lib/CodeGen/UnreachableBlockElim.cpp

   1 //===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This pass is an extremely simple version of the SimplifyCFG pass.  Its sole
  11 // job is to delete LLVM basic blocks that are not reachable from the entry
  12 // node.  To do this, it performs a simple depth first traversal of the CFG,
  13 // then deletes any unvisited nodes.
  14 //
  15 // Note that this pass is really a hack.  In particular, the instruction
  16 // selectors for various targets should just not generate code for unreachable
  17 // blocks.  Until LLVM has a more systematic way of defining instruction
  18 // selectors, however, we cannot really expect them to handle additional
  19 // complexity.
  20 //
  21 //===----------------------------------------------------------------------===//
  22
  23 #include "llvm/CodeGen/UnreachableBlockElim.h"
  24 #include "llvm/ADT/DepthFirstIterator.h"
  25 #include "llvm/ADT/SmallPtrSet.h"
  26 #include "llvm/CodeGen/MachineDominators.h"
  27 #include "llvm/CodeGen/MachineFunctionPass.h"
  28 #include "llvm/CodeGen/MachineInstrBuilder.h"
  29 #include "llvm/CodeGen/MachineLoopInfo.h"
  30 #include "llvm/CodeGen/MachineModuleInfo.h"
  31 #include "llvm/CodeGen/MachineRegisterInfo.h"
  32 #include "llvm/CodeGen/Passes.h"
  33 #include "llvm/CodeGen/TargetInstrInfo.h"
  34 #include "llvm/IR/CFG.h"
  35 #include "llvm/IR/Constant.h"
  36 #include "llvm/IR/Dominators.h"
  37 #include "llvm/IR/Function.h"
  38 #include "llvm/IR/Instructions.h"
  39 #include "llvm/IR/Type.h"
  40 #include "llvm/Pass.h"
  41 using namespace llvm;
  42
  43 static bool eliminateUnreachableBlock(Function &F) {
  44   df_iterator_default_set<BasicBlock*> Reachable;
  45
  46   // Mark all reachable blocks.
  47   for (BasicBlock *BB : depth_first_ext(&F, Reachable))
  48     (void)BB/* Mark all reachable blocks */;
  49
  50   // Loop over all dead blocks, remembering them and deleting all instructions
  51   // in them.
  52   std::vector<BasicBlock*> DeadBlocks;
  53   for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
  54     if (!Reachable.count(&*I)) {
  55       BasicBlock *BB = &*I;
  56       DeadBlocks.push_back(BB);
  57       while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
  58         PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
  59         BB->getInstList().pop_front();
  60       }
  61       for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
  62         (*SI)->removePredecessor(BB);
  63       BB->dropAllReferences();
  64     }
  65
  66   // Actually remove the blocks now.
  67   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
  68     DeadBlocks[i]->eraseFromParent();
  69   }
  70
  71   return !DeadBlocks.empty();
  72 }
  73
  74 namespace {
  75 class UnreachableBlockElimLegacyPass : public FunctionPass {
  76   bool runOnFunction(Function &F) override {
  77     return eliminateUnreachableBlock(F);
  78   }
  79
  80 public:
  81   static char ID; // Pass identification, replacement for typeid
  82   UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
  83     initializeUnreachableBlockElimLegacyPassPass(
  84         *PassRegistry::getPassRegistry());
  85   }
  86
  87   void getAnalysisUsage(AnalysisUsage &AU) const override {
  88     AU.addPreserved<DominatorTreeWrapperPass>();
  89   }
  90 };
  91 }
  92 char UnreachableBlockElimLegacyPass::ID = 0;
  93 INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
  94                 "Remove unreachable blocks from the CFG", false, false)
  95
  96 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
  97   return new UnreachableBlockElimLegacyPass();
  98 }
  99
 100 PreservedAnalyses UnreachableBlockElimPass::run(Function &F,
 101                                                 FunctionAnalysisManager &AM) {
 102   bool Changed = eliminateUnreachableBlock(F);
 103   if (!Changed)
 104     return PreservedAnalyses::all();
 105   PreservedAnalyses PA;
 106   PA.preserve<DominatorTreeAnalysis>();
 107   return PA;
 108 }
 109
 110 namespace {
 111   class UnreachableMachineBlockElim : public MachineFunctionPass {
 112     bool runOnMachineFunction(MachineFunction &F) override;
 113     void getAnalysisUsage(AnalysisUsage &AU) const override;
 114     MachineModuleInfo *MMI;
 115   public:
 116     static char ID; // Pass identification, replacement for typeid
 117     UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
 118   };
 119 }
 120 char UnreachableMachineBlockElim::ID = 0;
 121
 122 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
 123   "Remove unreachable machine basic blocks", false, false)
 124
 125 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
 126
 127 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
 128   AU.addPreserved<MachineLoopInfo>();
 129   AU.addPreserved<MachineDominatorTree>();
 130   MachineFunctionPass::getAnalysisUsage(AU);
 131 }
 132
 133 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
 134   df_iterator_default_set<MachineBasicBlock*> Reachable;
 135   bool ModifiedPHI = false;
 136
 137   MMI = getAnalysisIfAvailable<MachineModuleInfo>();
 138   MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
 139   MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
 140
 141   // Mark all reachable blocks.
 142   for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
 143     (void)BB/* Mark all reachable blocks */;
 144
 145   // Loop over all dead blocks, remembering them and deleting all instructions
 146   // in them.
 147   std::vector<MachineBasicBlock*> DeadBlocks;
 148   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
 149     MachineBasicBlock *BB = &*I;
 150
 151     // Test for deadness.
 152     if (!Reachable.count(BB)) {
 153       DeadBlocks.push_back(BB);
 154
 155       // Update dominator and loop info.
 156       if (MLI) MLI->removeBlock(BB);
 157       if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
 158
 159       while (BB->succ_begin() != BB->succ_end()) {
 160         MachineBasicBlock* succ = *BB->succ_begin();
 161
 162         MachineBasicBlock::iterator start = succ->begin();
 163         while (start != succ->end() && start->isPHI()) {
 164           for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
 165             if (start->getOperand(i).isMBB() &&
 166                 start->getOperand(i).getMBB() == BB) {
 167               start->RemoveOperand(i);
 168               start->RemoveOperand(i-1);
 169             }
 170
 171           start++;
 172         }
 173
 174         BB->removeSuccessor(BB->succ_begin());
 175       }
 176     }
 177   }
 178
 179   // Actually remove the blocks now.
 180   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
 181     DeadBlocks[i]->eraseFromParent();
 182
 183   // Cleanup PHI nodes.
 184   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
 185     MachineBasicBlock *BB = &*I;
 186     // Prune unneeded PHI entries.
 187     SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
 188                                              BB->pred_end());
 189     MachineBasicBlock::iterator phi = BB->begin();
 190     while (phi != BB->end() && phi->isPHI()) {
 191       for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
 192         if (!preds.count(phi->getOperand(i).getMBB())) {
 193           phi->RemoveOperand(i);
 194           phi->RemoveOperand(i-1);
 195           ModifiedPHI = true;
 196         }
 197
 198       if (phi->getNumOperands() == 3) {
 199         const MachineOperand &Input = phi->getOperand(1);
 200         const MachineOperand &Output = phi->getOperand(0);
 201         unsigned InputReg = Input.getReg();
 202         unsigned OutputReg = Output.getReg();
 203         assert(Output.getSubReg() == 0 && "Cannot have output subregister");
 204         ModifiedPHI = true;
 205
 206         if (InputReg != OutputReg) {
 207           MachineRegisterInfo &MRI = F.getRegInfo();
 208           unsigned InputSub = Input.getSubReg();
 209           if (InputSub == 0 &&
 210               MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
 211               !Input.isUndef()) {
 212             MRI.replaceRegWith(OutputReg, InputReg);
 213           } else {
 214             // The input register to the PHI has a subregister or it can't be
 215             // constrained to the proper register class or it is undef:
 216             // insert a COPY instead of simply replacing the output
 217             // with the input.
 218             const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
 219             BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(),
 220                     TII->get(TargetOpcode::COPY), OutputReg)
 221                 .addReg(InputReg, getRegState(Input), InputSub);
 222           }
 223           phi++->eraseFromParent();
 224         }
 225         continue;
 226       }
 227
 228       ++phi;
 229     }
 230   }
 231
 232   F.RenumberBlocks();
 233
 234   return (!DeadBlocks.empty() || ModifiedPHI);
 235 }