explanation. I guess I just assumed the interpreter
> would prefer a jump to the duplication, but that's a decision it's
> free to take!

-- 
Terry Jan Reedy