1 //===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass is an extremely simple version of the SimplifyCFG pass. Its sole
11 // job is to delete LLVM basic blocks that are not reachable from the entry
12 // node. To do this, it performs a simple depth first traversal of the CFG,
13 // then deletes any unvisited nodes.
14 //
15 // Note that this pass is really a hack. In particular, the instruction
16 // selectors for various targets should just not generate code for unreachable
17 // blocks. Until LLVM has a more systematic way of defining instruction
18 // selectors, however, we cannot really expect them to handle additional
19 // complexity.
20 //
21 //===----------------------------------------------------------------------===//
22
23 #include "llvm/CodeGen/UnreachableBlockElim.h"
24 #include "llvm/ADT/DepthFirstIterator.h"
25 #include "llvm/ADT/SmallPtrSet.h"
26 #include "llvm/CodeGen/MachineDominators.h"
27 #include "llvm/CodeGen/MachineFunctionPass.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/IR/CFG.h"
33 #include "llvm/IR/Constant.h"
34 #include "llvm/IR/Dominators.h"
35 #include "llvm/IR/Function.h"
36 #include "llvm/IR/Instructions.h"
37 #include "llvm/IR/Type.h"
38 #include "llvm/Pass.h"
39 #include "llvm/Target/TargetInstrInfo.h"
40 using namespace llvm;
41
eliminateUnreachableBlock(Function & F)42 static bool eliminateUnreachableBlock(Function &F) {
43 SmallPtrSet<BasicBlock*, 8> Reachable;
44
45 // Mark all reachable blocks.
46 for (BasicBlock *BB : depth_first_ext(&F, Reachable))
47 (void)BB/* Mark all reachable blocks */;
48
49 // Loop over all dead blocks, remembering them and deleting all instructions
50 // in them.
51 std::vector<BasicBlock*> DeadBlocks;
52 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
53 if (!Reachable.count(&*I)) {
54 BasicBlock *BB = &*I;
55 DeadBlocks.push_back(BB);
56 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
57 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
58 BB->getInstList().pop_front();
59 }
60 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
61 (*SI)->removePredecessor(BB);
62 BB->dropAllReferences();
63 }
64
65 // Actually remove the blocks now.
66 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
67 DeadBlocks[i]->eraseFromParent();
68 }
69
70 return !DeadBlocks.empty();
71 }
72
73 namespace {
74 class UnreachableBlockElimLegacyPass : public FunctionPass {
runOnFunction(Function & F)75 bool runOnFunction(Function &F) override {
76 return eliminateUnreachableBlock(F);
77 }
78
79 public:
80 static char ID; // Pass identification, replacement for typeid
UnreachableBlockElimLegacyPass()81 UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
82 initializeUnreachableBlockElimLegacyPassPass(
83 *PassRegistry::getPassRegistry());
84 }
85
getAnalysisUsage(AnalysisUsage & AU) const86 void getAnalysisUsage(AnalysisUsage &AU) const override {
87 AU.addPreserved<DominatorTreeWrapperPass>();
88 }
89 };
90 }
91 char UnreachableBlockElimLegacyPass::ID = 0;
92 INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
93 "Remove unreachable blocks from the CFG", false, false)
94
createUnreachableBlockEliminationPass()95 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
96 return new UnreachableBlockElimLegacyPass();
97 }
98
run(Function & F,FunctionAnalysisManager & AM)99 PreservedAnalyses UnreachableBlockElimPass::run(Function &F,
100 FunctionAnalysisManager &AM) {
101 bool Changed = eliminateUnreachableBlock(F);
102 if (!Changed)
103 return PreservedAnalyses::all();
104 PreservedAnalyses PA;
105 PA.preserve<DominatorTreeAnalysis>();
106 return PA;
107 }
108
109 namespace {
110 class UnreachableMachineBlockElim : public MachineFunctionPass {
111 bool runOnMachineFunction(MachineFunction &F) override;
112 void getAnalysisUsage(AnalysisUsage &AU) const override;
113 MachineModuleInfo *MMI;
114 public:
115 static char ID; // Pass identification, replacement for typeid
UnreachableMachineBlockElim()116 UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
117 };
118 }
119 char UnreachableMachineBlockElim::ID = 0;
120
121 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
122 "Remove unreachable machine basic blocks", false, false)
123
124 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
125
getAnalysisUsage(AnalysisUsage & AU) const126 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
127 AU.addPreserved<MachineLoopInfo>();
128 AU.addPreserved<MachineDominatorTree>();
129 MachineFunctionPass::getAnalysisUsage(AU);
130 }
131
runOnMachineFunction(MachineFunction & F)132 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
133 SmallPtrSet<MachineBasicBlock*, 8> Reachable;
134 bool ModifiedPHI = false;
135
136 MMI = getAnalysisIfAvailable<MachineModuleInfo>();
137 MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
138 MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
139
140 // Mark all reachable blocks.
141 for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
142 (void)BB/* Mark all reachable blocks */;
143
144 // Loop over all dead blocks, remembering them and deleting all instructions
145 // in them.
146 std::vector<MachineBasicBlock*> DeadBlocks;
147 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
148 MachineBasicBlock *BB = &*I;
149
150 // Test for deadness.
151 if (!Reachable.count(BB)) {
152 DeadBlocks.push_back(BB);
153
154 // Update dominator and loop info.
155 if (MLI) MLI->removeBlock(BB);
156 if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
157
158 while (BB->succ_begin() != BB->succ_end()) {
159 MachineBasicBlock* succ = *BB->succ_begin();
160
161 MachineBasicBlock::iterator start = succ->begin();
162 while (start != succ->end() && start->isPHI()) {
163 for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
164 if (start->getOperand(i).isMBB() &&
165 start->getOperand(i).getMBB() == BB) {
166 start->RemoveOperand(i);
167 start->RemoveOperand(i-1);
168 }
169
170 start++;
171 }
172
173 BB->removeSuccessor(BB->succ_begin());
174 }
175 }
176 }
177
178 // Actually remove the blocks now.
179 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
180 DeadBlocks[i]->eraseFromParent();
181
182 // Cleanup PHI nodes.
183 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
184 MachineBasicBlock *BB = &*I;
185 // Prune unneeded PHI entries.
186 SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
187 BB->pred_end());
188 MachineBasicBlock::iterator phi = BB->begin();
189 while (phi != BB->end() && phi->isPHI()) {
190 for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
191 if (!preds.count(phi->getOperand(i).getMBB())) {
192 phi->RemoveOperand(i);
193 phi->RemoveOperand(i-1);
194 ModifiedPHI = true;
195 }
196
197 if (phi->getNumOperands() == 3) {
198 unsigned Input = phi->getOperand(1).getReg();
199 unsigned Output = phi->getOperand(0).getReg();
200
201 phi++->eraseFromParent();
202 ModifiedPHI = true;
203
204 if (Input != Output) {
205 MachineRegisterInfo &MRI = F.getRegInfo();
206 MRI.constrainRegClass(Input, MRI.getRegClass(Output));
207 MRI.replaceRegWith(Output, Input);
208 }
209
210 continue;
211 }
212
213 ++phi;
214 }
215 }
216
217 F.RenumberBlocks();
218
219 return (!DeadBlocks.empty() || ModifiedPHI);
220 }
221