[llvm] [ADT] ImutAVLFactory: avoid overhead of vector for freelist (PR #196534)

[Christoph Cullmann via llvm-commits]

christoph-cullmann wrote:

Ok, with that code change the next I 'used' is just no longer there in the type: @Xazax-hun given you optimized that, do you think the free list optimization is still possible with the current variant?

https://github.com/llvm/llvm-project/pull/196534