1 //===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements "aggressive" dead code elimination. ADCE is DCe where
11 // values are assumed to be dead until proven otherwise. This is similar to
12 // SCCP, except applied to the liveness of values.
14 //===----------------------------------------------------------------------===//
16 #include "llvm/Transforms/Scalar.h"
17 #include "llvm/Constants.h"
18 #include "llvm/Instructions.h"
19 #include "llvm/Analysis/AliasAnalysis.h"
20 #include "llvm/Analysis/PostDominators.h"
21 #include "llvm/Support/CFG.h"
22 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
23 #include "llvm/Transforms/Utils/Local.h"
24 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
25 #include "llvm/Support/Debug.h"
26 #include "llvm/ADT/DepthFirstIterator.h"
27 #include "llvm/ADT/Statistic.h"
28 #include "llvm/ADT/STLExtras.h"
33 Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed");
34 Statistic<> NumInstRemoved ("adce", "Number of instructions removed");
35 Statistic<> NumCallRemoved ("adce", "Number of calls and invokes removed");
37 //===----------------------------------------------------------------------===//
40 // This class does all of the work of Aggressive Dead Code Elimination.
41 // It's public interface consists of a constructor and a doADCE() method.
43 class ADCE : public FunctionPass {
44 Function *Func; // The function that we are working on
45 std::vector<Instruction*> WorkList; // Instructions that just became live
46 std::set<Instruction*> LiveSet; // The set of live instructions
48 //===--------------------------------------------------------------------===//
49 // The public interface for this class
52 // Execute the Aggressive Dead Code Elimination Algorithm
54 virtual bool runOnFunction(Function &F) {
56 bool Changed = doADCE();
57 assert(WorkList.empty());
61 // getAnalysisUsage - We require post dominance frontiers (aka Control
63 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
64 // We require that all function nodes are unified, because otherwise code
65 // can be marked live that wouldn't necessarily be otherwise.
66 AU.addRequired<UnifyFunctionExitNodes>();
67 AU.addRequired<AliasAnalysis>();
68 AU.addRequired<PostDominatorTree>();
69 AU.addRequired<PostDominanceFrontier>();
73 //===--------------------------------------------------------------------===//
74 // The implementation of this class
77 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
78 // true if the function was modified.
82 void markBlockAlive(BasicBlock *BB);
85 // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in
86 // the specified basic block, deleting ones that are dead according to
88 bool deleteDeadInstructionsInLiveBlock(BasicBlock *BB);
90 TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
92 inline void markInstructionLive(Instruction *I) {
93 if (!LiveSet.insert(I).second) return;
94 DEBUG(std::cerr << "Insn Live: " << *I);
95 WorkList.push_back(I);
98 inline void markTerminatorLive(const BasicBlock *BB) {
99 DEBUG(std::cerr << "Terminator Live: " << *BB->getTerminator());
100 markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
104 RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
105 } // End of anonymous namespace
107 FunctionPass *llvm::createAggressiveDCEPass() { return new ADCE(); }
109 void ADCE::markBlockAlive(BasicBlock *BB) {
110 // Mark the basic block as being newly ALIVE... and mark all branches that
111 // this block is control dependent on as being alive also...
113 PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
115 PostDominanceFrontier::const_iterator It = CDG.find(BB);
116 if (It != CDG.end()) {
117 // Get the blocks that this node is control dependent on...
118 const PostDominanceFrontier::DomSetType &CDB = It->second;
119 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
120 bind_obj(this, &ADCE::markTerminatorLive));
123 // If this basic block is live, and it ends in an unconditional branch, then
124 // the branch is alive as well...
125 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
126 if (BI->isUnconditional())
127 markTerminatorLive(BB);
130 // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in the
131 // specified basic block, deleting ones that are dead according to LiveSet.
132 bool ADCE::deleteDeadInstructionsInLiveBlock(BasicBlock *BB) {
133 bool Changed = false;
134 for (BasicBlock::iterator II = BB->begin(), E = --BB->end(); II != E; ) {
135 Instruction *I = II++;
136 if (!LiveSet.count(I)) { // Is this instruction alive?
138 I->replaceAllUsesWith(UndefValue::get(I->getType()));
140 // Nope... remove the instruction from it's basic block...
141 if (isa<CallInst>(I))
145 BB->getInstList().erase(I);
153 /// convertToUnconditionalBranch - Transform this conditional terminator
154 /// instruction into an unconditional branch because we don't care which of the
155 /// successors it goes to. This eliminate a use of the condition as well.
157 TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
158 BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
159 BasicBlock *BB = TI->getParent();
161 // Remove entries from PHI nodes to avoid confusing ourself later...
162 for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
163 TI->getSuccessor(i)->removePredecessor(BB);
165 // Delete the old branch itself...
166 BB->getInstList().erase(TI);
171 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
172 // true if the function was modified.
174 bool ADCE::doADCE() {
175 bool MadeChanges = false;
177 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
180 // Iterate over all invokes in the function, turning invokes into calls if
181 // they cannot throw.
182 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
183 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
184 if (Function *F = II->getCalledFunction())
185 if (AA.onlyReadsMemory(F)) {
186 // The function cannot unwind. Convert it to a call with a branch
187 // after it to the normal destination.
188 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
189 std::string Name = II->getName(); II->setName("");
190 Instruction *NewCall = new CallInst(F, Args, Name, II);
191 II->replaceAllUsesWith(NewCall);
192 new BranchInst(II->getNormalDest(), II);
194 // Update PHI nodes in the unwind destination
195 II->getUnwindDest()->removePredecessor(BB);
196 BB->getInstList().erase(II);
198 if (NewCall->use_empty()) {
199 BB->getInstList().erase(NewCall);
204 // Iterate over all of the instructions in the function, eliminating trivially
205 // dead instructions, and marking instructions live that are known to be
206 // needed. Perform the walk in depth first order so that we avoid marking any
207 // instructions live in basic blocks that are unreachable. These blocks will
208 // be eliminated later, along with the instructions inside.
210 std::set<BasicBlock*> ReachableBBs;
211 for (df_ext_iterator<BasicBlock*>
212 BBI = df_ext_begin(&Func->front(), ReachableBBs),
213 BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) {
214 BasicBlock *BB = *BBI;
215 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
216 Instruction *I = II++;
217 if (CallInst *CI = dyn_cast<CallInst>(I)) {
218 Function *F = CI->getCalledFunction();
219 if (F && AA.onlyReadsMemory(F)) {
220 if (CI->use_empty()) {
221 BB->getInstList().erase(CI);
225 markInstructionLive(I);
227 } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
228 isa<UnwindInst>(I) || isa<UnreachableInst>(I)) {
229 // FIXME: Unreachable instructions should not be marked intrinsically
231 markInstructionLive(I);
232 } else if (isInstructionTriviallyDead(I)) {
233 // Remove the instruction from it's basic block...
234 BB->getInstList().erase(I);
240 // Check to ensure we have an exit node for this CFG. If we don't, we won't
241 // have any post-dominance information, thus we cannot perform our
242 // transformations safely.
244 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
245 if (DT[&Func->getEntryBlock()] == 0) {
250 // Scan the function marking blocks without post-dominance information as
251 // live. Blocks without post-dominance information occur when there is an
252 // infinite loop in the program. Because the infinite loop could contain a
253 // function which unwinds, exits or has side-effects, we don't want to delete
254 // the infinite loop or those blocks leading up to it.
255 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
256 if (DT[I] == 0 && ReachableBBs.count(I))
257 for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
258 markInstructionLive((*PI)->getTerminator());
260 DEBUG(std::cerr << "Processing work list\n");
262 // AliveBlocks - Set of basic blocks that we know have instructions that are
265 std::set<BasicBlock*> AliveBlocks;
267 // Process the work list of instructions that just became live... if they
268 // became live, then that means that all of their operands are necessary as
269 // well... make them live as well.
271 while (!WorkList.empty()) {
272 Instruction *I = WorkList.back(); // Get an instruction that became live...
275 BasicBlock *BB = I->getParent();
276 if (!ReachableBBs.count(BB)) continue;
277 if (AliveBlocks.insert(BB).second) // Basic block not alive yet.
278 markBlockAlive(BB); // Make it so now!
280 // PHI nodes are a special case, because the incoming values are actually
281 // defined in the predecessor nodes of this block, meaning that the PHI
282 // makes the predecessors alive.
284 if (PHINode *PN = dyn_cast<PHINode>(I)) {
285 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
286 // If the incoming edge is clearly dead, it won't have control
287 // dependence information. Do not mark it live.
288 BasicBlock *PredBB = PN->getIncomingBlock(i);
289 if (ReachableBBs.count(PredBB)) {
290 // FIXME: This should mark the control dependent edge as live, not
291 // necessarily the predecessor itself!
292 if (AliveBlocks.insert(PredBB).second)
293 markBlockAlive(PN->getIncomingBlock(i)); // Block is newly ALIVE!
294 if (Instruction *Op = dyn_cast<Instruction>(PN->getIncomingValue(i)))
295 markInstructionLive(Op);
299 // Loop over all of the operands of the live instruction, making sure that
300 // they are known to be alive as well.
302 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
303 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
304 markInstructionLive(Operand);
309 std::cerr << "Current Function: X = Live\n";
310 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
311 std::cerr << I->getName() << ":\t"
312 << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
313 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
314 if (LiveSet.count(BI)) std::cerr << "X ";
319 // All blocks being live is a common case, handle it specially.
320 if (AliveBlocks.size() == Func->size()) { // No dead blocks?
321 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
322 // Loop over all of the instructions in the function deleting instructions
323 // to drop their references.
324 deleteDeadInstructionsInLiveBlock(I);
326 // Check to make sure the terminator instruction is live. If it isn't,
327 // this means that the condition that it branches on (we know it is not an
328 // unconditional branch), is not needed to make the decision of where to
329 // go to, because all outgoing edges go to the same place. We must remove
330 // the use of the condition (because it's probably dead), so we convert
331 // the terminator to an unconditional branch.
333 TerminatorInst *TI = I->getTerminator();
334 if (!LiveSet.count(TI))
335 convertToUnconditionalBranch(TI);
342 // If the entry node is dead, insert a new entry node to eliminate the entry
343 // node as a special case.
345 if (!AliveBlocks.count(&Func->front())) {
346 BasicBlock *NewEntry = new BasicBlock();
347 new BranchInst(&Func->front(), NewEntry);
348 Func->getBasicBlockList().push_front(NewEntry);
349 AliveBlocks.insert(NewEntry); // This block is always alive!
350 LiveSet.insert(NewEntry->getTerminator()); // The branch is live
353 // Loop over all of the alive blocks in the function. If any successor
354 // blocks are not alive, we adjust the outgoing branches to branch to the
355 // first live postdominator of the live block, adjusting any PHI nodes in
356 // the block to reflect this.
358 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
359 if (AliveBlocks.count(I)) {
361 TerminatorInst *TI = BB->getTerminator();
363 // If the terminator instruction is alive, but the block it is contained
364 // in IS alive, this means that this terminator is a conditional branch on
365 // a condition that doesn't matter. Make it an unconditional branch to
366 // ONE of the successors. This has the side effect of dropping a use of
367 // the conditional value, which may also be dead.
368 if (!LiveSet.count(TI))
369 TI = convertToUnconditionalBranch(TI);
371 // Loop over all of the successors, looking for ones that are not alive.
372 // We cannot save the number of successors in the terminator instruction
373 // here because we may remove them if we don't have a postdominator.
375 for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
376 if (!AliveBlocks.count(TI->getSuccessor(i))) {
377 // Scan up the postdominator tree, looking for the first
378 // postdominator that is alive, and the last postdominator that is
381 PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
382 PostDominatorTree::Node *NextNode = 0;
385 NextNode = LastNode->getIDom();
386 while (!AliveBlocks.count(NextNode->getBlock())) {
388 NextNode = NextNode->getIDom();
396 // There is a special case here... if there IS no post-dominator for
397 // the block we have nowhere to point our branch to. Instead, convert
398 // it to a return. This can only happen if the code branched into an
399 // infinite loop. Note that this may not be desirable, because we
400 // _are_ altering the behavior of the code. This is a well known
401 // drawback of ADCE, so in the future if we choose to revisit the
402 // decision, this is where it should be.
404 if (LastNode == 0) { // No postdominator!
405 if (!isa<InvokeInst>(TI)) {
406 // Call RemoveSuccessor to transmogrify the terminator instruction
407 // to not contain the outgoing branch, or to create a new
408 // terminator if the form fundamentally changes (i.e.,
409 // unconditional branch to return). Note that this will change a
410 // branch into an infinite loop into a return instruction!
412 RemoveSuccessor(TI, i);
414 // RemoveSuccessor may replace TI... make sure we have a fresh
417 TI = BB->getTerminator();
419 // Rescan this successor...
425 // Get the basic blocks that we need...
426 BasicBlock *LastDead = LastNode->getBlock();
427 BasicBlock *NextAlive = NextNode->getBlock();
429 // Make the conditional branch now go to the next alive block...
430 TI->getSuccessor(i)->removePredecessor(BB);
431 TI->setSuccessor(i, NextAlive);
433 // If there are PHI nodes in NextAlive, we need to add entries to
434 // the PHI nodes for the new incoming edge. The incoming values
435 // should be identical to the incoming values for LastDead.
437 for (BasicBlock::iterator II = NextAlive->begin();
438 isa<PHINode>(II); ++II) {
439 PHINode *PN = cast<PHINode>(II);
440 if (LiveSet.count(PN)) { // Only modify live phi nodes
441 // Get the incoming value for LastDead...
442 int OldIdx = PN->getBasicBlockIndex(LastDead);
443 assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
444 Value *InVal = PN->getIncomingValue(OldIdx);
446 // Add an incoming value for BB now...
447 PN->addIncoming(InVal, BB);
453 // Now loop over all of the instructions in the basic block, deleting
454 // dead instructions. This is so that the next sweep over the program
455 // can safely delete dead instructions without other dead instructions
456 // still referring to them.
458 deleteDeadInstructionsInLiveBlock(BB);
461 // Loop over all of the basic blocks in the function, dropping references of
462 // the dead basic blocks. We must do this after the previous step to avoid
463 // dropping references to PHIs which still have entries...
465 std::vector<BasicBlock*> DeadBlocks;
466 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
467 if (!AliveBlocks.count(BB)) {
468 // Remove PHI node entries for this block in live successor blocks.
469 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
470 if (!SI->empty() && isa<PHINode>(SI->front()) && AliveBlocks.count(*SI))
471 (*SI)->removePredecessor(BB);
473 BB->dropAllReferences();
475 DeadBlocks.push_back(BB);
478 NumBlockRemoved += DeadBlocks.size();
480 // Now loop through all of the blocks and delete the dead ones. We can safely
481 // do this now because we know that there are no references to dead blocks
482 // (because they have dropped all of their references).
483 for (std::vector<BasicBlock*>::iterator I = DeadBlocks.begin(),
484 E = DeadBlocks.end(); I != E; ++I)
485 Func->getBasicBlockList().erase(*I);