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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/Analysis/Dominators.h"
     27 #include "llvm/Analysis/ProfileInfo.h"
     28 #include "llvm/CodeGen/MachineDominators.h"
     29 #include "llvm/CodeGen/MachineFunctionPass.h"
     30 #include "llvm/CodeGen/MachineLoopInfo.h"
     31 #include "llvm/CodeGen/MachineModuleInfo.h"
     32 #include "llvm/CodeGen/MachineRegisterInfo.h"
     33 #include "llvm/IR/Constant.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/Support/CFG.h"
     39 #include "llvm/Target/TargetInstrInfo.h"
     40 using namespace llvm;
     41 
     42 namespace {
     43   class UnreachableBlockElim : public FunctionPass {
     44     virtual bool runOnFunction(Function &F);
     45   public:
     46     static char ID; // Pass identification, replacement for typeid
     47     UnreachableBlockElim() : FunctionPass(ID) {
     48       initializeUnreachableBlockElimPass(*PassRegistry::getPassRegistry());
     49     }
     50 
     51     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
     52       AU.addPreserved<DominatorTree>();
     53       AU.addPreserved<ProfileInfo>();
     54     }
     55   };
     56 }
     57 char UnreachableBlockElim::ID = 0;
     58 INITIALIZE_PASS(UnreachableBlockElim, "unreachableblockelim",
     59                 "Remove unreachable blocks from the CFG", false, false)
     60 
     61 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
     62   return new UnreachableBlockElim();
     63 }
     64 
     65 bool UnreachableBlockElim::runOnFunction(Function &F) {
     66   SmallPtrSet<BasicBlock*, 8> Reachable;
     67 
     68   // Mark all reachable blocks.
     69   for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I =
     70        df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I)
     71     /* Mark all reachable blocks */;
     72 
     73   // Loop over all dead blocks, remembering them and deleting all instructions
     74   // in them.
     75   std::vector<BasicBlock*> DeadBlocks;
     76   for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
     77     if (!Reachable.count(I)) {
     78       BasicBlock *BB = I;
     79       DeadBlocks.push_back(BB);
     80       while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
     81         PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
     82         BB->getInstList().pop_front();
     83       }
     84       for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
     85         (*SI)->removePredecessor(BB);
     86       BB->dropAllReferences();
     87     }
     88 
     89   // Actually remove the blocks now.
     90   ProfileInfo *PI = getAnalysisIfAvailable<ProfileInfo>();
     91   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
     92     if (PI) PI->removeBlock(DeadBlocks[i]);
     93     DeadBlocks[i]->eraseFromParent();
     94   }
     95 
     96   return DeadBlocks.size();
     97 }
     98 
     99 
    100 namespace {
    101   class UnreachableMachineBlockElim : public MachineFunctionPass {
    102     virtual bool runOnMachineFunction(MachineFunction &F);
    103     virtual void getAnalysisUsage(AnalysisUsage &AU) const;
    104     MachineModuleInfo *MMI;
    105   public:
    106     static char ID; // Pass identification, replacement for typeid
    107     UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
    108   };
    109 }
    110 char UnreachableMachineBlockElim::ID = 0;
    111 
    112 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
    113   "Remove unreachable machine basic blocks", false, false)
    114 
    115 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
    116 
    117 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
    118   AU.addPreserved<MachineLoopInfo>();
    119   AU.addPreserved<MachineDominatorTree>();
    120   MachineFunctionPass::getAnalysisUsage(AU);
    121 }
    122 
    123 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
    124   SmallPtrSet<MachineBasicBlock*, 8> Reachable;
    125   bool ModifiedPHI = false;
    126 
    127   MMI = getAnalysisIfAvailable<MachineModuleInfo>();
    128   MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
    129   MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
    130 
    131   // Mark all reachable blocks.
    132   for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> >
    133        I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable);
    134        I != E; ++I)
    135     /* Mark all reachable blocks */;
    136 
    137   // Loop over all dead blocks, remembering them and deleting all instructions
    138   // in them.
    139   std::vector<MachineBasicBlock*> DeadBlocks;
    140   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
    141     MachineBasicBlock *BB = I;
    142 
    143     // Test for deadness.
    144     if (!Reachable.count(BB)) {
    145       DeadBlocks.push_back(BB);
    146 
    147       // Update dominator and loop info.
    148       if (MLI) MLI->removeBlock(BB);
    149       if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
    150 
    151       while (BB->succ_begin() != BB->succ_end()) {
    152         MachineBasicBlock* succ = *BB->succ_begin();
    153 
    154         MachineBasicBlock::iterator start = succ->begin();
    155         while (start != succ->end() && start->isPHI()) {
    156           for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
    157             if (start->getOperand(i).isMBB() &&
    158                 start->getOperand(i).getMBB() == BB) {
    159               start->RemoveOperand(i);
    160               start->RemoveOperand(i-1);
    161             }
    162 
    163           start++;
    164         }
    165 
    166         BB->removeSuccessor(BB->succ_begin());
    167       }
    168     }
    169   }
    170 
    171   // Actually remove the blocks now.
    172   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
    173     DeadBlocks[i]->eraseFromParent();
    174 
    175   // Cleanup PHI nodes.
    176   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
    177     MachineBasicBlock *BB = I;
    178     // Prune unneeded PHI entries.
    179     SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
    180                                              BB->pred_end());
    181     MachineBasicBlock::iterator phi = BB->begin();
    182     while (phi != BB->end() && phi->isPHI()) {
    183       for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
    184         if (!preds.count(phi->getOperand(i).getMBB())) {
    185           phi->RemoveOperand(i);
    186           phi->RemoveOperand(i-1);
    187           ModifiedPHI = true;
    188         }
    189 
    190       if (phi->getNumOperands() == 3) {
    191         unsigned Input = phi->getOperand(1).getReg();
    192         unsigned Output = phi->getOperand(0).getReg();
    193 
    194         MachineInstr* temp = phi;
    195         ++phi;
    196         temp->eraseFromParent();
    197         ModifiedPHI = true;
    198 
    199         if (Input != Output) {
    200           MachineRegisterInfo &MRI = F.getRegInfo();
    201           MRI.constrainRegClass(Input, MRI.getRegClass(Output));
    202           MRI.replaceRegWith(Output, Input);
    203         }
    204 
    205         continue;
    206       }
    207 
    208       ++phi;
    209     }
    210   }
    211 
    212   F.RenumberBlocks();
    213 
    214   return (DeadBlocks.size() || ModifiedPHI);
    215 }
    216