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/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()));
111 RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
112 } // End of anonymous namespace
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 = 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>();
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 for (df_ext_iterator<BasicBlock*>
195 BBI = df_ext_begin(&Func->front(), ReachableBBs),
196 BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) {
197 BasicBlock *BB = *BBI;
198 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
199 Instruction *I = II++;
200 if (CallInst *CI = dyn_cast<CallInst>(I)) {
201 Function *F = CI->getCalledFunction();
202 if (F && AA.onlyReadsMemory(F)) {
203 if (CI->use_empty()) {
204 BB->getInstList().erase(CI);
208 markInstructionLive(I);
210 } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
211 isa<UnwindInst>(I) || isa<UnreachableInst>(I)) {
212 // FIXME: Unreachable instructions should not be marked intrinsically
214 markInstructionLive(I);
215 } else if (isInstructionTriviallyDead(I)) {
216 // Remove the instruction from it's basic block...
217 BB->getInstList().erase(I);
223 // Check to ensure we have an exit node for this CFG. If we don't, we won't
224 // have any post-dominance information, thus we cannot perform our
225 // transformations safely.
227 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
228 if (DT[&Func->getEntryBlock()] == 0) {
233 // Scan the function marking blocks without post-dominance information as
234 // live. Blocks without post-dominance information occur when there is an
235 // infinite loop in the program. Because the infinite loop could contain a
236 // function which unwinds, exits or has side-effects, we don't want to delete
237 // the infinite loop or those blocks leading up to it.
238 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
239 if (DT[I] == 0 && ReachableBBs.count(I))
240 for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
241 markInstructionLive((*PI)->getTerminator());
243 DOUT << "Processing work list\n";
245 // AliveBlocks - Set of basic blocks that we know have instructions that are
248 std::set<BasicBlock*> AliveBlocks;
250 // Process the work list of instructions that just became live... if they
251 // became live, then that means that all of their operands are necessary as
252 // well... make them live as well.
254 while (!WorkList.empty()) {
255 Instruction *I = WorkList.back(); // Get an instruction that became live...
258 BasicBlock *BB = I->getParent();
259 if (!ReachableBBs.count(BB)) continue;
260 if (AliveBlocks.insert(BB).second) // Basic block not alive yet.
261 markBlockAlive(BB); // Make it so now!
263 // PHI nodes are a special case, because the incoming values are actually
264 // defined in the predecessor nodes of this block, meaning that the PHI
265 // makes the predecessors alive.
267 if (PHINode *PN = dyn_cast<PHINode>(I)) {
268 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
269 // If the incoming edge is clearly dead, it won't have control
270 // dependence information. Do not mark it live.
271 BasicBlock *PredBB = PN->getIncomingBlock(i);
272 if (ReachableBBs.count(PredBB)) {
273 // FIXME: This should mark the control dependent edge as live, not
274 // necessarily the predecessor itself!
275 if (AliveBlocks.insert(PredBB).second)
276 markBlockAlive(PN->getIncomingBlock(i)); // Block is newly ALIVE!
277 if (Instruction *Op = dyn_cast<Instruction>(PN->getIncomingValue(i)))
278 markInstructionLive(Op);
282 // Loop over all of the operands of the live instruction, making sure that
283 // they are known to be alive as well.
285 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
286 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
287 markInstructionLive(Operand);
292 DOUT << "Current Function: X = Live\n";
293 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
294 DOUT << I->getName() << ":\t"
295 << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
296 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
297 if (LiveSet.count(BI)) DOUT << "X ";
302 // All blocks being live is a common case, handle it specially.
303 if (AliveBlocks.size() == Func->size()) { // No dead blocks?
304 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
305 // Loop over all of the instructions in the function deleting instructions
306 // to drop their references.
307 deleteDeadInstructionsInLiveBlock(I);
309 // Check to make sure the terminator instruction is live. If it isn't,
310 // this means that the condition that it branches on (we know it is not an
311 // unconditional branch), is not needed to make the decision of where to
312 // go to, because all outgoing edges go to the same place. We must remove
313 // the use of the condition (because it's probably dead), so we convert
314 // the terminator to an unconditional branch.
316 TerminatorInst *TI = I->getTerminator();
317 if (!LiveSet.count(TI))
318 convertToUnconditionalBranch(TI);
325 // If the entry node is dead, insert a new entry node to eliminate the entry
326 // node as a special case.
328 if (!AliveBlocks.count(&Func->front())) {
329 BasicBlock *NewEntry = new BasicBlock();
330 new BranchInst(&Func->front(), NewEntry);
331 Func->getBasicBlockList().push_front(NewEntry);
332 AliveBlocks.insert(NewEntry); // This block is always alive!
333 LiveSet.insert(NewEntry->getTerminator()); // The branch is live
336 // Loop over all of the alive blocks in the function. If any successor
337 // blocks are not alive, we adjust the outgoing branches to branch to the
338 // first live postdominator of the live block, adjusting any PHI nodes in
339 // the block to reflect this.
341 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
342 if (AliveBlocks.count(I)) {
344 TerminatorInst *TI = BB->getTerminator();
346 // If the terminator instruction is alive, but the block it is contained
347 // in IS alive, this means that this terminator is a conditional branch on
348 // a condition that doesn't matter. Make it an unconditional branch to
349 // ONE of the successors. This has the side effect of dropping a use of
350 // the conditional value, which may also be dead.
351 if (!LiveSet.count(TI))
352 TI = convertToUnconditionalBranch(TI);
354 // Loop over all of the successors, looking for ones that are not alive.
355 // We cannot save the number of successors in the terminator instruction
356 // here because we may remove them if we don't have a postdominator.
358 for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
359 if (!AliveBlocks.count(TI->getSuccessor(i))) {
360 // Scan up the postdominator tree, looking for the first
361 // postdominator that is alive, and the last postdominator that is
364 DomTreeNode *LastNode = DT[TI->getSuccessor(i)];
365 DomTreeNode *NextNode = 0;
368 NextNode = LastNode->getIDom();
369 while (!AliveBlocks.count(NextNode->getBlock())) {
371 NextNode = NextNode->getIDom();
379 // There is a special case here... if there IS no post-dominator for
380 // the block we have nowhere to point our branch to. Instead, convert
381 // it to a return. This can only happen if the code branched into an
382 // infinite loop. Note that this may not be desirable, because we
383 // _are_ altering the behavior of the code. This is a well known
384 // drawback of ADCE, so in the future if we choose to revisit the
385 // decision, this is where it should be.
387 if (LastNode == 0) { // No postdominator!
388 if (!isa<InvokeInst>(TI)) {
389 // Call RemoveSuccessor to transmogrify the terminator instruction
390 // to not contain the outgoing branch, or to create a new
391 // terminator if the form fundamentally changes (i.e.,
392 // unconditional branch to return). Note that this will change a
393 // branch into an infinite loop into a return instruction!
395 RemoveSuccessor(TI, i);
397 // RemoveSuccessor may replace TI... make sure we have a fresh
400 TI = BB->getTerminator();
402 // Rescan this successor...
408 // Get the basic blocks that we need...
409 BasicBlock *LastDead = LastNode->getBlock();
410 BasicBlock *NextAlive = NextNode->getBlock();
412 // Make the conditional branch now go to the next alive block...
413 TI->getSuccessor(i)->removePredecessor(BB);
414 TI->setSuccessor(i, NextAlive);
416 // If there are PHI nodes in NextAlive, we need to add entries to
417 // the PHI nodes for the new incoming edge. The incoming values
418 // should be identical to the incoming values for LastDead.
420 for (BasicBlock::iterator II = NextAlive->begin();
421 isa<PHINode>(II); ++II) {
422 PHINode *PN = cast<PHINode>(II);
423 if (LiveSet.count(PN)) { // Only modify live phi nodes
424 // Get the incoming value for LastDead...
425 int OldIdx = PN->getBasicBlockIndex(LastDead);
426 assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
427 Value *InVal = PN->getIncomingValue(OldIdx);
429 // Add an incoming value for BB now...
430 PN->addIncoming(InVal, BB);
436 // Now loop over all of the instructions in the basic block, deleting
437 // dead instructions. This is so that the next sweep over the program
438 // can safely delete dead instructions without other dead instructions
439 // still referring to them.
441 deleteDeadInstructionsInLiveBlock(BB);
444 // Loop over all of the basic blocks in the function, dropping references of
445 // the dead basic blocks. We must do this after the previous step to avoid
446 // dropping references to PHIs which still have entries...
448 std::vector<BasicBlock*> DeadBlocks;
449 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
450 if (!AliveBlocks.count(BB)) {
451 // Remove PHI node entries for this block in live successor blocks.
452 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
453 if (!SI->empty() && isa<PHINode>(SI->front()) && AliveBlocks.count(*SI))
454 (*SI)->removePredecessor(BB);
456 BB->dropAllReferences();
458 DeadBlocks.push_back(BB);
461 NumBlockRemoved += DeadBlocks.size();
463 // Now loop through all of the blocks and delete the dead ones. We can safely
464 // do this now because we know that there are no references to dead blocks
465 // (because they have dropped all of their references).
466 for (std::vector<BasicBlock*>::iterator I = DeadBlocks.begin(),
467 E = DeadBlocks.end(); I != E; ++I)
468 Func->getBasicBlockList().erase(*I);