Ammar Askar <[email protected]> added the comment:
So I think this is a weird edge case with a set of opcode predictions (GET_ITER
-> FOR_ITER -> POP_BLOCK) going outside of a line boundary.
The disassembly of the reproducer above is:
4 0 SETUP_FINALLY 16 (to 18)
5 2 LOAD_CONST 1 (())
4 GET_ITER
>> 6 FOR_ITER 4 (to 12)
8 STORE_FAST 0 (i)
10 JUMP_ABSOLUTE 6
6 >> 12 POP_BLOCK
14 LOAD_CONST 2 (1)
16 RETURN_VALUE
When computed gotos are disabled and there is a tracing function, instructions
0, 2, 4, 14 and 16 hit the `fast_next_opcode` label and have a chance to be
traced as line hits. Note that `maybe_call_line_trace` will only cause a line
hit if the instruction that starts a line (POP_BLOCK in this case) is being
executed.
When computed gotos are enabled, DISPATCH is a no-op and there is a special
case for when tracing is enabled that causes every opcode to go through
`fast_next_opcode`:
https://github.com/python/cpython/blob/c3a651ad2544d7d1be389b63e9a4a58a92a31623/Python/ceval.c#L1054-L1059
When computed gotos are not enabled, there is no similar check for PREDICT (and
might be too costly to add) causing this issue:
https://github.com/python/cpython/blob/c3a651ad2544d7d1be389b63e9a4a58a92a31623/Python/ceval.c#L1131-L1141
----------
title: Windows and Linux execute the same code differently -> ceval traces code
differently based with USE_COMPUTED_GOTOS
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<https://bugs.python.org/issue41670>
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