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comparison lib/CodeGen/UnreachableBlockElim.cpp @ 0:95c75e76d11b LLVM3.4

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LLVM 3.4
author Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
date Thu, 12 Dec 2013 13:56:28 +0900
parents
children 54457678186b
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-1:000000000000 0:95c75e76d11b
1 //===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This pass is an extremely simple version of the SimplifyCFG pass. Its sole
11 // job is to delete LLVM basic blocks that are not reachable from the entry
12 // node. To do this, it performs a simple depth first traversal of the CFG,
13 // then deletes any unvisited nodes.
14 //
15 // Note that this pass is really a hack. In particular, the instruction
16 // selectors for various targets should just not generate code for unreachable
17 // blocks. Until LLVM has a more systematic way of defining instruction
18 // selectors, however, we cannot really expect them to handle additional
19 // complexity.
20 //
21 //===----------------------------------------------------------------------===//
22
23 #include "llvm/CodeGen/Passes.h"
24 #include "llvm/ADT/DepthFirstIterator.h"
25 #include "llvm/ADT/SmallPtrSet.h"
26 #include "llvm/Analysis/Dominators.h"
27 #include "llvm/CodeGen/MachineDominators.h"
28 #include "llvm/CodeGen/MachineFunctionPass.h"
29 #include "llvm/CodeGen/MachineLoopInfo.h"
30 #include "llvm/CodeGen/MachineModuleInfo.h"
31 #include "llvm/CodeGen/MachineRegisterInfo.h"
32 #include "llvm/IR/Constant.h"
33 #include "llvm/IR/Function.h"
34 #include "llvm/IR/Instructions.h"
35 #include "llvm/IR/Type.h"
36 #include "llvm/Pass.h"
37 #include "llvm/Support/CFG.h"
38 #include "llvm/Target/TargetInstrInfo.h"
39 using namespace llvm;
40
41 namespace {
42 class UnreachableBlockElim : public FunctionPass {
43 virtual bool runOnFunction(Function &F);
44 public:
45 static char ID; // Pass identification, replacement for typeid
46 UnreachableBlockElim() : FunctionPass(ID) {
47 initializeUnreachableBlockElimPass(*PassRegistry::getPassRegistry());
48 }
49
50 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
51 AU.addPreserved<DominatorTree>();
52 }
53 };
54 }
55 char UnreachableBlockElim::ID = 0;
56 INITIALIZE_PASS(UnreachableBlockElim, "unreachableblockelim",
57 "Remove unreachable blocks from the CFG", false, false)
58
59 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
60 return new UnreachableBlockElim();
61 }
62
63 bool UnreachableBlockElim::runOnFunction(Function &F) {
64 SmallPtrSet<BasicBlock*, 8> Reachable;
65
66 // Mark all reachable blocks.
67 for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I =
68 df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I)
69 /* Mark all reachable blocks */;
70
71 // Loop over all dead blocks, remembering them and deleting all instructions
72 // in them.
73 std::vector<BasicBlock*> DeadBlocks;
74 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
75 if (!Reachable.count(I)) {
76 BasicBlock *BB = I;
77 DeadBlocks.push_back(BB);
78 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
79 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
80 BB->getInstList().pop_front();
81 }
82 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
83 (*SI)->removePredecessor(BB);
84 BB->dropAllReferences();
85 }
86
87 // Actually remove the blocks now.
88 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
89 DeadBlocks[i]->eraseFromParent();
90 }
91
92 return DeadBlocks.size();
93 }
94
95
96 namespace {
97 class UnreachableMachineBlockElim : public MachineFunctionPass {
98 virtual bool runOnMachineFunction(MachineFunction &F);
99 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
100 MachineModuleInfo *MMI;
101 public:
102 static char ID; // Pass identification, replacement for typeid
103 UnreachableMachineBlockElim() : MachineFunctionPass(ID) {}
104 };
105 }
106 char UnreachableMachineBlockElim::ID = 0;
107
108 INITIALIZE_PASS(UnreachableMachineBlockElim, "unreachable-mbb-elimination",
109 "Remove unreachable machine basic blocks", false, false)
110
111 char &llvm::UnreachableMachineBlockElimID = UnreachableMachineBlockElim::ID;
112
113 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
114 AU.addPreserved<MachineLoopInfo>();
115 AU.addPreserved<MachineDominatorTree>();
116 MachineFunctionPass::getAnalysisUsage(AU);
117 }
118
119 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
120 SmallPtrSet<MachineBasicBlock*, 8> Reachable;
121 bool ModifiedPHI = false;
122
123 MMI = getAnalysisIfAvailable<MachineModuleInfo>();
124 MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
125 MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
126
127 // Mark all reachable blocks.
128 for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> >
129 I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable);
130 I != E; ++I)
131 /* Mark all reachable blocks */;
132
133 // Loop over all dead blocks, remembering them and deleting all instructions
134 // in them.
135 std::vector<MachineBasicBlock*> DeadBlocks;
136 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
137 MachineBasicBlock *BB = I;
138
139 // Test for deadness.
140 if (!Reachable.count(BB)) {
141 DeadBlocks.push_back(BB);
142
143 // Update dominator and loop info.
144 if (MLI) MLI->removeBlock(BB);
145 if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
146
147 while (BB->succ_begin() != BB->succ_end()) {
148 MachineBasicBlock* succ = *BB->succ_begin();
149
150 MachineBasicBlock::iterator start = succ->begin();
151 while (start != succ->end() && start->isPHI()) {
152 for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
153 if (start->getOperand(i).isMBB() &&
154 start->getOperand(i).getMBB() == BB) {
155 start->RemoveOperand(i);
156 start->RemoveOperand(i-1);
157 }
158
159 start++;
160 }
161
162 BB->removeSuccessor(BB->succ_begin());
163 }
164 }
165 }
166
167 // Actually remove the blocks now.
168 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
169 DeadBlocks[i]->eraseFromParent();
170
171 // Cleanup PHI nodes.
172 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
173 MachineBasicBlock *BB = I;
174 // Prune unneeded PHI entries.
175 SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
176 BB->pred_end());
177 MachineBasicBlock::iterator phi = BB->begin();
178 while (phi != BB->end() && phi->isPHI()) {
179 for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
180 if (!preds.count(phi->getOperand(i).getMBB())) {
181 phi->RemoveOperand(i);
182 phi->RemoveOperand(i-1);
183 ModifiedPHI = true;
184 }
185
186 if (phi->getNumOperands() == 3) {
187 unsigned Input = phi->getOperand(1).getReg();
188 unsigned Output = phi->getOperand(0).getReg();
189
190 MachineInstr* temp = phi;
191 ++phi;
192 temp->eraseFromParent();
193 ModifiedPHI = true;
194
195 if (Input != Output) {
196 MachineRegisterInfo &MRI = F.getRegInfo();
197 MRI.constrainRegClass(Input, MRI.getRegClass(Output));
198 MRI.replaceRegWith(Output, Input);
199 }
200
201 continue;
202 }
203
204 ++phi;
205 }
206 }
207
208 F.RenumberBlocks();
209
210 return (DeadBlocks.size() || ModifiedPHI);
211 }