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      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/MachineLoopInfo.h"
     29 #include "llvm/CodeGen/MachineModuleInfo.h"
     30 #include "llvm/CodeGen/MachineRegisterInfo.h"
     31 #include "llvm/CodeGen/Passes.h"
     32 #include "llvm/IR/CFG.h"
     33 #include "llvm/IR/Constant.h"
     34 #include "llvm/IR/Dominators.h"
     35 #include "llvm/IR/Function.h"
     36 #include "llvm/IR/Instructions.h"
     37 #include "llvm/IR/Type.h"
     38 #include "llvm/Pass.h"
     39 #include "llvm/Target/TargetInstrInfo.h"
     40 using namespace llvm;
     41 
     42 static bool eliminateUnreachableBlock(Function &F) {
     43   SmallPtrSet<BasicBlock*, 8> 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   SmallPtrSet<MachineBasicBlock*, 8> 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         unsigned Input = phi->getOperand(1).getReg();
    199         unsigned Output = phi->getOperand(0).getReg();
    200 
    201         phi++->eraseFromParent();
    202         ModifiedPHI = true;
    203 
    204         if (Input != Output) {
    205           MachineRegisterInfo &MRI = F.getRegInfo();
    206           MRI.constrainRegClass(Input, MRI.getRegClass(Output));
    207           MRI.replaceRegWith(Output, Input);
    208         }
    209 
    210         continue;
    211       }
    212 
    213       ++phi;
    214     }
    215   }
    216 
    217   F.RenumberBlocks();
    218 
    219   return (!DeadBlocks.empty() || ModifiedPHI);
    220 }
    221