[remat] Make prevent_cse's tuple form account for consts

[gspschmid]

@mattjj

[gemini-code-assist]

Summary of Changes

Hello @gspschmid, 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 refines the jax.remat checkpointing mechanism by ensuring that the prevent_cse option, when specified as a tuple, correctly applies to constants within the traced function's JAX expression program. This prevents unintended common subexpression elimination for constants, improving the robustness and predictability of remat's behavior. A corresponding test case has been updated to validate this change.

Highlights

• CSE Prevention for Constants: The prevent_cse mechanism in jax.remat now correctly accounts for constants when prevent_cse is provided as a tuple. Previously, the tuple form only considered arguments and keyword arguments, potentially leading to incorrect common subexpression elimination (CSE) behavior for constants.
• Test Case Enhancement: An existing test case (test_remat_partial_cse_prevention) was updated to include a constant in the jaxpr of the layer function. This ensures that the new logic for handling constants in prevent_cse tuples is properly validated.

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[gemini-code-assist]

Code Review

This pull request correctly adjusts prevent_cse in jax.remat to account for constants present in the traced function. Previously, when prevent_cse was a tuple, it only considered the function's arguments, leading to a mismatch if constants were also present. The fix prepends False for each constant, ensuring the prevent_cse tuple aligns with all inputs to the underlying primitive. The accompanying test has been updated to include a constant, properly validating this fix. The changes are correct and well-tested.

[mattjj]

Thanks!