[llvm] [CodeGen] Introduce `MachineDomTreeUpdater` (PR #95369)

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,62 @@
+//===- MachineDomTreeUpdater.cpp -----------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the MachineDomTreeUpdater class, which provides a
+// uniform way to update dominator tree related data structures.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/CodeGen/MachineDomTreeUpdater.h"
+#include "llvm/ADT/SmallSet.h"
+#include "llvm/CodeGen/MachinePostDominators.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/Support/GenericDomTree.h"
+#include <algorithm>
+#include <functional>
+#include <utility>
+
+namespace llvm {
+
+template class GenericDomTreeUpdater<
+    MachineDomTreeUpdater, MachineDominatorTree, MachinePostDominatorTree>;
+
+bool MachineDomTreeUpdater::forceFlushDeletedBB() {
+  if (DeletedBBs.empty())
+    return false;
+
+  for (auto *BB : DeletedBBs) {
+    eraseDelBBNode(BB);
+    BB->eraseFromParent();
+  }
+  DeletedBBs.clear();
+  return true;
+}
+
+// The DT and PDT require the nodes related to updates
+// are not deleted when update functions are called.
+// So MachineBasicBlock deletions must be pended when the
+// UpdateStrategy is Lazy. When the UpdateStrategy is
+// Eager, the MachineBasicBlock will be deleted immediately.
+void MachineDomTreeUpdater::deleteBB(MachineBasicBlock *DelBB) {
+  validateDeleteBB(DelBB);
+  if (Strategy == UpdateStrategy::Lazy) {
+    DeletedBBs.insert(DelBB);
+    return;
+  }
+
+  eraseDelBBNode(DelBB);
+  DelBB->eraseFromParent();
+}
+
+void MachineDomTreeUpdater::validateDeleteBB(MachineBasicBlock *DelBB) {
+  assert(DelBB && "Invalid push_back of nullptr DelBB.");
+  assert(DelBB->pred_empty() && "DelBB has one or more predecessors.");
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arsenm wrote:

unreachable typically corresponds to 0 instructions, unless the TrapOnUnreachable is used. Typically unreachable has no other instructions in the IR BB, which will codegen to an empty MBB 

https://github.com/llvm/llvm-project/pull/95369