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