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/InitializePasses.h"
40 #include "llvm/Pass.h"
41 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
42 using namespace llvm;
43
44 namespace {
45 class UnreachableBlockElimLegacyPass : public FunctionPass {
runOnFunction(Function & F)46 bool runOnFunction(Function &F) override {
47 return llvm::EliminateUnreachableBlocks(F);
48 }
49
50 public:
51 static char ID; // Pass identification, replacement for typeid
UnreachableBlockElimLegacyPass()52 UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
53 initializeUnreachableBlockElimLegacyPassPass(
54 *PassRegistry::getPassRegistry());
55 }
56
getAnalysisUsage(AnalysisUsage & AU) const57 void getAnalysisUsage(AnalysisUsage &AU) const override {
58 AU.addPreserved<DominatorTreeWrapperPass>();
59 }
60 };
61 }
62 char UnreachableBlockElimLegacyPass::ID = 0;
63 INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
64 "Remove unreachable blocks from the CFG", false, false)
65
createUnreachableBlockEliminationPass()66 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
67 return new UnreachableBlockElimLegacyPass();
68 }
69
run(Function & F,FunctionAnalysisManager & AM)70 PreservedAnalyses UnreachableBlockElimPass::run(Function &F,
71 FunctionAnalysisManager &AM) {
72 bool Changed = llvm::EliminateUnreachableBlocks(F);
73 if (!Changed)
74 return PreservedAnalyses::all();
75 PreservedAnalyses PA;
76 PA.preserve<DominatorTreeAnalysis>();
77 return PA;
78 }
79
80 namespace {
81 class UnreachableMachineBlockElim : public MachineFunctionPass {
82 bool runOnMachineFunction(MachineFunction &F) override;
83 void getAnalysisUsage(AnalysisUsage &AU) const override;
84
85 public:
86 static char ID; // Pass identification, replacement for typeid
UnreachableMachineBlockElim()87 UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
88 };
89 }
90 char UnreachableMachineBlockElim::ID = 0;
91
92 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
93 "Remove unreachable machine basic blocks", false, false)
94
95 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
96
getAnalysisUsage(AnalysisUsage & AU) const97 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
98 AU.addPreserved<MachineLoopInfo>();
99 AU.addPreserved<MachineDominatorTree>();
100 MachineFunctionPass::getAnalysisUsage(AU);
101 }
102
runOnMachineFunction(MachineFunction & F)103 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
104 df_iterator_default_set<MachineBasicBlock*> Reachable;
105 bool ModifiedPHI = false;
106
107 MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
108 MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
109
110 // Mark all reachable blocks.
111 for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
112 (void)BB/* Mark all reachable blocks */;
113
114 // Loop over all dead blocks, remembering them and deleting all instructions
115 // in them.
116 std::vector<MachineBasicBlock*> DeadBlocks;
117 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
118 MachineBasicBlock *BB = &*I;
119
120 // Test for deadness.
121 if (!Reachable.count(BB)) {
122 DeadBlocks.push_back(BB);
123
124 // Update dominator and loop info.
125 if (MLI) MLI->removeBlock(BB);
126 if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
127
128 while (BB->succ_begin() != BB->succ_end()) {
129 MachineBasicBlock* succ = *BB->succ_begin();
130
131 MachineBasicBlock::iterator start = succ->begin();
132 while (start != succ->end() && start->isPHI()) {
133 for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
134 if (start->getOperand(i).isMBB() &&
135 start->getOperand(i).getMBB() == BB) {
136 start->RemoveOperand(i);
137 start->RemoveOperand(i-1);
138 }
139
140 start++;
141 }
142
143 BB->removeSuccessor(BB->succ_begin());
144 }
145 }
146 }
147
148 // Actually remove the blocks now.
149 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
150 // Remove any call site information for calls in the block.
151 for (auto &I : DeadBlocks[i]->instrs())
152 if (I.shouldUpdateCallSiteInfo())
153 DeadBlocks[i]->getParent()->eraseCallSiteInfo(&I);
154
155 DeadBlocks[i]->eraseFromParent();
156 }
157
158 // Cleanup PHI nodes.
159 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
160 MachineBasicBlock *BB = &*I;
161 // Prune unneeded PHI entries.
162 SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
163 BB->pred_end());
164 MachineBasicBlock::iterator phi = BB->begin();
165 while (phi != BB->end() && phi->isPHI()) {
166 for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
167 if (!preds.count(phi->getOperand(i).getMBB())) {
168 phi->RemoveOperand(i);
169 phi->RemoveOperand(i-1);
170 ModifiedPHI = true;
171 }
172
173 if (phi->getNumOperands() == 3) {
174 const MachineOperand &Input = phi->getOperand(1);
175 const MachineOperand &Output = phi->getOperand(0);
176 Register InputReg = Input.getReg();
177 Register OutputReg = Output.getReg();
178 assert(Output.getSubReg() == 0 && "Cannot have output subregister");
179 ModifiedPHI = true;
180
181 if (InputReg != OutputReg) {
182 MachineRegisterInfo &MRI = F.getRegInfo();
183 unsigned InputSub = Input.getSubReg();
184 if (InputSub == 0 &&
185 MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
186 !Input.isUndef()) {
187 MRI.replaceRegWith(OutputReg, InputReg);
188 } else {
189 // The input register to the PHI has a subregister or it can't be
190 // constrained to the proper register class or it is undef:
191 // insert a COPY instead of simply replacing the output
192 // with the input.
193 const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
194 BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(),
195 TII->get(TargetOpcode::COPY), OutputReg)
196 .addReg(InputReg, getRegState(Input), InputSub);
197 }
198 phi++->eraseFromParent();
199 }
200 continue;
201 }
202
203 ++phi;
204 }
205 }
206
207 F.RenumberBlocks();
208
209 return (!DeadBlocks.empty() || ModifiedPHI);
210 }
211