[PATCH] D68298: [GlobalOpt] Pass DTU to removeUnreachableBlocks instead of recomputing.

[Florian Hahn via Phabricator via llvm-commits]

fhahn added a comment.

In D68298#1804128 <https://reviews.llvm.org/D68298#1804128>, @nikic wrote:

> I suspect that this change caused a major compile-time performance regression in GlobalOpt. Doesn't this essentially break the laziness of `LookupDomTree()` and force DT construction even if it will not be needed?


Do you have a test case handy? Assuming the DT for F is already computed, LookupDomTree should be free,  but looking at the preceding passes, it looks like IPSCCP does not preserve the DTs with the LegacyPassManager (When I tried I could not get a module pass to preserve individual function analysis).

Unless we can get the DTs preserved until here, it is probably best to use `getAnalysisIfAvailable<DominatorTreeWrapperPass>()` here instead.


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