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 const char *getPassName() const { return "Aggressive Dead Code Elimination"; }
48 // Execute the Aggressive Dead Code Elimination Algorithm
50 virtual bool runOnFunction(Function &F) {
52 bool Changed = doADCE();
53 assert(WorkList.empty());
57 // getAnalysisUsage - We require post dominance frontiers (aka Control
59 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
60 AU.addRequired(DominatorTree::PostDomID);
61 AU.addRequired(DominanceFrontier::PostDomID);
65 //===--------------------------------------------------------------------===//
66 // The implementation of this class
69 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
70 // true if the function was modified.
74 void markBlockAlive(BasicBlock *BB);
76 inline void markInstructionLive(Instruction *I) {
77 if (LiveSet.count(I)) return;
78 DEBUG(cerr << "Insn Live: " << I);
80 WorkList.push_back(I);
83 inline void markTerminatorLive(const BasicBlock *BB) {
84 DEBUG(cerr << "Terminat Live: " << BB->getTerminator());
85 markInstructionLive((Instruction*)BB->getTerminator());
89 } // End of anonymous namespace
91 Pass *createAggressiveDCEPass() { return new ADCE(); }
94 void ADCE::markBlockAlive(BasicBlock *BB) {
95 // Mark the basic block as being newly ALIVE... and mark all branches that
96 // this block is control dependant on as being alive also...
98 DominanceFrontier &CDG =
99 getAnalysis<DominanceFrontier>(DominanceFrontier::PostDomID);
101 DominanceFrontier::const_iterator It = CDG.find(BB);
102 if (It != CDG.end()) {
103 // Get the blocks that this node is control dependant on...
104 const DominanceFrontier::DomSetType &CDB = It->second;
105 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
106 bind_obj(this, &ADCE::markTerminatorLive));
109 // If this basic block is live, then the terminator must be as well!
110 markTerminatorLive(BB);
114 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
115 // true if the function was modified.
117 bool ADCE::doADCE() {
118 bool MadeChanges = false;
120 // Iterate over all of the instructions in the function, eliminating trivially
121 // dead instructions, and marking instructions live that are known to be
122 // needed. Perform the walk in depth first order so that we avoid marking any
123 // instructions live in basic blocks that are unreachable. These blocks will
124 // be eliminated later, along with the instructions inside.
126 for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
128 BasicBlock *BB = *BBI;
129 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
130 if (II->hasSideEffects() || II->getOpcode() == Instruction::Ret) {
131 markInstructionLive(II);
132 ++II; // Increment the inst iterator if the inst wasn't deleted
133 } else if (isInstructionTriviallyDead(II)) {
134 // Remove the instruction from it's basic block...
135 II = BB->getInstList().erase(II);
139 ++II; // Increment the inst iterator if the inst wasn't deleted
144 DEBUG(cerr << "Processing work list\n");
146 // AliveBlocks - Set of basic blocks that we know have instructions that are
149 std::set<BasicBlock*> AliveBlocks;
151 // Process the work list of instructions that just became live... if they
152 // became live, then that means that all of their operands are neccesary as
153 // well... make them live as well.
155 while (!WorkList.empty()) {
156 Instruction *I = WorkList.back(); // Get an instruction that became live...
159 BasicBlock *BB = I->getParent();
160 if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
161 AliveBlocks.insert(BB); // Block is now ALIVE!
162 markBlockAlive(BB); // Make it so now!
165 // PHI nodes are a special case, because the incoming values are actually
166 // defined in the predecessor nodes of this block, meaning that the PHI
167 // makes the predecessors alive.
169 if (PHINode *PN = dyn_cast<PHINode>(I))
170 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
171 if (!AliveBlocks.count(*PI)) {
172 AliveBlocks.insert(BB); // Block is now ALIVE!
176 // Loop over all of the operands of the live instruction, making sure that
177 // they are known to be alive as well...
179 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
180 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
181 markInstructionLive(Operand);
185 cerr << "Current Function: X = Live\n";
186 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
187 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
188 if (LiveSet.count(BI)) cerr << "X ";
193 // Find the first postdominator of the entry node that is alive. Make it the
196 DominatorTree &DT = getAnalysis<DominatorTree>(DominatorTree::PostDomID);
198 // If there are some blocks dead...
199 if (AliveBlocks.size() != Func->size()) {
200 // Insert a new entry node to eliminate the entry node as a special case.
201 BasicBlock *NewEntry = new BasicBlock();
202 NewEntry->getInstList().push_back(new BranchInst(&Func->front()));
203 Func->getBasicBlockList().push_front(NewEntry);
204 AliveBlocks.insert(NewEntry); // This block is always alive!
206 // Loop over all of the alive blocks in the function. If any successor
207 // blocks are not alive, we adjust the outgoing branches to branch to the
208 // first live postdominator of the live block, adjusting any PHI nodes in
209 // the block to reflect this.
211 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
212 if (AliveBlocks.count(I)) {
214 TerminatorInst *TI = BB->getTerminator();
216 // Loop over all of the successors, looking for ones that are not alive
217 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
218 if (!AliveBlocks.count(TI->getSuccessor(i))) {
219 // Scan up the postdominator tree, looking for the first
220 // postdominator that is alive, and the last postdominator that is
223 DominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
224 DominatorTree::Node *NextNode = LastNode->getIDom();
225 while (!AliveBlocks.count(NextNode->getNode())) {
227 NextNode = NextNode->getIDom();
230 // Get the basic blocks that we need...
231 BasicBlock *LastDead = LastNode->getNode();
232 BasicBlock *NextAlive = NextNode->getNode();
234 // Make the conditional branch now go to the next alive block...
235 TI->getSuccessor(i)->removePredecessor(BB);
236 TI->setSuccessor(i, NextAlive);
238 // If there are PHI nodes in NextAlive, we need to add entries to
239 // the PHI nodes for the new incoming edge. The incoming values
240 // should be identical to the incoming values for LastDead.
242 for (BasicBlock::iterator II = NextAlive->begin();
243 PHINode *PN = dyn_cast<PHINode>(&*II); ++II) {
244 // Get the incoming value for LastDead...
245 int OldIdx = PN->getBasicBlockIndex(LastDead);
246 assert(OldIdx != -1 && "LastDead is not a pred of NextAlive!");
247 Value *InVal = PN->getIncomingValue(OldIdx);
249 // Add an incoming value for BB now...
250 PN->addIncoming(InVal, BB);
254 // Now loop over all of the instructions in the basic block, telling
255 // dead instructions to drop their references. This is so that the next
256 // sweep over the program can safely delete dead instructions without
257 // other dead instructions still refering to them.
259 for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; ++I)
260 if (!LiveSet.count(I)) // Is this instruction alive?
261 I->dropAllReferences(); // Nope, drop references...
265 // Loop over all of the basic blocks in the function, dropping references of
266 // the dead basic blocks
268 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) {
269 if (!AliveBlocks.count(BB)) {
270 // Remove all outgoing edges from this basic block and convert the
271 // terminator into a return instruction.
272 vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
274 if (!Succs.empty()) {
275 // Loop over all of the successors, removing this block from PHI node
276 // entries that might be in the block...
277 while (!Succs.empty()) {
278 Succs.back()->removePredecessor(BB);
282 // Delete the old terminator instruction...
283 BB->getInstList().pop_back();
284 const Type *RetTy = Func->getReturnType();
285 Instruction *New = new ReturnInst(RetTy != Type::VoidTy ?
286 Constant::getNullValue(RetTy) : 0);
287 BB->getInstList().push_back(New);
290 BB->dropAllReferences();
296 // Now loop through all of the blocks and delete the dead ones. We can safely
297 // do this now because we know that there are no references to dead blocks
298 // (because they have dropped all of their references... we also remove dead
299 // instructions from alive blocks.
301 for (Function::iterator BI = Func->begin(); BI != Func->end(); )
302 if (!AliveBlocks.count(BI))
303 BI = Func->getBasicBlockList().erase(BI);
305 for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
306 if (!LiveSet.count(II)) { // Is this instruction alive?
307 // Nope... remove the instruction from it's basic block...
308 II = BI->getInstList().erase(II);
315 ++BI; // Increment iterator...