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 #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/Statistic.h"
29 #include "llvm/ADT/STLExtras.h"
30 #include "llvm/Support/Compiler.h"
34 STATISTIC(NumBlockRemoved, "Number of basic blocks removed");
35 STATISTIC(NumInstRemoved , "Number of instructions removed");
36 STATISTIC(NumCallRemoved , "Number of calls and invokes removed");
39 //===----------------------------------------------------------------------===//
42 // This class does all of the work of Aggressive Dead Code Elimination.
43 // It's public interface consists of a constructor and a doADCE() method.
45 class VISIBILITY_HIDDEN ADCE : public FunctionPass {
46 Function *Func; // The function that we are working on
47 std::vector<Instruction*> WorkList; // Instructions that just became live
48 std::set<Instruction*> LiveSet; // The set of live instructions
50 //===--------------------------------------------------------------------===//
51 // The public interface for this class
54 // Execute the Aggressive Dead Code Elimination Algorithm
56 virtual bool runOnFunction(Function &F) {
58 bool Changed = doADCE();
59 assert(WorkList.empty());
63 // getAnalysisUsage - We require post dominance frontiers (aka Control
65 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
66 // We require that all function nodes are unified, because otherwise code
67 // can be marked live that wouldn't necessarily be otherwise.
68 AU.addRequired<UnifyFunctionExitNodes>();
69 AU.addRequired<AliasAnalysis>();
70 AU.addRequired<PostDominatorTree>();
71 AU.addRequired<PostDominanceFrontier>();
75 //===--------------------------------------------------------------------===//
76 // The implementation of this class
79 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
80 // true if the function was modified.
84 void markBlockAlive(BasicBlock *BB);
87 // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in
88 // the specified basic block, deleting ones that are dead according to
90 bool deleteDeadInstructionsInLiveBlock(BasicBlock *BB);
92 TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
94 inline void markInstructionLive(Instruction *I) {
95 if (!LiveSet.insert(I).second) return;
96 DOUT << "Insn Live: " << *I;
97 WorkList.push_back(I);
100 inline void markTerminatorLive(const BasicBlock *BB) {
101 DOUT << "Terminator Live: " << *BB->getTerminator();
102 markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
106 RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
107 } // End of anonymous namespace
109 FunctionPass *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 (PostDominanceFrontier::DomSetType::const_iterator I =
122 CDB.begin(), E = CDB.end(); I != E; ++I)
123 markTerminatorLive(*I); // Mark all their terminators as live
126 // If this basic block is live, and it ends in an unconditional branch, then
127 // the branch is alive as well...
128 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
129 if (BI->isUnconditional())
130 markTerminatorLive(BB);
133 // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in the
134 // specified basic block, deleting ones that are dead according to LiveSet.
135 bool ADCE::deleteDeadInstructionsInLiveBlock(BasicBlock *BB) {
136 bool Changed = false;
137 for (BasicBlock::iterator II = BB->begin(), E = --BB->end(); II != E; ) {
138 Instruction *I = II++;
139 if (!LiveSet.count(I)) { // Is this instruction alive?
141 I->replaceAllUsesWith(UndefValue::get(I->getType()));
143 // Nope... remove the instruction from it's basic block...
144 if (isa<CallInst>(I))
148 BB->getInstList().erase(I);
156 /// convertToUnconditionalBranch - Transform this conditional terminator
157 /// instruction into an unconditional branch because we don't care which of the
158 /// successors it goes to. This eliminate a use of the condition as well.
160 TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
161 BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
162 BasicBlock *BB = TI->getParent();
164 // Remove entries from PHI nodes to avoid confusing ourself later...
165 for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
166 TI->getSuccessor(i)->removePredecessor(BB);
168 // Delete the old branch itself...
169 BB->getInstList().erase(TI);
174 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
175 // true if the function was modified.
177 bool ADCE::doADCE() {
178 bool MadeChanges = false;
180 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
183 // Iterate over all invokes in the function, turning invokes into calls if
184 // they cannot throw.
185 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
186 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
187 if (Function *F = II->getCalledFunction())
188 if (AA.onlyReadsMemory(F)) {
189 // The function cannot unwind. Convert it to a call with a branch
190 // after it to the normal destination.
191 std::vector<Value*> Args(II->op_begin()+3, II->op_end());
192 CallInst *NewCall = new CallInst(F, Args, "", II);
193 NewCall->takeName(II);
194 NewCall->setCallingConv(II->getCallingConv());
195 II->replaceAllUsesWith(NewCall);
196 new BranchInst(II->getNormalDest(), II);
198 // Update PHI nodes in the unwind destination
199 II->getUnwindDest()->removePredecessor(BB);
200 BB->getInstList().erase(II);
202 if (NewCall->use_empty()) {
203 BB->getInstList().erase(NewCall);
208 // Iterate over all of the instructions in the function, eliminating trivially
209 // dead instructions, and marking instructions live that are known to be
210 // needed. Perform the walk in depth first order so that we avoid marking any
211 // instructions live in basic blocks that are unreachable. These blocks will
212 // be eliminated later, along with the instructions inside.
214 std::set<BasicBlock*> ReachableBBs;
215 for (df_ext_iterator<BasicBlock*>
216 BBI = df_ext_begin(&Func->front(), ReachableBBs),
217 BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) {
218 BasicBlock *BB = *BBI;
219 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
220 Instruction *I = II++;
221 if (CallInst *CI = dyn_cast<CallInst>(I)) {
222 Function *F = CI->getCalledFunction();
223 if (F && AA.onlyReadsMemory(F)) {
224 if (CI->use_empty()) {
225 BB->getInstList().erase(CI);
229 markInstructionLive(I);
231 } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
232 isa<UnwindInst>(I) || isa<UnreachableInst>(I)) {
233 // FIXME: Unreachable instructions should not be marked intrinsically
235 markInstructionLive(I);
236 } else if (isInstructionTriviallyDead(I)) {
237 // Remove the instruction from it's basic block...
238 BB->getInstList().erase(I);
244 // Check to ensure we have an exit node for this CFG. If we don't, we won't
245 // have any post-dominance information, thus we cannot perform our
246 // transformations safely.
248 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
249 if (DT[&Func->getEntryBlock()] == 0) {
254 // Scan the function marking blocks without post-dominance information as
255 // live. Blocks without post-dominance information occur when there is an
256 // infinite loop in the program. Because the infinite loop could contain a
257 // function which unwinds, exits or has side-effects, we don't want to delete
258 // the infinite loop or those blocks leading up to it.
259 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
260 if (DT[I] == 0 && ReachableBBs.count(I))
261 for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
262 markInstructionLive((*PI)->getTerminator());
264 DOUT << "Processing work list\n";
266 // AliveBlocks - Set of basic blocks that we know have instructions that are
269 std::set<BasicBlock*> AliveBlocks;
271 // Process the work list of instructions that just became live... if they
272 // became live, then that means that all of their operands are necessary as
273 // well... make them live as well.
275 while (!WorkList.empty()) {
276 Instruction *I = WorkList.back(); // Get an instruction that became live...
279 BasicBlock *BB = I->getParent();
280 if (!ReachableBBs.count(BB)) continue;
281 if (AliveBlocks.insert(BB).second) // Basic block not alive yet.
282 markBlockAlive(BB); // Make it so now!
284 // PHI nodes are a special case, because the incoming values are actually
285 // defined in the predecessor nodes of this block, meaning that the PHI
286 // makes the predecessors alive.
288 if (PHINode *PN = dyn_cast<PHINode>(I)) {
289 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
290 // If the incoming edge is clearly dead, it won't have control
291 // dependence information. Do not mark it live.
292 BasicBlock *PredBB = PN->getIncomingBlock(i);
293 if (ReachableBBs.count(PredBB)) {
294 // FIXME: This should mark the control dependent edge as live, not
295 // necessarily the predecessor itself!
296 if (AliveBlocks.insert(PredBB).second)
297 markBlockAlive(PN->getIncomingBlock(i)); // Block is newly ALIVE!
298 if (Instruction *Op = dyn_cast<Instruction>(PN->getIncomingValue(i)))
299 markInstructionLive(Op);
303 // Loop over all of the operands of the live instruction, making sure that
304 // they are known to be alive as well.
306 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
307 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
308 markInstructionLive(Operand);
313 DOUT << "Current Function: X = Live\n";
314 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
315 DOUT << I->getName() << ":\t"
316 << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
317 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
318 if (LiveSet.count(BI)) DOUT << "X ";
323 // All blocks being live is a common case, handle it specially.
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 deleting instructions
327 // to drop their references.
328 deleteDeadInstructionsInLiveBlock(I);
330 // Check to make sure the terminator instruction is live. If it isn't,
331 // this means that the condition that it branches on (we know it is not an
332 // unconditional branch), is not needed to make the decision of where to
333 // go to, because all outgoing edges go to the same place. We must remove
334 // the use of the condition (because it's probably dead), so we convert
335 // the terminator to an unconditional branch.
337 TerminatorInst *TI = I->getTerminator();
338 if (!LiveSet.count(TI))
339 convertToUnconditionalBranch(TI);
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 on
369 // a condition that doesn't matter. Make it an unconditional branch to
370 // ONE of the successors. This has the side effect of dropping a use of
371 // 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)];
386 PostDominatorTree::Node *NextNode = 0;
389 NextNode = LastNode->getIDom();
390 while (!AliveBlocks.count(NextNode->getBlock())) {
392 NextNode = NextNode->getIDom();
400 // There is a special case here... if there IS no post-dominator for
401 // the block we have nowhere to point our branch to. Instead, convert
402 // it to a return. This can only happen if the code branched into an
403 // infinite loop. Note that this may not be desirable, because we
404 // _are_ altering the behavior of the code. This is a well known
405 // drawback of ADCE, so in the future if we choose to revisit the
406 // decision, this is where it should be.
408 if (LastNode == 0) { // No postdominator!
409 if (!isa<InvokeInst>(TI)) {
410 // Call RemoveSuccessor to transmogrify the terminator instruction
411 // to not contain the outgoing branch, or to create a new
412 // terminator if the form fundamentally changes (i.e.,
413 // unconditional branch to return). Note that this will change a
414 // branch into an infinite loop into a return instruction!
416 RemoveSuccessor(TI, i);
418 // RemoveSuccessor may replace TI... make sure we have a fresh
421 TI = BB->getTerminator();
423 // Rescan this successor...
429 // Get the basic blocks that we need...
430 BasicBlock *LastDead = LastNode->getBlock();
431 BasicBlock *NextAlive = NextNode->getBlock();
433 // Make the conditional branch now go to the next alive block...
434 TI->getSuccessor(i)->removePredecessor(BB);
435 TI->setSuccessor(i, NextAlive);
437 // If there are PHI nodes in NextAlive, we need to add entries to
438 // the PHI nodes for the new incoming edge. The incoming values
439 // should be identical to the incoming values for LastDead.
441 for (BasicBlock::iterator II = NextAlive->begin();
442 isa<PHINode>(II); ++II) {
443 PHINode *PN = cast<PHINode>(II);
444 if (LiveSet.count(PN)) { // Only modify live phi nodes
445 // Get the incoming value for LastDead...
446 int OldIdx = PN->getBasicBlockIndex(LastDead);
447 assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
448 Value *InVal = PN->getIncomingValue(OldIdx);
450 // Add an incoming value for BB now...
451 PN->addIncoming(InVal, BB);
457 // Now loop over all of the instructions in the basic block, deleting
458 // dead instructions. This is so that the next sweep over the program
459 // can safely delete dead instructions without other dead instructions
460 // still referring to them.
462 deleteDeadInstructionsInLiveBlock(BB);
465 // Loop over all of the basic blocks in the function, dropping references of
466 // the dead basic blocks. We must do this after the previous step to avoid
467 // dropping references to PHIs which still have entries...
469 std::vector<BasicBlock*> DeadBlocks;
470 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
471 if (!AliveBlocks.count(BB)) {
472 // Remove PHI node entries for this block in live successor blocks.
473 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
474 if (!SI->empty() && isa<PHINode>(SI->front()) && AliveBlocks.count(*SI))
475 (*SI)->removePredecessor(BB);
477 BB->dropAllReferences();
479 DeadBlocks.push_back(BB);
482 NumBlockRemoved += DeadBlocks.size();
484 // Now loop through all of the blocks and delete the dead ones. We can safely
485 // do this now because we know that there are no references to dead blocks
486 // (because they have dropped all of their references).
487 for (std::vector<BasicBlock*>::iterator I = DeadBlocks.begin(),
488 E = DeadBlocks.end(); I != E; ++I)
489 Func->getBasicBlockList().erase(*I);