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 DOUT << "Insn Live: " << *I;
95 WorkList.push_back(I);
98 inline void markTerminatorLive(const BasicBlock *BB) {
99 DOUT << "Terminator Live: " << *BB->getTerminator();
100 markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
104 RegisterPass<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 (PostDominanceFrontier::DomSetType::const_iterator I =
120 CDB.begin(), E = CDB.end(); I != E; ++I)
121 markTerminatorLive(*I); // Mark all their terminators as live
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 // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in the
132 // specified basic block, deleting ones that are dead according to LiveSet.
133 bool ADCE::deleteDeadInstructionsInLiveBlock(BasicBlock *BB) {
134 bool Changed = false;
135 for (BasicBlock::iterator II = BB->begin(), E = --BB->end(); II != E; ) {
136 Instruction *I = II++;
137 if (!LiveSet.count(I)) { // Is this instruction alive?
139 I->replaceAllUsesWith(UndefValue::get(I->getType()));
141 // Nope... remove the instruction from it's basic block...
142 if (isa<CallInst>(I))
146 BB->getInstList().erase(I);
154 /// convertToUnconditionalBranch - Transform this conditional terminator
155 /// instruction into an unconditional branch because we don't care which of the
156 /// successors it goes to. This eliminate a use of the condition as well.
158 TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
159 BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
160 BasicBlock *BB = TI->getParent();
162 // Remove entries from PHI nodes to avoid confusing ourself later...
163 for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
164 TI->getSuccessor(i)->removePredecessor(BB);
166 // Delete the old branch itself...
167 BB->getInstList().erase(TI);
172 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
173 // true if the function was modified.
175 bool ADCE::doADCE() {
176 bool MadeChanges = false;
178 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
181 // Iterate over all invokes in the function, turning invokes into calls if
182 // they cannot throw.
183 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
184 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
185 if (Function *F = II->getCalledFunction())
186 if (AA.onlyReadsMemory(F)) {
187 // The function cannot unwind. Convert it to a call with a branch
188 // after it to the normal destination.
189 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
190 std::string Name = II->getName(); II->setName("");
191 CallInst *NewCall = new CallInst(F, Args, Name, II);
192 NewCall->setCallingConv(II->getCallingConv());
193 II->replaceAllUsesWith(NewCall);
194 new BranchInst(II->getNormalDest(), II);
196 // Update PHI nodes in the unwind destination
197 II->getUnwindDest()->removePredecessor(BB);
198 BB->getInstList().erase(II);
200 if (NewCall->use_empty()) {
201 BB->getInstList().erase(NewCall);
206 // Iterate over all of the instructions in the function, eliminating trivially
207 // dead instructions, and marking instructions live that are known to be
208 // needed. Perform the walk in depth first order so that we avoid marking any
209 // instructions live in basic blocks that are unreachable. These blocks will
210 // be eliminated later, along with the instructions inside.
212 std::set<BasicBlock*> ReachableBBs;
213 for (df_ext_iterator<BasicBlock*>
214 BBI = df_ext_begin(&Func->front(), ReachableBBs),
215 BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) {
216 BasicBlock *BB = *BBI;
217 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
218 Instruction *I = II++;
219 if (CallInst *CI = dyn_cast<CallInst>(I)) {
220 Function *F = CI->getCalledFunction();
221 if (F && AA.onlyReadsMemory(F)) {
222 if (CI->use_empty()) {
223 BB->getInstList().erase(CI);
227 markInstructionLive(I);
229 } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
230 isa<UnwindInst>(I) || isa<UnreachableInst>(I)) {
231 // FIXME: Unreachable instructions should not be marked intrinsically
233 markInstructionLive(I);
234 } else if (isInstructionTriviallyDead(I)) {
235 // Remove the instruction from it's basic block...
236 BB->getInstList().erase(I);
242 // Check to ensure we have an exit node for this CFG. If we don't, we won't
243 // have any post-dominance information, thus we cannot perform our
244 // transformations safely.
246 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
247 if (DT[&Func->getEntryBlock()] == 0) {
252 // Scan the function marking blocks without post-dominance information as
253 // live. Blocks without post-dominance information occur when there is an
254 // infinite loop in the program. Because the infinite loop could contain a
255 // function which unwinds, exits or has side-effects, we don't want to delete
256 // the infinite loop or those blocks leading up to it.
257 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
258 if (DT[I] == 0 && ReachableBBs.count(I))
259 for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
260 markInstructionLive((*PI)->getTerminator());
262 DOUT << "Processing work list\n";
264 // AliveBlocks - Set of basic blocks that we know have instructions that are
267 std::set<BasicBlock*> AliveBlocks;
269 // Process the work list of instructions that just became live... if they
270 // became live, then that means that all of their operands are necessary as
271 // well... make them live as well.
273 while (!WorkList.empty()) {
274 Instruction *I = WorkList.back(); // Get an instruction that became live...
277 BasicBlock *BB = I->getParent();
278 if (!ReachableBBs.count(BB)) continue;
279 if (AliveBlocks.insert(BB).second) // Basic block not alive yet.
280 markBlockAlive(BB); // Make it so now!
282 // PHI nodes are a special case, because the incoming values are actually
283 // defined in the predecessor nodes of this block, meaning that the PHI
284 // makes the predecessors alive.
286 if (PHINode *PN = dyn_cast<PHINode>(I)) {
287 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
288 // If the incoming edge is clearly dead, it won't have control
289 // dependence information. Do not mark it live.
290 BasicBlock *PredBB = PN->getIncomingBlock(i);
291 if (ReachableBBs.count(PredBB)) {
292 // FIXME: This should mark the control dependent edge as live, not
293 // necessarily the predecessor itself!
294 if (AliveBlocks.insert(PredBB).second)
295 markBlockAlive(PN->getIncomingBlock(i)); // Block is newly ALIVE!
296 if (Instruction *Op = dyn_cast<Instruction>(PN->getIncomingValue(i)))
297 markInstructionLive(Op);
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);
311 DOUT << "Current Function: X = Live\n";
312 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
313 DOUT << 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)) DOUT << "X ";
321 // All blocks being live is a common case, handle it specially.
322 if (AliveBlocks.size() == Func->size()) { // No dead blocks?
323 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
324 // Loop over all of the instructions in the function deleting instructions
325 // to drop their references.
326 deleteDeadInstructionsInLiveBlock(I);
328 // Check to make sure the terminator instruction is live. If it isn't,
329 // this means that the condition that it branches on (we know it is not an
330 // unconditional branch), is not needed to make the decision of where to
331 // go to, because all outgoing edges go to the same place. We must remove
332 // the use of the condition (because it's probably dead), so we convert
333 // the terminator to an unconditional branch.
335 TerminatorInst *TI = I->getTerminator();
336 if (!LiveSet.count(TI))
337 convertToUnconditionalBranch(TI);
344 // If the entry node is dead, insert a new entry node to eliminate the entry
345 // node as a special case.
347 if (!AliveBlocks.count(&Func->front())) {
348 BasicBlock *NewEntry = new BasicBlock();
349 new BranchInst(&Func->front(), NewEntry);
350 Func->getBasicBlockList().push_front(NewEntry);
351 AliveBlocks.insert(NewEntry); // This block is always alive!
352 LiveSet.insert(NewEntry->getTerminator()); // The branch is live
355 // Loop over all of the alive blocks in the function. If any successor
356 // blocks are not alive, we adjust the outgoing branches to branch to the
357 // first live postdominator of the live block, adjusting any PHI nodes in
358 // the block to reflect this.
360 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
361 if (AliveBlocks.count(I)) {
363 TerminatorInst *TI = BB->getTerminator();
365 // If the terminator instruction is alive, but the block it is contained
366 // in IS alive, this means that this terminator is a conditional branch on
367 // a condition that doesn't matter. Make it an unconditional branch to
368 // ONE of the successors. This has the side effect of dropping a use of
369 // the conditional value, which may also be dead.
370 if (!LiveSet.count(TI))
371 TI = convertToUnconditionalBranch(TI);
373 // Loop over all of the successors, looking for ones that are not alive.
374 // We cannot save the number of successors in the terminator instruction
375 // here because we may remove them if we don't have a postdominator.
377 for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
378 if (!AliveBlocks.count(TI->getSuccessor(i))) {
379 // Scan up the postdominator tree, looking for the first
380 // postdominator that is alive, and the last postdominator that is
383 PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
384 PostDominatorTree::Node *NextNode = 0;
387 NextNode = LastNode->getIDom();
388 while (!AliveBlocks.count(NextNode->getBlock())) {
390 NextNode = NextNode->getIDom();
398 // There is a special case here... if there IS no post-dominator for
399 // the block we have nowhere to point our branch to. Instead, convert
400 // it to a return. This can only happen if the code branched into an
401 // infinite loop. Note that this may not be desirable, because we
402 // _are_ altering the behavior of the code. This is a well known
403 // drawback of ADCE, so in the future if we choose to revisit the
404 // decision, this is where it should be.
406 if (LastNode == 0) { // No postdominator!
407 if (!isa<InvokeInst>(TI)) {
408 // Call RemoveSuccessor to transmogrify the terminator instruction
409 // to not contain the outgoing branch, or to create a new
410 // terminator if the form fundamentally changes (i.e.,
411 // unconditional branch to return). Note that this will change a
412 // branch into an infinite loop into a return instruction!
414 RemoveSuccessor(TI, i);
416 // RemoveSuccessor may replace TI... make sure we have a fresh
419 TI = BB->getTerminator();
421 // Rescan this successor...
427 // Get the basic blocks that we need...
428 BasicBlock *LastDead = LastNode->getBlock();
429 BasicBlock *NextAlive = NextNode->getBlock();
431 // Make the conditional branch now go to the next alive block...
432 TI->getSuccessor(i)->removePredecessor(BB);
433 TI->setSuccessor(i, NextAlive);
435 // If there are PHI nodes in NextAlive, we need to add entries to
436 // the PHI nodes for the new incoming edge. The incoming values
437 // should be identical to the incoming values for LastDead.
439 for (BasicBlock::iterator II = NextAlive->begin();
440 isa<PHINode>(II); ++II) {
441 PHINode *PN = cast<PHINode>(II);
442 if (LiveSet.count(PN)) { // Only modify live phi nodes
443 // Get the incoming value for LastDead...
444 int OldIdx = PN->getBasicBlockIndex(LastDead);
445 assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
446 Value *InVal = PN->getIncomingValue(OldIdx);
448 // Add an incoming value for BB now...
449 PN->addIncoming(InVal, BB);
455 // Now loop over all of the instructions in the basic block, deleting
456 // dead instructions. This is so that the next sweep over the program
457 // can safely delete dead instructions without other dead instructions
458 // still referring to them.
460 deleteDeadInstructionsInLiveBlock(BB);
463 // Loop over all of the basic blocks in the function, dropping references of
464 // the dead basic blocks. We must do this after the previous step to avoid
465 // dropping references to PHIs which still have entries...
467 std::vector<BasicBlock*> DeadBlocks;
468 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
469 if (!AliveBlocks.count(BB)) {
470 // Remove PHI node entries for this block in live successor blocks.
471 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
472 if (!SI->empty() && isa<PHINode>(SI->front()) && AliveBlocks.count(*SI))
473 (*SI)->removePredecessor(BB);
475 BB->dropAllReferences();
477 DeadBlocks.push_back(BB);
480 NumBlockRemoved += DeadBlocks.size();
482 // Now loop through all of the blocks and delete the dead ones. We can safely
483 // do this now because we know that there are no references to dead blocks
484 // (because they have dropped all of their references).
485 for (std::vector<BasicBlock*>::iterator I = DeadBlocks.begin(),
486 E = DeadBlocks.end(); I != E; ++I)
487 Func->getBasicBlockList().erase(*I);