[Arith] Gate canonical-simplify LT Case 2 on extra scale == +1

[jinhongyii]

Summary

CanonicalSimplifier::Impl::VisitExpr_(LTNode) Case 2 rewrites

S + xn < 0  ⇔  S/d + xn // d < 0      where d = gcd(scales)

The Case 1 derivation only works when xn ≥ 0. With scale = -1 the equivalence becomes ≤ rather than <, and the rewrite silently strengthens the predicate by dropping the boundary S/d == xn // d.

After CSE/inlining, a comparison such as 2*(tx%4) < 16*warp + (tx%32)//4 (where row and col are independent projections of the same lane id) reaches canonical_simplify with the divided projection on the LHS (scale = -1), and Case 2 folds it to a plain 0 < warp_id — zeroing every thread that should have written val in warp 0. The same path also folds other configurations (e.g. 0 < (tx%32) - 8*warp) all the way to False.

The fix gates Case 2 with extra->args[0]->scale == 1. The original target shape (yn % m with positive scale and lower_factor=1, plus the scale = +1 / lower_factor > 1 generalization) is unchanged; truly-always-true comparisons still fold to True.

Test plan

• New regression test test_simplify_le_negative_scale_extra in tests/python/arith/test_arith_canonical_simplify.py — asserts on simplified PrimExpr, no GPU required; pre-fix fails, post-fix passes. It also pins the buggy scale = -1 shapes to their unsimplified form, confirms the scale = +1 Case 2 path still optimizes, and re-asserts the truly-always-true variant still folds to True.
• Existing test_simplify_le (the original Case 2 target with scale = +1) still passes.
• tests/python/arith/test_arith_canonical_simplify.py — 16 passed.
• Full tests/python/arith/ — 932 passed (1 pre-existing flaky random-seed failure in test_arith_solve_linear_equations.py unrelated to this change, passes on rerun).

[gemini-code-assist]

Code Review

This pull request restricts Case 2 of the LT-with-divisible-coeffs rewrite in the canonical simplifier to only fire when the leftover split term has a positive scale (scale == 1). This prevents incorrect simplifications when the scale is negative, which previously led to silently dropping boundary conditions. A regression test has been added to verify this behavior. The reviewer suggested adding a defensive check against potential signed integer overflow during the multiplication of gcd and lower_factor to avoid undefined behavior.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

[jinhongyii]

cc: @tqchen