[PATCH] D97401: [basicaa] Recurse through a single phi input

[Philip Reames via Phabricator via llvm-commits]

reames added a comment.

In D97401#2585880 <https://reviews.llvm.org/D97401#2585880>, @nikic wrote:

> I'm rather leery of relaxing this until we have a reliably complexity limit for BasicAA (D96647 <https://reviews.llvm.org/D96647> or similar). From past experience, it's very easy to open up new pathological compile-time cases in BasicAA, because it walks a really thin line to avoid exponential queries in the absence of complexity cutoffs.

I understand your hesitancy here, but let me explain why I think this should go forward regardless.  Specifically, with the most restricted form of the patch as updated.

1. With the updated patch, we specifically only continue analysis when there's a single phi value being recursed on.  Given that, this patch definitely can not create an exponential number of queries.  It can still expose potential exponential that already exist by reaching deeper into the graph, but that's a separate issue.

2. This is a really critical case.  Specifically, it's the case of a pointer induction variable in a loop nest.  It seems really embarrassing to have LLVM miss this.

3. I'm general hesitant to block on progress on fixing an exponential issue.  This isn't the only one we have and it isn't the first time we've hit one.  In general, different contributors will be working on different corpus, and what shows in one place doesn't in another.  We certainly don't want to take changes which make a problem markedly worse on anyone's corpus, but something that is orthogonal in practice (if not in theory) seems good to move forward with.


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