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/Pass.h"
40 using namespace llvm;
41 
42 static bool eliminateUnreachableBlock(Function &F) {
43   df_iterator_default_set<BasicBlock*> 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 {
75   bool runOnFunction(Function &F) override {
76     return eliminateUnreachableBlock(F);
77   }
78 
79 public:
80   static char ID; // Pass identification, replacement for typeid
81   UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
82     initializeUnreachableBlockElimLegacyPassPass(
83         *PassRegistry::getPassRegistry());
84   }
85 
86   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 
95 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
96   return new UnreachableBlockElimLegacyPass();
97 }
98 
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
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 
126 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
127   AU.addPreserved<MachineLoopInfo>();
128   AU.addPreserved<MachineDominatorTree>();
129   MachineFunctionPass::getAnalysisUsage(AU);
130 }
131 
132 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
133   df_iterator_default_set<MachineBasicBlock*> 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         const MachineOperand &Input = phi->getOperand(1);
199         const MachineOperand &Output = phi->getOperand(0);
200         unsigned InputReg = Input.getReg();
201         unsigned OutputReg = Output.getReg();
202         assert(Output.getSubReg() == 0 && "Cannot have output subregister");
203         ModifiedPHI = true;
204 
205         if (InputReg != OutputReg) {
206           MachineRegisterInfo &MRI = F.getRegInfo();
207           unsigned InputSub = Input.getSubReg();
208           if (InputSub == 0 &&
209               MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
210               !Input.isUndef()) {
211             MRI.replaceRegWith(OutputReg, InputReg);
212           } else {
213             // The input register to the PHI has a subregister or it can't be
214             // constrained to the proper register class or it is undef:
215             // insert a COPY instead of simply replacing the output
216             // with the input.
217             const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
218             BuildMI(*BB, BB->getFirstNonPHI(), phi->getDebugLoc(),
219                     TII->get(TargetOpcode::COPY), OutputReg)
220                 .addReg(InputReg, getRegState(Input), InputSub);
221           }
222           phi++->eraseFromParent();
223         }
224         continue;
225       }
226 
227       ++phi;
228     }
229   }
230 
231   F.RenumberBlocks();
232 
233   return (!DeadBlocks.empty() || ModifiedPHI);
234 }
235