100 lines · cpp
1//===-- GCEmptyBasicBlocks.cpp ----------------------------------*- C++ -*-===//2//3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.4// See https://llvm.org/LICENSE.txt for license information.5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception6//7//===----------------------------------------------------------------------===//8///9/// \file10/// This file contains the implementation of empty blocks garbage collection11/// pass.12///13//===----------------------------------------------------------------------===//14#include "llvm/ADT/SmallVector.h"15#include "llvm/ADT/Statistic.h"16#include "llvm/CodeGen/MachineBasicBlock.h"17#include "llvm/CodeGen/MachineFunction.h"18#include "llvm/CodeGen/MachineFunctionPass.h"19#include "llvm/CodeGen/MachineJumpTableInfo.h"20#include "llvm/CodeGen/Passes.h"21#include "llvm/InitializePasses.h"22 23using namespace llvm;24 25#define DEBUG_TYPE "gc-empty-basic-blocks"26 27STATISTIC(NumEmptyBlocksRemoved, "Number of empty blocks removed");28 29class GCEmptyBasicBlocks : public MachineFunctionPass {30public:31 static char ID;32 33 GCEmptyBasicBlocks() : MachineFunctionPass(ID) {34 initializeGCEmptyBasicBlocksPass(*PassRegistry::getPassRegistry());35 }36 37 StringRef getPassName() const override {38 return "Remove Empty Basic Blocks.";39 }40 41 bool runOnMachineFunction(MachineFunction &MF) override;42};43 44bool GCEmptyBasicBlocks::runOnMachineFunction(MachineFunction &MF) {45 if (MF.size() < 2)46 return false;47 MachineJumpTableInfo *JTI = MF.getJumpTableInfo();48 int NumRemoved = 0;49 50 // Iterate over all blocks except the last one. We can't remove the last block51 // since it has no fallthrough block to rewire its predecessors to.52 for (MachineFunction::iterator MBB = MF.begin(),53 LastMBB = MachineFunction::iterator(MF.back()),54 NextMBB;55 MBB != LastMBB; MBB = NextMBB) {56 NextMBB = std::next(MBB);57 // TODO If a block is an eh pad, or it has address taken, we don't remove58 // it. Removing such blocks is possible, but it probably requires a more59 // complex logic.60 if (MBB->isEHPad() || MBB->hasAddressTaken())61 continue;62 // Skip blocks with real code.63 bool HasAnyRealCode = llvm::any_of(*MBB, [](const MachineInstr &MI) {64 return !MI.isPosition() && !MI.isImplicitDef() && !MI.isKill() &&65 !MI.isDebugInstr();66 });67 if (HasAnyRealCode)68 continue;69 70 LLVM_DEBUG(dbgs() << "Removing basic block " << MBB->getName()71 << " in function " << MF.getName() << ":\n"72 << *MBB << "\n");73 SmallVector<MachineBasicBlock *, 8> Preds(MBB->predecessors());74 // Rewire the predecessors of this block to use the next block.75 for (auto &Pred : Preds)76 Pred->ReplaceUsesOfBlockWith(&*MBB, &*NextMBB);77 // Update the jump tables.78 if (JTI)79 JTI->ReplaceMBBInJumpTables(&*MBB, &*NextMBB);80 // Remove this block from predecessors of all its successors.81 while (!MBB->succ_empty())82 MBB->removeSuccessor(MBB->succ_end() - 1);83 // Finally, remove the block from the function.84 MBB->eraseFromParent();85 ++NumRemoved;86 }87 NumEmptyBlocksRemoved += NumRemoved;88 return NumRemoved != 0;89}90 91char GCEmptyBasicBlocks::ID = 0;92INITIALIZE_PASS(GCEmptyBasicBlocks, "gc-empty-basic-blocks",93 "Removes empty basic blocks and redirects their uses to their "94 "fallthrough blocks.",95 false, false)96 97MachineFunctionPass *llvm::createGCEmptyBasicBlocksPass() {98 return new GCEmptyBasicBlocks();99}100