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