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