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/Pass.h"
  40 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
  41 using namespace llvm;
  42
  43 namespace {
  44 class UnreachableBlockElimLegacyPass : public FunctionPass {
  45   bool runOnFunction(Function &F) override {
  46     return llvm::EliminateUnreachableBlocks(F);
  47   }
  48
  49 public:
  50   static char ID; // Pass identification, replacement for typeid
  51   UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
  52     initializeUnreachableBlockElimLegacyPassPass(
  53         *PassRegistry::getPassRegistry());
  54   }
  55
  56   void getAnalysisUsage(AnalysisUsage &AU) const override {
  57     AU.addPreserved<DominatorTreeWrapperPass>();
  58   }
  59 };
  60 }
  61 char UnreachableBlockElimLegacyPass::ID = 0;
  62 INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
  63                 "Remove unreachable blocks from the CFG", false, false)
  64
  65 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
  66   return new UnreachableBlockElimLegacyPass();
  67 }
  68
  69 PreservedAnalyses UnreachableBlockElimPass::run(Function &F,
  70                                                 FunctionAnalysisManager &AM) {
  71   bool Changed = llvm::EliminateUnreachableBlocks(F);
  72   if (!Changed)
  73     return PreservedAnalyses::all();
  74   PreservedAnalyses PA;
  75   PA.preserve<DominatorTreeAnalysis>();
  76   return PA;
  77 }
  78
  79 namespace {
  80   class UnreachableMachineBlockElim : public MachineFunctionPass {
  81     bool runOnMachineFunction(MachineFunction &F) override;
  82     void getAnalysisUsage(AnalysisUsage &AU) const override;
  83     MachineModuleInfo *MMI;
  84   public:
  85     static char ID; // Pass identification, replacement for typeid
  86     UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
  87   };
  88 }
  89 char UnreachableMachineBlockElim::ID = 0;
  90
  91 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
  92   "Remove unreachable machine basic blocks", false, false)
  93
  94 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
  95
  96 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
  97   AU.addPreserved<MachineLoopInfo>();
  98   AU.addPreserved<MachineDominatorTree>();
  99   MachineFunctionPass::getAnalysisUsage(AU);
 100 }
 101
 102 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
 103   df_iterator_default_set<MachineBasicBlock*> Reachable;
 104   bool ModifiedPHI = false;
 105
 106   auto *MMIWP = getAnalysisIfAvailable<MachineModuleInfoWrapperPass>();
 107   MMI = MMIWP ? &MMIWP->getMMI() : nullptr;
 108   MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
 109   MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
 110
 111   // Mark all reachable blocks.
 112   for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
 113     (void)BB/* Mark all reachable blocks */;
 114
 115   // Loop over all dead blocks, remembering them and deleting all instructions
 116   // in them.
 117   std::vector<MachineBasicBlock*> DeadBlocks;
 118   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
 119     MachineBasicBlock *BB = &*I;
 120
 121     // Test for deadness.
 122     if (!Reachable.count(BB)) {
 123       DeadBlocks.push_back(BB);
 124
 125       // Update dominator and loop info.
 126       if (MLI) MLI->removeBlock(BB);
 127       if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
 128
 129       while (BB->succ_begin() != BB->succ_end()) {
 130         MachineBasicBlock* succ = *BB->succ_begin();
 131
 132         MachineBasicBlock::iterator start = succ->begin();
 133         while (start != succ->end() && start->isPHI()) {
 134           for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
 135             if (start->getOperand(i).isMBB() &&
 136                 start->getOperand(i).getMBB() == BB) {
 137               start->RemoveOperand(i);
 138               start->RemoveOperand(i-1);
 139             }
 140
 141           start++;
 142         }
 143
 144         BB->removeSuccessor(BB->succ_begin());
 145       }
 146     }
 147   }
 148
 149   // Actually remove the blocks now.
 150   for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
 151     // Remove any call site information for calls in the block.
 152     for (auto &I : DeadBlocks[i]->instrs())
 153       if (I.isCall(MachineInstr::IgnoreBundle))
 154         DeadBlocks[i]->getParent()->eraseCallSiteInfo(&I);
 155
 156     DeadBlocks[i]->eraseFromParent();
 157   }
 158
 159   // Cleanup PHI nodes.
 160   for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
 161     MachineBasicBlock *BB = &*I;
 162     // Prune unneeded PHI entries.
 163     SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
 164                                              BB->pred_end());
 165     MachineBasicBlock::iterator phi = BB->begin();
 166     while (phi != BB->end() && phi->isPHI()) {
 167       for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
 168         if (!preds.count(phi->getOperand(i).getMBB())) {
 169           phi->RemoveOperand(i);
 170           phi->RemoveOperand(i-1);
 171           ModifiedPHI = true;
 172         }
 173
 174       if (phi->getNumOperands() == 3) {
 175         const MachineOperand &Input = phi->getOperand(1);
 176         const MachineOperand &Output = phi->getOperand(0);
 177         Register InputReg = Input.getReg();
 178         Register OutputReg = Output.getReg();
 179         assert(Output.getSubReg() == 0 && "Cannot have output subregister");
 180         ModifiedPHI = true;
 181
 182         if (InputReg != OutputReg) {
 183           MachineRegisterInfo &MRI = F.getRegInfo();
 184           unsigned InputSub = Input.getSubReg();
 185           if (InputSub == 0 &&
 186               MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
 187               !Input.isUndef()) {
 188             MRI.replaceRegWith(OutputReg, InputReg);
 189           } else {
 190             // The input register to the PHI has a subregister or it can't be
 191             // constrained to the proper register class or it is undef:
 192             // insert a COPY instead of simply replacing the output
 193             // with the input.
 194             const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
 195             BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(),
 196                     TII->get(TargetOpcode::COPY), OutputReg)
 197                 .addReg(InputReg, getRegState(Input), InputSub);
 198           }
 199           phi++->eraseFromParent();
 200         }
 201         continue;
 202       }
 203
 204       ++phi;
 205     }
 206   }
 207
 208   F.RenumberBlocks();
 209
 210   return (!DeadBlocks.empty() || ModifiedPHI);
 211 }