1 //===-- UnreachableBlockElim.cpp - Remove unreachable blocks for codegen --===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
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
21 //===----------------------------------------------------------------------===//
23 #include "llvm/CodeGen/Passes.h"
24 #include "llvm/Constant.h"
25 #include "llvm/Instructions.h"
26 #include "llvm/Function.h"
27 #include "llvm/Pass.h"
28 #include "llvm/Type.h"
29 #include "llvm/Analysis/ProfileInfo.h"
30 #include "llvm/CodeGen/MachineDominators.h"
31 #include "llvm/CodeGen/MachineFunctionPass.h"
32 #include "llvm/CodeGen/MachineModuleInfo.h"
33 #include "llvm/CodeGen/MachineLoopInfo.h"
34 #include "llvm/CodeGen/MachineRegisterInfo.h"
35 #include "llvm/Support/CFG.h"
36 #include "llvm/Target/TargetInstrInfo.h"
37 #include "llvm/ADT/DepthFirstIterator.h"
38 #include "llvm/ADT/SmallPtrSet.h"
42 class UnreachableBlockElim : public FunctionPass {
43 virtual bool runOnFunction(Function &F);
45 static char ID; // Pass identification, replacement for typeid
46 UnreachableBlockElim() : FunctionPass(&ID) {}
48 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
49 AU.addPreserved<ProfileInfo>();
53 char UnreachableBlockElim::ID = 0;
54 static RegisterPass<UnreachableBlockElim>
55 X("unreachableblockelim", "Remove unreachable blocks from the CFG");
57 FunctionPass *llvm::createUnreachableBlockEliminationPass() {
58 return new UnreachableBlockElim();
61 static void MarkReachableFrom(BasicBlock *BB,
62 SmallPtrSet<BasicBlock*, 8> &Reachable) {
63 for (df_ext_iterator<BasicBlock*, SmallPtrSet<BasicBlock*, 8> > I =
64 df_ext_begin(BB, Reachable), E = df_ext_end(BB, Reachable); I != E; ++I)
65 ; // Mark all reachable blocks.
68 bool UnreachableBlockElim::runOnFunction(Function &F) {
69 SmallPtrSet<BasicBlock*, 8> Reachable;
71 // Mark all reachable blocks.
72 MarkReachableFrom(&F.getEntryBlock(), Reachable);
74 // Mark any address-taken blocks. We don't want codegen to delete these
75 // because the address may already be referenced by another function and the
76 // label may be referenced.
77 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
78 if (I->hasAddressTaken() && !Reachable.count(I))
79 MarkReachableFrom(I, Reachable);
81 // Loop over all dead blocks, remembering them and deleting all instructions
83 std::vector<BasicBlock*> DeadBlocks;
84 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
85 if (!Reachable.count(I)) {
87 DeadBlocks.push_back(BB);
88 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
89 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
90 BB->getInstList().pop_front();
92 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
93 (*SI)->removePredecessor(BB);
94 BB->dropAllReferences();
97 // Actually remove the blocks now.
98 ProfileInfo *PI = getAnalysisIfAvailable<ProfileInfo>();
99 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
100 if (PI) PI->removeBlock(DeadBlocks[i]);
101 DeadBlocks[i]->eraseFromParent();
104 return DeadBlocks.size();
109 class UnreachableMachineBlockElim : public MachineFunctionPass {
110 virtual bool runOnMachineFunction(MachineFunction &F);
111 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
112 MachineModuleInfo *MMI;
114 static char ID; // Pass identification, replacement for typeid
115 UnreachableMachineBlockElim() : MachineFunctionPass(&ID) {}
118 char UnreachableMachineBlockElim::ID = 0;
120 static RegisterPass<UnreachableMachineBlockElim>
121 Y("unreachable-mbb-elimination",
122 "Remove unreachable machine basic blocks");
124 const PassInfo *const llvm::UnreachableMachineBlockElimID = &Y;
126 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
127 AU.addPreserved<MachineLoopInfo>();
128 AU.addPreserved<MachineDominatorTree>();
129 MachineFunctionPass::getAnalysisUsage(AU);
132 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
133 SmallPtrSet<MachineBasicBlock*, 8> Reachable;
135 MMI = getAnalysisIfAvailable<MachineModuleInfo>();
136 MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
137 MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
139 // Mark all reachable blocks.
140 for (df_ext_iterator<MachineFunction*, SmallPtrSet<MachineBasicBlock*, 8> >
141 I = df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable);
143 /* Mark all reachable blocks */;
145 // Loop over all dead blocks, remembering them and deleting all instructions
147 std::vector<MachineBasicBlock*> DeadBlocks;
148 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
149 MachineBasicBlock *BB = I;
151 // Test for deadness.
152 if (!Reachable.count(BB)) {
153 DeadBlocks.push_back(BB);
155 // Update dominator and loop info.
156 if (MLI) MLI->removeBlock(BB);
157 if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
159 while (BB->succ_begin() != BB->succ_end()) {
160 MachineBasicBlock* succ = *BB->succ_begin();
162 MachineBasicBlock::iterator start = succ->begin();
163 while (start != succ->end() && start->isPHI()) {
164 for (unsigned i = start->getNumOperands() - 1; i >= 2; i-=2)
165 if (start->getOperand(i).isMBB() &&
166 start->getOperand(i).getMBB() == BB) {
167 start->RemoveOperand(i);
168 start->RemoveOperand(i-1);
174 BB->removeSuccessor(BB->succ_begin());
179 // Actually remove the blocks now.
180 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i)
181 DeadBlocks[i]->eraseFromParent();
183 // Cleanup PHI nodes.
184 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
185 MachineBasicBlock *BB = I;
186 // Prune unneeded PHI entries.
187 SmallPtrSet<MachineBasicBlock*, 8> preds(BB->pred_begin(),
189 MachineBasicBlock::iterator phi = BB->begin();
190 while (phi != BB->end() && phi->isPHI()) {
191 for (unsigned i = phi->getNumOperands() - 1; i >= 2; i-=2)
192 if (!preds.count(phi->getOperand(i).getMBB())) {
193 phi->RemoveOperand(i);
194 phi->RemoveOperand(i-1);
197 if (phi->getNumOperands() == 3) {
198 unsigned Input = phi->getOperand(1).getReg();
199 unsigned Output = phi->getOperand(0).getReg();
201 MachineInstr* temp = phi;
203 temp->eraseFromParent();
206 F.getRegInfo().replaceRegWith(Output, Input);
217 return DeadBlocks.size();