[BugFix][Relax] Add structural_equal verification to subroutine cache lookup

[3em0]

Summary

• SubroutineMixin._get_subroutine() used structural_hash as the sole cache key without structural_equal verification. If two different arg_sinfo values produced the same 64-bit hash (collision), the cache would return a previously compiled function with mismatched parameter shapes, leading to silently incorrect compiled output.
• Changed the cache to store a list of (arg_sinfo, result) pairs per hash bucket and verify with structural_equal on lookup, consistent with the pattern in block_builder.cc.
• Added a security advisory document and regression test.

Root Cause

The subroutine cache (cls._gvar) was keyed by (structural_hash(arg_sinfo), is_dataflow). A hash match was treated as proof of structural equality, skipping the necessary structural_equal check. This is a hash-only lookup anti-pattern — hash determines the bucket, but equality must confirm the match.

For comparison, block_builder.cc correctly uses StructuralHash + StructuralEqual together as the hash and equality functions for std::unordered_map.

Test plan

• Existing test test_linear passes (no regression)
• New test test_different_shapes_produce_distinct_subroutines passes — verifies that the same Module class with different input shapes generates distinct subroutines

🤖 Generated with Claude Code

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses a critical correctness defect in the Relax frontend's subroutine caching mechanism. Previously, the cache relied solely on structural_hash for lookups, which could lead to silent errors if hash collisions occurred between different input shapes. The changes implement a robust cache lookup strategy by incorporating structural_equal verification, ensuring that cached subroutines are always structurally identical to the requested ones. This significantly improves the reliability of compiled output and includes a new security advisory and a regression test to validate the fix.

Highlights

• Cache Correction: Modified the SubroutineMixin._get_subroutine() cache to use structural_equal verification alongside structural_hash to prevent incorrect cache hits due to hash collisions.
• Cache Structure Update: Updated the cache to store a list of (arg_sinfo, result) pairs per hash bucket, ensuring proper equality checks.
• Security Advisory: Added a new security advisory document detailing the hash collision vulnerability and its implications.
• Regression Test: Introduced a new test case to confirm that distinct subroutines are generated for different input shapes, preventing the reintroduction of the bug.

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[gemini-code-assist]

Code Review

This pull request addresses a security advisory concerning a subroutine cache hash collision in SubroutineMixin._get_subroutine(). The fix modifies the caching mechanism to use structural_equal verification after an initial hash lookup, preventing incorrect compiled output when different input shapes produce the same hash. A new test case has been added to validate this behavior. The review comments suggest minor readability improvements, such as using a temporary variable for a tuple and refactoring a loop into a list comprehension.