(feat): def site analysis based on forward dataflow

[eytan-starkware]

Summary

Adds a DefLocation enum and a DefSiteAnalysis pass to the lowering analysis framework. DefLocation distinguishes between variables defined at block entry (function parameters, goto remapping destinations, match arm bindings) and variables defined as outputs of a statement. DefSiteAnalysis implements the DataflowAnalyzer trait using the existing forward dataflow framework and produces a DefSites result mapping each variable (by arena index) to its DefLocation.

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Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

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Why is this change needed?

Downstream optimization passes and semantic checks need to know where each variable in the lowered IR is first defined. Previously there was no dedicated analysis for this; callers had to reconstruct def-site information ad hoc. A shared, tested analysis avoids duplicated logic and provides a consistent representation.

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What was the behavior or documentation before?

There was no DefLocation type and no DefSiteAnalysis pass. Consumers of the lowered IR had no standard way to query where a variable was defined.

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What is the behavior or documentation after?

Calling DefSiteAnalysis::analyze(lowered) returns a DefSites value (a Vec<DefLocation> indexed by variable arena index). Each entry is either:

• DefLocation::BlockEntry(block) — the variable is a function parameter, a goto remapping destination, or a match arm binding introduced at the entry of block.
• DefLocation::Statement((block, stmt_idx)) — the variable is produced as an output of statement stmt_idx in block.

File-based tests covering linear functions, match arms, and goto remappings are included under src/analysis/test_data/def_site.

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Related issue or discussion (if any)

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Additional context

The DefSiteAnalysis state is accumulated globally (not per-block) because each variable has exactly one definition site in SSA-form lowered IR, so no per-block lattice state is needed. The Info type is () and merge is a no-op.

[reviewable-StarkWare]

This change is 

[eytan-starkware]

• (optimization): Using equality analysis for variable forwarding #9874
• (feat): Added an optimization dump for each phase under a special log target #9873
• (feat): merging analysis tests #9872
• (feat): dominator analysis using forward dataflow #9871
• (feat): def site analysis based on forward dataflow #9870 👈 (View in Graphite)
• (refactor): preserve partial struct construct info during merge with FieldVar placeholders #9804
• refactor: unify hashcons reverse maps and class_info into a single ClassInfo struct #9848
• main

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[cursor]

PR Summary

Low Risk
Adds an isolated new analysis pass and tests without changing existing lowering or optimization behavior; main risk is incorrect def-site attribution affecting future consumers.

Overview
Adds a new DefLocation enum plus a DefSiteAnalysis forward dataflow pass that computes, for each variable arena index, whether it is defined at a BlockEntry (params, match-arm bindings, goto remap destinations) or by a specific Statement output.

Introduces file-based tests and fixtures for linear code, match arms, goto remappings, multi-output statements, and a regression test that ensures the analysis panics if any variable remains UNRESOLVED.

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[chatgpt-codex-connector]

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[orizi]

@orizi reviewed 1 file and all commit messages, and made 2 comments.
Reviewable status: 1 of 4 files reviewed, 4 unresolved discussions (waiting on eytan-starkware and TomerStarkware).

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crates/cairo-lang-lowering/src/analysis/def_site.rs line 25 at r1 (raw file):

}

impl fmt::Display for DefSites {

implement Debug for DefLocation and use the debug string of the derive here.
format!("{:#?}")

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crates/cairo-lang-lowering/src/analysis/def_site.rs line 107 at r1 (raw file):

    ) {
        for &output in stmt.outputs() {
            self.def_sites[output.index()] = (block_id, stmt_idx + 1);

just use the DefLocation to begin with.

Code quote:

        for &output in stmt.outputs() {
            self.def_sites[output.index()] = (block_id, stmt_idx + 1);

[eytan-starkware]

@eytan-starkware made 3 comments.
Reviewable status: 1 of 4 files reviewed, 4 unresolved discussions (waiting on orizi and TomerStarkware).

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crates/cairo-lang-lowering/src/analysis/def_site.rs line 25 at r1 (raw file):

Previously, orizi wrote…

implement Debug for DefLocation and use the debug string of the derive here.
format!("{:#?}")

Done.

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crates/cairo-lang-lowering/src/analysis/def_site.rs line 107 at r1 (raw file):

Previously, orizi wrote…

just use the DefLocation to begin with.

Done.

[orizi]

@orizi reviewed 2 files, made 2 comments, and resolved 1 discussion.
Reviewable status: 2 of 4 files reviewed, 3 unresolved discussions (waiting on eytan-starkware and TomerStarkware).

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crates/cairo-lang-lowering/src/analysis/def_site.rs line 25 at r1 (raw file):

Previously, eytan-starkware wrote…

Done.

i think you can just remove this entire impl and do 1 other suggestion I added instead

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crates/cairo-lang-lowering/src/analysis/def_site_test.rs line 37 at r2 (raw file):

        let lowering_str = formatted_lowered(db, Some(&lowered));
        let result_str = DefSiteAnalysis::analyze(&lowered).to_string();
        (lowering_str, result_str)

Suggestion:

        (
            formatted_lowered(db, Some(&lowered)),
            format!("{:#?}", DefSiteAnalysis::analyze(&lowered).0
        )

[eytan-starkware]

@eytan-starkware made 2 comments.
Reviewable status: 2 of 4 files reviewed, 2 unresolved discussions (waiting on orizi and TomerStarkware).

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crates/cairo-lang-lowering/src/analysis/def_site.rs line 25 at r1 (raw file):

Previously, orizi wrote…

i think you can just remove this entire impl and do 1 other suggestion I added instead

Agreed

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crates/cairo-lang-lowering/src/analysis/def_site_test.rs line 37 at r2 (raw file):

        let lowering_str = formatted_lowered(db, Some(&lowered));
        let result_str = DefSiteAnalysis::analyze(&lowered).to_string();
        (lowering_str, result_str)

Done.

[orizi]

@orizi reviewed 3 files and all commit messages, made 1 comment, and resolved 2 discussions.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on TomerStarkware).

[eytan-starkware]

@eytan-starkware reviewed 1 file and all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on TomerStarkware).