[PATCH] D110057: [LoopFlatten] Move it to a LoopPassManager

[Sjoerd Meijer via Phabricator via llvm-commits]

SjoerdMeijer added a comment.

In D110057#3010889 <https://reviews.llvm.org/D110057#3010889>, @asbirlea wrote:

> In D109958 <https://reviews.llvm.org/D109958> I missed the fact that LoopFlatten does not preserve any analyses. This is, I assume, the reason it was not added alongside other Loop passes before and probably also why I'm seeing a crash when adding it as part of LPM2.
> The pass should not misbehave on its own in a LoopPassManager because it's not using the Updater to readd loops for reprocessing, otherwise it would be incorrect alone too.
> So either LoopFlatten needs to be remain in it's own LPM or taught to preserve analyses (LoopInfo, DominatorTree, etc)

I think there are 2 separate things going on here:

- A recent patch is causing a problem (as reported in D109958 <https://reviews.llvm.org/D109958>), which I am investigating and fixing. I suspect this is the same crash that you're seeing.
- In addition, we have the question of preserved analysis. LoopFlatten removes an inner-loop, so it sounds right to me that LoopFlatten doesn't preserve LoopInfo, DomTree, or readd loops, etc. I was assuming that when a pass doesn't preserve an analysis, the pass manager reruns it when a subsequent pass requires it. But please advise if LoopFlatten is fine where it is here in this patch, or if it needs moving to its own LPM.


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https://reviews.llvm.org/D110057