| //===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===// |
| // |
| // 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 pass is an extremely simple version of the SimplifyCFG pass. Its sole |
| // job is to delete LLVM basic blocks that are not reachable from the entry |
| // node. To do this, it performs a simple depth first traversal of the CFG, |
| // then deletes any unvisited nodes. |
| // |
| // Note that this pass is really a hack. In particular, the instruction |
| // selectors for various targets should just not generate code for unreachable |
| // blocks. Until LLVM has a more systematic way of defining instruction |
| // selectors, however, we cannot really expect them to handle additional |
| // complexity. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/CodeGen/UnreachableBlockElim.h" |
| #include "llvm/ADT/DepthFirstIterator.h" |
| #include "llvm/ADT/SmallPtrSet.h" |
| #include "llvm/CodeGen/MachineDominators.h" |
| #include "llvm/CodeGen/MachineFunctionPass.h" |
| #include "llvm/CodeGen/MachineInstrBuilder.h" |
| #include "llvm/CodeGen/MachineLoopInfo.h" |
| #include "llvm/CodeGen/MachineModuleInfo.h" |
| #include "llvm/CodeGen/MachineRegisterInfo.h" |
| #include "llvm/CodeGen/Passes.h" |
| #include "llvm/CodeGen/TargetInstrInfo.h" |
| #include "llvm/IR/CFG.h" |
| #include "llvm/IR/Constant.h" |
| #include "llvm/IR/Dominators.h" |
| #include "llvm/IR/Function.h" |
| #include "llvm/IR/Instructions.h" |
| #include "llvm/IR/Type.h" |
| #include "llvm/Pass.h" |
| using namespace llvm; |
| |
| static bool eliminateUnreachableBlock(Function &F) { |
| df_iterator_default_set<BasicBlock*> Reachable; |
| |
| // Mark all reachable blocks. |
| for (BasicBlock *BB : depth_first_ext(&F, Reachable)) |
| (void)BB/* Mark all reachable blocks */; |
| |
| // Loop over all dead blocks, remembering them and deleting all instructions |
| // in them. |
| std::vector<BasicBlock*> DeadBlocks; |
| for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) |
| if (!Reachable.count(&*I)) { |
| BasicBlock *BB = &*I; |
| DeadBlocks.push_back(BB); |
| while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) { |
| PN->replaceAllUsesWith(Constant::getNullValue(PN->getType())); |
| BB->getInstList().pop_front(); |
| } |
| for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) |
| (*SI)->removePredecessor(BB); |
| BB->dropAllReferences(); |
| } |
| |
| // Actually remove the blocks now. |
| for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) { |
| DeadBlocks[i]->eraseFromParent(); |
| } |
| |
| return !DeadBlocks.empty(); |
| } |
| |
| namespace { |
| class UnreachableBlockElimLegacyPass : public FunctionPass { |
| bool runOnFunction(Function &F) override { |
| return eliminateUnreachableBlock(F); |
| } |
| |
| public: |
| static char ID; // Pass identification, replacement for typeid |
| UnreachableBlockElimLegacyPass() : FunctionPass(ID) { |
| initializeUnreachableBlockElimLegacyPassPass( |
| *PassRegistry::getPassRegistry()); |
| } |
| |
| void getAnalysisUsage(AnalysisUsage &AU) const override { |
| AU.addPreserved<DominatorTreeWrapperPass>(); |
| } |
| }; |
| } |
| char UnreachableBlockElimLegacyPass::ID = 0; |
| INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim", |
| "Remove unreachable blocks from the CFG", false, false) |
| |
| FunctionPass *llvm::createUnreachableBlockEliminationPass() { |
| return new UnreachableBlockElimLegacyPass(); |
| } |
| |
| PreservedAnalyses UnreachableBlockElimPass::run(Function &F, |
| FunctionAnalysisManager &AM) { |
| bool Changed = eliminateUnreachableBlock(F); |
| if (!Changed) |
| return PreservedAnalyses::all(); |
| PreservedAnalyses PA; |
| PA.preserve<DominatorTreeAnalysis>(); |
| return PA; |
| } |
| |
| namespace { |
| class UnreachableMachineBlockElim : public MachineFunctionPass { |
| bool runOnMachineFunction(MachineFunction &F) override; |
| void getAnalysisUsage(AnalysisUsage &AU) const override; |
| MachineModuleInfo *MMI; |
| public: |
| static char ID; // Pass identification, replacement for typeid |
| UnreachableMachineBlockElim() : MachineFunctionPass(ID) {} |
| }; |
| } |
| char UnreachableMachineBlockElim::ID = 0; |
| |
| INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination", |
| "Remove unreachable machine basic blocks", false, false) |
| |
| char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID; |
| |
| void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.addPreserved<MachineLoopInfo>(); |
| AU.addPreserved<MachineDominatorTree>(); |
| MachineFunctionPass::getAnalysisUsage(AU); |
| } |
| |
| bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) { |
| df_iterator_default_set<MachineBasicBlock*> Reachable; |
| bool ModifiedPHI = false; |
| |
| MMI = getAnalysisIfAvailable<MachineModuleInfo>(); |
| MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>(); |
| MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>(); |
| |
| // Mark all reachable blocks. |
| for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable)) |
| (void)BB/* Mark all reachable blocks */; |
| |
| // Loop over all dead blocks, remembering them and deleting all instructions |
| // in them. |
| std::vector<MachineBasicBlock*> DeadBlocks; |
| for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) { |
| MachineBasicBlock *BB = &*I; |
| |
| // Test for deadness. |
| if (!Reachable.count(BB)) { |
| DeadBlocks.push_back(BB); |
| |
| // Update dominator and loop info. |
| if (MLI) MLI->removeBlock(BB); |
| if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB); |
| |
| while (BB->succ_begin() != BB->succ_end()) { |
| MachineBasicBlock* succ = *BB->succ_begin(); |
| |
| MachineBasicBlock::iterator start = succ->begin(); |
| while (start != succ->end() && start->isPHI()) { |
| for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2) |
| if (start->getOperand(i).isMBB() && |
| start->getOperand(i).getMBB() == BB) { |
| start->RemoveOperand(i); |
| start->RemoveOperand(i-1); |
| } |
| |
| start++; |
| } |
| |
| BB->removeSuccessor(BB->succ_begin()); |
| } |
| } |
| } |
| |
| // Actually remove the blocks now. |
| for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) |
| DeadBlocks[i]->eraseFromParent(); |
| |
| // Cleanup PHI nodes. |
| for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) { |
| MachineBasicBlock *BB = &*I; |
| // Prune unneeded PHI entries. |
| SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(), |
| BB->pred_end()); |
| MachineBasicBlock::iterator phi = BB->begin(); |
| while (phi != BB->end() && phi->isPHI()) { |
| for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2) |
| if (!preds.count(phi->getOperand(i).getMBB())) { |
| phi->RemoveOperand(i); |
| phi->RemoveOperand(i-1); |
| ModifiedPHI = true; |
| } |
| |
| if (phi->getNumOperands() == 3) { |
| const MachineOperand &Input = phi->getOperand(1); |
| const MachineOperand &Output = phi->getOperand(0); |
| unsigned InputReg = Input.getReg(); |
| unsigned OutputReg = Output.getReg(); |
| assert(Output.getSubReg() == 0 && "Cannot have output subregister"); |
| ModifiedPHI = true; |
| |
| if (InputReg != OutputReg) { |
| MachineRegisterInfo &MRI = F.getRegInfo(); |
| unsigned InputSub = Input.getSubReg(); |
| if (InputSub == 0 && |
| MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) && |
| !Input.isUndef()) { |
| MRI.replaceRegWith(OutputReg, InputReg); |
| } else { |
| // The input register to the PHI has a subregister or it can't be |
| // constrained to the proper register class or it is undef: |
| // insert a COPY instead of simply replacing the output |
| // with the input. |
| const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo(); |
| BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(), |
| TII->get(TargetOpcode::COPY), OutputReg) |
| .addReg(InputReg, getRegState(Input), InputSub); |
| } |
| phi++->eraseFromParent(); |
| } |
| continue; |
| } |
| |
| ++phi; |
| } |
| } |
| |
| F.RenumberBlocks(); |
| |
| return (!DeadBlocks.empty() || ModifiedPHI); |
| } |