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/Transforms/Utils/BasicBlockUtils.h"
12 #include "llvm/Type.h"
13 #include "llvm/Analysis/PostDominators.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/Statistic.h"
26 Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed");
27 Statistic<> NumInstRemoved ("adce", "Number of instructions removed");
29 //===----------------------------------------------------------------------===//
32 // This class does all of the work of Aggressive Dead Code Elimination.
33 // It's public interface consists of a constructor and a doADCE() method.
35 class ADCE : public FunctionPass {
36 Function *Func; // The function that we are working on
37 std::vector<Instruction*> WorkList; // Instructions that just became live
38 std::set<Instruction*> LiveSet; // The set of live instructions
40 //===--------------------------------------------------------------------===//
41 // The public interface for this class
44 // Execute the Aggressive Dead Code Elimination Algorithm
46 virtual bool runOnFunction(Function &F) {
48 bool Changed = doADCE();
49 assert(WorkList.empty());
53 // getAnalysisUsage - We require post dominance frontiers (aka Control
55 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
56 AU.addRequired<PostDominatorTree>();
57 AU.addRequired<PostDominanceFrontier>();
61 //===--------------------------------------------------------------------===//
62 // The implementation of this class
65 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
66 // true if the function was modified.
70 void markBlockAlive(BasicBlock *BB);
73 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
74 // instructions in the specified basic block, dropping references on
75 // instructions that are dead according to LiveSet.
76 bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB);
78 inline void markInstructionLive(Instruction *I) {
79 if (LiveSet.count(I)) return;
80 DEBUG(cerr << "Insn Live: " << I);
82 WorkList.push_back(I);
85 inline void markTerminatorLive(const BasicBlock *BB) {
86 DEBUG(cerr << "Terminat Live: " << BB->getTerminator());
87 markInstructionLive((Instruction*)BB->getTerminator());
91 RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
92 } // End of anonymous namespace
94 Pass *createAggressiveDCEPass() { return new ADCE(); }
96 void ADCE::markBlockAlive(BasicBlock *BB) {
97 // Mark the basic block as being newly ALIVE... and mark all branches that
98 // this block is control dependant on as being alive also...
100 PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
102 PostDominanceFrontier::const_iterator It = CDG.find(BB);
103 if (It != CDG.end()) {
104 // Get the blocks that this node is control dependant on...
105 const PostDominanceFrontier::DomSetType &CDB = It->second;
106 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
107 bind_obj(this, &ADCE::markTerminatorLive));
110 // If this basic block is live, then the terminator must be as well!
111 markTerminatorLive(BB);
114 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
115 // instructions in the specified basic block, dropping references on
116 // instructions that are dead according to LiveSet.
117 bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) {
118 bool Changed = false;
119 for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; )
120 if (!LiveSet.count(I)) { // Is this instruction alive?
121 I->dropAllReferences(); // Nope, drop references...
122 if (PHINode *PN = dyn_cast<PHINode>(&*I)) {
123 // We don't want to leave PHI nodes in the program that have
124 // #arguments != #predecessors, so we remove them now.
126 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
128 // Delete the instruction...
129 I = BB->getInstList().erase(I);
141 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
142 // true if the function was modified.
144 bool ADCE::doADCE() {
145 bool MadeChanges = false;
147 // Iterate over all of the instructions in the function, eliminating trivially
148 // dead instructions, and marking instructions live that are known to be
149 // needed. Perform the walk in depth first order so that we avoid marking any
150 // instructions live in basic blocks that are unreachable. These blocks will
151 // be eliminated later, along with the instructions inside.
153 for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
155 BasicBlock *BB = *BBI;
156 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
157 if (II->hasSideEffects() || II->getOpcode() == Instruction::Ret) {
158 markInstructionLive(II);
159 ++II; // Increment the inst iterator if the inst wasn't deleted
160 } else if (isInstructionTriviallyDead(II)) {
161 // Remove the instruction from it's basic block...
162 II = BB->getInstList().erase(II);
166 ++II; // Increment the inst iterator if the inst wasn't deleted
171 DEBUG(cerr << "Processing work list\n");
173 // AliveBlocks - Set of basic blocks that we know have instructions that are
176 std::set<BasicBlock*> AliveBlocks;
178 // Process the work list of instructions that just became live... if they
179 // became live, then that means that all of their operands are neccesary as
180 // well... make them live as well.
182 while (!WorkList.empty()) {
183 Instruction *I = WorkList.back(); // Get an instruction that became live...
186 BasicBlock *BB = I->getParent();
187 if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
188 AliveBlocks.insert(BB); // Block is now ALIVE!
189 markBlockAlive(BB); // Make it so now!
192 // PHI nodes are a special case, because the incoming values are actually
193 // defined in the predecessor nodes of this block, meaning that the PHI
194 // makes the predecessors alive.
196 if (PHINode *PN = dyn_cast<PHINode>(I))
197 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
198 if (!AliveBlocks.count(*PI)) {
199 AliveBlocks.insert(BB); // Block is now ALIVE!
203 // Loop over all of the operands of the live instruction, making sure that
204 // they are known to be alive as well...
206 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
207 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
208 markInstructionLive(Operand);
212 cerr << "Current Function: X = Live\n";
213 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
214 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
215 if (LiveSet.count(BI)) cerr << "X ";
220 // Find the first postdominator of the entry node that is alive. Make it the
223 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
226 if (AliveBlocks.size() == Func->size()) { // No dead blocks?
227 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
228 // Loop over all of the instructions in the function, telling dead
229 // instructions to drop their references. This is so that the next sweep
230 // over the program can safely delete dead instructions without other dead
231 // instructions still refering to them.
233 dropReferencesOfDeadInstructionsInLiveBlock(I);
235 } else { // If there are some blocks dead...
236 // If the entry node is dead, insert a new entry node to eliminate the entry
237 // node as a special case.
239 if (!AliveBlocks.count(&Func->front())) {
240 BasicBlock *NewEntry = new BasicBlock();
241 NewEntry->getInstList().push_back(new BranchInst(&Func->front()));
242 Func->getBasicBlockList().push_front(NewEntry);
243 AliveBlocks.insert(NewEntry); // This block is always alive!
246 // Loop over all of the alive blocks in the function. If any successor
247 // blocks are not alive, we adjust the outgoing branches to branch to the
248 // first live postdominator of the live block, adjusting any PHI nodes in
249 // the block to reflect this.
251 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
252 if (AliveBlocks.count(I)) {
254 TerminatorInst *TI = BB->getTerminator();
256 // Loop over all of the successors, looking for ones that are not alive.
257 // We cannot save the number of successors in the terminator instruction
258 // here because we may remove them if we don't have a postdominator...
260 for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
261 if (!AliveBlocks.count(TI->getSuccessor(i))) {
262 // Scan up the postdominator tree, looking for the first
263 // postdominator that is alive, and the last postdominator that is
266 PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
268 // There is a special case here... if there IS no post-dominator for
269 // the block we have no owhere to point our branch to. Instead,
270 // convert it to a return. This can only happen if the code
271 // branched into an infinite loop. Note that this may not be
272 // desirable, because we _are_ altering the behavior of the code.
273 // This is a well known drawback of ADCE, so in the future if we
274 // choose to revisit the decision, this is where it should be.
276 if (LastNode == 0) { // No postdominator!
277 // Call RemoveSuccessor to transmogrify the terminator instruction
278 // to not contain the outgoing branch, or to create a new
279 // terminator if the form fundementally changes (ie unconditional
280 // branch to return). Note that this will change a branch into an
281 // infinite loop into a return instruction!
283 RemoveSuccessor(TI, i);
285 // RemoveSuccessor may replace TI... make sure we have a fresh
286 // pointer... and e variable.
288 TI = BB->getTerminator();
290 // Rescan this successor...
293 PostDominatorTree::Node *NextNode = LastNode->getIDom();
295 while (!AliveBlocks.count(NextNode->getNode())) {
297 NextNode = NextNode->getIDom();
300 // Get the basic blocks that we need...
301 BasicBlock *LastDead = LastNode->getNode();
302 BasicBlock *NextAlive = NextNode->getNode();
304 // Make the conditional branch now go to the next alive block...
305 TI->getSuccessor(i)->removePredecessor(BB);
306 TI->setSuccessor(i, NextAlive);
308 // If there are PHI nodes in NextAlive, we need to add entries to
309 // the PHI nodes for the new incoming edge. The incoming values
310 // should be identical to the incoming values for LastDead.
312 for (BasicBlock::iterator II = NextAlive->begin();
313 PHINode *PN = dyn_cast<PHINode>(&*II); ++II) {
314 // Get the incoming value for LastDead...
315 int OldIdx = PN->getBasicBlockIndex(LastDead);
316 assert(OldIdx != -1 && "LastDead is not a pred of NextAlive!");
317 Value *InVal = PN->getIncomingValue(OldIdx);
319 // Add an incoming value for BB now...
320 PN->addIncoming(InVal, BB);
325 // Now loop over all of the instructions in the basic block, telling
326 // dead instructions to drop their references. This is so that the next
327 // sweep over the program can safely delete dead instructions without
328 // other dead instructions still refering to them.
330 dropReferencesOfDeadInstructionsInLiveBlock(BB);
334 // Loop over all of the basic blocks in the function, dropping references of
335 // the dead basic blocks
337 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) {
338 if (!AliveBlocks.count(BB)) {
339 // Remove all outgoing edges from this basic block and convert the
340 // terminator into a return instruction.
341 vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
343 if (!Succs.empty()) {
344 // Loop over all of the successors, removing this block from PHI node
345 // entries that might be in the block...
346 while (!Succs.empty()) {
347 Succs.back()->removePredecessor(BB);
351 // Delete the old terminator instruction...
352 BB->getInstList().pop_back();
353 const Type *RetTy = Func->getReturnType();
354 BB->getInstList().push_back(new ReturnInst(RetTy != Type::VoidTy ?
355 Constant::getNullValue(RetTy) : 0));
358 BB->dropAllReferences();
364 // Now loop through all of the blocks and delete the dead ones. We can safely
365 // do this now because we know that there are no references to dead blocks
366 // (because they have dropped all of their references... we also remove dead
367 // instructions from alive blocks.
369 for (Function::iterator BI = Func->begin(); BI != Func->end(); )
370 if (!AliveBlocks.count(BI)) { // Delete dead blocks...
371 BI = Func->getBasicBlockList().erase(BI);
372 } else { // Scan alive blocks...
373 for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
374 if (!LiveSet.count(II)) { // Is this instruction alive?
375 // Nope... remove the instruction from it's basic block...
376 II = BI->getInstList().erase(II);
383 ++BI; // Increment iterator...