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/Analysis/ProfileInfo.h"
  30 #include "llvm/CodeGen/MachineDominators.h"
  31 #include "llvm/CodeGen/MachineFunctionPass.h"
  32 #include "llvm/CodeGen/MachineModuleInfo.h"
  33 #include "llvm/CodeGen/MachineLoopInfo.h"
  34 #include "llvm/CodeGen/MachineRegisterInfo.h"
  35 #include "llvm/Support/CFG.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 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       initializeUnreachableBlockElimPass(*PassRegistry::getPassRegistry());
  48     }
  49
  50     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  51       AU.addPreserved<ProfileInfo>();
  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   ProfileInfo *PI = getAnalysisIfAvailable<ProfileInfo>();
  89   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
  90     if (PI) PI->removeBlock(DeadBlocks[i]);
  91     DeadBlocks[i]->eraseFromParent();
  92   }
  93
  94   return DeadBlocks.size();
  95 }
  96
  97
  98 namespace {
  99   class UnreachableMachineBlockElim : public MachineFunctionPass {
 100     virtual bool runOnMachineFunction(MachineFunction &F);
 101     virtual void getAnalysisUsage(AnalysisUsage &AU) const;
 102     MachineModuleInfo *MMI;
 103   public:
 104     static char ID; // Pass identification, replacement for typeid
 105     UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
 106   };
 107 }
 108 char UnreachableMachineBlockElim::ID = 0;
 109
 110 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
 111   "Remove unreachable machine basic blocks", false, false)
 112
 113 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
 114
 115 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
 116   AU.addPreserved<MachineLoopInfo>();
 117   AU.addPreserved<MachineDominatorTree>();
 118   MachineFunctionPass::getAnalysisUsage(AU);
 119 }
 120
 121 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
 122   SmallPtrSet<MachineBasicBlock*, 8> Reachable;
 123   bool ModifiedPHI = false;
 124
 125   MMI = getAnalysisIfAvailable<MachineModuleInfo>();
 126   MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
 127   MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
 128
 129   // Mark all reachable blocks.
 130   for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> >
 131        I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable);
 132        I != E; ++I)
 133     /* Mark all reachable blocks */;
 134
 135   // Loop over all dead blocks, remembering them and deleting all instructions
 136   // in them.
 137   std::vector<MachineBasicBlock*> DeadBlocks;
 138   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
 139     MachineBasicBlock *BB = I;
 140
 141     // Test for deadness.
 142     if (!Reachable.count(BB)) {
 143       DeadBlocks.push_back(BB);
 144
 145       // Update dominator and loop info.
 146       if (MLI) MLI->removeBlock(BB);
 147       if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
 148
 149       while (BB->succ_begin() != BB->succ_end()) {
 150         MachineBasicBlock* succ = *BB->succ_begin();
 151
 152         MachineBasicBlock::iterator start = succ->begin();
 153         while (start != succ->end() && start->isPHI()) {
 154           for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
 155             if (start->getOperand(i).isMBB() &&
 156                 start->getOperand(i).getMBB() == BB) {
 157               start->RemoveOperand(i);
 158               start->RemoveOperand(i-1);
 159             }
 160
 161           start++;
 162         }
 163
 164         BB->removeSuccessor(BB->succ_begin());
 165       }
 166     }
 167   }
 168
 169   // Actually remove the blocks now.
 170   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
 171     DeadBlocks[i]->eraseFromParent();
 172
 173   // Cleanup PHI nodes.
 174   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
 175     MachineBasicBlock *BB = I;
 176     // Prune unneeded PHI entries.
 177     SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
 178                                              BB->pred_end());
 179     MachineBasicBlock::iterator phi = BB->begin();
 180     while (phi != BB->end() && phi->isPHI()) {
 181       for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
 182         if (!preds.count(phi->getOperand(i).getMBB())) {
 183           phi->RemoveOperand(i);
 184           phi->RemoveOperand(i-1);
 185           ModifiedPHI = true;
 186         }
 187
 188       if (phi->getNumOperands() == 3) {
 189         unsigned Input = phi->getOperand(1).getReg();
 190         unsigned Output = phi->getOperand(0).getReg();
 191
 192         MachineInstr* temp = phi;
 193         ++phi;
 194         temp->eraseFromParent();
 195         ModifiedPHI = true;
 196
 197         if (Input != Output)
 198           F.getRegInfo().replaceRegWith(Output, Input);
 199
 200         continue;
 201       }
 202
 203       ++phi;
 204     }
 205   }
 206
 207   F.RenumberBlocks();
 208
 209   return (DeadBlocks.size() || ModifiedPHI);
 210 }