[llvm-dev] RFC: Replace usage of Alias Set Tracker with MemorySSA in LICM (original) (raw)
Alina Sbirlea via llvm-dev llvm-dev at lists.llvm.org
Mon Nov 20 13:01:51 PST 2017
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Hi,
I wanted to give an update on this topic, as a heads up for what I'd like the plan to be going forward.
I would like to split the current patch doing the replacement of Alias Set Tracker (AST) with MemorySSA (D35741 <https://reviews.llvm.org/D35741>) into more manageable pieces. (Note, while the current patch is not the prettiest, it covers all the final functionality. I will rebase this patch on the intermediate patches planned below.)
A few reasons for the split:
- Preliminary performance results on pathological test cases show that building MemorySSA as a dependency without using it in LICM (i.e., build MemorySSA and also build AST!) leads to prohibitively large compile times, making this a non-option. No surprise here. Hence the flag to enable MemorySSA will be: -> false (do not build MemorySSA, build Alias Set Tracker) -> true (build MemorySSA and use it, do not build Alias Set Tracker)
- Doing sink/hoist in LICM with MemorySSA is straight-forward, while promotion is difficult to tackle.
- Having loop passes preserve MemorySSA is a necessary long term step.
- Easier to review, of course.
Rough next steps:
- Adding MemorySSA as a dependency, to be built under a flag in both new and legacy pass managers.
- With flag enabled, update MemorySSA in LICM, use it to do sink and hoist, do not do promotion.
- With flag enabled, do promotion, add MemorySSA sets. This piece can later be replaced, with a different promotion mechanism.
- Update MemorySSA in LoopUnswitch and LoopRotate
- Update MemorySSA in IndVarsimplify, LoopIdiomRecognize, LoopDeletion
Patches coming up soon, and feedback welcome.
Thanks, Alina
On Tue, May 30, 2017 at 10:07 AM, Alina Sbirlea <alina.sbirlea at gmail.com> wrote:
Hi,
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. Motivation: 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: 1. As a first step, I'm looking at having MemorySSA as a dependency only for LICM, in the old pass manager. 2. 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. 3. 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. Best, Alina -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20171120/636f73d4/attachment.html>
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