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"
36 using namespace llvm;
37 
38 namespace {
39 class UnreachableBlockElimLegacyPass : public FunctionPass {
40   bool runOnFunction(Function &F) override {
41     return llvm::EliminateUnreachableBlocks(F);
42   }
43 
44 public:
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 }
56 char UnreachableBlockElimLegacyPass::ID = 0;
57 INITIALIZE_PASS(UnreachableBlockElimLegacyPass, "unreachableblockelim",
58                 "Remove unreachable blocks from the CFG", false, false)
59 
60 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
61   return new UnreachableBlockElimLegacyPass();
62 }
63 
64 PreservedAnalyses 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 
74 namespace {
75   class UnreachableMachineBlockElim : public MachineFunctionPass {
76     bool runOnMachineFunction(MachineFunction &F) override;
77     void getAnalysisUsage(AnalysisUsage &AU) const override;
78 
79   public:
80     static char ID; // Pass identification, replacement for typeid
81     UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
82   };
83 }
84 char UnreachableMachineBlockElim::ID = 0;
85 
86 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
87   "Remove unreachable machine basic blocks", false, false)
88 
89 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
90 
91 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
92   AU.addPreserved<MachineLoopInfo>();
93   AU.addPreserved<MachineDominatorTree>();
94   MachineFunctionPass::getAnalysisUsage(AU);
95 }
96 
97 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
98   df_iterator_default_set<MachineBasicBlock*> Reachable;
99   bool ModifiedPHI = false;
100 
101   MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
102   MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
103 
104   // Mark all reachable blocks.
105   for (MachineBasicBlock *BB : depth_first_ext(&F, Reachable))
106     (void)BB/* Mark all reachable blocks */;
107 
108   // Loop over all dead blocks, remembering them and deleting all instructions
109   // in them.
110   std::vector<MachineBasicBlock*> DeadBlocks;
111   for (MachineBasicBlock &BB : F) {
112     // Test for deadness.
113     if (!Reachable.count(&BB)) {
114       DeadBlocks.push_back(&BB);
115 
116       // Update dominator and loop info.
117       if (MLI) MLI->removeBlock(&BB);
118       if (MDT && MDT->getNode(&BB)) MDT->eraseNode(&BB);
119 
120       while (BB.succ_begin() != BB.succ_end()) {
121         MachineBasicBlock* succ = *BB.succ_begin();
122 
123         MachineBasicBlock::iterator start = succ->begin();
124         while (start != succ->end() && start->isPHI()) {
125           for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
126             if (start->getOperand(i).isMBB() &&
127                 start->getOperand(i).getMBB() == &BB) {
128               start->removeOperand(i);
129               start->removeOperand(i-1);
130             }
131 
132           start++;
133         }
134 
135         BB.removeSuccessor(BB.succ_begin());
136       }
137     }
138   }
139 
140   // Actually remove the blocks now.
141   for (MachineBasicBlock *BB : DeadBlocks) {
142     // Remove any call site information for calls in the block.
143     for (auto &I : BB->instrs())
144       if (I.shouldUpdateCallSiteInfo())
145         BB->getParent()->eraseCallSiteInfo(&I);
146 
147     BB->eraseFromParent();
148   }
149 
150   // Cleanup PHI nodes.
151   for (MachineBasicBlock &BB : F) {
152     // Prune unneeded PHI entries.
153     SmallPtrSet<MachineBasicBlock*, 8> preds(BB.pred_begin(),
154                                              BB.pred_end());
155     MachineBasicBlock::iterator phi = BB.begin();
156     while (phi != BB.end() && phi->isPHI()) {
157       for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
158         if (!preds.count(phi->getOperand(i).getMBB())) {
159           phi->removeOperand(i);
160           phi->removeOperand(i-1);
161           ModifiedPHI = true;
162         }
163 
164       if (phi->getNumOperands() == 3) {
165         const MachineOperand &Input = phi->getOperand(1);
166         const MachineOperand &Output = phi->getOperand(0);
167         Register InputReg = Input.getReg();
168         Register OutputReg = Output.getReg();
169         assert(Output.getSubReg() == 0 && "Cannot have output subregister");
170         ModifiedPHI = true;
171 
172         if (InputReg != OutputReg) {
173           MachineRegisterInfo &MRI = F.getRegInfo();
174           unsigned InputSub = Input.getSubReg();
175           if (InputSub == 0 &&
176               MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
177               !Input.isUndef()) {
178             MRI.replaceRegWith(OutputReg, InputReg);
179           } else {
180             // The input register to the PHI has a subregister or it can't be
181             // constrained to the proper register class or it is undef:
182             // insert a COPY instead of simply replacing the output
183             // with the input.
184             const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
185             BuildMI(BB, BB.getFirstNonPHI(), phi->getDebugLoc(),
186                     TII->get(TargetOpcode::COPY), OutputReg)
187                 .addReg(InputReg, getRegState(Input), InputSub);
188           }
189           phi++->eraseFromParent();
190         }
191         continue;
192       }
193 
194       ++phi;
195     }
196   }
197 
198   F.RenumberBlocks();
199 
200   return (!DeadBlocks.empty() || ModifiedPHI);
201 }
202