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/MachineModuleInfo.h"
  30 #include "llvm/CodeGen/MachineRegisterInfo.h"
  31 #include "llvm/CodeGen/Passes.h"
  32 #include "llvm/CodeGen/TargetInstrInfo.h"
  33 #include "llvm/IR/CFG.h"
  34 #include "llvm/IR/Constant.h"
  35 #include "llvm/IR/Dominators.h"
  36 #include "llvm/IR/Function.h"
  37 #include "llvm/IR/Instructions.h"
  38 #include "llvm/IR/Type.h"
  39 #include "llvm/InitializePasses.h"
  40 #include "llvm/Pass.h"
  41 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  42 using namespace llvm;
  43
  44 namespace {
  45 class UnreachableBlockElimLegacyPass : public FunctionPass {
  46   bool runOnFunction(Function &F) override {
  47     return llvm::EliminateUnreachableBlocks(F);
  48   }
  49
  50 public:
  51   static char ID; // Pass identification, replacement for typeid
  52   UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
  53     initializeUnreachableBlockElimLegacyPassPass(
  54         *PassRegistry::getPassRegistry());
  55   }
  56
  57   void getAnalysisUsage(AnalysisUsage &AU) const override {
  58     AU.addPreserved<DominatorTreeWrapperPass>();
  59   }
  60 };
  61 }
  62 char UnreachableBlockElimLegacyPass::ID = 0;
  63 INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
  64                 "Remove unreachable blocks from the CFG", false, false)
  65
  66 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
  67   return new UnreachableBlockElimLegacyPass();
  68 }
  69
  70 PreservedAnalyses UnreachableBlockElimPass::run(Function &F,
  71                                                 FunctionAnalysisManager &AM) {
  72   bool Changed = llvm::EliminateUnreachableBlocks(F);
  73   if (!Changed)
  74     return PreservedAnalyses::all();
  75   PreservedAnalyses PA;
  76   PA.preserve<DominatorTreeAnalysis>();
  77   return PA;
  78 }
  79
  80 namespace {
  81   class UnreachableMachineBlockElim : public MachineFunctionPass {
  82     bool runOnMachineFunction(MachineFunction &F) override;
  83     void getAnalysisUsage(AnalysisUsage &AU) const override;
  84
  85   public:
  86     static char ID; // Pass identification, replacement for typeid
  87     UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
  88   };
  89 }
  90 char UnreachableMachineBlockElim::ID = 0;
  91
  92 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
  93   "Remove unreachable machine basic blocks", false, false)
  94
  95 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
  96
  97 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
  98   AU.addPreserved<MachineLoopInfo>();
  99   AU.addPreserved<MachineDominatorTree>();
 100   MachineFunctionPass::getAnalysisUsage(AU);
 101 }
 102
 103 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
 104   df_iterator_default_set<MachineBasicBlock*> Reachable;
 105   bool ModifiedPHI = false;
 106
 107   MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
 108   MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
 109
 110   // Mark all reachable blocks.
 111   for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
 112     (void)BB/* Mark all reachable blocks */;
 113
 114   // Loop over all dead blocks, remembering them and deleting all instructions
 115   // in them.
 116   std::vector<MachineBasicBlock*> DeadBlocks;
 117   for (MachineBasicBlock &BB : F) {
 118     // Test for deadness.
 119     if (!Reachable.count(&BB)) {
 120       DeadBlocks.push_back(&BB);
 121
 122       // Update dominator and loop info.
 123       if (MLI) MLI->removeBlock(&BB);
 124       if (MDT && MDT->getNode(&BB)) MDT->eraseNode(&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() && start->isPHI()) {
 131           for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
 132             if (start->getOperand(i).isMBB() &&
 133                 start->getOperand(i).getMBB() == &BB) {
 134               start->RemoveOperand(i);
 135               start->RemoveOperand(i-1);
 136             }
 137
 138           start++;
 139         }
 140
 141         BB.removeSuccessor(BB.succ_begin());
 142       }
 143     }
 144   }
 145
 146   // Actually remove the blocks now.
 147   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
 148     // Remove any call site information for calls in the block.
 149     for (auto &I : DeadBlocks[i]->instrs())
 150       if (I.shouldUpdateCallSiteInfo())
 151         DeadBlocks[i]->getParent()->eraseCallSiteInfo(&I);
 152
 153     DeadBlocks[i]->eraseFromParent();
 154   }
 155
 156   // Cleanup PHI nodes.
 157   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
 158     MachineBasicBlock *BB = &*I;
 159     // Prune unneeded PHI entries.
 160     SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
 161                                              BB->pred_end());
 162     MachineBasicBlock::iterator phi = BB->begin();
 163     while (phi != BB->end() && phi->isPHI()) {
 164       for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
 165         if (!preds.count(phi->getOperand(i).getMBB())) {
 166           phi->RemoveOperand(i);
 167           phi->RemoveOperand(i-1);
 168           ModifiedPHI = true;
 169         }
 170
 171       if (phi->getNumOperands() == 3) {
 172         const MachineOperand &Input = phi->getOperand(1);
 173         const MachineOperand &Output = phi->getOperand(0);
 174         Register InputReg = Input.getReg();
 175         Register OutputReg = Output.getReg();
 176         assert(Output.getSubReg() == 0 && "Cannot have output subregister");
 177         ModifiedPHI = true;
 178
 179         if (InputReg != OutputReg) {
 180           MachineRegisterInfo &MRI = F.getRegInfo();
 181           unsigned InputSub = Input.getSubReg();
 182           if (InputSub == 0 &&
 183               MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
 184               !Input.isUndef()) {
 185             MRI.replaceRegWith(OutputReg, InputReg);
 186           } else {
 187             // The input register to the PHI has a subregister or it can't be
 188             // constrained to the proper register class or it is undef:
 189             // insert a COPY instead of simply replacing the output
 190             // with the input.
 191             const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
 192             BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(),
 193                     TII->get(TargetOpcode::COPY), OutputReg)
 194                 .addReg(InputReg, getRegState(Input), InputSub);
 195           }
 196           phi++->eraseFromParent();
 197         }
 198         continue;
 199       }
 200
 201       ++phi;
 202     }
 203   }
 204
 205   F.RenumberBlocks();
 206
 207   return (!DeadBlocks.empty() || ModifiedPHI);
 208 }