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"
34 Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed");
35 Statistic<> NumInstRemoved ("adce", "Number of instructions removed");
36 Statistic<> NumCallRemoved ("adce", "Number of calls and invokes removed");
38 //===----------------------------------------------------------------------===//
41 // This class does all of the work of Aggressive Dead Code Elimination.
42 // It's public interface consists of a constructor and a doADCE() method.
44 class ADCE : public FunctionPass {
45 Function *Func; // The function that we are working on
46 std::vector<Instruction*> WorkList; // Instructions that just became live
47 std::set<Instruction*> LiveSet; // The set of live instructions
49 //===--------------------------------------------------------------------===//
50 // The public interface for this class
53 // Execute the Aggressive Dead Code Elimination Algorithm
55 virtual bool runOnFunction(Function &F) {
57 bool Changed = doADCE();
58 assert(WorkList.empty());
62 // getAnalysisUsage - We require post dominance frontiers (aka Control
64 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
65 // We require that all function nodes are unified, because otherwise code
66 // can be marked live that wouldn't necessarily be otherwise.
67 AU.addRequired<UnifyFunctionExitNodes>();
68 AU.addRequired<AliasAnalysis>();
69 AU.addRequired<PostDominatorTree>();
70 AU.addRequired<PostDominanceFrontier>();
74 //===--------------------------------------------------------------------===//
75 // The implementation of this class
78 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
79 // true if the function was modified.
83 void markBlockAlive(BasicBlock *BB);
86 // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in
87 // the specified basic block, deleting ones that are dead according to
89 bool deleteDeadInstructionsInLiveBlock(BasicBlock *BB);
91 TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
93 inline void markInstructionLive(Instruction *I) {
94 if (!LiveSet.insert(I).second) return;
95 DEBUG(std::cerr << "Insn Live: " << *I);
96 WorkList.push_back(I);
99 inline void markTerminatorLive(const BasicBlock *BB) {
100 DEBUG(std::cerr << "Terminator Live: " << *BB->getTerminator());
101 markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
105 RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
106 } // End of anonymous namespace
108 FunctionPass *llvm::createAggressiveDCEPass() { return new ADCE(); }
110 void ADCE::markBlockAlive(BasicBlock *BB) {
111 // Mark the basic block as being newly ALIVE... and mark all branches that
112 // this block is control dependent on as being alive also...
114 PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
116 PostDominanceFrontier::const_iterator It = CDG.find(BB);
117 if (It != CDG.end()) {
118 // Get the blocks that this node is control dependent on...
119 const PostDominanceFrontier::DomSetType &CDB = It->second;
120 for (PostDominanceFrontier::DomSetType::const_iterator I =
121 CDB.begin(), E = CDB.end(); I != E; ++I)
122 markTerminatorLive(*I); // Mark all their terminators as live
125 // If this basic block is live, and it ends in an unconditional branch, then
126 // the branch is alive as well...
127 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
128 if (BI->isUnconditional())
129 markTerminatorLive(BB);
132 // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in the
133 // specified basic block, deleting ones that are dead according to LiveSet.
134 bool ADCE::deleteDeadInstructionsInLiveBlock(BasicBlock *BB) {
135 bool Changed = false;
136 for (BasicBlock::iterator II = BB->begin(), E = --BB->end(); II != E; ) {
137 Instruction *I = II++;
138 if (!LiveSet.count(I)) { // Is this instruction alive?
140 I->replaceAllUsesWith(UndefValue::get(I->getType()));
142 // Nope... remove the instruction from it's basic block...
143 if (isa<CallInst>(I))
147 BB->getInstList().erase(I);
155 /// convertToUnconditionalBranch - Transform this conditional terminator
156 /// instruction into an unconditional branch because we don't care which of the
157 /// successors it goes to. This eliminate a use of the condition as well.
159 TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
160 BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
161 BasicBlock *BB = TI->getParent();
163 // Remove entries from PHI nodes to avoid confusing ourself later...
164 for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
165 TI->getSuccessor(i)->removePredecessor(BB);
167 // Delete the old branch itself...
168 BB->getInstList().erase(TI);
173 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
174 // true if the function was modified.
176 bool ADCE::doADCE() {
177 bool MadeChanges = false;
179 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
182 // Iterate over all invokes in the function, turning invokes into calls if
183 // they cannot throw.
184 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
185 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
186 if (Function *F = II->getCalledFunction())
187 if (AA.onlyReadsMemory(F)) {
188 // The function cannot unwind. Convert it to a call with a branch
189 // after it to the normal destination.
190 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
191 std::string Name = II->getName(); II->setName("");
192 CallInst *NewCall = new CallInst(F, Args, Name, II);
193 NewCall->setCallingConv(II->getCallingConv());
194 II->replaceAllUsesWith(NewCall);
195 new BranchInst(II->getNormalDest(), II);
197 // Update PHI nodes in the unwind destination
198 II->getUnwindDest()->removePredecessor(BB);
199 BB->getInstList().erase(II);
201 if (NewCall->use_empty()) {
202 BB->getInstList().erase(NewCall);
207 // Iterate over all of the instructions in the function, eliminating trivially
208 // dead instructions, and marking instructions live that are known to be
209 // needed. Perform the walk in depth first order so that we avoid marking any
210 // instructions live in basic blocks that are unreachable. These blocks will
211 // be eliminated later, along with the instructions inside.
213 std::set<BasicBlock*> ReachableBBs;
214 for (df_ext_iterator<BasicBlock*>
215 BBI = df_ext_begin(&Func->front(), ReachableBBs),
216 BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) {
217 BasicBlock *BB = *BBI;
218 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
219 Instruction *I = II++;
220 if (CallInst *CI = dyn_cast<CallInst>(I)) {
221 Function *F = CI->getCalledFunction();
222 if (F && AA.onlyReadsMemory(F)) {
223 if (CI->use_empty()) {
224 BB->getInstList().erase(CI);
228 markInstructionLive(I);
230 } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
231 isa<UnwindInst>(I) || isa<UnreachableInst>(I)) {
232 // FIXME: Unreachable instructions should not be marked intrinsically
234 markInstructionLive(I);
235 } else if (isInstructionTriviallyDead(I)) {
236 // Remove the instruction from it's basic block...
237 BB->getInstList().erase(I);
243 // Check to ensure we have an exit node for this CFG. If we don't, we won't
244 // have any post-dominance information, thus we cannot perform our
245 // transformations safely.
247 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
248 if (DT[&Func->getEntryBlock()] == 0) {
253 // Scan the function marking blocks without post-dominance information as
254 // live. Blocks without post-dominance information occur when there is an
255 // infinite loop in the program. Because the infinite loop could contain a
256 // function which unwinds, exits or has side-effects, we don't want to delete
257 // the infinite loop or those blocks leading up to it.
258 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
259 if (DT[I] == 0 && ReachableBBs.count(I))
260 for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
261 markInstructionLive((*PI)->getTerminator());
263 DEBUG(std::cerr << "Processing work list\n");
265 // AliveBlocks - Set of basic blocks that we know have instructions that are
268 std::set<BasicBlock*> AliveBlocks;
270 // Process the work list of instructions that just became live... if they
271 // became live, then that means that all of their operands are necessary as
272 // well... make them live as well.
274 while (!WorkList.empty()) {
275 Instruction *I = WorkList.back(); // Get an instruction that became live...
278 BasicBlock *BB = I->getParent();
279 if (!ReachableBBs.count(BB)) continue;
280 if (AliveBlocks.insert(BB).second) // Basic block not alive yet.
281 markBlockAlive(BB); // Make it so now!
283 // PHI nodes are a special case, because the incoming values are actually
284 // defined in the predecessor nodes of this block, meaning that the PHI
285 // makes the predecessors alive.
287 if (PHINode *PN = dyn_cast<PHINode>(I)) {
288 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
289 // If the incoming edge is clearly dead, it won't have control
290 // dependence information. Do not mark it live.
291 BasicBlock *PredBB = PN->getIncomingBlock(i);
292 if (ReachableBBs.count(PredBB)) {
293 // FIXME: This should mark the control dependent edge as live, not
294 // necessarily the predecessor itself!
295 if (AliveBlocks.insert(PredBB).second)
296 markBlockAlive(PN->getIncomingBlock(i)); // Block is newly ALIVE!
297 if (Instruction *Op = dyn_cast<Instruction>(PN->getIncomingValue(i)))
298 markInstructionLive(Op);
302 // Loop over all of the operands of the live instruction, making sure that
303 // they are known to be alive as well.
305 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
306 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
307 markInstructionLive(Operand);
312 std::cerr << "Current Function: X = Live\n";
313 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
314 std::cerr << I->getName() << ":\t"
315 << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
316 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
317 if (LiveSet.count(BI)) std::cerr << "X ";
322 // All blocks being live is a common case, handle it specially.
323 if (AliveBlocks.size() == Func->size()) { // No dead blocks?
324 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
325 // Loop over all of the instructions in the function deleting instructions
326 // to drop their references.
327 deleteDeadInstructionsInLiveBlock(I);
329 // Check to make sure the terminator instruction is live. If it isn't,
330 // this means that the condition that it branches on (we know it is not an
331 // unconditional branch), is not needed to make the decision of where to
332 // go to, because all outgoing edges go to the same place. We must remove
333 // the use of the condition (because it's probably dead), so we convert
334 // the terminator to an unconditional branch.
336 TerminatorInst *TI = I->getTerminator();
337 if (!LiveSet.count(TI))
338 convertToUnconditionalBranch(TI);
345 // If the entry node is dead, insert a new entry node to eliminate the entry
346 // node as a special case.
348 if (!AliveBlocks.count(&Func->front())) {
349 BasicBlock *NewEntry = new BasicBlock();
350 new BranchInst(&Func->front(), NewEntry);
351 Func->getBasicBlockList().push_front(NewEntry);
352 AliveBlocks.insert(NewEntry); // This block is always alive!
353 LiveSet.insert(NewEntry->getTerminator()); // The branch is live
356 // Loop over all of the alive blocks in the function. If any successor
357 // blocks are not alive, we adjust the outgoing branches to branch to the
358 // first live postdominator of the live block, adjusting any PHI nodes in
359 // the block to reflect this.
361 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
362 if (AliveBlocks.count(I)) {
364 TerminatorInst *TI = BB->getTerminator();
366 // If the terminator instruction is alive, but the block it is contained
367 // in IS alive, this means that this terminator is a conditional branch on
368 // a condition that doesn't matter. Make it an unconditional branch to
369 // ONE of the successors. This has the side effect of dropping a use of
370 // the conditional value, which may also be dead.
371 if (!LiveSet.count(TI))
372 TI = convertToUnconditionalBranch(TI);
374 // Loop over all of the successors, looking for ones that are not alive.
375 // We cannot save the number of successors in the terminator instruction
376 // here because we may remove them if we don't have a postdominator.
378 for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
379 if (!AliveBlocks.count(TI->getSuccessor(i))) {
380 // Scan up the postdominator tree, looking for the first
381 // postdominator that is alive, and the last postdominator that is
384 PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
385 PostDominatorTree::Node *NextNode = 0;
388 NextNode = LastNode->getIDom();
389 while (!AliveBlocks.count(NextNode->getBlock())) {
391 NextNode = NextNode->getIDom();
399 // There is a special case here... if there IS no post-dominator for
400 // the block we have nowhere to point our branch to. Instead, convert
401 // it to a return. This can only happen if the code branched into an
402 // infinite loop. Note that this may not be desirable, because we
403 // _are_ altering the behavior of the code. This is a well known
404 // drawback of ADCE, so in the future if we choose to revisit the
405 // decision, this is where it should be.
407 if (LastNode == 0) { // No postdominator!
408 if (!isa<InvokeInst>(TI)) {
409 // Call RemoveSuccessor to transmogrify the terminator instruction
410 // to not contain the outgoing branch, or to create a new
411 // terminator if the form fundamentally changes (i.e.,
412 // unconditional branch to return). Note that this will change a
413 // branch into an infinite loop into a return instruction!
415 RemoveSuccessor(TI, i);
417 // RemoveSuccessor may replace TI... make sure we have a fresh
420 TI = BB->getTerminator();
422 // Rescan this successor...
428 // Get the basic blocks that we need...
429 BasicBlock *LastDead = LastNode->getBlock();
430 BasicBlock *NextAlive = NextNode->getBlock();
432 // Make the conditional branch now go to the next alive block...
433 TI->getSuccessor(i)->removePredecessor(BB);
434 TI->setSuccessor(i, NextAlive);
436 // If there are PHI nodes in NextAlive, we need to add entries to
437 // the PHI nodes for the new incoming edge. The incoming values
438 // should be identical to the incoming values for LastDead.
440 for (BasicBlock::iterator II = NextAlive->begin();
441 isa<PHINode>(II); ++II) {
442 PHINode *PN = cast<PHINode>(II);
443 if (LiveSet.count(PN)) { // Only modify live phi nodes
444 // Get the incoming value for LastDead...
445 int OldIdx = PN->getBasicBlockIndex(LastDead);
446 assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
447 Value *InVal = PN->getIncomingValue(OldIdx);
449 // Add an incoming value for BB now...
450 PN->addIncoming(InVal, BB);
456 // Now loop over all of the instructions in the basic block, deleting
457 // dead instructions. This is so that the next sweep over the program
458 // can safely delete dead instructions without other dead instructions
459 // still referring to them.
461 deleteDeadInstructionsInLiveBlock(BB);
464 // Loop over all of the basic blocks in the function, dropping references of
465 // the dead basic blocks. We must do this after the previous step to avoid
466 // dropping references to PHIs which still have entries...
468 std::vector<BasicBlock*> DeadBlocks;
469 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
470 if (!AliveBlocks.count(BB)) {
471 // Remove PHI node entries for this block in live successor blocks.
472 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
473 if (!SI->empty() && isa<PHINode>(SI->front()) && AliveBlocks.count(*SI))
474 (*SI)->removePredecessor(BB);
476 BB->dropAllReferences();
478 DeadBlocks.push_back(BB);
481 NumBlockRemoved += DeadBlocks.size();
483 // Now loop through all of the blocks and delete the dead ones. We can safely
484 // do this now because we know that there are no references to dead blocks
485 // (because they have dropped all of their references).
486 for (std::vector<BasicBlock*>::iterator I = DeadBlocks.begin(),
487 E = DeadBlocks.end(); I != E; ++I)
488 Func->getBasicBlockList().erase(*I);