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 and invokes 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 // Execute the Aggressive Dead Code Elimination Algorithm
57 virtual bool runOnFunction(Function &F) {
59 bool Changed = doADCE();
60 assert(WorkList.empty());
64 // getAnalysisUsage - We require post dominance frontiers (aka Control
66 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
67 // We require that all function nodes are unified, because otherwise code
68 // can be marked live that wouldn't necessarily be otherwise.
69 AU.addRequired<UnifyFunctionExitNodes>();
70 AU.addRequired<AliasAnalysis>();
71 AU.addRequired<PostDominatorTree>();
72 AU.addRequired<PostDominanceFrontier>();
76 //===--------------------------------------------------------------------===//
77 // The implementation of this class
80 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
81 // true if the function was modified.
85 void markBlockAlive(BasicBlock *BB);
88 // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in
89 // the specified basic block, deleting ones that are dead according to
91 bool deleteDeadInstructionsInLiveBlock(BasicBlock *BB);
93 TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
95 inline void markInstructionLive(Instruction *I) {
96 if (!LiveSet.insert(I).second) return;
97 DOUT << "Insn Live: " << *I;
98 WorkList.push_back(I);
101 inline void markTerminatorLive(const BasicBlock *BB) {
102 DOUT << "Terminator Live: " << *BB->getTerminator();
103 markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
107 RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
108 } // End of anonymous namespace
110 FunctionPass *llvm::createAggressiveDCEPass() { return new ADCE(); }
112 void ADCE::markBlockAlive(BasicBlock *BB) {
113 // Mark the basic block as being newly ALIVE... and mark all branches that
114 // this block is control dependent on as being alive also...
116 PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
118 PostDominanceFrontier::const_iterator It = CDG.find(BB);
119 if (It != CDG.end()) {
120 // Get the blocks that this node is control dependent on...
121 const PostDominanceFrontier::DomSetType &CDB = It->second;
122 for (PostDominanceFrontier::DomSetType::const_iterator I =
123 CDB.begin(), E = CDB.end(); I != E; ++I)
124 markTerminatorLive(*I); // Mark all their terminators as live
127 // If this basic block is live, and it ends in an unconditional branch, then
128 // the branch is alive as well...
129 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
130 if (BI->isUnconditional())
131 markTerminatorLive(BB);
134 // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in the
135 // specified basic block, deleting ones that are dead according to LiveSet.
136 bool ADCE::deleteDeadInstructionsInLiveBlock(BasicBlock *BB) {
137 bool Changed = false;
138 for (BasicBlock::iterator II = BB->begin(), E = --BB->end(); II != E; ) {
139 Instruction *I = II++;
140 if (!LiveSet.count(I)) { // Is this instruction alive?
142 I->replaceAllUsesWith(UndefValue::get(I->getType()));
144 // Nope... remove the instruction from it's basic block...
145 if (isa<CallInst>(I))
149 BB->getInstList().erase(I);
157 /// convertToUnconditionalBranch - Transform this conditional terminator
158 /// instruction into an unconditional branch because we don't care which of the
159 /// successors it goes to. This eliminate a use of the condition as well.
161 TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
162 BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
163 BasicBlock *BB = TI->getParent();
165 // Remove entries from PHI nodes to avoid confusing ourself later...
166 for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
167 TI->getSuccessor(i)->removePredecessor(BB);
169 // Delete the old branch itself...
170 BB->getInstList().erase(TI);
175 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
176 // true if the function was modified.
178 bool ADCE::doADCE() {
179 bool MadeChanges = false;
181 AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
184 // Iterate over all invokes in the function, turning invokes into calls if
185 // they cannot throw.
186 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
187 if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
188 if (Function *F = II->getCalledFunction())
189 if (AA.onlyReadsMemory(F)) {
190 // The function cannot unwind. Convert it to a call with a branch
191 // after it to the normal destination.
192 SmallVector<Value*, 8> Args(II->op_begin()+3, II->op_end());
193 CallInst *NewCall = new CallInst(F, &Args[0], Args.size(), "", II);
194 NewCall->takeName(II);
195 NewCall->setCallingConv(II->getCallingConv());
196 II->replaceAllUsesWith(NewCall);
197 new BranchInst(II->getNormalDest(), II);
199 // Update PHI nodes in the unwind destination
200 II->getUnwindDest()->removePredecessor(BB);
201 BB->getInstList().erase(II);
203 if (NewCall->use_empty()) {
204 BB->getInstList().erase(NewCall);
209 // Iterate over all of the instructions in the function, eliminating trivially
210 // dead instructions, and marking instructions live that are known to be
211 // needed. Perform the walk in depth first order so that we avoid marking any
212 // instructions live in basic blocks that are unreachable. These blocks will
213 // be eliminated later, along with the instructions inside.
215 std::set<BasicBlock*> ReachableBBs;
216 for (df_ext_iterator<BasicBlock*>
217 BBI = df_ext_begin(&Func->front(), ReachableBBs),
218 BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) {
219 BasicBlock *BB = *BBI;
220 for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
221 Instruction *I = II++;
222 if (CallInst *CI = dyn_cast<CallInst>(I)) {
223 Function *F = CI->getCalledFunction();
224 if (F && AA.onlyReadsMemory(F)) {
225 if (CI->use_empty()) {
226 BB->getInstList().erase(CI);
230 markInstructionLive(I);
232 } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
233 isa<UnwindInst>(I) || isa<UnreachableInst>(I)) {
234 // FIXME: Unreachable instructions should not be marked intrinsically
236 markInstructionLive(I);
237 } else if (isInstructionTriviallyDead(I)) {
238 // Remove the instruction from it's basic block...
239 BB->getInstList().erase(I);
245 // Check to ensure we have an exit node for this CFG. If we don't, we won't
246 // have any post-dominance information, thus we cannot perform our
247 // transformations safely.
249 PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
250 if (DT[&Func->getEntryBlock()] == 0) {
255 // Scan the function marking blocks without post-dominance information as
256 // live. Blocks without post-dominance information occur when there is an
257 // infinite loop in the program. Because the infinite loop could contain a
258 // function which unwinds, exits or has side-effects, we don't want to delete
259 // the infinite loop or those blocks leading up to it.
260 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
261 if (DT[I] == 0 && ReachableBBs.count(I))
262 for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
263 markInstructionLive((*PI)->getTerminator());
265 DOUT << "Processing work list\n";
267 // AliveBlocks - Set of basic blocks that we know have instructions that are
270 std::set<BasicBlock*> AliveBlocks;
272 // Process the work list of instructions that just became live... if they
273 // became live, then that means that all of their operands are necessary as
274 // well... make them live as well.
276 while (!WorkList.empty()) {
277 Instruction *I = WorkList.back(); // Get an instruction that became live...
280 BasicBlock *BB = I->getParent();
281 if (!ReachableBBs.count(BB)) continue;
282 if (AliveBlocks.insert(BB).second) // Basic block not alive yet.
283 markBlockAlive(BB); // Make it so now!
285 // PHI nodes are a special case, because the incoming values are actually
286 // defined in the predecessor nodes of this block, meaning that the PHI
287 // makes the predecessors alive.
289 if (PHINode *PN = dyn_cast<PHINode>(I)) {
290 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
291 // If the incoming edge is clearly dead, it won't have control
292 // dependence information. Do not mark it live.
293 BasicBlock *PredBB = PN->getIncomingBlock(i);
294 if (ReachableBBs.count(PredBB)) {
295 // FIXME: This should mark the control dependent edge as live, not
296 // necessarily the predecessor itself!
297 if (AliveBlocks.insert(PredBB).second)
298 markBlockAlive(PN->getIncomingBlock(i)); // Block is newly ALIVE!
299 if (Instruction *Op = dyn_cast<Instruction>(PN->getIncomingValue(i)))
300 markInstructionLive(Op);
304 // Loop over all of the operands of the live instruction, making sure that
305 // they are known to be alive as well.
307 for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
308 if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
309 markInstructionLive(Operand);
314 DOUT << "Current Function: X = Live\n";
315 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
316 DOUT << I->getName() << ":\t"
317 << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
318 for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
319 if (LiveSet.count(BI)) DOUT << "X ";
324 // All blocks being live is a common case, handle it specially.
325 if (AliveBlocks.size() == Func->size()) { // No dead blocks?
326 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
327 // Loop over all of the instructions in the function deleting instructions
328 // to drop their references.
329 deleteDeadInstructionsInLiveBlock(I);
331 // Check to make sure the terminator instruction is live. If it isn't,
332 // this means that the condition that it branches on (we know it is not an
333 // unconditional branch), is not needed to make the decision of where to
334 // go to, because all outgoing edges go to the same place. We must remove
335 // the use of the condition (because it's probably dead), so we convert
336 // the terminator to an unconditional branch.
338 TerminatorInst *TI = I->getTerminator();
339 if (!LiveSet.count(TI))
340 convertToUnconditionalBranch(TI);
347 // If the entry node is dead, insert a new entry node to eliminate the entry
348 // node as a special case.
350 if (!AliveBlocks.count(&Func->front())) {
351 BasicBlock *NewEntry = new BasicBlock();
352 new BranchInst(&Func->front(), NewEntry);
353 Func->getBasicBlockList().push_front(NewEntry);
354 AliveBlocks.insert(NewEntry); // This block is always alive!
355 LiveSet.insert(NewEntry->getTerminator()); // The branch is live
358 // Loop over all of the alive blocks in the function. If any successor
359 // blocks are not alive, we adjust the outgoing branches to branch to the
360 // first live postdominator of the live block, adjusting any PHI nodes in
361 // the block to reflect this.
363 for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
364 if (AliveBlocks.count(I)) {
366 TerminatorInst *TI = BB->getTerminator();
368 // If the terminator instruction is alive, but the block it is contained
369 // in IS alive, this means that this terminator is a conditional branch on
370 // a condition that doesn't matter. Make it an unconditional branch to
371 // ONE of the successors. This has the side effect of dropping a use of
372 // the conditional value, which may also be dead.
373 if (!LiveSet.count(TI))
374 TI = convertToUnconditionalBranch(TI);
376 // Loop over all of the successors, looking for ones that are not alive.
377 // We cannot save the number of successors in the terminator instruction
378 // here because we may remove them if we don't have a postdominator.
380 for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
381 if (!AliveBlocks.count(TI->getSuccessor(i))) {
382 // Scan up the postdominator tree, looking for the first
383 // postdominator that is alive, and the last postdominator that is
386 PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
387 PostDominatorTree::Node *NextNode = 0;
390 NextNode = LastNode->getIDom();
391 while (!AliveBlocks.count(NextNode->getBlock())) {
393 NextNode = NextNode->getIDom();
401 // There is a special case here... if there IS no post-dominator for
402 // the block we have nowhere to point our branch to. Instead, convert
403 // it to a return. This can only happen if the code branched into an
404 // infinite loop. Note that this may not be desirable, because we
405 // _are_ altering the behavior of the code. This is a well known
406 // drawback of ADCE, so in the future if we choose to revisit the
407 // decision, this is where it should be.
409 if (LastNode == 0) { // No postdominator!
410 if (!isa<InvokeInst>(TI)) {
411 // Call RemoveSuccessor to transmogrify the terminator instruction
412 // to not contain the outgoing branch, or to create a new
413 // terminator if the form fundamentally changes (i.e.,
414 // unconditional branch to return). Note that this will change a
415 // branch into an infinite loop into a return instruction!
417 RemoveSuccessor(TI, i);
419 // RemoveSuccessor may replace TI... make sure we have a fresh
422 TI = BB->getTerminator();
424 // Rescan this successor...
430 // Get the basic blocks that we need...
431 BasicBlock *LastDead = LastNode->getBlock();
432 BasicBlock *NextAlive = NextNode->getBlock();
434 // Make the conditional branch now go to the next alive block...
435 TI->getSuccessor(i)->removePredecessor(BB);
436 TI->setSuccessor(i, NextAlive);
438 // If there are PHI nodes in NextAlive, we need to add entries to
439 // the PHI nodes for the new incoming edge. The incoming values
440 // should be identical to the incoming values for LastDead.
442 for (BasicBlock::iterator II = NextAlive->begin();
443 isa<PHINode>(II); ++II) {
444 PHINode *PN = cast<PHINode>(II);
445 if (LiveSet.count(PN)) { // Only modify live phi nodes
446 // Get the incoming value for LastDead...
447 int OldIdx = PN->getBasicBlockIndex(LastDead);
448 assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
449 Value *InVal = PN->getIncomingValue(OldIdx);
451 // Add an incoming value for BB now...
452 PN->addIncoming(InVal, BB);
458 // Now loop over all of the instructions in the basic block, deleting
459 // dead instructions. This is so that the next sweep over the program
460 // can safely delete dead instructions without other dead instructions
461 // still referring to them.
463 deleteDeadInstructionsInLiveBlock(BB);
466 // Loop over all of the basic blocks in the function, dropping references of
467 // the dead basic blocks. We must do this after the previous step to avoid
468 // dropping references to PHIs which still have entries...
470 std::vector<BasicBlock*> DeadBlocks;
471 for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
472 if (!AliveBlocks.count(BB)) {
473 // Remove PHI node entries for this block in live successor blocks.
474 for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
475 if (!SI->empty() && isa<PHINode>(SI->front()) && AliveBlocks.count(*SI))
476 (*SI)->removePredecessor(BB);
478 BB->dropAllReferences();
480 DeadBlocks.push_back(BB);
483 NumBlockRemoved += DeadBlocks.size();
485 // Now loop through all of the blocks and delete the dead ones. We can safely
486 // do this now because we know that there are no references to dead blocks
487 // (because they have dropped all of their references).
488 for (std::vector<BasicBlock*>::iterator I = DeadBlocks.begin(),
489 E = DeadBlocks.end(); I != E; ++I)
490 Func->getBasicBlockList().erase(*I);