Revert "[BasicBlockUtils] Fix dominator tree update for entry block in splitBlockBefore() (#178895)" (#179373)

This reverts commit ad8d534.

LLVM Buildbot detected a failure,
https://lab.llvm.org/buildbot/#/builders/210/builds/8229

Author: Enna1

llvm/lib/Transforms/Utils/BasicBlockUtils.cpp

@@ -1054,6 +1054,50 @@ BasicBlock *llvm::SplitBlock(BasicBlock *Old, BasicBlock::iterator SplitPt,
   return SplitBlockImpl(Old, SplitPt, DTU, /*DT=*/nullptr, LI, MSSAU, BBName);
 }
 
+BasicBlock *llvm::splitBlockBefore(BasicBlock *Old, BasicBlock::iterator SplitPt,
+                                   DomTreeUpdater *DTU, LoopInfo *LI,
+                                   MemorySSAUpdater *MSSAU,
+                                   const Twine &BBName) {
+
+  BasicBlock::iterator SplitIt = SplitPt;
+  while (isa<PHINode>(SplitIt) || SplitIt->isEHPad())
+    ++SplitIt;
+  std::string Name = BBName.str();
+  BasicBlock *New = Old->splitBasicBlockBefore(
+      SplitIt, Name.empty() ? Old->getName() + ".split" : Name);
+
+  // The new block lives in whichever loop the old one did. This preserves
+  // LCSSA as well, because we force the split point to be after any PHI nodes.
+  if (LI)
+    if (Loop *L = LI->getLoopFor(Old))
+      L->addBasicBlockToLoop(New, *LI);
+
+  if (DTU) {
+    SmallVector<DominatorTree::UpdateType, 8> DTUpdates;
+    // New dominates Old. The predecessor nodes of the Old node dominate
+    // New node.
+    SmallPtrSet<BasicBlock *, 8> UniquePredecessorsOfOld;
+    DTUpdates.push_back({DominatorTree::Insert, New, Old});
+    DTUpdates.reserve(DTUpdates.size() + 2 * pred_size(New));
+    for (BasicBlock *PredecessorOfOld : predecessors(New))
+      if (UniquePredecessorsOfOld.insert(PredecessorOfOld).second) {
+        DTUpdates.push_back({DominatorTree::Insert, PredecessorOfOld, New});
+        DTUpdates.push_back({DominatorTree::Delete, PredecessorOfOld, Old});
+      }
+
+    DTU->applyUpdates(DTUpdates);
+
+    // Move MemoryAccesses still tracked in Old, but part of New now.
+    // Update accesses in successor blocks accordingly.
+    if (MSSAU) {
+      MSSAU->applyUpdates(DTUpdates, DTU->getDomTree());
+      if (VerifyMemorySSA)
+        MSSAU->getMemorySSA()->verifyMemorySSA();
+    }
+  }
+  return New;
+}
+
 /// Update DominatorTree, LoopInfo, and LCCSA analysis information.
 /// Invalidates DFS Numbering when DTU or DT is provided.
 static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
@@ -1168,25 +1212,6 @@ static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
   }
 }
 
-BasicBlock *llvm::splitBlockBefore(BasicBlock *Old,
-                                   BasicBlock::iterator SplitPt,
-                                   DomTreeUpdater *DTU, LoopInfo *LI,
-                                   MemorySSAUpdater *MSSAU,
-                                   const Twine &BBName) {
-  BasicBlock::iterator SplitIt = SplitPt;
-  while (isa<PHINode>(SplitIt) || SplitIt->isEHPad())
-    ++SplitIt;
-  SmallVector<BasicBlock *, 4> Preds(predecessors(Old));
-  BasicBlock *New = Old->splitBasicBlockBefore(
-      SplitIt, BBName.isTriviallyEmpty() ? Old->getName() + ".split" : BBName);
-
-  bool HasLoopExit = false;
-  UpdateAnalysisInformation(Old, New, Preds, DTU, nullptr, LI, MSSAU, false,
-                            HasLoopExit);
-
-  return New;
-}
-
 /// Update the PHI nodes in OrigBB to include the values coming from NewBB.
 /// This also updates AliasAnalysis, if available.
 static void UpdatePHINodes(BasicBlock *OrigBB, BasicBlock *NewBB,

llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp

@@ -378,31 +378,6 @@ define i32 @no_unreachable(i1 %cond) {
   EXPECT_TRUE(DT.verify());
 }
 
-TEST(BasicBlockUtils, splitBlockBefore) {
-  LLVMContext C;
-  std::unique_ptr<Module> M = parseIR(C, R"IR(
-define i32 @basic_func(i1 %cond) {
-entry:
-  br i1 %cond, label %bb0, label %bb1
-bb0:
-  br label %bb1
-bb1:
-  %phi = phi i32 [ 0, %entry ], [ 1, %bb0 ]
-  ret i32 %phi
-}
-)IR");
-  Function *F = M->getFunction("basic_func");
-  DominatorTree DT(*F);
-  DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
-  BasicBlock *EntryBB = &F->getEntryBlock();
-  Instruction *TI = EntryBB->getTerminator();
-
-  // Make sure the dominator tree is properly updated if calling this on the
-  // entry block.
-  splitBlockBefore(EntryBB, TI, &DTU, nullptr, nullptr);
-  EXPECT_TRUE(DTU.getDomTree().verify());
-}
-
 TEST(BasicBlockUtils, SplitBlockPredecessors) {
   LLVMContext C;
   std::unique_ptr<Module> M = parseIR(C, R"IR(