[dynamo 3.11] implement 3.11 exceptiontable (#96511)
Summary of changes:
- Add CPython exceptiontable parsing/assembling functions in torch/_dynamo/bytecode_transformation.py, based on https://github.com/python/cpython/blob/3.11/Objects/exception_handling_notes.txt.
- Add optional `exn_tab_entry` field to dynamo `Instruction`s in torch/_dynamo/bytecode_transformation.py in order to virtualize exception table entries (start, end, target instructions).
- Add checks guarding against duplicate instructions in dynamo, so that jump/exceptiontable targets are unambiguous. See `get_indexof` in torch/_dynamo/bytecode_analysis.py. Ensure that bytecode generation throughout dynamo does not generate duplicate instructions.
- Allow dynamo bytecode generation logic to generate nested exception table entries for developer convenience. CPython expects entries to not overlap, so we flatten nested entries during assembly in torch/_dynamo/bytecode_transformation.py:compute_exception_table.
- Simulate the block stack in torch/_dynamo/symbolic_convert.py. CPython removed the block stack in 3.11, but dynamo needs it in order to keep track of active contexts. So we simulate the block stack as before by looking at exceptiontable entries in order to determine the current blocks.
- Update context codegen in torch/_dynamo/resume_execution.py. The `SETUP_FINALLY` bytecode, which conveniently had a jump target to the finally block, was removed in 3.11, so we need to keep track of the jump target of the finally block using exceptiontables. Generating resume functions is more difficult since the original exceptiontable entries pointing to old cleanup code need to be modified to point to new cleanup code.
- Fix a push_null bug in torch/_dynamo/variables/functions.py introduced by https://github.com/pytorch/pytorch/pull/98699
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96511
Approved by: https://github.com/jansel, https://github.com/yanboliang, https://github.com/albanD
