lib/CodeGen/UnreachableBlockElim.cpp

   1 //===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
   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-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This pass is an extremely simple version of the SimplifyCFG pass.  Its sole
  10 // job is to delete LLVM basic blocks that are not reachable from the entry
  11 // node.  To do this, it performs a simple depth first traversal of the CFG,
  12 // then deletes any unvisited nodes.
  13 //
  14 // Note that this pass is really a hack.  In particular, the instruction
  15 // selectors for various targets should just not generate code for unreachable
  16 // blocks.  Until LLVM has a more systematic way of defining instruction
  17 // selectors, however, we cannot really expect them to handle additional
  18 // complexity.
  19 //
  20 //===----------------------------------------------------------------------===//
  21
  22 #include "llvm/CodeGen/UnreachableBlockElim.h"
  23 #include "llvm/ADT/DepthFirstIterator.h"
  24 #include "llvm/ADT/SmallPtrSet.h"
  25 #include "llvm/CodeGen/MachineDominators.h"
  26 #include "llvm/CodeGen/MachineFunctionPass.h"
  27 #include "llvm/CodeGen/MachineInstrBuilder.h"
  28 #include "llvm/CodeGen/MachineLoopInfo.h"
  29 #include "llvm/CodeGen/MachineModuleInfo.h"
  30 #include "llvm/CodeGen/MachineRegisterInfo.h"
  31 #include "llvm/CodeGen/Passes.h"
  32 #include "llvm/CodeGen/TargetInstrInfo.h"
  33 #include "llvm/IR/CFG.h"
  34 #include "llvm/IR/Constant.h"
  35 #include "llvm/IR/Dominators.h"
  36 #include "llvm/IR/Function.h"
  37 #include "llvm/IR/Instructions.h"
  38 #include "llvm/IR/Type.h"
  39 #include "llvm/Pass.h"
  40 using namespace llvm;
  41
  42 static bool eliminateUnreachableBlock(Function &F) {
  43   df_iterator_default_set<BasicBlock*> Reachable;
  44
  45   // Mark all reachable blocks.
  46   for (BasicBlock *BB : depth_first_ext(&F, Reachable))
  47     (void)BB/* Mark all reachable blocks */;
  48
  49   // Loop over all dead blocks, remembering them and deleting all instructions
  50   // in them.
  51   std::vector<BasicBlock*> DeadBlocks;
  52   for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
  53     if (!Reachable.count(&*I)) {
  54       BasicBlock *BB = &*I;
  55       DeadBlocks.push_back(BB);
  56       while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
  57         PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
  58         BB->getInstList().pop_front();
  59       }
  60       for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
  61         (*SI)->removePredecessor(BB);
  62       BB->dropAllReferences();
  63     }
  64
  65   // Actually remove the blocks now.
  66   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
  67     DeadBlocks[i]->eraseFromParent();
  68   }
  69
  70   return !DeadBlocks.empty();
  71 }
  72
  73 namespace {
  74 class UnreachableBlockElimLegacyPass : public FunctionPass {
  75   bool runOnFunction(Function &F) override {
  76     return eliminateUnreachableBlock(F);
  77   }
  78
  79 public:
  80   static char ID; // Pass identification, replacement for typeid
  81   UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
  82     initializeUnreachableBlockElimLegacyPassPass(
  83         *PassRegistry::getPassRegistry());
  84   }
  85
  86   void getAnalysisUsage(AnalysisUsage &AU) const override {
  87     AU.addPreserved<DominatorTreeWrapperPass>();
  88   }
  89 };
  90 }
  91 char UnreachableBlockElimLegacyPass::ID = 0;
  92 INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
  93                 "Remove unreachable blocks from the CFG", false, false)
  94
  95 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
  96   return new UnreachableBlockElimLegacyPass();
  97 }
  98
  99 PreservedAnalyses UnreachableBlockElimPass::run(Function &F,
 100                                                 FunctionAnalysisManager &AM) {
 101   bool Changed = eliminateUnreachableBlock(F);
 102   if (!Changed)
 103     return PreservedAnalyses::all();
 104   PreservedAnalyses PA;
 105   PA.preserve<DominatorTreeAnalysis>();
 106   return PA;
 107 }
 108
 109 namespace {
 110   class UnreachableMachineBlockElim : public MachineFunctionPass {
 111     bool runOnMachineFunction(MachineFunction &F) override;
 112     void getAnalysisUsage(AnalysisUsage &AU) const override;
 113     MachineModuleInfo *MMI;
 114   public:
 115     static char ID; // Pass identification, replacement for typeid
 116     UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
 117   };
 118 }
 119 char UnreachableMachineBlockElim::ID = 0;
 120
 121 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
 122   "Remove unreachable machine basic blocks", false, false)
 123
 124 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
 125
 126 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
 127   AU.addPreserved<MachineLoopInfo>();
 128   AU.addPreserved<MachineDominatorTree>();
 129   MachineFunctionPass::getAnalysisUsage(AU);
 130 }
 131
 132 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
 133   df_iterator_default_set<MachineBasicBlock*> Reachable;
 134   bool ModifiedPHI = false;
 135
 136   MMI = getAnalysisIfAvailable<MachineModuleInfo>();
 137   MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
 138   MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
 139
 140   // Mark all reachable blocks.
 141   for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
 142     (void)BB/* Mark all reachable blocks */;
 143
 144   // Loop over all dead blocks, remembering them and deleting all instructions
 145   // in them.
 146   std::vector<MachineBasicBlock*> DeadBlocks;
 147   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
 148     MachineBasicBlock *BB = &*I;
 149
 150     // Test for deadness.
 151     if (!Reachable.count(BB)) {
 152       DeadBlocks.push_back(BB);
 153
 154       // Update dominator and loop info.
 155       if (MLI) MLI->removeBlock(BB);
 156       if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
 157
 158       while (BB->succ_begin() != BB->succ_end()) {
 159         MachineBasicBlock* succ = *BB->succ_begin();
 160
 161         MachineBasicBlock::iterator start = succ->begin();
 162         while (start != succ->end() && start->isPHI()) {
 163           for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
 164             if (start->getOperand(i).isMBB() &&
 165                 start->getOperand(i).getMBB() == BB) {
 166               start->RemoveOperand(i);
 167               start->RemoveOperand(i-1);
 168             }
 169
 170           start++;
 171         }
 172
 173         BB->removeSuccessor(BB->succ_begin());
 174       }
 175     }
 176   }
 177
 178   // Actually remove the blocks now.
 179   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
 180     DeadBlocks[i]->eraseFromParent();
 181
 182   // Cleanup PHI nodes.
 183   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
 184     MachineBasicBlock *BB = &*I;
 185     // Prune unneeded PHI entries.
 186     SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
 187                                              BB->pred_end());
 188     MachineBasicBlock::iterator phi = BB->begin();
 189     while (phi != BB->end() && phi->isPHI()) {
 190       for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
 191         if (!preds.count(phi->getOperand(i).getMBB())) {
 192           phi->RemoveOperand(i);
 193           phi->RemoveOperand(i-1);
 194           ModifiedPHI = true;
 195         }
 196
 197       if (phi->getNumOperands() == 3) {
 198         const MachineOperand &Input = phi->getOperand(1);
 199         const MachineOperand &Output = phi->getOperand(0);
 200         unsigned InputReg = Input.getReg();
 201         unsigned OutputReg = Output.getReg();
 202         assert(Output.getSubReg() == 0 && "Cannot have output subregister");
 203         ModifiedPHI = true;
 204
 205         if (InputReg != OutputReg) {
 206           MachineRegisterInfo &MRI = F.getRegInfo();
 207           unsigned InputSub = Input.getSubReg();
 208           if (InputSub == 0 &&
 209               MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
 210               !Input.isUndef()) {
 211             MRI.replaceRegWith(OutputReg, InputReg);
 212           } else {
 213             // The input register to the PHI has a subregister or it can't be
 214             // constrained to the proper register class or it is undef:
 215             // insert a COPY instead of simply replacing the output
 216             // with the input.
 217             const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
 218             BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(),
 219                     TII->get(TargetOpcode::COPY), OutputReg)
 220                 .addReg(InputReg, getRegState(Input), InputSub);
 221           }
 222           phi++->eraseFromParent();
 223         }
 224         continue;
 225       }
 226
 227       ++phi;
 228     }
 229   }
 230
 231   F.RenumberBlocks();
 232
 233   return (!DeadBlocks.empty() || ModifiedPHI);
 234 }