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/Constant.h"
18 #include "llvm/Instructions.h"
19 #include "llvm/Type.h"
20 #include "llvm/Analysis/AliasAnalysis.h"
21 #include "llvm/Analysis/PostDominators.h"
22 #include "llvm/Support/CFG.h"
23 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
24 #include "llvm/Transforms/Utils/Local.h"
25 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
26 #include "llvm/Support/Debug.h"
27 #include "llvm/ADT/DepthFirstIterator.h"
28 #include "llvm/ADT/Statistic.h"
29 #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 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
87 // instructions in the specified basic block, dropping references on
88 // instructions that are dead according to LiveSet.
89 bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB);
91 TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
93 inline void markInstructionLive(Instruction *I) {
94 if (LiveSet.count(I)) return;
95 DEBUG(std::cerr << "Insn Live: " << *I);
97 WorkList.push_back(I);
100 inline void markTerminatorLive(const BasicBlock *BB) {
101 DEBUG(std::cerr << "Terminator Live: " << *BB->getTerminator());
102 markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
106 RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
107 } // End of anonymous namespace
109 Pass *llvm::createAggressiveDCEPass() { return new ADCE(); }
111 void ADCE::markBlockAlive(BasicBlock *BB) {
112 // Mark the basic block as being newly ALIVE... and mark all branches that
113 // this block is control dependent on as being alive also...
115 PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
117 PostDominanceFrontier::const_iterator It = CDG.find(BB);
118 if (It != CDG.end()) {
119 // Get the blocks that this node is control dependent on...
120 const PostDominanceFrontier::DomSetType &CDB = It->second;
121 for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
122 bind_obj(this, &ADCE::markTerminatorLive));
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 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
133 // instructions in the specified basic block, dropping references on
134 // instructions that are dead according to LiveSet.
135 bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) {
136 bool Changed = false;
137 for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; )
138 if (!LiveSet.count(I)) { // Is this instruction alive?
139 I->dropAllReferences(); // Nope, drop references...
140 if (PHINode *PN = dyn_cast<PHINode>(I)) {
141 // We don't want to leave PHI nodes in the program that have
142 // #arguments != #predecessors, so we remove them now.
144 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
146 // Delete the instruction...
148 BB->getInstList().erase(PN);
161 /// convertToUnconditionalBranch - Transform this conditional terminator
162 /// instruction into an unconditional branch because we don't care which of the
163 /// successors it goes to. This eliminate a use of the condition as well.
165 TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
166 BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
167 BasicBlock *BB = TI->getParent();
169 // Remove entries from PHI nodes to avoid confusing ourself later...
170 for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
171 TI->getSuccessor(i)->removePredecessor(BB);
173 // Delete the old branch itself...
174 BB->getInstList().erase(TI);
179 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
180 // true if the function was modified.
182 bool ADCE::doADCE() {
183 bool MadeChanges = false;
185 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
188 // Iterate over all invokes in the function, turning invokes into calls if
189 // they cannot throw.
190 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
191 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
192 if (Function *F = II->getCalledFunction())
193 if (AA.onlyReadsMemory(F)) {
194 // The function cannot unwind. Convert it to a call with a branch
195 // after it to the normal destination.
196 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
197 std::string Name = II->getName(); II->setName("");
198 Instruction *NewCall = new CallInst(F, Args, Name, II);
199 II->replaceAllUsesWith(NewCall);
200 new BranchInst(II->getNormalDest(), II);
202 // Update PHI nodes in the unwind destination
203 II->getUnwindDest()->removePredecessor(BB);
204 BB->getInstList().erase(II);
206 if (NewCall->use_empty()) {
207 BB->getInstList().erase(NewCall);
212 // Iterate over all of the instructions in the function, eliminating trivially
213 // dead instructions, and marking instructions live that are known to be
214 // needed. Perform the walk in depth first order so that we avoid marking any
215 // instructions live in basic blocks that are unreachable. These blocks will
216 // be eliminated later, along with the instructions inside.
218 std::set<BasicBlock*> ReachableBBs;
219 for (df_ext_iterator<BasicBlock*>
220 BBI = df_ext_begin(&Func->front(), ReachableBBs),
221 BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) {
222 BasicBlock *BB = *BBI;
223 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
224 Instruction *I = II++;
225 if (CallInst *CI = dyn_cast<CallInst>(I)) {
226 Function *F = CI->getCalledFunction();
227 if (F && AA.onlyReadsMemory(F)) {
228 if (CI->use_empty()) {
229 BB->getInstList().erase(CI);
233 markInstructionLive(I);
235 } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
236 isa<UnwindInst>(I)) {
237 markInstructionLive(I);
238 } else if (isInstructionTriviallyDead(I)) {
239 // Remove the instruction from it's basic block...
240 BB->getInstList().erase(I);
246 // Check to ensure we have an exit node for this CFG. If we don't, we won't
247 // have any post-dominance information, thus we cannot perform our
248 // transformations safely.
250 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
251 if (DT[&Func->getEntryBlock()] == 0) {
256 // Scan the function marking blocks without post-dominance information as
257 // live. Blocks without post-dominance information occur when there is an
258 // infinite loop in the program. Because the infinite loop could contain a
259 // function which unwinds, exits or has side-effects, we don't want to delete
260 // the infinite loop or those blocks leading up to it.
261 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
263 for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
264 markInstructionLive((*PI)->getTerminator());
268 DEBUG(std::cerr << "Processing work list\n");
270 // AliveBlocks - Set of basic blocks that we know have instructions that are
273 std::set<BasicBlock*> AliveBlocks;
275 // Process the work list of instructions that just became live... if they
276 // became live, then that means that all of their operands are necessary as
277 // well... make them live as well.
279 while (!WorkList.empty()) {
280 Instruction *I = WorkList.back(); // Get an instruction that became live...
283 BasicBlock *BB = I->getParent();
284 if (!ReachableBBs.count(BB)) continue;
285 if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
286 AliveBlocks.insert(BB); // Block is now ALIVE!
287 markBlockAlive(BB); // Make it so now!
290 // PHI nodes are a special case, because the incoming values are actually
291 // defined in the predecessor nodes of this block, meaning that the PHI
292 // makes the predecessors alive.
294 if (PHINode *PN = dyn_cast<PHINode>(I))
295 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
296 if (!AliveBlocks.count(*PI)) {
297 AliveBlocks.insert(BB); // Block is now ALIVE!
301 // Loop over all of the operands of the live instruction, making sure that
302 // they are known to be alive as well...
304 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
305 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
306 markInstructionLive(Operand);
310 std::cerr << "Current Function: X = Live\n";
311 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
312 std::cerr << I->getName() << ":\t"
313 << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
314 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
315 if (LiveSet.count(BI)) std::cerr << "X ";
320 // Find the first postdominator of the entry node that is alive. Make it the
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, telling dead
326 // instructions to drop their references. This is so that the next sweep
327 // over the program can safely delete dead instructions without other dead
328 // instructions still referring to them.
330 dropReferencesOfDeadInstructionsInLiveBlock(I);
332 // Check to make sure the terminator instruction is live. If it isn't,
333 // this means that the condition that it branches on (we know it is not an
334 // unconditional branch), is not needed to make the decision of where to
335 // go to, because all outgoing edges go to the same place. We must remove
336 // the use of the condition (because it's probably dead), so we convert
337 // the terminator to a conditional branch.
339 TerminatorInst *TI = I->getTerminator();
340 if (!LiveSet.count(TI))
341 convertToUnconditionalBranch(TI);
344 } else { // If there are some blocks dead...
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
368 // on a condition that doesn't matter. Make it an unconditional branch
369 // to ONE of the successors. This has the side effect of dropping a use
370 // of 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)];
386 // There is a special case here... if there IS no post-dominator for
387 // the block we have no owhere to point our branch to. Instead,
388 // convert it to a return. This can only happen if the code
389 // branched into an infinite loop. Note that this may not be
390 // desirable, because we _are_ altering the behavior of the code.
391 // This is a well known drawback of ADCE, so in the future if we
392 // choose to revisit the decision, this is where it should be.
394 if (LastNode == 0) { // No postdominator!
395 // Call RemoveSuccessor to transmogrify the terminator instruction
396 // to not contain the outgoing branch, or to create a new
397 // terminator if the form fundamentally changes (i.e.,
398 // unconditional branch to return). Note that this will change a
399 // branch into an infinite loop into a return instruction!
401 RemoveSuccessor(TI, i);
403 // RemoveSuccessor may replace TI... make sure we have a fresh
404 // pointer... and e variable.
406 TI = BB->getTerminator();
408 // Rescan this successor...
411 PostDominatorTree::Node *NextNode = LastNode->getIDom();
413 while (!AliveBlocks.count(NextNode->getBlock())) {
415 NextNode = NextNode->getIDom();
418 // Get the basic blocks that we need...
419 BasicBlock *LastDead = LastNode->getBlock();
420 BasicBlock *NextAlive = NextNode->getBlock();
422 // Make the conditional branch now go to the next alive block...
423 TI->getSuccessor(i)->removePredecessor(BB);
424 TI->setSuccessor(i, NextAlive);
426 // If there are PHI nodes in NextAlive, we need to add entries to
427 // the PHI nodes for the new incoming edge. The incoming values
428 // should be identical to the incoming values for LastDead.
430 for (BasicBlock::iterator II = NextAlive->begin();
431 isa<PHINode>(II); ++II) {
432 PHINode *PN = cast<PHINode>(II);
433 if (LiveSet.count(PN)) { // Only modify live phi nodes
434 // Get the incoming value for LastDead...
435 int OldIdx = PN->getBasicBlockIndex(LastDead);
436 assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
437 Value *InVal = PN->getIncomingValue(OldIdx);
439 // Add an incoming value for BB now...
440 PN->addIncoming(InVal, BB);
446 // Now loop over all of the instructions in the basic block, telling
447 // dead instructions to drop their references. This is so that the next
448 // sweep over the program can safely delete dead instructions without
449 // other dead instructions still referring to them.
451 dropReferencesOfDeadInstructionsInLiveBlock(BB);
455 // We make changes if there are any dead blocks in the function...
456 if (unsigned NumDeadBlocks = Func->size() - AliveBlocks.size()) {
458 NumBlockRemoved += NumDeadBlocks;
461 // Loop over all of the basic blocks in the function, removing control flow
462 // edges to live blocks (also eliminating any entries in PHI functions in
463 // referenced blocks).
465 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
466 if (!AliveBlocks.count(BB)) {
467 // Remove all outgoing edges from this basic block and convert the
468 // terminator into a return instruction.
469 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
471 if (!Succs.empty()) {
472 // Loop over all of the successors, removing this block from PHI node
473 // entries that might be in the block...
474 while (!Succs.empty()) {
475 Succs.back()->removePredecessor(BB);
479 // Delete the old terminator instruction...
480 const Type *TermTy = BB->getTerminator()->getType();
481 if (TermTy != Type::VoidTy)
482 BB->getTerminator()->replaceAllUsesWith(
483 Constant::getNullValue(TermTy));
484 BB->getInstList().pop_back();
485 const Type *RetTy = Func->getReturnType();
486 new ReturnInst(RetTy != Type::VoidTy ?
487 Constant::getNullValue(RetTy) : 0, BB);
492 // Loop over all of the basic blocks in the function, dropping references of
493 // the dead basic blocks. We must do this after the previous step to avoid
494 // dropping references to PHIs which still have entries...
496 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
497 if (!AliveBlocks.count(BB))
498 BB->dropAllReferences();
500 // Now loop through all of the blocks and delete the dead ones. We can safely
501 // do this now because we know that there are no references to dead blocks
502 // (because they have dropped all of their references... we also remove dead
503 // instructions from alive blocks.
505 for (Function::iterator BI = Func->begin(); BI != Func->end(); )
506 if (!AliveBlocks.count(BI)) { // Delete dead blocks...
507 BI = Func->getBasicBlockList().erase(BI);
508 } else { // Scan alive blocks...
509 for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
510 if (!LiveSet.count(II)) { // Is this instruction alive?
511 // Nope... remove the instruction from it's basic block...
512 if (isa<CallInst>(II))
516 II = BI->getInstList().erase(II);
522 ++BI; // Increment iterator...