| //===-- 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->isMachineBlockAddressTaken()) |
| 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(); |
| } |