JDK-8371122 : C2 Allocation Elimination: handle some mismatched accesses to arrays
  • Type: Enhancement
  • Component: hotspot
  • Sub-Component: compiler
  • Affected Version: 26
  • Priority: P4
  • Status: Open
  • Resolution: Unresolved
  • Submitted: 2025-11-03
  • Updated: 2025-11-03
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tbdUnresolved
Related Reports
Relates :  
JDK-8370936 - C2 MergeStores: move process_for_merge_stores_igvn after allocation elimination
Relates :  
JDK-8370405 - C2: mismatched store from MergeStores wrongly scalarized in allocation elimination
Description
Currently, we don't handle mismatched stores. But for arrays, it could be useful to also eliminate some allocations of arrays that have mismatched stores. For example via Unsafe, or MemorySegment.

Note: this was a suggestion by [~mhaessig], as an alternative solution for JDK-8370405:
https://github.com/openjdk/jdk/pull/27997#pullrequestreview-3392451330
https://github.com/openjdk/jdk/pull/27997#issuecomment-3460642090

We have to be very careful and write a lot of tests for this.

This could also be an alternative for JDK-8370936.

Basic idea:

Imagine we have an int-array, where we currently only allow StoreI that are exactly aligned to an int-element.
We could now use Unsafe or MemorySegment to do StoreL over two int-elements. That produces a mismatched StoreL that prevents allocation elimination.

Harder cases, maybe not worth it: where array elements are only partially overlapped by a store. E.g. StoreB to an int-element.