Align bytecode CFG cleanup with CPython by youknowone · Pull Request #7781 · RustPython/RustPython

youknowone

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📝 Walkthrough

Walkthrough

One cohesive PR: add a percent-format AST preprocessor; restrict constant folding and pre-evaluate boolean prefixes; refactor try/with/f-string emission; add layout-aware IR NOP/layout helpers, short-circuit-aware jump threading, and a fast-scan path; unify symboltable scope handling; add tests and dictionary entries.

Changes

Codegen Control-Flow and IR Optimization Refactoring

Layer / File(s)	Summary
Preprocess: percent-format -> f-string `crates/codegen/src/preprocess.rs`, `crates/codegen/src/lib.rs`	Add `preprocess_mod` and `AstPreprocessor` to rewrite simple `%`-format expressions into `Expr::FString` with parsed interpolations and basic format-specs; register `mod preprocess;`.
Symbol Table Scope Unification `crates/codegen/src/symboltable.rs`	Centralize scope-type determination via `is_function_like_scope`; pass explicit `CompilerScope` into `enter_scope_with_parameters`; track unevaluated annotations and extend named-expression scoping to match CPython.
Compile entry & constant folding `crates/codegen/src/compile.rs`	Wire `preprocess_mod` into `compile_top`; restrict constant folding to expressions reported constant; ensure boolean-op prefix-values are compiled before short-circuit threading; add nested-with tail detection helper.
Try/Except end-path & With cleanup NOPs `crates/codegen/src/compile.rs`	Refactor try normal-exit to route through a `cleanup_block` and directly compile `orelse`; compute `preserve_outer_cleanup_target_nop` from nested-with tail preservation and terminal suppress-exit predecessor detection to decide NOP retention.
F-string literal no-location tracking `crates/codegen/src/compile.rs`	Thread `pending_literal_no_location` through f-string buffering/join paths and call `set_no_location()` when emitting marked pending literal fragments.
IR: Finalize & Peephole `crates/codegen/src/ir.rs`	Replace exception-specific materialization with `materialize_empty_conditional_exit_targets`; convert peephole optimizer to indexed iteration and preserve pure self-loop block-start NOP anchors; update debug tracing.
IR: Layout-aware NOP Removal & Helpers `crates/codegen/src/ir.rs`	Compute layout predecessors and add predicates (`keep_target_start_no_location_nop`, `layout_predecessor_ends_with_pop_iter`, `is_async_loop_cleanup_block`, `block_has_simple_scope_exit`, `leading_bool_guard_has_scope_exit_successor`) to drive NOP removal/retention and named-cleanup deopt logic.
IR: Jump Threading Short-Circuit Rules `crates/codegen/src/ir.rs`	Add `short_circuit_stub_conditional`, `opposite_short_circuit_target`, and `same_short_circuit_target` and use them in `jump_threading_impl` to retarget conditional jumps for CPython-style boolean patterns; unit tests added.
IR: Fast-scan for Many Locals `crates/codegen/src/ir.rs`	Add `fast_scan_many_locals` and route uninitialized-load checks through it when `nlocals > 64`, then clamp back to the smaller path’s supported range.
Tests / Validation `crates/codegen/src/compile.rs`, `crates/codegen/src/ir.rs`	New tests for percent-format preprocess effects, fast-scan parameter initialization behavior, boolean short-circuit threading, loop header/backedge NOP ordering, return-in-loop cleanup sequencing, nested-with NOP retention/drop, and for-loop conditional-raise ordering.
Spell dictionary `.cspell.dict/cpython.txt`	Add CPython-specific identifiers used in code/tests.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~60 minutes

Possibly related PRs

RustPython/RustPython#7773: Overlapping edits to codegen/IR and symboltable scope handling.
RustPython/RustPython#7721: Related edits touching compile folding, try/with emission, and NOP/jump handling.
RustPython/RustPython#7541: Related changes around f-string lowering, constant folding, and control-flow emission.

Suggested reviewers

fanninpm

Poem

🐰 I hopped the AST with eager cheer,

turned percent signs to f-strings clear,
folded constants only when told,
threaded jumps and kept anchors bold,
tests clapped softly — off I steer.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 35.90% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Align bytecode CFG cleanup with CPython' is directly related to the main changes across multiple files that refactor and align bytecode control-flow-graph cleanup behavior with CPython standards.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

Create PR with unit tests

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