Align load-fast borrow barriers with CPython by youknowone · Pull Request #7930 · RustPython/RustPython

youknowone

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Reviewing files that changed from the base of the PR and between 01d82fa and 388a620.

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crates/codegen/src/compile.rs

📝 Walkthrough

Walkthrough

Refines LOAD_FAST_BORROW optimization to match CPython empty-label reuse by adding a Block passthrough flag, CFG fallthrough/barrier analysis, stack/AST helpers, and compile-time preservation for folded ifexp, try/finally, loop tails, and with-cleanup; includes many tests.

Changes

Load-Fast Label Reuse Passthrough Refinement

Layer / File(s)	Summary
IR block field and folded-load escape detection `crates/codegen/src/ir.rs`	Block struct gains `load_fast_label_reuse_passthrough`. Adds `folded_load_escapes_block` and integrates per-instruction folded-escape checks into `optimize_load_fast_borrow`.
Fallthrough predecessor matching & empty-try barrier `crates/codegen/src/ir.rs`	Adds `has_fallthrough_predecessor_matching` and rewrites `has_cpython_empty_try_end_barrier` to consider finally-cleanup fallthrough and refined predicates; tightens `canonical_target` traversal.
Passthrough redirection predicates `crates/codegen/src/ir.rs`	`redirect_load_fast_passthrough_targets` extended with warm-fallthrough, assertion NOP/failure, try-else/orelse entry, and labeled-successor predicates; `passthrough_target` rewired to use these and the new flag.
Compiler helpers and AST/stack inspection `crates/codegen/src/compile.rs`	Adds `current_block_stack_depth`, `mark_load_fast_label_reuse_passthrough_block`, expands `statements_contain_with` for nested forms, and adds `statements_end_with_try_finally_finalbody_with`.
Constant-folded ifexp assignment handling `crates/codegen/src/compile.rs`	When assignment RHS is constant-foldable `ifexp`, detects empty end blocks using stack inspection and inserts load-fast barriers plus continuation blocks to preserve successor load strength.
Try-finally exit preservation and marking `crates/codegen/src/compile.rs`	Extends preservation checks to consider preserved empty labels for try-finally exits and switches marking to `mark_load_fast_label_reuse_passthrough_block` at relevant points.
While-loop / try-orelse / with tail preservation `crates/codegen/src/compile.rs`	Adds flags for preserving successor-strong loads after nested try/orelse and with/orelse ends, preserves empty test-prefix boundaries, disables load-fast borrowing when needed, and marks passthrough targets on normal/with-cleanup exits.
Flag rename, OR-merge update, tests, and cspell `crates/codegen/src/compile.rs`, `crates/codegen/src/ir.rs`, `.cspell.json`	Renames `load_fast_passthrough` → `load_fast_label_reuse_passthrough`, updates OR-merge logic, adds many unit tests validating LoadFast vs LoadFastBorrow across control-flow shapes, and adds `ifexp` to cspell allowlist.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~60 minutes

Possibly related PRs

RustPython/RustPython#7870: Modifies optimize_load_fast_borrow / empty-label fallthrough and CFG traversal behavior in the same LOAD_FAST_BORROW parity area.
RustPython/RustPython#7773: Also changes crates/codegen/src/ir.rs optimize_load_fast* traversal and passthrough/barrier handling affecting borrow vs strong load decisions.

Poem

🐰 I hopped through labels, folds, and tries,
I nudged the stack where mystery lies,
With passthrough flags and careful art,
Strong loads keep strength and play their part,
A tiny rabbit's bytecode prize.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 35.21% 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 clearly summarizes the main objective of the pull request: aligning load-fast borrow barriers with CPython behavior. The changes across three files (compile.rs, ir.rs, and .cspell.json) all work toward this central goal of improving CPython parity in the bytecode compiler.
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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