llvm/lib/CodeGen/UnreachableBlockElim.cpp

   1 //===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This pass is an extremely simple version of the SimplifyCFG pass.  Its sole
  10 // job is to delete LLVM basic blocks that are not reachable from the entry
  11 // node.  To do this, it performs a simple depth first traversal of the CFG,
  12 // then deletes any unvisited nodes.
  13 //
  14 // Note that this pass is really a hack.  In particular, the instruction
  15 // selectors for various targets should just not generate code for unreachable
  16 // blocks.  Until LLVM has a more systematic way of defining instruction
  17 // selectors, however, we cannot really expect them to handle additional
  18 // complexity.
  19 //
  20 //===----------------------------------------------------------------------===//
  21
  22 #include "llvm/CodeGen/UnreachableBlockElim.h"
  23 #include "llvm/ADT/DepthFirstIterator.h"
  24 #include "llvm/ADT/SmallPtrSet.h"
  25 #include "llvm/CodeGen/MachineDominators.h"
  26 #include "llvm/CodeGen/MachineFunctionPass.h"
  27 #include "llvm/CodeGen/MachineInstrBuilder.h"
  28 #include "llvm/CodeGen/MachineLoopInfo.h"
  29 #include "llvm/CodeGen/MachineRegisterInfo.h"
  30 #include "llvm/CodeGen/Passes.h"
  31 #include "llvm/CodeGen/TargetInstrInfo.h"
  32 #include "llvm/IR/Dominators.h"
  33 #include "llvm/InitializePasses.h"
  34 #include "llvm/Pass.h"
  35 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  36 using namespace llvm;
  37
  38 namespace {
  39 class UnreachableBlockElimLegacyPass : public FunctionPass {
  40   bool runOnFunction(Function &F) override {
  41     return llvm::EliminateUnreachableBlocks(F);
  42   }
  43
  44 public:
  45   static char ID; // Pass identification, replacement for typeid
  46   UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
  47     initializeUnreachableBlockElimLegacyPassPass(
  48         *PassRegistry::getPassRegistry());
  49   }
  50
  51   void getAnalysisUsage(AnalysisUsage &AU) const override {
  52     AU.addPreserved<DominatorTreeWrapperPass>();
  53   }
  54 };
  55 }
  56 char UnreachableBlockElimLegacyPass::ID = 0;
  57 INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
  58                 "Remove unreachable blocks from the CFG", false, false)
  59
  60 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
  61   return new UnreachableBlockElimLegacyPass();
  62 }
  63
  64 PreservedAnalyses UnreachableBlockElimPass::run(Function &F,
  65                                                 FunctionAnalysisManager &AM) {
  66   bool Changed = llvm::EliminateUnreachableBlocks(F);
  67   if (!Changed)
  68     return PreservedAnalyses::all();
  69   PreservedAnalyses PA;
  70   PA.preserve<DominatorTreeAnalysis>();
  71   return PA;
  72 }
  73
  74 namespace {
  75   class UnreachableMachineBlockElim : public MachineFunctionPass {
  76     bool runOnMachineFunction(MachineFunction &F) override;
  77     void getAnalysisUsage(AnalysisUsage &AU) const override;
  78
  79   public:
  80     static char ID; // Pass identification, replacement for typeid
  81     UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
  82   };
  83 }
  84 char UnreachableMachineBlockElim::ID = 0;
  85
  86 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
  87   "Remove unreachable machine basic blocks", false, false)
  88
  89 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
  90
  91 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
  92   AU.addPreserved<MachineLoopInfo>();
  93   AU.addPreserved<MachineDominatorTree>();
  94   MachineFunctionPass::getAnalysisUsage(AU);
  95 }
  96
  97 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
  98   df_iterator_default_set<MachineBasicBlock*> Reachable;
  99   bool ModifiedPHI = false;
 100
 101   MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
 102   MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
 103
 104   // Mark all reachable blocks.
 105   for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
 106     (void)BB/* Mark all reachable blocks */;
 107
 108   // Loop over all dead blocks, remembering them and deleting all instructions
 109   // in them.
 110   std::vector<MachineBasicBlock*> DeadBlocks;
 111   for (MachineBasicBlock &BB : F) {
 112     // Test for deadness.
 113     if (!Reachable.count(&BB)) {
 114       DeadBlocks.push_back(&BB);
 115
 116       // Update dominator and loop info.
 117       if (MLI) MLI->removeBlock(&BB);
 118       if (MDT && MDT->getNode(&BB)) MDT->eraseNode(&BB);
 119
 120       while (BB.succ_begin() != BB.succ_end()) {
 121         MachineBasicBlock* succ = *BB.succ_begin();
 122
 123         MachineBasicBlock::iterator start = succ->begin();
 124         while (start != succ->end() && start->isPHI()) {
 125           for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
 126             if (start->getOperand(i).isMBB() &&
 127                 start->getOperand(i).getMBB() == &BB) {
 128               start->removeOperand(i);
 129               start->removeOperand(i-1);
 130             }
 131
 132           start++;
 133         }
 134
 135         BB.removeSuccessor(BB.succ_begin());
 136       }
 137     }
 138   }
 139
 140   // Actually remove the blocks now.
 141   for (MachineBasicBlock *BB : DeadBlocks) {
 142     // Remove any call site information for calls in the block.
 143     for (auto &I : BB->instrs())
 144       if (I.shouldUpdateCallSiteInfo())
 145         BB->getParent()->eraseCallSiteInfo(&I);
 146
 147     BB->eraseFromParent();
 148   }
 149
 150   // Cleanup PHI nodes.
 151   for (MachineBasicBlock &BB : F) {
 152     // Prune unneeded PHI entries.
 153     SmallPtrSet<MachineBasicBlock*, 8> preds(BB.pred_begin(),
 154                                              BB.pred_end());
 155     MachineBasicBlock::iterator phi = BB.begin();
 156     while (phi != BB.end() && phi->isPHI()) {
 157       for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
 158         if (!preds.count(phi->getOperand(i).getMBB())) {
 159           phi->removeOperand(i);
 160           phi->removeOperand(i-1);
 161           ModifiedPHI = true;
 162         }
 163
 164       if (phi->getNumOperands() == 3) {
 165         const MachineOperand &Input = phi->getOperand(1);
 166         const MachineOperand &Output = phi->getOperand(0);
 167         Register InputReg = Input.getReg();
 168         Register OutputReg = Output.getReg();
 169         assert(Output.getSubReg() == 0 && "Cannot have output subregister");
 170         ModifiedPHI = true;
 171
 172         if (InputReg != OutputReg) {
 173           MachineRegisterInfo &MRI = F.getRegInfo();
 174           unsigned InputSub = Input.getSubReg();
 175           if (InputSub == 0 &&
 176               MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
 177               !Input.isUndef()) {
 178             MRI.replaceRegWith(OutputReg, InputReg);
 179           } else {
 180             // The input register to the PHI has a subregister or it can't be
 181             // constrained to the proper register class or it is undef:
 182             // insert a COPY instead of simply replacing the output
 183             // with the input.
 184             const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
 185             BuildMI(BB, BB.getFirstNonPHI(), phi->getDebugLoc(),
 186                     TII->get(TargetOpcode::COPY), OutputReg)
 187                 .addReg(InputReg, getRegState(Input), InputSub);
 188           }
 189           phi++->eraseFromParent();
 190         }
 191         continue;
 192       }
 193
 194       ++phi;
 195     }
 196   }
 197
 198   F.RenumberBlocks();
 199
 200   return (!DeadBlocks.empty() || ModifiedPHI);
 201 }