I wanted to give a heads-up that I’ve been looking into replacing the AliasSetTracker(AST) with MemorySSA in the Loop Invariant Code Motion (LICM) pass.
I would love to get feedback on the best way to incrementally push in this change.
There has been an outstanding issue with using the Alias Set Tracker due to its expensive construction time (quadratic).
We’ve had test cases take forever to compile, where the largest time was spent constructing the AST in LICM.
We’ve also had to degrade the AST results as a temporary fix to resolve this (D23432).
The hope is that by using MemorySSA instead, which has linear construction time, we can avoid this issues and decrease compile times.
Some initial discussion points:
- As a first step, I’m looking at having MemorySSA as a dependency only for LICM, in the old pass manager.
- In the future, MemorySSA may become one of the loop passes that’s readily available for all loops in the new pass manager. This will be a more extensive change and it may require analyzing the overhead of preserving MemorySSA by all loop passes.
- As was pointed out in an earlier thread, MemorySSA could be used to implement AliasSetTracker. I am not looking into that. I’m looking into explicitly using MemorySSA, instead of the AliasSetTracker.
If there are issues with this choice, please raise them here.
As this is still work in progress, the feedback received will help shape how the replacement gets implemented and merged in.