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
23using namespace llvm;
24
25#define DEBUG_TYPE "gc-empty-basic-blocks"
26
27STATISTIC(NumEmptyBlocksRemoved, "Number of empty blocks removed");
28
29class GCEmptyBasicBlocks : public MachineFunctionPass {
30public:
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
44bool 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(x: 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(Range&: *MBB, P: [](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(Old: &*MBB, New: &*NextMBB);
77 // Update the jump tables.
78 if (JTI)
79 JTI->ReplaceMBBInJumpTables(Old: &*MBB, New: &*NextMBB);
80 // Remove this block from predecessors of all its successors.
81 while (!MBB->succ_empty())
82 MBB->removeSuccessor(I: 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
91char GCEmptyBasicBlocks::ID = 0;
92INITIALIZE_PASS(GCEmptyBasicBlocks, "gc-empty-basic-blocks",
93 "Removes empty basic blocks and redirects their uses to their "
94 "fallthrough blocks.",
95 false, false)
96
97MachineFunctionPass *llvm::createGCEmptyBasicBlocksPass() {
98 return new GCEmptyBasicBlocks();
99}
100