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 {
46   bool runOnFunction(Function &F) override {
47     return llvm::EliminateUnreachableBlocks(F);
48   }
49 
50 public:
51   static char ID; // Pass identification, replacement for typeid
52   UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
53     initializeUnreachableBlockElimLegacyPassPass(
54         *PassRegistry::getPassRegistry());
55   }
56 
57   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 
66 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
67   return new UnreachableBlockElimLegacyPass();
68 }
69 
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
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 
97 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
98   AU.addPreserved<MachineLoopInfo>();
99   AU.addPreserved<MachineDominatorTree>();
100   MachineFunctionPass::getAnalysisUsage(AU);
101 }
102 
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