[BugFix][TIR] Fix incorrect optimization when lowering floordiv and f…

[guocju]

…loormod

This patch fixes an issue in the LowerIntrin pass where incorrect optimizations were applied to floordiv and floormod operations.

The root cause is that the pass attempts to find an equivalent representation for floordiv(a, b) by calculating the expression (op->b - 1) - a_min. This expression, when subjected to constant folding, can potentially overflow the range of int32 or int16. When this overflow occurs, the transformation becomes invalid and no longer equivalent to the original operation.

To fix this, we enhanced the condition under which the transformation is applied. The new condition ensures that the transformation is only performed when (b_max - a_min) is less than INT_MAX + 2. If this condition is not met, the transformation is skipped and the common lowering steps are followed to ensure correctness.

A regression test has been added to cover this case.

Fixes #18684

[gemini-code-assist]

Summary of Changes

Hello @guocju, 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 bug within the LowerIntrin pass that caused erroneous optimizations for floordiv and floormod operations. The root cause was an integer overflow during constant folding of an intermediate expression, leading to invalid transformations. The solution involves refining the conditions under which these optimizations are applied, specifically by adding a check to prevent the transformation when the bounds could lead to overflow. This ensures that the lowering steps are correctly followed, and a new test case has been included to validate the fix.

Highlights

• Bug Fix for Floordiv/Floormod Optimization: This pull request resolves an issue in the LowerIntrin pass where incorrect optimizations were applied to floordiv and floormod operations due to potential integer overflow during constant folding.
• Improved Optimization Condition: The fix introduces an enhanced condition for applying the optimization, ensuring it only proceeds when (b_max - a_min) is less than INT_MAX + 2, thereby preventing overflow and maintaining correctness.
• Regression Test Added: A new regression test has been added to specifically cover the scenario that previously led to the incorrect optimization, ensuring the fix is robust.

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[gemini-code-assist]

Code Review

This pull request addresses a critical bug in the LowerIntrin pass where an optimization for floordiv and floormod could cause an integer overflow. The fix correctly introduces a new condition to safeguard against this overflow by checking the bounds of the operands before applying the transformation. The logic is sound and has been applied to both floordiv and floormod operations. Additionally, new regression tests have been added to cover the fixed cases. My review includes a couple of minor suggestions to improve code readability by extracting a repeated complex expression into a local variable.

[tqchen]

Thanks @guocju please cross check genimi review, and add explicit comment for context, the ci issue should be fixed by #18695 thanks a lot for the contribution

[guocju]

Yes, I’ve updated the check to explicitly extract max_value_of_dtype as a variable, and added a comment documenting why the condition is <= max_value_of_dtype + 1.

[tqchen]

/gemini review

[gemini-code-assist]

Code Review

This pull request addresses a bug in the LowerIntrin pass where an optimization for floordiv and floormod could lead to integer overflows and incorrect results. The fix introduces a more robust safety check that considers the bounds of both the numerator and denominator, ensuring the optimization is only applied when safe. This change is correctly applied to both floordiv and floormod lowering logic. Additionally, new regression tests are added to verify the correctness of the fallback path when the optimization is skipped. The fix appears correct and effectively resolves the issue. I have one suggestion to improve code maintainability by refactoring some duplicated logic.

[tqchen]

Thanks @guocju for the bugfix!

[tqchen]

#18699 contains some followups to further clean things up and robustify the scenario. thanks @guocju for bringing up the initial case