[PATCH] D141810: [LoopUnroll] Don't update DT for changeToUnreachable.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: nikic, reames, mkazantsev, asbirlea, kuhar.
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There is no need to update the DT here, because there must be a unique
latch. Hence if the latch is not exiting it must directly branch back
to the original loop header and does not dominate any nodes.

Skipping a DT update here simplifies D141487 <https://reviews.llvm.org/D141487>.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D141810

Files:
  llvm/lib/Transforms/Utils/LoopUnroll.cpp


Index: llvm/lib/Transforms/Utils/LoopUnroll.cpp
===================================================================
--- llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -756,8 +756,13 @@
   }
 
   // When completely unrolling, the last latch becomes unreachable.
-  if (!LatchIsExiting && CompletelyUnroll)
-    changeToUnreachable(Latches.back()->getTerminator(), PreserveLCSSA, &DTU);
+  if (!LatchIsExiting && CompletelyUnroll) {
+    // There is no need to update the DT here, because there must be a unique
+    // latch. Hence if the latch is not exiting it must directly branch back to
+    // the original loop header and does not dominate any nodes.
+    assert(LatchBlock->getSingleSuccessor() && "Loop with multiple latches?");
+    changeToUnreachable(Latches.back()->getTerminator(), PreserveLCSSA);
+  }
 
   // Merge adjacent basic blocks, if possible.
   for (BasicBlock *Latch : Latches) {


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