1 //===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
3 // This file implements "aggressive" dead code elimination. ADCE is DCe where
4 // values are assumed to be dead until proven otherwise. This is similar to
5 // SCCP, except applied to the liveness of values.
7 //===----------------------------------------------------------------------===//
9 #include "llvm/Transforms/Scalar.h"
10 #include "llvm/Transforms/Utils/Local.h"
11 #include "llvm/Type.h"
12 #include "llvm/Analysis/Dominators.h"
13 #include "llvm/Analysis/Writer.h"
14 #include "llvm/iTerminators.h"
15 #include "llvm/iPHINode.h"
16 #include "llvm/Constant.h"
17 #include "llvm/Support/CFG.h"
18 #include "Support/STLExtras.h"
19 #include "Support/DepthFirstIterator.h"
20 #include "Support/StatisticReporter.h"
26 static Statistic<> NumBlockRemoved("adce\t\t- Number of basic blocks removed");
27 static Statistic<> NumInstRemoved ("adce\t\t- Number of instructions removed");
31 //===----------------------------------------------------------------------===//
34 // This class does all of the work of Aggressive Dead Code Elimination.
35 // It's public interface consists of a constructor and a doADCE() method.
37 class ADCE : public FunctionPass {
38 Function *Func; // The function that we are working on
39 std::vector<Instruction*> WorkList; // Instructions that just became live
40 std::set<Instruction*> LiveSet; // The set of live instructions
42 //===--------------------------------------------------------------------===//
43 // The public interface for this class
46 // Execute the Aggressive Dead Code Elimination Algorithm
48 virtual bool runOnFunction(Function &F) {
50 bool Changed = doADCE();
51 assert(WorkList.empty());
55 // getAnalysisUsage - We require post dominance frontiers (aka Control
57 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
58 AU.addRequired(PostDominatorTree::ID);
59 AU.addRequired(PostDominanceFrontier::ID);
63 //===--------------------------------------------------------------------===//
64 // The implementation of this class
67 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
68 // true if the function was modified.
72 void markBlockAlive(BasicBlock *BB);
74 inline void markInstructionLive(Instruction *I) {
75 if (LiveSet.count(I)) return;
76 DEBUG(cerr << "Insn Live: " << I);
78 WorkList.push_back(I);
81 inline void markTerminatorLive(const BasicBlock *BB) {
82 DEBUG(cerr << "Terminat Live: " << BB->getTerminator());
83 markInstructionLive((Instruction*)BB->getTerminator());
87 RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
88 } // End of anonymous namespace
90 Pass *createAggressiveDCEPass() { return new ADCE(); }
92 void ADCE::markBlockAlive(BasicBlock *BB) {
93 // Mark the basic block as being newly ALIVE... and mark all branches that
94 // this block is control dependant on as being alive also...
96 PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
98 PostDominanceFrontier::const_iterator It = CDG.find(BB);
99 if (It != CDG.end()) {
100 // Get the blocks that this node is control dependant on...
101 const PostDominanceFrontier::DomSetType &CDB = It->second;
102 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
103 bind_obj(this, &ADCE::markTerminatorLive));
106 // If this basic block is live, then the terminator must be as well!
107 markTerminatorLive(BB);
111 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
112 // true if the function was modified.
114 bool ADCE::doADCE() {
115 bool MadeChanges = false;
117 // Iterate over all of the instructions in the function, eliminating trivially
118 // dead instructions, and marking instructions live that are known to be
119 // needed. Perform the walk in depth first order so that we avoid marking any
120 // instructions live in basic blocks that are unreachable. These blocks will
121 // be eliminated later, along with the instructions inside.
123 for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
125 BasicBlock *BB = *BBI;
126 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
127 if (II->hasSideEffects() || II->getOpcode() == Instruction::Ret) {
128 markInstructionLive(II);
129 ++II; // Increment the inst iterator if the inst wasn't deleted
130 } else if (isInstructionTriviallyDead(II)) {
131 // Remove the instruction from it's basic block...
132 II = BB->getInstList().erase(II);
136 ++II; // Increment the inst iterator if the inst wasn't deleted
141 DEBUG(cerr << "Processing work list\n");
143 // AliveBlocks - Set of basic blocks that we know have instructions that are
146 std::set<BasicBlock*> AliveBlocks;
148 // Process the work list of instructions that just became live... if they
149 // became live, then that means that all of their operands are neccesary as
150 // well... make them live as well.
152 while (!WorkList.empty()) {
153 Instruction *I = WorkList.back(); // Get an instruction that became live...
156 BasicBlock *BB = I->getParent();
157 if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
158 AliveBlocks.insert(BB); // Block is now ALIVE!
159 markBlockAlive(BB); // Make it so now!
162 // PHI nodes are a special case, because the incoming values are actually
163 // defined in the predecessor nodes of this block, meaning that the PHI
164 // makes the predecessors alive.
166 if (PHINode *PN = dyn_cast<PHINode>(I))
167 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
168 if (!AliveBlocks.count(*PI)) {
169 AliveBlocks.insert(BB); // Block is now ALIVE!
173 // Loop over all of the operands of the live instruction, making sure that
174 // they are known to be alive as well...
176 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
177 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
178 markInstructionLive(Operand);
182 cerr << "Current Function: X = Live\n";
183 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
184 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
185 if (LiveSet.count(BI)) cerr << "X ";
190 // Find the first postdominator of the entry node that is alive. Make it the
193 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
195 // If there are some blocks dead...
196 if (AliveBlocks.size() != Func->size()) {
197 // Insert a new entry node to eliminate the entry node as a special case.
198 BasicBlock *NewEntry = new BasicBlock();
199 NewEntry->getInstList().push_back(new BranchInst(&Func->front()));
200 Func->getBasicBlockList().push_front(NewEntry);
201 AliveBlocks.insert(NewEntry); // This block is always alive!
203 // Loop over all of the alive blocks in the function. If any successor
204 // blocks are not alive, we adjust the outgoing branches to branch to the
205 // first live postdominator of the live block, adjusting any PHI nodes in
206 // the block to reflect this.
208 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
209 if (AliveBlocks.count(I)) {
211 TerminatorInst *TI = BB->getTerminator();
213 // Loop over all of the successors, looking for ones that are not alive
214 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
215 if (!AliveBlocks.count(TI->getSuccessor(i))) {
216 // Scan up the postdominator tree, looking for the first
217 // postdominator that is alive, and the last postdominator that is
220 PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
221 PostDominatorTree::Node *NextNode = LastNode->getIDom();
222 while (!AliveBlocks.count(NextNode->getNode())) {
224 NextNode = NextNode->getIDom();
227 // Get the basic blocks that we need...
228 BasicBlock *LastDead = LastNode->getNode();
229 BasicBlock *NextAlive = NextNode->getNode();
231 // Make the conditional branch now go to the next alive block...
232 TI->getSuccessor(i)->removePredecessor(BB);
233 TI->setSuccessor(i, NextAlive);
235 // If there are PHI nodes in NextAlive, we need to add entries to
236 // the PHI nodes for the new incoming edge. The incoming values
237 // should be identical to the incoming values for LastDead.
239 for (BasicBlock::iterator II = NextAlive->begin();
240 PHINode *PN = dyn_cast<PHINode>(&*II); ++II) {
241 // Get the incoming value for LastDead...
242 int OldIdx = PN->getBasicBlockIndex(LastDead);
243 assert(OldIdx != -1 && "LastDead is not a pred of NextAlive!");
244 Value *InVal = PN->getIncomingValue(OldIdx);
246 // Add an incoming value for BB now...
247 PN->addIncoming(InVal, BB);
251 // Now loop over all of the instructions in the basic block, telling
252 // dead instructions to drop their references. This is so that the next
253 // sweep over the program can safely delete dead instructions without
254 // other dead instructions still refering to them.
256 for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; ++I)
257 if (!LiveSet.count(I)) // Is this instruction alive?
258 I->dropAllReferences(); // Nope, drop references...
262 // Loop over all of the basic blocks in the function, dropping references of
263 // the dead basic blocks
265 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) {
266 if (!AliveBlocks.count(BB)) {
267 // Remove all outgoing edges from this basic block and convert the
268 // terminator into a return instruction.
269 vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
271 if (!Succs.empty()) {
272 // Loop over all of the successors, removing this block from PHI node
273 // entries that might be in the block...
274 while (!Succs.empty()) {
275 Succs.back()->removePredecessor(BB);
279 // Delete the old terminator instruction...
280 BB->getInstList().pop_back();
281 const Type *RetTy = Func->getReturnType();
282 Instruction *New = new ReturnInst(RetTy != Type::VoidTy ?
283 Constant::getNullValue(RetTy) : 0);
284 BB->getInstList().push_back(New);
287 BB->dropAllReferences();
293 // Now loop through all of the blocks and delete the dead ones. We can safely
294 // do this now because we know that there are no references to dead blocks
295 // (because they have dropped all of their references... we also remove dead
296 // instructions from alive blocks.
298 for (Function::iterator BI = Func->begin(); BI != Func->end(); )
299 if (!AliveBlocks.count(BI))
300 BI = Func->getBasicBlockList().erase(BI);
302 for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
303 if (!LiveSet.count(II)) { // Is this instruction alive?
304 // Nope... remove the instruction from it's basic block...
305 II = BI->getInstList().erase(II);
312 ++BI; // Increment iterator...