1 files changed, 100 insertions, 0 deletions
diff --git a/llvm/lib/CodeGen/GCEmptyBasicBlocks.cpp b/llvm/lib/CodeGen/GCEmptyBasicBlocks.cpp
new file mode 100644
index 000000000000..598be26e40c8
--- /dev/null
+++ b/llvm/lib/CodeGen/GCEmptyBasicBlocks.cpp
@@ -0,0 +1,100 @@
+//===-- GCEmptyBasicBlocks.cpp ----------------------------------*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// This file contains the implementation of empty blocks garbage collection
+/// pass.
+///
+//===----------------------------------------------------------------------===//
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/CodeGen/MachineBasicBlock.h"
+#include "llvm/CodeGen/MachineFunction.h"
+#include "llvm/CodeGen/MachineFunctionPass.h"
+#include "llvm/CodeGen/MachineJumpTableInfo.h"
+#include "llvm/CodeGen/Passes.h"
+#include "llvm/CodeGen/TargetInstrInfo.h"
+#include "llvm/InitializePasses.h"
+
+using namespace llvm;
+
+#define DEBUG_TYPE "gc-empty-basic-blocks"
+
+STATISTIC(NumEmptyBlocksRemoved, "Number of empty blocks removed");
+
+class GCEmptyBasicBlocks : public MachineFunctionPass {
+public:
+  static char ID;
+
+  GCEmptyBasicBlocks() : MachineFunctionPass(ID) {
+    initializeGCEmptyBasicBlocksPass(*PassRegistry::getPassRegistry());
+  }
+
+  StringRef getPassName() const override {
+    return "Remove Empty Basic Blocks.";
+  }
+
+  bool runOnMachineFunction(MachineFunction &MF) override;
+};
+
+bool GCEmptyBasicBlocks::runOnMachineFunction(MachineFunction &MF) {
+  if (MF.size() < 2)
+    return false;
+  MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
+  int NumRemoved = 0;
+
+  // Iterate over all blocks except the last one. We can't remove the last block
+  // since it has no fallthrough block to rewire its predecessors to.
+  for (MachineFunction::iterator MBB = MF.begin(),
+                                 LastMBB = MachineFunction::iterator(MF.back()),
+                                 NextMBB;
+       MBB != LastMBB; MBB = NextMBB) {
+    NextMBB = std::next(MBB);
+    // TODO If a block is an eh pad, or it has address taken, we don't remove
+    // it. Removing such blocks is possible, but it probably requires a more
+    // complex logic.
+    if (MBB->isEHPad() || MBB->hasAddressTaken())
+      continue;
+    // Skip blocks with real code.
+    bool HasAnyRealCode = llvm::any_of(*MBB, [](const MachineInstr &MI) {
+      return !MI.isPosition() && !MI.isImplicitDef() && !MI.isKill() &&
+             !MI.isDebugInstr();
+    });
+    if (HasAnyRealCode)
+      continue;
+
+    LLVM_DEBUG(dbgs() << "Removing basic block " << MBB->getName()
+                      << " in function " << MF.getName() << ":\n"
+                      << *MBB << "\n");
+    SmallVector<MachineBasicBlock *, 8> Preds(MBB->predecessors());
+    // Rewire the predecessors of this block to use the next block.
+    for (auto &Pred : Preds)
+      Pred->ReplaceUsesOfBlockWith(&*MBB, &*NextMBB);
+    // Update the jump tables.
+    if (JTI)
+      JTI->ReplaceMBBInJumpTables(&*MBB, &*NextMBB);
+    // Remove this block from predecessors of all its successors.
+    while (!MBB->succ_empty())
+      MBB->removeSuccessor(MBB->succ_end() - 1);
+    // Finally, remove the block from the function.
+    MBB->eraseFromParent();
+    ++NumRemoved;
+  }
+  NumEmptyBlocksRemoved += NumRemoved;
+  return NumRemoved != 0;
+}
+
+char GCEmptyBasicBlocks::ID = 0;
+INITIALIZE_PASS(GCEmptyBasicBlocks, "gc-empty-basic-blocks",
+                "Removes empty basic blocks and redirects their uses to their "
+                "fallthrough blocks.",
+                false, false)
+
+MachineFunctionPass *llvm::createGCEmptyBasicBlocksPass() {
+  return new GCEmptyBasicBlocks();
+}