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