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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/Passes.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/IR/CFG.h"
     32 #include "llvm/IR/Constant.h"
     33 #include "llvm/IR/Dominators.h"
     34 #include "llvm/IR/Function.h"
     35 #include "llvm/IR/Instructions.h"
     36 #include "llvm/IR/Type.h"
     37 #include "llvm/Pass.h"
     38 #include "llvm/Target/TargetInstrInfo.h"
     39 using namespace llvm;
     40 
     41 namespace {
     42   class UnreachableBlockElim : public FunctionPass {
     43     bool runOnFunction(Function &F) override;
     44   public:
     45     static char ID; // Pass identification, replacement for typeid
     46     UnreachableBlockElim() : FunctionPass(ID) {
     47       initializeUnreachableBlockElimPass(*PassRegistry::getPassRegistry());
     48     }
     49 
     50     void getAnalysisUsage(AnalysisUsage &AU) const override {
     51       AU.addPreserved<DominatorTreeWrapperPass>();
     52     }
     53   };
     54 }
     55 char UnreachableBlockElim::ID = 0;
     56 INITIALIZE_PASS(UnreachableBlockElim, "unreachableblockelim",
     57                 "Remove unreachable blocks from the CFG", false, false)
     58 
     59 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
     60   return new UnreachableBlockElim();
     61 }
     62 
     63 bool UnreachableBlockElim::runOnFunction(Function &F) {
     64   SmallPtrSet<BasicBlock*, 8> Reachable;
     65 
     66   // Mark all reachable blocks.
     67   for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I =
     68        df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I)
     69     /* Mark all reachable blocks */;
     70 
     71   // Loop over all dead blocks, remembering them and deleting all instructions
     72   // in them.
     73   std::vector<BasicBlock*> DeadBlocks;
     74   for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
     75     if (!Reachable.count(I)) {
     76       BasicBlock *BB = I;
     77       DeadBlocks.push_back(BB);
     78       while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
     79         PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
     80         BB->getInstList().pop_front();
     81       }
     82       for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
     83         (*SI)->removePredecessor(BB);
     84       BB->dropAllReferences();
     85     }
     86 
     87   // Actually remove the blocks now.
     88   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
     89     DeadBlocks[i]->eraseFromParent();
     90   }
     91 
     92   return DeadBlocks.size();
     93 }
     94 
     95 
     96 namespace {
     97   class UnreachableMachineBlockElim : public MachineFunctionPass {
     98     bool runOnMachineFunction(MachineFunction &F) override;
     99     void getAnalysisUsage(AnalysisUsage &AU) const override;
    100     MachineModuleInfo *MMI;
    101   public:
    102     static char ID; // Pass identification, replacement for typeid
    103     UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
    104   };
    105 }
    106 char UnreachableMachineBlockElim::ID = 0;
    107 
    108 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
    109   "Remove unreachable machine basic blocks", false, false)
    110 
    111 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
    112 
    113 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
    114   AU.addPreserved<MachineLoopInfo>();
    115   AU.addPreserved<MachineDominatorTree>();
    116   MachineFunctionPass::getAnalysisUsage(AU);
    117 }
    118 
    119 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
    120   SmallPtrSet<MachineBasicBlock*, 8> Reachable;
    121   bool ModifiedPHI = false;
    122 
    123   MMI = getAnalysisIfAvailable<MachineModuleInfo>();
    124   MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
    125   MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
    126 
    127   // Mark all reachable blocks.
    128   for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> >
    129        I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable);
    130        I != E; ++I)
    131     /* Mark all reachable blocks */;
    132 
    133   // Loop over all dead blocks, remembering them and deleting all instructions
    134   // in them.
    135   std::vector<MachineBasicBlock*> DeadBlocks;
    136   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
    137     MachineBasicBlock *BB = I;
    138 
    139     // Test for deadness.
    140     if (!Reachable.count(BB)) {
    141       DeadBlocks.push_back(BB);
    142 
    143       // Update dominator and loop info.
    144       if (MLI) MLI->removeBlock(BB);
    145       if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
    146 
    147       while (BB->succ_begin() != BB->succ_end()) {
    148         MachineBasicBlock* succ = *BB->succ_begin();
    149 
    150         MachineBasicBlock::iterator start = succ->begin();
    151         while (start != succ->end() && start->isPHI()) {
    152           for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
    153             if (start->getOperand(i).isMBB() &&
    154                 start->getOperand(i).getMBB() == BB) {
    155               start->RemoveOperand(i);
    156               start->RemoveOperand(i-1);
    157             }
    158 
    159           start++;
    160         }
    161 
    162         BB->removeSuccessor(BB->succ_begin());
    163       }
    164     }
    165   }
    166 
    167   // Actually remove the blocks now.
    168   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
    169     DeadBlocks[i]->eraseFromParent();
    170 
    171   // Cleanup PHI nodes.
    172   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
    173     MachineBasicBlock *BB = I;
    174     // Prune unneeded PHI entries.
    175     SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
    176                                              BB->pred_end());
    177     MachineBasicBlock::iterator phi = BB->begin();
    178     while (phi != BB->end() && phi->isPHI()) {
    179       for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
    180         if (!preds.count(phi->getOperand(i).getMBB())) {
    181           phi->RemoveOperand(i);
    182           phi->RemoveOperand(i-1);
    183           ModifiedPHI = true;
    184         }
    185 
    186       if (phi->getNumOperands() == 3) {
    187         unsigned Input = phi->getOperand(1).getReg();
    188         unsigned Output = phi->getOperand(0).getReg();
    189 
    190         MachineInstr* temp = phi;
    191         ++phi;
    192         temp->eraseFromParent();
    193         ModifiedPHI = true;
    194 
    195         if (Input != Output) {
    196           MachineRegisterInfo &MRI = F.getRegInfo();
    197           MRI.constrainRegClass(Input, MRI.getRegClass(Output));
    198           MRI.replaceRegWith(Output, Input);
    199         }
    200 
    201         continue;
    202       }
    203 
    204       ++phi;
    205     }
    206   }
    207 
    208   F.RenumberBlocks();
    209 
    210   return (DeadBlocks.size() || ModifiedPHI);
    211 }
    212