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The Wire · Showcase

JAX FIXES CRITICAL SCAN TRANSPOSE BUG THAT BROKE REF ACCUMULATORS

By RepoJournal · Filed · About Google

A foundational bug in JAX's scan transpose logic is fixed after the code assumed accumulators would follow a neat type-sorted pattern they never actually did.

The scan transpose implementation was making a dangerous assumption about how ref accumulators would be arranged in memory [1]. The code expected them to be sorted and contiguous by type, but real-world usage scattered them arbitrarily across the accumulator space. This created silent correctness issues that the new canonicalization logic now handles properly, complete with systematic test coverage to prevent regression [1]. In parallel, JAX is restoring cumulative sum performance on GPU by re-enabling CHLO lowering after a recent disable [2]. XLA dependencies were also bumped to the latest revision to align with upstream improvements [3].

Action items

References

  1. [1] [vjp3][mutable-arrays] fix scan transpose bugs with refs ↗ google/jax
  2. [2] [JAX] Enable CHLO lowering for cumulative sums on GPU again. ↗ google/jax
  3. [3] Update XLA dependency to use revision http://github.com/openxla/xla/commit/42d2e6ac6c715184ab7a64855f5162f03c561b29 google/jax

FAQ

What changed in Google on July 5, 2026?
A foundational bug in JAX's scan transpose logic is fixed after the code assumed accumulators would follow a neat type-sorted pattern they never actually did.
What should Google teams do about it?
Pull the latest JAX commit with ref accumulator fix [ref:1] if you use scan with mutable refs • Test cumulative sum operations on GPU to verify performance is back to baseline
Which Google repositories shipped on July 5, 2026?
google/jax

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