[LoopUnroll] Use changeToUnreachable() (NFC)

When fulling unrolling with a non-latch exit, the latch block is
folded to unreachable. Replace this folding with the existing
changeToUnreachable() helper, rather than performing it manually.

This also moves the fold to happen after the manual DT update
for exit blocks. I believe this is correct in that the conversion
of an unconditional backedge into unreachable should not affect
the DT at all.

Differential Revision: https://reviews.llvm.org/D103340

diff --git a/llvm/lib/Transforms/Utils/LoopUnroll.cpp b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
index cb84eb6..3574abe 100644 (file)
--- a/llvm/lib/Transforms/Utils/LoopUnroll.cpp
+++ b/llvm/lib/Transforms/Utils/LoopUnroll.cpp
@@ -763,13 +763,6 @@ LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
     }
   }
 
-  // When completely unrolling, the last latch becomes unreachable.
-  if (!LatchIsExiting && CompletelyUnroll) {
-    BranchInst *Term = cast<BranchInst>(Latches.back()->getTerminator());
-    new UnreachableInst(Term->getContext(), Term);
-    Term->eraseFromParent();
-  }
-
   // Update dominators of blocks we might reach through exits.
   // Immediate dominator of such block might change, because we add more
   // routes which can lead to the exit: we can now reach it from the copied
@@ -819,6 +812,12 @@ LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
          DT->verify(DominatorTree::VerificationLevel::Fast));
 
   DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
+
+  // When completely unrolling, the last latch becomes unreachable.
+  if (!LatchIsExiting && CompletelyUnroll)
+    changeToUnreachable(Latches.back()->getTerminator(), /* UseTrap */ false,
+                        PreserveLCSSA, &DTU);
+
   // Merge adjacent basic blocks, if possible.
   for (BasicBlock *Latch : Latches) {
     BranchInst *Term = dyn_cast<BranchInst>(Latch->getTerminator());