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