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.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 RegisterOpt<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_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
121 bind_obj(this, &ADCE::markTerminatorLive));
124 // If this basic block is live, and it ends in an unconditional branch, then
125 // the branch is alive as well...
126 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
127 if (BI->isUnconditional())
128 markTerminatorLive(BB);
131 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
132 // instructions in the specified basic block, dropping references on
133 // instructions that are dead according to LiveSet.
134 bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) {
135 bool Changed = false;
136 for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; )
137 if (!LiveSet.count(I)) { // Is this instruction alive?
138 I->dropAllReferences(); // Nope, drop references...
139 if (PHINode *PN = dyn_cast<PHINode>(I)) {
140 // We don't want to leave PHI nodes in the program that have
141 // #arguments != #predecessors, so we remove them now.
143 PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
145 // Delete the instruction...
147 BB->getInstList().erase(PN);
160 /// convertToUnconditionalBranch - Transform this conditional terminator
161 /// instruction into an unconditional branch because we don't care which of the
162 /// successors it goes to. This eliminate a use of the condition as well.
164 TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
165 BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
166 BasicBlock *BB = TI->getParent();
168 // Remove entries from PHI nodes to avoid confusing ourself later...
169 for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
170 TI->getSuccessor(i)->removePredecessor(BB);
172 // Delete the old branch itself...
173 BB->getInstList().erase(TI);
178 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
179 // true if the function was modified.
181 bool ADCE::doADCE() {
182 bool MadeChanges = false;
184 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
187 // Iterate over all invokes in the function, turning invokes into calls if
188 // they cannot throw.
189 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
190 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
191 if (Function *F = II->getCalledFunction())
192 if (AA.onlyReadsMemory(F)) {
193 // The function cannot unwind. Convert it to a call with a branch
194 // after it to the normal destination.
195 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
196 std::string Name = II->getName(); II->setName("");
197 Instruction *NewCall = new CallInst(F, Args, Name, II);
198 II->replaceAllUsesWith(NewCall);
199 new BranchInst(II->getNormalDest(), II);
201 // Update PHI nodes in the unwind destination
202 II->getUnwindDest()->removePredecessor(BB);
203 BB->getInstList().erase(II);
205 if (NewCall->use_empty()) {
206 BB->getInstList().erase(NewCall);
211 // Iterate over all of the instructions in the function, eliminating trivially
212 // dead instructions, and marking instructions live that are known to be
213 // needed. Perform the walk in depth first order so that we avoid marking any
214 // instructions live in basic blocks that are unreachable. These blocks will
215 // be eliminated later, along with the instructions inside.
217 std::set<BasicBlock*> ReachableBBs;
218 for (df_ext_iterator<BasicBlock*>
219 BBI = df_ext_begin(&Func->front(), ReachableBBs),
220 BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) {
221 BasicBlock *BB = *BBI;
222 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
223 Instruction *I = II++;
224 if (CallInst *CI = dyn_cast<CallInst>(I)) {
225 Function *F = CI->getCalledFunction();
226 if (F && AA.onlyReadsMemory(F)) {
227 if (CI->use_empty()) {
228 BB->getInstList().erase(CI);
232 markInstructionLive(I);
234 } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
235 isa<UnwindInst>(I) || isa<UnreachableInst>(I)) {
236 // FIXME: Unreachable instructions should not be marked intrinsically
238 markInstructionLive(I);
239 } else if (isInstructionTriviallyDead(I)) {
240 // Remove the instruction from it's basic block...
241 BB->getInstList().erase(I);
247 // Check to ensure we have an exit node for this CFG. If we don't, we won't
248 // have any post-dominance information, thus we cannot perform our
249 // transformations safely.
251 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
252 if (DT[&Func->getEntryBlock()] == 0) {
257 // Scan the function marking blocks without post-dominance information as
258 // live. Blocks without post-dominance information occur when there is an
259 // infinite loop in the program. Because the infinite loop could contain a
260 // function which unwinds, exits or has side-effects, we don't want to delete
261 // the infinite loop or those blocks leading up to it.
262 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
264 for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
265 markInstructionLive((*PI)->getTerminator());
269 DEBUG(std::cerr << "Processing work list\n");
271 // AliveBlocks - Set of basic blocks that we know have instructions that are
274 std::set<BasicBlock*> AliveBlocks;
276 // Process the work list of instructions that just became live... if they
277 // became live, then that means that all of their operands are necessary as
278 // well... make them live as well.
280 while (!WorkList.empty()) {
281 Instruction *I = WorkList.back(); // Get an instruction that became live...
284 BasicBlock *BB = I->getParent();
285 if (!ReachableBBs.count(BB)) continue;
286 if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
287 AliveBlocks.insert(BB); // Block is now ALIVE!
288 markBlockAlive(BB); // Make it so now!
291 // PHI nodes are a special case, because the incoming values are actually
292 // defined in the predecessor nodes of this block, meaning that the PHI
293 // makes the predecessors alive.
295 if (PHINode *PN = dyn_cast<PHINode>(I))
296 for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
297 if (!AliveBlocks.count(*PI)) {
298 AliveBlocks.insert(BB); // Block is now ALIVE!
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);
311 std::cerr << "Current Function: X = Live\n";
312 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
313 std::cerr << I->getName() << ":\t"
314 << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
315 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
316 if (LiveSet.count(BI)) std::cerr << "X ";
321 // Find the first postdominator of the entry node that is alive. Make it the
324 if (AliveBlocks.size() == Func->size()) { // No dead blocks?
325 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
326 // Loop over all of the instructions in the function, telling dead
327 // instructions to drop their references. This is so that the next sweep
328 // over the program can safely delete dead instructions without other dead
329 // instructions still referring to them.
331 dropReferencesOfDeadInstructionsInLiveBlock(I);
333 // Check to make sure the terminator instruction is live. If it isn't,
334 // this means that the condition that it branches on (we know it is not an
335 // unconditional branch), is not needed to make the decision of where to
336 // go to, because all outgoing edges go to the same place. We must remove
337 // the use of the condition (because it's probably dead), so we convert
338 // the terminator to a conditional branch.
340 TerminatorInst *TI = I->getTerminator();
341 if (!LiveSet.count(TI))
342 convertToUnconditionalBranch(TI);
345 } else { // If there are some blocks dead...
346 // If the entry node is dead, insert a new entry node to eliminate the entry
347 // node as a special case.
349 if (!AliveBlocks.count(&Func->front())) {
350 BasicBlock *NewEntry = new BasicBlock();
351 new BranchInst(&Func->front(), NewEntry);
352 Func->getBasicBlockList().push_front(NewEntry);
353 AliveBlocks.insert(NewEntry); // This block is always alive!
354 LiveSet.insert(NewEntry->getTerminator()); // The branch is live
357 // Loop over all of the alive blocks in the function. If any successor
358 // blocks are not alive, we adjust the outgoing branches to branch to the
359 // first live postdominator of the live block, adjusting any PHI nodes in
360 // the block to reflect this.
362 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
363 if (AliveBlocks.count(I)) {
365 TerminatorInst *TI = BB->getTerminator();
367 // If the terminator instruction is alive, but the block it is contained
368 // in IS alive, this means that this terminator is a conditional branch
369 // on a condition that doesn't matter. Make it an unconditional branch
370 // to ONE of the successors. This has the side effect of dropping a use
371 // of the conditional value, which may also be dead.
372 if (!LiveSet.count(TI))
373 TI = convertToUnconditionalBranch(TI);
375 // Loop over all of the successors, looking for ones that are not alive.
376 // We cannot save the number of successors in the terminator instruction
377 // here because we may remove them if we don't have a postdominator...
379 for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
380 if (!AliveBlocks.count(TI->getSuccessor(i))) {
381 // Scan up the postdominator tree, looking for the first
382 // postdominator that is alive, and the last postdominator that is
385 PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
387 // There is a special case here... if there IS no post-dominator for
388 // the block we have no owhere to point our branch to. Instead,
389 // convert it to a return. This can only happen if the code
390 // branched into an infinite loop. Note that this may not be
391 // desirable, because we _are_ altering the behavior of the code.
392 // This is a well known drawback of ADCE, so in the future if we
393 // choose to revisit the decision, this is where it should be.
395 if (LastNode == 0) { // No postdominator!
396 // Call RemoveSuccessor to transmogrify the terminator instruction
397 // to not contain the outgoing branch, or to create a new
398 // terminator if the form fundamentally changes (i.e.,
399 // unconditional branch to return). Note that this will change a
400 // branch into an infinite loop into a return instruction!
402 RemoveSuccessor(TI, i);
404 // RemoveSuccessor may replace TI... make sure we have a fresh
405 // pointer... and e variable.
407 TI = BB->getTerminator();
409 // Rescan this successor...
412 PostDominatorTree::Node *NextNode = LastNode->getIDom();
414 while (!AliveBlocks.count(NextNode->getBlock())) {
416 NextNode = NextNode->getIDom();
419 // Get the basic blocks that we need...
420 BasicBlock *LastDead = LastNode->getBlock();
421 BasicBlock *NextAlive = NextNode->getBlock();
423 // Make the conditional branch now go to the next alive block...
424 TI->getSuccessor(i)->removePredecessor(BB);
425 TI->setSuccessor(i, NextAlive);
427 // If there are PHI nodes in NextAlive, we need to add entries to
428 // the PHI nodes for the new incoming edge. The incoming values
429 // should be identical to the incoming values for LastDead.
431 for (BasicBlock::iterator II = NextAlive->begin();
432 isa<PHINode>(II); ++II) {
433 PHINode *PN = cast<PHINode>(II);
434 if (LiveSet.count(PN)) { // Only modify live phi nodes
435 // Get the incoming value for LastDead...
436 int OldIdx = PN->getBasicBlockIndex(LastDead);
437 assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
438 Value *InVal = PN->getIncomingValue(OldIdx);
440 // Add an incoming value for BB now...
441 PN->addIncoming(InVal, BB);
447 // Now loop over all of the instructions in the basic block, telling
448 // dead instructions to drop their references. This is so that the next
449 // sweep over the program can safely delete dead instructions without
450 // other dead instructions still referring to them.
452 dropReferencesOfDeadInstructionsInLiveBlock(BB);
456 // We make changes if there are any dead blocks in the function...
457 if (unsigned NumDeadBlocks = Func->size() - AliveBlocks.size()) {
459 NumBlockRemoved += NumDeadBlocks;
462 // Loop over all of the basic blocks in the function, removing control flow
463 // edges to live blocks (also eliminating any entries in PHI functions in
464 // referenced blocks).
466 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
467 if (!AliveBlocks.count(BB)) {
468 // Remove all outgoing edges from this basic block and convert the
469 // terminator into a return instruction.
470 std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
472 if (!Succs.empty()) {
473 // Loop over all of the successors, removing this block from PHI node
474 // entries that might be in the block...
475 while (!Succs.empty()) {
476 Succs.back()->removePredecessor(BB);
480 // Delete the old terminator instruction...
481 const Type *TermTy = BB->getTerminator()->getType();
482 if (TermTy != Type::VoidTy)
483 BB->getTerminator()->replaceAllUsesWith(
484 Constant::getNullValue(TermTy));
485 BB->getInstList().pop_back();
486 const Type *RetTy = Func->getReturnType();
487 new ReturnInst(RetTy != Type::VoidTy ?
488 Constant::getNullValue(RetTy) : 0, BB);
493 // Loop over all of the basic blocks in the function, dropping references of
494 // the dead basic blocks. We must do this after the previous step to avoid
495 // dropping references to PHIs which still have entries...
497 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
498 if (!AliveBlocks.count(BB))
499 BB->dropAllReferences();
501 // Now loop through all of the blocks and delete the dead ones. We can safely
502 // do this now because we know that there are no references to dead blocks
503 // (because they have dropped all of their references... we also remove dead
504 // instructions from alive blocks.
506 for (Function::iterator BI = Func->begin(); BI != Func->end(); )
507 if (!AliveBlocks.count(BI)) { // Delete dead blocks...
508 BI = Func->getBasicBlockList().erase(BI);
509 } else { // Scan alive blocks...
510 for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
511 if (!LiveSet.count(II)) { // Is this instruction alive?
512 // Nope... remove the instruction from it's basic block...
513 if (isa<CallInst>(II))
517 II = BI->getInstList().erase(II);
523 ++BI; // Increment iterator...