[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 tried similar optimizations in the past and this resulted in carrying very subtle update bugs in trunk for a few months until someone was able to trace miscompiles back to domtree. For this reason, I'm very hesitant to violate the contract that DTU knows about all the updates.

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