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         for (MachineInstr &Phi : succ->phis()) {
 124           for (unsigned i = Phi.getNumOperands() - 1; i >= 2; i -= 2) {
 125             if (Phi.getOperand(i).isMBB() &&
 126                 Phi.getOperand(i).getMBB() == &BB) {
 127               Phi.removeOperand(i);
 128               Phi.removeOperand(i - 1);
 129             }
 130           }
 131         }
 132
 133         BB.removeSuccessor(BB.succ_begin());
 134       }
 135     }
 136   }
 137
 138   // Actually remove the blocks now.
 139   for (MachineBasicBlock *BB : DeadBlocks) {
 140     // Remove any call site information for calls in the block.
 141     for (auto &I : BB->instrs())
 142       if (I.shouldUpdateCallSiteInfo())
 143         BB->getParent()->eraseCallSiteInfo(&I);
 144
 145     BB->eraseFromParent();
 146   }
 147
 148   // Cleanup PHI nodes.
 149   for (MachineBasicBlock &BB : F) {
 150     // Prune unneeded PHI entries.
 151     SmallPtrSet<MachineBasicBlock*, 8> preds(BB.pred_begin(),
 152                                              BB.pred_end());
 153     for (MachineInstr &Phi : make_early_inc_range(BB.phis())) {
 154       for (unsigned i = Phi.getNumOperands() - 1; i >= 2; i -= 2) {
 155         if (!preds.count(Phi.getOperand(i).getMBB())) {
 156           Phi.removeOperand(i);
 157           Phi.removeOperand(i - 1);
 158           ModifiedPHI = true;
 159         }
 160       }
 161
 162       if (Phi.getNumOperands() == 3) {
 163         const MachineOperand &Input = Phi.getOperand(1);
 164         const MachineOperand &Output = Phi.getOperand(0);
 165         Register InputReg = Input.getReg();
 166         Register OutputReg = Output.getReg();
 167         assert(Output.getSubReg() == 0 && "Cannot have output subregister");
 168         ModifiedPHI = true;
 169
 170         if (InputReg != OutputReg) {
 171           MachineRegisterInfo &MRI = F.getRegInfo();
 172           unsigned InputSub = Input.getSubReg();
 173           if (InputSub == 0 &&
 174               MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
 175               !Input.isUndef()) {
 176             MRI.replaceRegWith(OutputReg, InputReg);
 177           } else {
 178             // The input register to the PHI has a subregister or it can't be
 179             // constrained to the proper register class or it is undef:
 180             // insert a COPY instead of simply replacing the output
 181             // with the input.
 182             const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
 183             BuildMI(BB, BB.getFirstNonPHI(), Phi.getDebugLoc(),
 184                     TII->get(TargetOpcode::COPY), OutputReg)
 185                 .addReg(InputReg, getRegState(Input), InputSub);
 186           }
 187           Phi.eraseFromParent();
 188         }
 189       }
 190     }
 191   }
 192
 193   F.RenumberBlocks();
 194
 195   return (!DeadBlocks.empty() || ModifiedPHI);
 196 }