[PATCH] D101142: [SimplifyCFG/JumpThreading] Do not simplify empty blocks with unconditional branches if this causes loop metadata confusion.

[Michael Kruse via Phabricator via llvm-commits]

Meinersbur added a comment.

In D101142#2711283 <https://reviews.llvm.org/D101142#2711283>, @jeroen.dobbelaere wrote:

> Did you verify the effect of this on the performance ? In a similar (but different) trial where I tried to block merging of blocks when that would result in a conflict with loop annotations, I noticed unacceptable regressions.

I did not. This is a correctness fix. Arguing for performance over correctness feels weird.

In D101142#2711289 <https://reviews.llvm.org/D101142#2711289>, @lebedev.ri wrote:

> Patch's subject is wrong. This doesn't "Transfer loop metadata to new latch.", it prevents folding.

Fixed title.

> if you already have metadata, why do you need `LoopHeaders` ?

To identify which blocks are latches. If moving metadata from another loop there may cause correctness problems, for instance if a `mustprogress` annotation is associated with a loop that does not need to progress.

Unfortunately the metadata itself is not sufficient. Let A and B be to blocks which are latches of two different loops. A has metadata, but B has not. Combining the blocks and using the metadata of A for it will be wrong, as it would apply the same metadata (e.g. `mustprogress`) to both loops, as happened in the PR50060. The alternative would be to not combine any blocks if any of the two has loop metadata. Or dropping both metadata.


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