Match CPython CFG normalization and jump cleanup in late codegen passes #25

Summary

A large share of the remaining bytecode drift comes from control-flow graph normalization, jump threading, and
cleanup-tail layout differences. This affects multiple files, including Lib/test/test_buffer.py, Lib/test/ test_dis.py, Lib/test/test_math.py, Lib/test/test_tarfile.py, and Lib/dataclasses.py.

Evidence

Representative mismatches include:

• POP_JUMP_IF_TRUE vs POP_JUMP_IF_FALSE polarity flips
• FOR_ITER target drift
• JUMP_FORWARD vs duplicated inline tail blocks
• CPython NOP anchors vs RustPython LOAD_CONST None
• shared exit blocks being duplicated or threaded differently from CPython

Top affected tracked files include:

• Lib/test/test_buffer.py: 5131 diffs
• Lib/test/test_dis.py: 3194 diffs
• Lib/test/test_math.py: 2368 diffs
• Lib/test/test_tarfile.py: 2149 diffs
• Lib/dataclasses.py: 1577 diffs

Expected direction

Late CFG passes should be aligned with CPython’s codegen.c and flowgraph.c behavior, especially for:

• conditional-chain normalization
• jump threading
• empty-block cleanup
• exit duplication
• synthetic NOP / no-location anchor handling

Likely implementation areas

• crates/codegen/src/ir.rs
• crates/codegen/src/compile.rs

Done when

Representative functions such as jumpy, dataclasses._asdict_inner, dataclasses._replace, tarfile
context-manager tests, and the larger test_buffer.py helpers converge to CPython bytecode structure instead
of only matching outcome.