llvm/lib/CodeGen/GCEmptyBasicBlocks.cpp

   1 //===-- GCEmptyBasicBlocks.cpp ----------------------------------*- C++ -*-===//
   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 /// \file
  10 /// This file contains the implementation of empty blocks garbage collection
  11 /// pass.
  12 ///
  13 //===----------------------------------------------------------------------===//
  14 #include "llvm/ADT/SmallVector.h"
  15 #include "llvm/ADT/Statistic.h"
  16 #include "llvm/CodeGen/MachineBasicBlock.h"
  17 #include "llvm/CodeGen/MachineFunction.h"
  18 #include "llvm/CodeGen/MachineFunctionPass.h"
  19 #include "llvm/CodeGen/MachineJumpTableInfo.h"
  20 #include "llvm/CodeGen/Passes.h"
  21 #include "llvm/CodeGen/TargetInstrInfo.h"
  22 #include "llvm/InitializePasses.h"
  23
  24 using namespace llvm;
  25
  26 #define DEBUG_TYPE "gc-empty-basic-blocks"
  27
  28 STATISTIC(NumEmptyBlocksRemoved, "Number of empty blocks removed");
  29
  30 class GCEmptyBasicBlocks : public MachineFunctionPass {
  31 public:
  32   static char ID;
  33
  34   GCEmptyBasicBlocks() : MachineFunctionPass(ID) {
  35     initializeGCEmptyBasicBlocksPass(*PassRegistry::getPassRegistry());
  36   }
  37
  38   StringRef getPassName() const override {
  39     return "Remove Empty Basic Blocks.";
  40   }
  41
  42   bool runOnMachineFunction(MachineFunction &MF) override;
  43 };
  44
  45 bool GCEmptyBasicBlocks::runOnMachineFunction(MachineFunction &MF) {
  46   if (MF.size() < 2)
  47     return false;
  48   MachineJumpTableInfo *JTI = MF.getJumpTableInfo();
  49   int NumRemoved = 0;
  50
  51   // Iterate over all blocks except the last one. We can't remove the last block
  52   // since it has no fallthrough block to rewire its predecessors to.
  53   for (MachineFunction::iterator MBB = MF.begin(),
  54                                  LastMBB = MachineFunction::iterator(MF.back()),
  55                                  NextMBB;
  56        MBB != LastMBB; MBB = NextMBB) {
  57     NextMBB = std::next(MBB);
  58     // TODO If a block is an eh pad, or it has address taken, we don't remove
  59     // it. Removing such blocks is possible, but it probably requires a more
  60     // complex logic.
  61     if (MBB->isEHPad() || MBB->hasAddressTaken())
  62       continue;
  63     // Skip blocks with real code.
  64     bool HasAnyRealCode = llvm::any_of(*MBB, [](const MachineInstr &MI) {
  65       return !MI.isPosition() && !MI.isImplicitDef() && !MI.isKill() &&
  66              !MI.isDebugInstr();
  67     });
  68     if (HasAnyRealCode)
  69       continue;
  70
  71     LLVM_DEBUG(dbgs() << "Removing basic block " << MBB->getName()
  72                       << " in function " << MF.getName() << ":\n"
  73                       << *MBB << "\n");
  74     SmallVector<MachineBasicBlock *, 8> Preds(MBB->predecessors());
  75     // Rewire the predecessors of this block to use the next block.
  76     for (auto &Pred : Preds)
  77       Pred->ReplaceUsesOfBlockWith(&*MBB, &*NextMBB);
  78     // Update the jump tables.
  79     if (JTI)
  80       JTI->ReplaceMBBInJumpTables(&*MBB, &*NextMBB);
  81     // Remove this block from predecessors of all its successors.
  82     while (!MBB->succ_empty())
  83       MBB->removeSuccessor(MBB->succ_end() - 1);
  84     // Finally, remove the block from the function.
  85     MBB->eraseFromParent();
  86     ++NumRemoved;
  87   }
  88   NumEmptyBlocksRemoved += NumRemoved;
  89   return NumRemoved != 0;
  90 }
  91
  92 char GCEmptyBasicBlocks::ID = 0;
  93 INITIALIZE_PASS(GCEmptyBasicBlocks, "gc-empty-basic-blocks",
  94                 "Removes empty basic blocks and redirects their uses to their "
  95                 "fallthrough blocks.",
  96                 false, false)
  97
  98 MachineFunctionPass *llvm::createGCEmptyBasicBlocksPass() {
  99   return new GCEmptyBasicBlocks();
 100 }