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/Support/Compiler.h"
  37 #include "llvm/Target/TargetInstrInfo.h"
  38 #include "llvm/ADT/DepthFirstIterator.h"
  39 #include "llvm/ADT/SmallPtrSet.h"
  40 using namespace llvm;
  41
  42 namespace {
  43   class VISIBILITY_HIDDEN 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
  49     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
  50       AU.addPreserved<ProfileInfo>();
  51     }
  52   };
  53 }
  54 char UnreachableBlockElim::ID = 0;
  55 static RegisterPass<UnreachableBlockElim>
  56 X("unreachableblockelim", "Remove unreachable blocks from the CFG");
  57
  58 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
  59   return new UnreachableBlockElim();
  60 }
  61
  62 bool UnreachableBlockElim::runOnFunction(Function &F) {
  63   SmallPtrSet<BasicBlock*, 8> Reachable;
  64
  65   // Mark all reachable blocks.
  66   for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I =
  67        df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I)
  68     /* 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   ProfileInfo *PI = getAnalysisIfAvailable<ProfileInfo>();
  88   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
  89     if (PI) PI->removeBlock(DeadBlocks[i]);
  90     DeadBlocks[i]->eraseFromParent();
  91   }
  92
  93   return DeadBlocks.size();
  94 }
  95
  96
  97 namespace {
  98   class VISIBILITY_HIDDEN UnreachableMachineBlockElim :
  99         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 static RegisterPass<UnreachableMachineBlockElim>
 111 Y("unreachable-mbb-elimination",
 112   "Remove unreachable machine basic blocks");
 113
 114 const PassInfo *const llvm::UnreachableMachineBlockElimID = &Y;
 115
 116 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
 117   AU.addPreserved<MachineLoopInfo>();
 118   AU.addPreserved<MachineDominatorTree>();
 119   MachineFunctionPass::getAnalysisUsage(AU);
 120 }
 121
 122 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
 123   SmallPtrSet<MachineBasicBlock*, 8> Reachable;
 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() &&
 154                start->getOpcode() == TargetInstrInfo::PHI) {
 155           for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
 156             if (start->getOperand(i).isMBB() &&
 157                 start->getOperand(i).getMBB() == BB) {
 158               start->RemoveOperand(i);
 159               start->RemoveOperand(i-1);
 160             }
 161
 162           start++;
 163         }
 164
 165         BB->removeSuccessor(BB->succ_begin());
 166       }
 167     }
 168   }
 169
 170   // Actually remove the blocks now.
 171   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
 172     MachineBasicBlock *MBB = DeadBlocks[i];
 173     // If there are any labels in the basic block, unregister them from
 174     // MachineModuleInfo.
 175     if (MMI && !MBB->empty()) {
 176       for (MachineBasicBlock::iterator I = MBB->begin(),
 177              E = MBB->end(); I != E; ++I) {
 178         if (I->isLabel())
 179           // The label ID # is always operand #0, an immediate.
 180           MMI->InvalidateLabel(I->getOperand(0).getImm());
 181       }
 182     }
 183     MBB->eraseFromParent();
 184   }
 185
 186   // Cleanup PHI nodes.
 187   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
 188     MachineBasicBlock *BB = I;
 189     // Prune unneeded PHI entries.
 190     SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
 191                                              BB->pred_end());
 192     MachineBasicBlock::iterator phi = BB->begin();
 193     while (phi != BB->end() &&
 194            phi->getOpcode() == TargetInstrInfo::PHI) {
 195       for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
 196         if (!preds.count(phi->getOperand(i).getMBB())) {
 197           phi->RemoveOperand(i);
 198           phi->RemoveOperand(i-1);
 199         }
 200
 201       if (phi->getNumOperands() == 3) {
 202         unsigned Input = phi->getOperand(1).getReg();
 203         unsigned Output = phi->getOperand(0).getReg();
 204
 205         MachineInstr* temp = phi;
 206         ++phi;
 207         temp->eraseFromParent();
 208
 209         if (Input != Output)
 210           F.getRegInfo().replaceRegWith(Output, Input);
 211
 212         continue;
 213       }
 214
 215       ++phi;
 216     }
 217   }
 218
 219   F.RenumberBlocks();
 220
 221   return DeadBlocks.size();
 222 }