(feat): add array support to equality analysis

[eytan-starkware]

Summary

Extends the equality analysis to track array operations (array_new, array_append, array_pop_front, array_snapshot_pop_front, array_snapshot_pop_back) by reusing the existing struct hashcons infrastructure. Arrays are now represented as (TypeId, Vec<VariableId>) mappings, and pop operations act as destructures that can recover original elements and remaining arrays.

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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?

The equality analysis previously only tracked snapshot/desnap, box/unbox, and struct construct relationships. Array operations were not analyzed, meaning the system couldn't detect when array elements could be recovered from pop operations or when two arrays with equivalent elements should be considered equivalent. This limited optimization opportunities for array-heavy code.

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What was the behavior or documentation before?

The equality analysis ignored array operations entirely. Calls to array_new, array_append, and array pop functions were treated as opaque operations with no tracked relationships between inputs and outputs.

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What is the behavior or documentation after?

The analysis now:

• Tracks array_new() as creating an empty array construct
• Tracks array_append(arr, elem) as extending the array's element list
• Handles array_pop_front by recovering the first element (as boxed) and remaining array
• Handles array_snapshot_pop_front and array_snapshot_pop_back by properly managing snapshot/box relationships
• Detects when two arrays with identical element sequences are equivalent

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Related issue or discussion (if any)

None specified.

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Additional context

The implementation cleverly reuses the existing struct hashcons infrastructure since both structs and arrays map (TypeId, Vec<VariableId>). The analysis correctly handles the complex snapshot and boxing relationships that occur during array pop operations, ensuring that unboxing popped elements yields the correct snapshot types rather than falsely equating snapshot and non-snapshot values.

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Note

Medium Risk
Extends core dataflow/equality analysis logic to model array construction and pop semantics, which can affect downstream optimizations and correctness if relationships are inferred incorrectly. Adds new handling paths for extern calls/matches (including snapshot/boxed elements), increasing surface area for edge cases.

Overview
Equality analysis now models arrays as structured values. It reuses the existing struct hashcons to track array_new/array_append chains as (TypeId, elements...) -> array constructs, allowing equivalence to propagate between arrays built from equivalent elements.

Extern match handling is extended for array pop APIs. On array_pop_front/array_pop_front_consume and snapshot variants (array_snapshot_pop_front/array_snapshot_pop_back), the analysis treats pops as destructures: it relates the popped element (boxed, or boxed-snapshot) to the corresponding element and records the remaining array construct.

Tests are updated and expanded. The equality analysis test harness now runs selected optimization phases (notably inlining) so ArrayTrait calls lower to the relevant externs, and new golden tests cover array new/append, pop-front merge behavior, snapshot pop-front/pop-back, and the None-arm empty-array case.

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[reviewable-StarkWare]

This change is 

[eytan-starkware]

• (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 #9803
• (feat): add array support to equality analysis #9802 👈 (View in Graphite)
• main

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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[cursor]

Unchecked index access may panic on empty var_ids

Medium Severity

The else branch for array_snapshot_pop_front/array_snapshot_pop_back unconditionally accesses arm.var_ids[0] without checking that the vector is non-empty. The parallel code for array_pop_front/array_pop_front_consume correctly guards this with if arm.var_ids.len() == 1 before indexing. If arm.var_ids is ever empty (e.g., due to a future "consume" variant of snapshot pop or unexpected IR), this will panic at runtime.

Additional Locations (1)

• crates/cairo-lang-lowering/src/analysis/equality_analysis.rs#L396-L411

 

[eytan-starkware]

@eytan-starkware resolved 1 discussion.
Reviewable status: 0 of 3 files reviewed, all discussions resolved (waiting on orizi and TomerStarkware).