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 {
runOnFunction(Function & F)40 bool runOnFunction(Function &F) override {
41 return llvm::EliminateUnreachableBlocks(F);
42 }
43
44 public:
45 static char ID; // Pass identification, replacement for typeid
UnreachableBlockElimLegacyPass()46 UnreachableBlockElimLegacyPass() : FunctionPass(ID) {
47 initializeUnreachableBlockElimLegacyPassPass(
48 *PassRegistry::getPassRegistry());
49 }
50
getAnalysisUsage(AnalysisUsage & AU) const51 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
createUnreachableBlockEliminationPass()60 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
61 return new UnreachableBlockElimLegacyPass();
62 }
63
run(Function & F,FunctionAnalysisManager & AM)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
UnreachableMachineBlockElim()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
getAnalysisUsage(AnalysisUsage & AU) const91 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
92 AU.addPreserved<MachineLoopInfo>();
93 AU.addPreserved<MachineDominatorTree>();
94 MachineFunctionPass::getAnalysisUsage(AU);
95 }
96
runOnMachineFunction(MachineFunction & F)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