1 //===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // 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 #define DEBUG_TYPE "adce"
17 #include "llvm/Transforms/Scalar.h"
18 #include "llvm/Constants.h"
19 #include "llvm/Instructions.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/SmallVector.h"
29 #include "llvm/ADT/Statistic.h"
30 #include "llvm/ADT/STLExtras.h"
31 #include "llvm/Support/Compiler.h"
35 STATISTIC(NumBlockRemoved, "Number of basic blocks removed");
36 STATISTIC(NumInstRemoved , "Number of instructions removed");
37 STATISTIC(NumCallRemoved , "Number of calls removed");
40 //===----------------------------------------------------------------------===//
43 // This class does all of the work of Aggressive Dead Code Elimination.
44 // It's public interface consists of a constructor and a doADCE() method.
46 class VISIBILITY_HIDDEN ADCE : public FunctionPass {
47 Function *Func; // The function that we are working on
48 std::vector<Instruction*> WorkList; // Instructions that just became live
49 std::set<Instruction*> LiveSet; // The set of live instructions
51 //===--------------------------------------------------------------------===//
52 // The public interface for this class
55 static char ID; // Pass identification, replacement for typeid
56 ADCE() : FunctionPass((intptr_t)&ID) {}
58 // Execute the Aggressive Dead Code Elimination Algorithm
60 virtual bool runOnFunction(Function &F) {
62 bool Changed = doADCE();
63 assert(WorkList.empty());
67 // getAnalysisUsage - We require post dominance frontiers (aka Control
69 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
70 // We require that all function nodes are unified, because otherwise code
71 // can be marked live that wouldn't necessarily be otherwise.
72 AU.addRequired<UnifyFunctionExitNodes>();
73 AU.addRequired<AliasAnalysis>();
74 AU.addRequired<PostDominatorTree>();
75 AU.addRequired<PostDominanceFrontier>();
79 //===--------------------------------------------------------------------===//
80 // The implementation of this class
83 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
84 // true if the function was modified.
88 void markBlockAlive(BasicBlock *BB);
91 // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in
92 // the specified basic block, deleting ones that are dead according to
94 bool deleteDeadInstructionsInLiveBlock(BasicBlock *BB);
96 TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
98 inline void markInstructionLive(Instruction *I) {
99 if (!LiveSet.insert(I).second) return;
100 DOUT << "Insn Live: " << *I;
101 WorkList.push_back(I);
104 inline void markTerminatorLive(const BasicBlock *BB) {
105 DOUT << "Terminator Live: " << *BB->getTerminator();
106 markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
109 } // End of anonymous namespace
112 static RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
114 FunctionPass *llvm::createAggressiveDCEPass() { return new ADCE(); }
116 void ADCE::markBlockAlive(BasicBlock *BB) {
117 // Mark the basic block as being newly ALIVE... and mark all branches that
118 // this block is control dependent on as being alive also...
120 PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
122 PostDominanceFrontier::const_iterator It = CDG.find(BB);
123 if (It != CDG.end()) {
124 // Get the blocks that this node is control dependent on...
125 const PostDominanceFrontier::DomSetType &CDB = It->second;
126 for (PostDominanceFrontier::DomSetType::const_iterator I =
127 CDB.begin(), E = CDB.end(); I != E; ++I)
128 markTerminatorLive(*I); // Mark all their terminators as live
131 // If this basic block is live, and it ends in an unconditional branch, then
132 // the branch is alive as well...
133 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
134 if (BI->isUnconditional())
135 markTerminatorLive(BB);
138 // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in the
139 // specified basic block, deleting ones that are dead according to LiveSet.
140 bool ADCE::deleteDeadInstructionsInLiveBlock(BasicBlock *BB) {
141 bool Changed = false;
142 for (BasicBlock::iterator II = BB->begin(), E = --BB->end(); II != E; ) {
143 Instruction *I = II++;
144 if (!LiveSet.count(I)) { // Is this instruction alive?
146 I->replaceAllUsesWith(UndefValue::get(I->getType()));
148 // Nope... remove the instruction from it's basic block...
149 if (isa<CallInst>(I))
153 BB->getInstList().erase(I);
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 = BranchInst::Create(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>();
187 // Iterate over all of the instructions in the function, eliminating trivially
188 // dead instructions, and marking instructions live that are known to be
189 // needed. Perform the walk in depth first order so that we avoid marking any
190 // instructions live in basic blocks that are unreachable. These blocks will
191 // be eliminated later, along with the instructions inside.
193 std::set<BasicBlock*> ReachableBBs;
194 std::vector<BasicBlock*> Stack;
195 Stack.push_back(&Func->getEntryBlock());
197 while (!Stack.empty()) {
198 BasicBlock* BB = Stack.back();
199 if (ReachableBBs.count(BB)) {
203 ReachableBBs.insert(BB);
206 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
207 Instruction *I = II++;
208 if (CallInst *CI = dyn_cast<CallInst>(I)) {
209 if (AA.onlyReadsMemory(CI)) {
210 if (CI->use_empty()) {
211 BB->getInstList().erase(CI);
215 markInstructionLive(I);
217 } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
218 isa<UnwindInst>(I) || isa<UnreachableInst>(I)) {
219 // FIXME: Unreachable instructions should not be marked intrinsically
221 markInstructionLive(I);
222 } else if (isInstructionTriviallyDead(I)) {
223 // Remove the instruction from it's basic block...
224 BB->getInstList().erase(I);
229 for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI) {
230 // Back edges (as opposed to cross edges) indicate loops, so implicitly
232 if (std::find(Stack.begin(), Stack.end(), *SI) != Stack.end())
233 markInstructionLive(BB->getTerminator());
234 if (!ReachableBBs.count(*SI))
235 Stack.push_back(*SI);
239 // Check to ensure we have an exit node for this CFG. If we don't, we won't
240 // have any post-dominance information, thus we cannot perform our
241 // transformations safely.
243 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
244 if (DT[&Func->getEntryBlock()] == 0) {
249 // Scan the function marking blocks without post-dominance information as
250 // live. Blocks without post-dominance information occur when there is an
251 // infinite loop in the program. Because the infinite loop could contain a
252 // function which unwinds, exits or has side-effects, we don't want to delete
253 // the infinite loop or those blocks leading up to it.
254 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
255 if (DT[I] == 0 && ReachableBBs.count(I))
256 for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
257 markInstructionLive((*PI)->getTerminator());
259 DOUT << "Processing work list\n";
261 // AliveBlocks - Set of basic blocks that we know have instructions that are
264 std::set<BasicBlock*> AliveBlocks;
266 // Process the work list of instructions that just became live... if they
267 // became live, then that means that all of their operands are necessary as
268 // well... make them live as well.
270 while (!WorkList.empty()) {
271 Instruction *I = WorkList.back(); // Get an instruction that became live...
274 BasicBlock *BB = I->getParent();
275 if (!ReachableBBs.count(BB)) continue;
276 if (AliveBlocks.insert(BB).second) // Basic block not alive yet.
277 markBlockAlive(BB); // Make it so now!
279 // PHI nodes are a special case, because the incoming values are actually
280 // defined in the predecessor nodes of this block, meaning that the PHI
281 // makes the predecessors alive.
283 if (PHINode *PN = dyn_cast<PHINode>(I)) {
284 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
285 // If the incoming edge is clearly dead, it won't have control
286 // dependence information. Do not mark it live.
287 BasicBlock *PredBB = PN->getIncomingBlock(i);
288 if (ReachableBBs.count(PredBB)) {
289 // FIXME: This should mark the control dependent edge as live, not
290 // necessarily the predecessor itself!
291 if (AliveBlocks.insert(PredBB).second)
292 markBlockAlive(PN->getIncomingBlock(i)); // Block is newly ALIVE!
293 if (Instruction *Op = dyn_cast<Instruction>(PN->getIncomingValue(i)))
294 markInstructionLive(Op);
298 // Loop over all of the operands of the live instruction, making sure that
299 // they are known to be alive as well.
301 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
302 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
303 markInstructionLive(Operand);
308 DOUT << "Current Function: X = Live\n";
309 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
310 DOUT << I->getName() << ":\t"
311 << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
312 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
313 if (LiveSet.count(BI)) DOUT << "X ";
318 // All blocks being live is a common case, handle it specially.
319 if (AliveBlocks.size() == Func->size()) { // No dead blocks?
320 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
321 // Loop over all of the instructions in the function deleting instructions
322 // to drop their references.
323 deleteDeadInstructionsInLiveBlock(I);
325 // Check to make sure the terminator instruction is live. If it isn't,
326 // this means that the condition that it branches on (we know it is not an
327 // unconditional branch), is not needed to make the decision of where to
328 // go to, because all outgoing edges go to the same place. We must remove
329 // the use of the condition (because it's probably dead), so we convert
330 // the terminator to an unconditional branch.
332 TerminatorInst *TI = I->getTerminator();
333 if (!LiveSet.count(TI))
334 convertToUnconditionalBranch(TI);
341 // If the entry node is dead, insert a new entry node to eliminate the entry
342 // node as a special case.
344 if (!AliveBlocks.count(&Func->front())) {
345 BasicBlock *NewEntry = BasicBlock::Create();
346 BranchInst::Create(&Func->front(), NewEntry);
347 Func->getBasicBlockList().push_front(NewEntry);
348 AliveBlocks.insert(NewEntry); // This block is always alive!
349 LiveSet.insert(NewEntry->getTerminator()); // The branch is live
352 // Loop over all of the alive blocks in the function. If any successor
353 // blocks are not alive, we adjust the outgoing branches to branch to the
354 // first live postdominator of the live block, adjusting any PHI nodes in
355 // the block to reflect this.
357 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
358 if (AliveBlocks.count(I)) {
360 TerminatorInst *TI = BB->getTerminator();
362 // If the terminator instruction is alive, but the block it is contained
363 // in IS alive, this means that this terminator is a conditional branch on
364 // a condition that doesn't matter. Make it an unconditional branch to
365 // ONE of the successors. This has the side effect of dropping a use of
366 // the conditional value, which may also be dead.
367 if (!LiveSet.count(TI))
368 TI = convertToUnconditionalBranch(TI);
370 // Loop over all of the successors, looking for ones that are not alive.
371 // We cannot save the number of successors in the terminator instruction
372 // here because we may remove them if we don't have a postdominator.
374 for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
375 if (!AliveBlocks.count(TI->getSuccessor(i))) {
376 // Scan up the postdominator tree, looking for the first
377 // postdominator that is alive, and the last postdominator that is
380 DomTreeNode *LastNode = DT[TI->getSuccessor(i)];
381 DomTreeNode *NextNode = 0;
384 NextNode = LastNode->getIDom();
385 while (!AliveBlocks.count(NextNode->getBlock())) {
387 NextNode = NextNode->getIDom();
395 // There is a special case here... if there IS no post-dominator for
396 // the block we have nowhere to point our branch to. Instead, convert
397 // it to a return. This can only happen if the code branched into an
398 // infinite loop. Note that this may not be desirable, because we
399 // _are_ altering the behavior of the code. This is a well known
400 // drawback of ADCE, so in the future if we choose to revisit the
401 // decision, this is where it should be.
403 if (LastNode == 0) { // No postdominator!
404 if (!isa<InvokeInst>(TI)) {
405 // Call RemoveSuccessor to transmogrify the terminator instruction
406 // to not contain the outgoing branch, or to create a new
407 // terminator if the form fundamentally changes (i.e.,
408 // unconditional branch to return). Note that this will change a
409 // branch into an infinite loop into a return instruction!
411 RemoveSuccessor(TI, i);
413 // RemoveSuccessor may replace TI... make sure we have a fresh
416 TI = BB->getTerminator();
418 // Rescan this successor...
424 // Get the basic blocks that we need...
425 BasicBlock *LastDead = LastNode->getBlock();
426 BasicBlock *NextAlive = NextNode->getBlock();
428 // Make the conditional branch now go to the next alive block...
429 TI->getSuccessor(i)->removePredecessor(BB);
430 TI->setSuccessor(i, NextAlive);
432 // If there are PHI nodes in NextAlive, we need to add entries to
433 // the PHI nodes for the new incoming edge. The incoming values
434 // should be identical to the incoming values for LastDead.
436 for (BasicBlock::iterator II = NextAlive->begin();
437 isa<PHINode>(II); ++II) {
438 PHINode *PN = cast<PHINode>(II);
439 if (LiveSet.count(PN)) { // Only modify live phi nodes
440 // Get the incoming value for LastDead...
441 int OldIdx = PN->getBasicBlockIndex(LastDead);
442 assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
443 Value *InVal = PN->getIncomingValue(OldIdx);
445 // Add an incoming value for BB now...
446 PN->addIncoming(InVal, BB);
452 // Now loop over all of the instructions in the basic block, deleting
453 // dead instructions. This is so that the next sweep over the program
454 // can safely delete dead instructions without other dead instructions
455 // still referring to them.
457 deleteDeadInstructionsInLiveBlock(BB);
460 // Loop over all of the basic blocks in the function, dropping references of
461 // the dead basic blocks. We must do this after the previous step to avoid
462 // dropping references to PHIs which still have entries...
464 std::vector<BasicBlock*> DeadBlocks;
465 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
466 if (!AliveBlocks.count(BB)) {
467 // Remove PHI node entries for this block in live successor blocks.
468 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
469 if (!SI->empty() && isa<PHINode>(SI->front()) && AliveBlocks.count(*SI))
470 (*SI)->removePredecessor(BB);
472 BB->dropAllReferences();
474 DeadBlocks.push_back(BB);
477 NumBlockRemoved += DeadBlocks.size();
479 // Now loop through all of the blocks and delete the dead ones. We can safely
480 // do this now because we know that there are no references to dead blocks
481 // (because they have dropped all of their references).
482 for (std::vector<BasicBlock*>::iterator I = DeadBlocks.begin(),
483 E = DeadBlocks.end(); I != E; ++I)
484 Func->getBasicBlockList().erase(*I);