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"
36using namespace llvm;
37
38namespace {
39class UnreachableBlockElimLegacyPass : public FunctionPass {
40 bool runOnFunction(Function &F) override {
41 return llvm::EliminateUnreachableBlocks(F);
42 }
43
44public:
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}
56char UnreachableBlockElimLegacyPass::ID = 0;
57INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
58 "Remove unreachable blocks from the CFG", false, false)
59
60FunctionPass *llvm::createUnreachableBlockEliminationPass() {
61 return new UnreachableBlockElimLegacyPass();
62}
63
64PreservedAnalyses 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
74namespace {
75class UnreachableMachineBlockElim {
76 MachineDominatorTree *MDT;
77 MachineLoopInfo *MLI;
78
79public:
80 UnreachableMachineBlockElim(MachineDominatorTree *MDT, MachineLoopInfo *MLI)
81 : MDT(MDT), MLI(MLI) {}
82 bool run(MachineFunction &MF);
83};
84
85class UnreachableMachineBlockElimLegacy : public MachineFunctionPass {
86 bool runOnMachineFunction(MachineFunction &F) override;
87 void getAnalysisUsage(AnalysisUsage &AU) const override;
88
89public:
90 static char ID; // Pass identification, replacement for typeid
91 UnreachableMachineBlockElimLegacy() : MachineFunctionPass(ID) {}
92};
93} // namespace
94
95char UnreachableMachineBlockElimLegacy::ID = 0;
96
97INITIALIZE_PASS(UnreachableMachineBlockElimLegacy,
98 "unreachable-mbb-elimination",
99 "Remove unreachable machine basic blocks", false, false)
100
101char &llvm::UnreachableMachineBlockElimID =
102 UnreachableMachineBlockElimLegacy::ID;
103
104void UnreachableMachineBlockElimLegacy::getAnalysisUsage(
105 AnalysisUsage &AU) const {
106 AU.addPreserved<MachineLoopInfoWrapperPass>();
107 AU.addPreserved<MachineDominatorTreeWrapperPass>();
108 MachineFunctionPass::getAnalysisUsage(AU);
109}
110
111PreservedAnalyses
112UnreachableMachineBlockElimPass::run(MachineFunction &MF,
113 MachineFunctionAnalysisManager &AM) {
114 auto *MDT = AM.getCachedResult<MachineDominatorTreeAnalysis>(IR&: MF);
115 auto *MLI = AM.getCachedResult<MachineLoopAnalysis>(IR&: MF);
116
117 if (!UnreachableMachineBlockElim(MDT, MLI).run(MF))
118 return PreservedAnalyses::all();
119
120 return getMachineFunctionPassPreservedAnalyses()
121 .preserve<MachineLoopAnalysis>()
122 .preserve<MachineDominatorTreeAnalysis>();
123}
124
125bool UnreachableMachineBlockElimLegacy::runOnMachineFunction(
126 MachineFunction &MF) {
127 MachineDominatorTreeWrapperPass *MDTWrapper =
128 getAnalysisIfAvailable<MachineDominatorTreeWrapperPass>();
129 MachineDominatorTree *MDT = MDTWrapper ? &MDTWrapper->getDomTree() : nullptr;
130 MachineLoopInfoWrapperPass *MLIWrapper =
131 getAnalysisIfAvailable<MachineLoopInfoWrapperPass>();
132 MachineLoopInfo *MLI = MLIWrapper ? &MLIWrapper->getLI() : nullptr;
133
134 return UnreachableMachineBlockElim(MDT, MLI).run(MF);
135}
136
137bool UnreachableMachineBlockElim::run(MachineFunction &F) {
138 df_iterator_default_set<MachineBasicBlock *> Reachable;
139 bool ModifiedPHI = false;
140
141 // Mark all reachable blocks.
142 for (MachineBasicBlock *BB : depth_first_ext(G: &F, S&: Reachable))
143 (void)BB/* Mark all reachable blocks */;
144
145 // Loop over all dead blocks, remembering them and deleting all instructions
146 // in them.
147 std::vector<MachineBasicBlock*> DeadBlocks;
148 for (MachineBasicBlock &BB : F) {
149 // Test for deadness.
150 if (!Reachable.count(Ptr: &BB)) {
151 DeadBlocks.push_back(x: &BB);
152
153 // Update dominator and loop info.
154 if (MLI) MLI->removeBlock(BB: &BB);
155 if (MDT && MDT->getNode(BB: &BB)) MDT->eraseNode(BB: &BB);
156
157 while (!BB.succ_empty()) {
158 MachineBasicBlock* succ = *BB.succ_begin();
159
160 for (MachineInstr &Phi : succ->phis()) {
161 for (unsigned i = Phi.getNumOperands() - 1; i >= 2; i -= 2) {
162 if (Phi.getOperand(i).isMBB() &&
163 Phi.getOperand(i).getMBB() == &BB) {
164 Phi.removeOperand(OpNo: i);
165 Phi.removeOperand(OpNo: i - 1);
166 }
167 }
168 }
169
170 BB.removeSuccessor(I: BB.succ_begin());
171 }
172 }
173 }
174
175 // Actually remove the blocks now.
176 for (MachineBasicBlock *BB : DeadBlocks) {
177 // Remove any call information for calls in the block.
178 for (auto &I : BB->instrs())
179 if (I.shouldUpdateAdditionalCallInfo())
180 BB->getParent()->eraseAdditionalCallInfo(MI: &I);
181
182 BB->eraseFromParent();
183 }
184
185 // Cleanup PHI nodes.
186 for (MachineBasicBlock &BB : F) {
187 // Prune unneeded PHI entries.
188 SmallPtrSet<MachineBasicBlock *, 8> preds(llvm::from_range,
189 BB.predecessors());
190 for (MachineInstr &Phi : make_early_inc_range(Range: BB.phis())) {
191 for (unsigned i = Phi.getNumOperands() - 1; i >= 2; i -= 2) {
192 if (!preds.count(Ptr: Phi.getOperand(i).getMBB())) {
193 Phi.removeOperand(OpNo: i);
194 Phi.removeOperand(OpNo: i - 1);
195 ModifiedPHI = true;
196 }
197 }
198
199 if (Phi.getNumOperands() == 3) {
200 const MachineOperand &Input = Phi.getOperand(i: 1);
201 const MachineOperand &Output = Phi.getOperand(i: 0);
202 Register InputReg = Input.getReg();
203 Register OutputReg = Output.getReg();
204 assert(Output.getSubReg() == 0 && "Cannot have output subregister");
205 ModifiedPHI = true;
206
207 if (InputReg != OutputReg) {
208 MachineRegisterInfo &MRI = F.getRegInfo();
209 unsigned InputSub = Input.getSubReg();
210 if (InputSub == 0 &&
211 MRI.constrainRegClass(Reg: InputReg, RC: MRI.getRegClass(Reg: OutputReg)) &&
212 !Input.isUndef()) {
213 MRI.replaceRegWith(FromReg: OutputReg, ToReg: InputReg);
214 } else {
215 // The input register to the PHI has a subregister or it can't be
216 // constrained to the proper register class or it is undef:
217 // insert a COPY instead of simply replacing the output
218 // with the input.
219 const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
220 BuildMI(BB, I: BB.getFirstNonPHI(), MIMD: Phi.getDebugLoc(),
221 MCID: TII->get(Opcode: TargetOpcode::COPY), DestReg: OutputReg)
222 .addReg(RegNo: InputReg, flags: getRegState(RegOp: Input), SubReg: InputSub);
223 }
224 Phi.eraseFromParent();
225 }
226 }
227 }
228 }
229
230 F.RenumberBlocks();
231 if (MDT)
232 MDT->updateBlockNumbers();
233
234 return (!DeadBlocks.empty() || ModifiedPHI);
235}
236