[llvm] [BasicBlockUtils] Fix dominator tree update for entry block in splitBlockBefore() (PR #178895)

[Mingjie Xu via llvm-commits]

https://github.com/Enna1 created https://github.com/llvm/llvm-project/pull/178895

06dfbb50d70eea4ae38d655842626a0b9b224d5c fixed domoninator update for entry block in `SplitBlockPredecessors()`, this patch fixes dominator tree update for entry block in `splitBlockBefore()` with `UpdateAnalysisInformation()`.

>From cd32e5b36c524cc0f713927a5f10090b971aea60 Mon Sep 17 00:00:00 2001
From: Enna1 <xumingjie.enna1 at bytedance.com>
Date: Fri, 30 Jan 2026 21:38:20 +0800
Subject: [PATCH 1/2] [BasicBlockUtils] Fix dominator tree update for entry
 block in splitBlockBefore()

Domoninator update for entry block in `SplitBlockPredecessors()` was fixed by
06dfbb50d70eea4ae38d655842626a0b9b224d5c , this patch fixes dominator
tree update for entry block in `splitBlockBefore()` with
`UpdateAnalysisInformation()`.
---
 llvm/lib/Transforms/Utils/BasicBlockUtils.cpp | 66 ++++++-------------
 1 file changed, 21 insertions(+), 45 deletions(-)

diff --git a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
index b0c04086f5e84..12206e14ca8fd 100644
--- a/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
+++ b/llvm/lib/Transforms/Utils/BasicBlockUtils.cpp
@@ -1064,51 +1064,6 @@ BasicBlock *llvm::SplitBlock(BasicBlock *Old, BasicBlock::iterator SplitPt,
                         Before);
 }
 
-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->splitBasicBlock(
-      SplitIt, Name.empty() ? Old->getName() + ".split" : Name,
-      /* Before=*/true);
-
-  // 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,
@@ -1223,6 +1178,27 @@ 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));
+  std::string Name = BBName.str();
+  BasicBlock *New = Old->splitBasicBlock(
+      SplitIt, Name.empty() ? Old->getName() + ".split" : Name,
+      /* Before=*/true);
+
+  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,

>From 0cf9493bf6611ebecbbd4a6a895fd5c4d1413b85 Mon Sep 17 00:00:00 2001
From: Enna1 <xumingjie.enna1 at bytedance.com>
Date: Fri, 30 Jan 2026 21:38:28 +0800
Subject: [PATCH 2/2] add test

---
 .../Transforms/Utils/BasicBlockUtilsTest.cpp  | 25 +++++++++++++++++++
 1 file changed, 25 insertions(+)

diff --git a/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp b/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
index 00d9e9ff81e05..d5b4f6b6d6593 100644
--- a/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
+++ b/llvm/unittests/Transforms/Utils/BasicBlockUtilsTest.cpp
@@ -378,6 +378,31 @@ 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(