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 bool UnreachableBlockElim::runOnFunction(Function &F) {
62 SmallPtrSet<BasicBlock*, 8> Reachable;
64 // Mark all reachable blocks.
65 for (df_ext_iterator<Function*, SmallPtrSet<BasicBlock*, 8> > I =
66 df_ext_begin(&F, Reachable), E = df_ext_end(&F, Reachable); I != E; ++I)
67 /* Mark all reachable blocks */;
69 // Loop over all dead blocks, remembering them and deleting all instructions
71 std::vector<BasicBlock*> DeadBlocks;
72 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
73 if (!Reachable.count(I)) {
75 DeadBlocks.push_back(BB);
76 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
77 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
78 BB->getInstList().pop_front();
80 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
81 (*SI)->removePredecessor(BB);
82 BB->dropAllReferences();
85 // Actually remove the blocks now.
86 ProfileInfo *PI = getAnalysisIfAvailable<ProfileInfo>();
87 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
88 if (PI) PI->removeBlock(DeadBlocks[i]);
89 DeadBlocks[i]->eraseFromParent();
92 return DeadBlocks.size();
97 class UnreachableMachineBlockElim : public MachineFunctionPass {
98 virtual bool runOnMachineFunction(MachineFunction &F);
99 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
100 MachineModuleInfo *MMI;
102 static char ID; // Pass identification, replacement for typeid
103 UnreachableMachineBlockElim() : MachineFunctionPass(&ID) {}
106 char UnreachableMachineBlockElim::ID = 0;
108 static RegisterPass<UnreachableMachineBlockElim>
109 Y("unreachable-mbb-elimination",
110 "Remove unreachable machine basic blocks");
112 const PassInfo *const llvm::UnreachableMachineBlockElimID = &Y;
114 void UnreachableMachineBlockElim::getAnalysisUsage(AnalysisUsage &AU) const {
115 AU.addPreserved<MachineLoopInfo>();
116 AU.addPreserved<MachineDominatorTree>();
117 MachineFunctionPass::getAnalysisUsage(AU);
120 bool UnreachableMachineBlockElim::runOnMachineFunction(MachineFunction &F) {
121 SmallPtrSet<MachineBasicBlock*, 8> Reachable;
123 MMI = getAnalysisIfAvailable<MachineModuleInfo>();
124 MachineDominatorTree *MDT = getAnalysisIfAvailable<MachineDominatorTree>();
125 MachineLoopInfo *MLI = getAnalysisIfAvailable<MachineLoopInfo>();
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);
131 /* Mark all reachable blocks */;
133 // Loop over all dead blocks, remembering them and deleting all instructions
135 std::vector<MachineBasicBlock*> DeadBlocks;
136 for (MachineFunction::iterator I = F.begin(), E = F.end(); I != E; ++I) {
137 MachineBasicBlock *BB = I;
139 // Test for deadness.
140 if (!Reachable.count(BB)) {
141 DeadBlocks.push_back(BB);
143 // Update dominator and loop info.
144 if (MLI) MLI->removeBlock(BB);
145 if (MDT && MDT->getNode(BB)) MDT->eraseNode(BB);
147 while (BB->succ_begin() != BB->succ_end()) {
148 MachineBasicBlock* succ = *BB->succ_begin();
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);
162 BB->removeSuccessor(BB->succ_begin());
167 // Actually remove the blocks now.
168 for (unsigned i = 0, e = DeadBlocks.size(); i != e; ++i) {
169 MachineBasicBlock *MBB = DeadBlocks[i];
170 // If there are any labels in the basic block, unregister them from
171 // MachineModuleInfo.
172 if (MMI && !MBB->empty()) {
173 for (MachineBasicBlock::iterator I = MBB->begin(),
174 E = MBB->end(); I != E; ++I) {
176 // The label ID # is always operand #0, an immediate.
177 MMI->InvalidateLabel(I->getOperand(0).getImm());
180 MBB->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();