[llvm] [LoopIdiomVectorize] Remove redundant DomTreeUpdates (PR #94681)

[Jakub Kuderski via llvm-commits]

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@@ -391,6 +392,14 @@ Value *LoopIdiomVectorize::expandFindMismatch(
   BasicBlock *LoopIncBlock = BasicBlock::Create(
       Ctx, "mismatch_loop_inc", EndBlock->getParent(), EndBlock);
 
+  // This is actually one of the only two DTU updates we need. The reason being
+  // that we're splitting `mismatch_end` out of the preheader and put
+  // most of the stuff we create later between the preheader and
+  // `mismatch_end`. Now when DTU removes an edge, it simply recalculates
+  // everything in between. In this case, it will be the prehedaer and
+  // `mismatch_end`, along with the aforementioned content. Therefore we don't
+  // need to insert additional DTU updates for new control flow edges
+  // added in this region.
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kuhar wrote:

I don't follow the exact logic in this file, but in the general case DTU should be notified of all changes to the underlying graph. Skipping some of the updates based on assumptions of the exact implementation makes things confusing and difficult to debug. Is it guaranteed it's going to work if we ever change the DTU implementation?

It sounds to me like optimization sounds like something that should be implemented in the updater itself. Please let me know if I missed something here. (Concrete examples would help.)

https://github.com/llvm/llvm-project/pull/94681