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/Dominators.h"
  30 #include "llvm/Analysis/ProfileInfo.h"
  31 #include "llvm/CodeGen/MachineDominators.h"
  32 #include "llvm/CodeGen/MachineFunctionPass.h"
  33 #include "llvm/CodeGen/MachineModuleInfo.h"
  34 #include "llvm/CodeGen/MachineLoopInfo.h"
  35 #include "llvm/CodeGen/MachineRegisterInfo.h"
  36 #include "llvm/Support/CFG.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 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 }