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         for (MachineInstr &Phi : succ->phis()) {
124           for (unsigned i = Phi.getNumOperands() - 1; i >= 2; i -= 2) {
125             if (Phi.getOperand(i).isMBB() &&
126                 Phi.getOperand(i).getMBB() == &BB) {
127               Phi.removeOperand(i);
128               Phi.removeOperand(i - 1);
129             }
130           }
131         }
132 
133         BB.removeSuccessor(BB.succ_begin());
134       }
135     }
136   }
137 
138   // Actually remove the blocks now.
139   for (MachineBasicBlock *BB : DeadBlocks) {
140     // Remove any call site information for calls in the block.
141     for (auto &I : BB->instrs())
142       if (I.shouldUpdateCallSiteInfo())
143         BB->getParent()->eraseCallSiteInfo(&I);
144 
145     BB->eraseFromParent();
146   }
147 
148   // Cleanup PHI nodes.
149   for (MachineBasicBlock &BB : F) {
150     // Prune unneeded PHI entries.
151     SmallPtrSet<MachineBasicBlock*, 8> preds(BB.pred_begin(),
152                                              BB.pred_end());
153     for (MachineInstr &Phi : make_early_inc_range(BB.phis())) {
154       for (unsigned i = Phi.getNumOperands() - 1; i >= 2; i -= 2) {
155         if (!preds.count(Phi.getOperand(i).getMBB())) {
156           Phi.removeOperand(i);
157           Phi.removeOperand(i - 1);
158           ModifiedPHI = true;
159         }
160       }
161 
162       if (Phi.getNumOperands() == 3) {
163         const MachineOperand &Input = Phi.getOperand(1);
164         const MachineOperand &Output = Phi.getOperand(0);
165         Register InputReg = Input.getReg();
166         Register OutputReg = Output.getReg();
167         assert(Output.getSubReg() == 0 && "Cannot have output subregister");
168         ModifiedPHI = true;
169 
170         if (InputReg != OutputReg) {
171           MachineRegisterInfo &MRI = F.getRegInfo();
172           unsigned InputSub = Input.getSubReg();
173           if (InputSub == 0 &&
174               MRI.constrainRegClass(InputReg, MRI.getRegClass(OutputReg)) &&
175               !Input.isUndef()) {
176             MRI.replaceRegWith(OutputReg, InputReg);
177           } else {
178             // The input register to the PHI has a subregister or it can't be
179             // constrained to the proper register class or it is undef:
180             // insert a COPY instead of simply replacing the output
181             // with the input.
182             const TargetInstrInfo *TII = F.getSubtarget().getInstrInfo();
183             BuildMI(BB, BB.getFirstNonPHI(), Phi.getDebugLoc(),
184                     TII->get(TargetOpcode::COPY), OutputReg)
185                 .addReg(InputReg, getRegState(Input), InputSub);
186           }
187           Phi.eraseFromParent();
188         }
189       }
190     }
191   }
192 
193   F.RenumberBlocks();
194 
195   return (!DeadBlocks.empty() || ModifiedPHI);
196 }
197