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/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>();
59 AU.addRequired<PostDominanceFrontier>();
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);
75 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
76 // instructions in the specified basic block, dropping references on
77 // instructions that are dead according to LiveSet.
78 bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB);
80 inline void markInstructionLive(Instruction *I) {
81 if (LiveSet.count(I)) return;
82 DEBUG(cerr << "Insn Live: " << I);
84 WorkList.push_back(I);
87 inline void markTerminatorLive(const BasicBlock *BB) {
88 DEBUG(cerr << "Terminat Live: " << BB->getTerminator());
89 markInstructionLive((Instruction*)BB->getTerminator());
93 RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
94 } // End of anonymous namespace
96 Pass *createAggressiveDCEPass() { return new ADCE(); }
98 void ADCE::markBlockAlive(BasicBlock *BB) {
99 // Mark the basic block as being newly ALIVE... and mark all branches that
100 // this block is control dependant on as being alive also...
102 PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
104 PostDominanceFrontier::const_iterator It = CDG.find(BB);
105 if (It != CDG.end()) {
106 // Get the blocks that this node is control dependant on...
107 const PostDominanceFrontier::DomSetType &CDB = It->second;
108 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
109 bind_obj(this, &ADCE::markTerminatorLive));
112 // If this basic block is live, then the terminator must be as well!
113 markTerminatorLive(BB);
116 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
117 // instructions in the specified basic block, dropping references on
118 // instructions that are dead according to LiveSet.
119 bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) {
120 bool Changed = false;
121 for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; )
122 if (!LiveSet.count(I)) { // Is this instruction alive?
123 I->dropAllReferences(); // Nope, drop references...
124 if (PHINode *PN = dyn_cast<PHINode>(&*I)) {
125 // We don't want to leave PHI nodes in the program that have
126 // #arguments != #predecessors, so we remove them now.
128 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
130 // Delete the instruction...
131 I = BB->getInstList().erase(I);
143 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
144 // true if the function was modified.
146 bool ADCE::doADCE() {
147 bool MadeChanges = false;
149 // Iterate over all of the instructions in the function, eliminating trivially
150 // dead instructions, and marking instructions live that are known to be
151 // needed. Perform the walk in depth first order so that we avoid marking any
152 // instructions live in basic blocks that are unreachable. These blocks will
153 // be eliminated later, along with the instructions inside.
155 for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
157 BasicBlock *BB = *BBI;
158 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
159 if (II->hasSideEffects() || II->getOpcode() == Instruction::Ret) {
160 markInstructionLive(II);
161 ++II; // Increment the inst iterator if the inst wasn't deleted
162 } else if (isInstructionTriviallyDead(II)) {
163 // Remove the instruction from it's basic block...
164 II = BB->getInstList().erase(II);
168 ++II; // Increment the inst iterator if the inst wasn't deleted
173 DEBUG(cerr << "Processing work list\n");
175 // AliveBlocks - Set of basic blocks that we know have instructions that are
178 std::set<BasicBlock*> AliveBlocks;
180 // Process the work list of instructions that just became live... if they
181 // became live, then that means that all of their operands are neccesary as
182 // well... make them live as well.
184 while (!WorkList.empty()) {
185 Instruction *I = WorkList.back(); // Get an instruction that became live...
188 BasicBlock *BB = I->getParent();
189 if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
190 AliveBlocks.insert(BB); // Block is now ALIVE!
191 markBlockAlive(BB); // Make it so now!
194 // PHI nodes are a special case, because the incoming values are actually
195 // defined in the predecessor nodes of this block, meaning that the PHI
196 // makes the predecessors alive.
198 if (PHINode *PN = dyn_cast<PHINode>(I))
199 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
200 if (!AliveBlocks.count(*PI)) {
201 AliveBlocks.insert(BB); // Block is now ALIVE!
205 // Loop over all of the operands of the live instruction, making sure that
206 // they are known to be alive as well...
208 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
209 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
210 markInstructionLive(Operand);
214 cerr << "Current Function: X = Live\n";
215 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
216 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
217 if (LiveSet.count(BI)) cerr << "X ";
222 // Find the first postdominator of the entry node that is alive. Make it the
225 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
228 if (AliveBlocks.size() == Func->size()) { // No dead blocks?
229 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
230 // Loop over all of the instructions in the function, telling dead
231 // instructions to drop their references. This is so that the next sweep
232 // over the program can safely delete dead instructions without other dead
233 // instructions still refering to them.
235 dropReferencesOfDeadInstructionsInLiveBlock(I);
237 } else { // If there are some blocks dead...
238 // If the entry node is dead, insert a new entry node to eliminate the entry
239 // node as a special case.
241 if (!AliveBlocks.count(&Func->front())) {
242 BasicBlock *NewEntry = new BasicBlock();
243 NewEntry->getInstList().push_back(new BranchInst(&Func->front()));
244 Func->getBasicBlockList().push_front(NewEntry);
245 AliveBlocks.insert(NewEntry); // This block is always alive!
248 // Loop over all of the alive blocks in the function. If any successor
249 // blocks are not alive, we adjust the outgoing branches to branch to the
250 // first live postdominator of the live block, adjusting any PHI nodes in
251 // the block to reflect this.
253 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
254 if (AliveBlocks.count(I)) {
256 TerminatorInst *TI = BB->getTerminator();
258 // Loop over all of the successors, looking for ones that are not alive.
259 // We cannot save the number of successors in the terminator instruction
260 // here because we may remove them if we don't have a postdominator...
262 for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
263 if (!AliveBlocks.count(TI->getSuccessor(i))) {
264 // Scan up the postdominator tree, looking for the first
265 // postdominator that is alive, and the last postdominator that is
268 PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
270 // There is a special case here... if there IS no post-dominator for
271 // the block we have no owhere to point our branch to. Instead,
272 // convert it to a return. This can only happen if the code
273 // branched into an infinite loop. Note that this may not be
274 // desirable, because we _are_ altering the behavior of the code.
275 // This is a well known drawback of ADCE, so in the future if we
276 // choose to revisit the decision, this is where it should be.
278 if (LastNode == 0) { // No postdominator!
279 // Call RemoveSuccessor to transmogrify the terminator instruction
280 // to not contain the outgoing branch, or to create a new
281 // terminator if the form fundementally changes (ie unconditional
282 // branch to return). Note that this will change a branch into an
283 // infinite loop into a return instruction!
285 RemoveSuccessor(TI, i);
287 // RemoveSuccessor may replace TI... make sure we have a fresh
288 // pointer... and e variable.
290 TI = BB->getTerminator();
292 // Rescan this successor...
295 PostDominatorTree::Node *NextNode = LastNode->getIDom();
297 while (!AliveBlocks.count(NextNode->getNode())) {
299 NextNode = NextNode->getIDom();
302 // Get the basic blocks that we need...
303 BasicBlock *LastDead = LastNode->getNode();
304 BasicBlock *NextAlive = NextNode->getNode();
306 // Make the conditional branch now go to the next alive block...
307 TI->getSuccessor(i)->removePredecessor(BB);
308 TI->setSuccessor(i, NextAlive);
310 // If there are PHI nodes in NextAlive, we need to add entries to
311 // the PHI nodes for the new incoming edge. The incoming values
312 // should be identical to the incoming values for LastDead.
314 for (BasicBlock::iterator II = NextAlive->begin();
315 PHINode *PN = dyn_cast<PHINode>(&*II); ++II) {
316 // Get the incoming value for LastDead...
317 int OldIdx = PN->getBasicBlockIndex(LastDead);
318 assert(OldIdx != -1 && "LastDead is not a pred of NextAlive!");
319 Value *InVal = PN->getIncomingValue(OldIdx);
321 // Add an incoming value for BB now...
322 PN->addIncoming(InVal, BB);
327 // Now loop over all of the instructions in the basic block, telling
328 // dead instructions to drop their references. This is so that the next
329 // sweep over the program can safely delete dead instructions without
330 // other dead instructions still refering to them.
332 dropReferencesOfDeadInstructionsInLiveBlock(BB);
336 // Loop over all of the basic blocks in the function, dropping references of
337 // the dead basic blocks
339 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) {
340 if (!AliveBlocks.count(BB)) {
341 // Remove all outgoing edges from this basic block and convert the
342 // terminator into a return instruction.
343 vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
345 if (!Succs.empty()) {
346 // Loop over all of the successors, removing this block from PHI node
347 // entries that might be in the block...
348 while (!Succs.empty()) {
349 Succs.back()->removePredecessor(BB);
353 // Delete the old terminator instruction...
354 BB->getInstList().pop_back();
355 const Type *RetTy = Func->getReturnType();
356 BB->getInstList().push_back(new ReturnInst(RetTy != Type::VoidTy ?
357 Constant::getNullValue(RetTy) : 0));
360 BB->dropAllReferences();
366 // Now loop through all of the blocks and delete the dead ones. We can safely
367 // do this now because we know that there are no references to dead blocks
368 // (because they have dropped all of their references... we also remove dead
369 // instructions from alive blocks.
371 for (Function::iterator BI = Func->begin(); BI != Func->end(); )
372 if (!AliveBlocks.count(BI)) { // Delete dead blocks...
373 BI = Func->getBasicBlockList().erase(BI);
374 } else { // Scan alive blocks...
375 for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
376 if (!LiveSet.count(II)) { // Is this instruction alive?
377 // Nope... remove the instruction from it's basic block...
378 II = BI->getInstList().erase(II);
385 ++BI; // Increment iterator...