1 //===- ADCE.cpp - Code to perform 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 the Aggressive Dead Code Elimination pass. This pass
11 // optimistically assumes that all instructions are dead until proven otherwise,
12 // allowing it to eliminate dead computations that other DCE passes do not
13 // catch, particularly involving loop computations.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Transforms/Scalar/ADCE.h"
19 #include "llvm/ADT/DepthFirstIterator.h"
20 #include "llvm/ADT/PostOrderIterator.h"
21 #include "llvm/ADT/SmallPtrSet.h"
22 #include "llvm/ADT/SmallVector.h"
23 #include "llvm/ADT/Statistic.h"
24 #include "llvm/Analysis/GlobalsModRef.h"
25 #include "llvm/Analysis/IteratedDominanceFrontier.h"
26 #include "llvm/Analysis/PostDominators.h"
27 #include "llvm/IR/BasicBlock.h"
28 #include "llvm/IR/CFG.h"
29 #include "llvm/IR/DebugInfoMetadata.h"
30 #include "llvm/IR/IRBuilder.h"
31 #include "llvm/IR/InstIterator.h"
32 #include "llvm/IR/Instructions.h"
33 #include "llvm/IR/IntrinsicInst.h"
34 #include "llvm/Pass.h"
35 #include "llvm/ProfileData/InstrProf.h"
36 #include "llvm/Transforms/Scalar.h"
39 #define DEBUG_TYPE "adce"
41 STATISTIC(NumRemoved, "Number of instructions removed");
42 STATISTIC(NumBranchesRemoved, "Number of branch instructions removed");
44 // This is a temporary option until we change the interface to this pass based
45 // on optimization level.
46 static cl::opt<bool> RemoveControlFlowFlag("adce-remove-control-flow",
47 cl::init(true), cl::Hidden);
49 // This option enables removing of may-be-infinite loops which have no other
51 static cl::opt<bool> RemoveLoops("adce-remove-loops", cl::init(false),
55 /// Information about Instructions
57 /// True if the associated instruction is live.
59 /// Quick access to information for block containing associated Instruction.
60 struct BlockInfoType *Block = nullptr;
63 /// Information about basic blocks relevant to dead code elimination.
64 struct BlockInfoType {
65 /// True when this block contains a live instructions.
67 /// True when this block ends in an unconditional branch.
68 bool UnconditionalBranch = false;
69 /// True when this block is known to have live PHI nodes.
70 bool HasLivePhiNodes = false;
71 /// Control dependence sources need to be live for this block.
74 /// Quick access to the LiveInfo for the terminator,
75 /// holds the value &InstInfo[Terminator]
76 InstInfoType *TerminatorLiveInfo = nullptr;
78 bool terminatorIsLive() const { return TerminatorLiveInfo->Live; }
80 /// Corresponding BasicBlock.
81 BasicBlock *BB = nullptr;
83 /// Cache of BB->getTerminator().
84 TerminatorInst *Terminator = nullptr;
86 /// Post-order numbering of reverse control flow graph.
90 class AggressiveDeadCodeElimination {
92 PostDominatorTree &PDT;
94 /// Mapping of blocks to associated information, an element in BlockInfoVec.
95 DenseMap<BasicBlock *, BlockInfoType> BlockInfo;
96 bool isLive(BasicBlock *BB) { return BlockInfo[BB].Live; }
98 /// Mapping of instructions to associated information.
99 DenseMap<Instruction *, InstInfoType> InstInfo;
100 bool isLive(Instruction *I) { return InstInfo[I].Live; }
102 /// Instructions known to be live where we need to mark
103 /// reaching definitions as live.
104 SmallVector<Instruction *, 128> Worklist;
105 /// Debug info scopes around a live instruction.
106 SmallPtrSet<const Metadata *, 32> AliveScopes;
108 /// Set of blocks with not known to have live terminators.
109 SmallPtrSet<BasicBlock *, 16> BlocksWithDeadTerminators;
111 /// The set of blocks which we have determined whose control
112 /// dependence sources must be live and which have not had
113 /// those dependences analyzed.
114 SmallPtrSet<BasicBlock *, 16> NewLiveBlocks;
116 /// Set up auxiliary data structures for Instructions and BasicBlocks and
117 /// initialize the Worklist to the set of must-be-live Instruscions.
119 /// Return true for operations which are always treated as live.
120 bool isAlwaysLive(Instruction &I);
121 /// Return true for instrumentation instructions for value profiling.
122 bool isInstrumentsConstant(Instruction &I);
124 /// Propagate liveness to reaching definitions.
125 void markLiveInstructions();
126 /// Mark an instruction as live.
127 void markLive(Instruction *I);
128 /// Mark a block as live.
129 void markLive(BlockInfoType &BB);
130 void markLive(BasicBlock *BB) { markLive(BlockInfo[BB]); }
132 /// Mark terminators of control predecessors of a PHI node live.
133 void markPhiLive(PHINode *PN);
135 /// Record the Debug Scopes which surround live debug information.
136 void collectLiveScopes(const DILocalScope &LS);
137 void collectLiveScopes(const DILocation &DL);
139 /// Analyze dead branches to find those whose branches are the sources
140 /// of control dependences impacting a live block. Those branches are
142 void markLiveBranchesFromControlDependences();
144 /// Remove instructions not marked live, return if any any instruction
146 bool removeDeadInstructions();
148 /// Identify connected sections of the control flow graph which have
149 /// dead terminators and rewrite the control flow graph to remove them.
150 void updateDeadRegions();
152 /// Set the BlockInfo::PostOrder field based on a post-order
153 /// numbering of the reverse control flow graph.
154 void computeReversePostOrder();
156 /// Make the terminator of this block an unconditional branch to \p Target.
157 void makeUnconditional(BasicBlock *BB, BasicBlock *Target);
160 AggressiveDeadCodeElimination(Function &F, PostDominatorTree &PDT)
162 bool performDeadCodeElimination();
166 bool AggressiveDeadCodeElimination::performDeadCodeElimination() {
168 markLiveInstructions();
169 return removeDeadInstructions();
172 static bool isUnconditionalBranch(TerminatorInst *Term) {
173 auto *BR = dyn_cast<BranchInst>(Term);
174 return BR && BR->isUnconditional();
177 void AggressiveDeadCodeElimination::initialize() {
179 auto NumBlocks = F.size();
181 // We will have an entry in the map for each block so we grow the
182 // structure to twice that size to keep the load factor low in the hash table.
183 BlockInfo.reserve(NumBlocks);
186 // Iterate over blocks and initialize BlockInfoVec entries, count
187 // instructions to size the InstInfo hash table.
189 NumInsts += BB.size();
190 auto &Info = BlockInfo[&BB];
192 Info.Terminator = BB.getTerminator();
193 Info.UnconditionalBranch = isUnconditionalBranch(Info.Terminator);
196 // Initialize instruction map and set pointers to block info.
197 InstInfo.reserve(NumInsts);
198 for (auto &BBInfo : BlockInfo)
199 for (Instruction &I : *BBInfo.second.BB)
200 InstInfo[&I].Block = &BBInfo.second;
202 // Since BlockInfoVec holds pointers into InstInfo and vice-versa, we may not
203 // add any more elements to either after this point.
204 for (auto &BBInfo : BlockInfo)
205 BBInfo.second.TerminatorLiveInfo = &InstInfo[BBInfo.second.Terminator];
207 // Collect the set of "root" instructions that are known live.
208 for (Instruction &I : instructions(F))
212 if (!RemoveControlFlowFlag)
216 // This stores state for the depth-first iterator. In addition
217 // to recording which nodes have been visited we also record whether
218 // a node is currently on the "stack" of active ancestors of the current
220 typedef DenseMap<BasicBlock *, bool> StatusMap ;
221 class DFState : public StatusMap {
223 std::pair<StatusMap::iterator, bool> insert(BasicBlock *BB) {
224 return StatusMap::insert(std::make_pair(BB, true));
227 // Invoked after we have visited all children of a node.
228 void completed(BasicBlock *BB) { (*this)[BB] = false; }
230 // Return true if \p BB is currently on the active stack
232 bool onStack(BasicBlock *BB) {
233 auto Iter = find(BB);
234 return Iter != end() && Iter->second;
238 State.reserve(F.size());
239 // Iterate over blocks in depth-first pre-order and
240 // treat all edges to a block already seen as loop back edges
241 // and mark the branch live it if there is a back edge.
242 for (auto *BB: depth_first_ext(&F.getEntryBlock(), State)) {
243 TerminatorInst *Term = BB->getTerminator();
247 for (auto *Succ : successors(BB))
248 if (State.onStack(Succ)) {
256 // Mark blocks live if there is no path from the block to the
257 // return of the function or a successor for which this is true.
258 // This protects IDFCalculator which cannot handle such blocks.
259 for (auto &BBInfoPair : BlockInfo) {
260 auto &BBInfo = BBInfoPair.second;
261 if (BBInfo.terminatorIsLive())
263 auto *BB = BBInfo.BB;
264 if (!PDT.getNode(BB)) {
265 DEBUG(dbgs() << "Not post-dominated by return: " << BB->getName()
267 markLive(BBInfo.Terminator);
270 for (auto *Succ : successors(BB))
271 if (!PDT.getNode(Succ)) {
272 DEBUG(dbgs() << "Successor not post-dominated by return: "
273 << BB->getName() << '\n';);
274 markLive(BBInfo.Terminator);
279 // Treat the entry block as always live
280 auto *BB = &F.getEntryBlock();
281 auto &EntryInfo = BlockInfo[BB];
282 EntryInfo.Live = true;
283 if (EntryInfo.UnconditionalBranch)
284 markLive(EntryInfo.Terminator);
286 // Build initial collection of blocks with dead terminators
287 for (auto &BBInfo : BlockInfo)
288 if (!BBInfo.second.terminatorIsLive())
289 BlocksWithDeadTerminators.insert(BBInfo.second.BB);
292 bool AggressiveDeadCodeElimination::isAlwaysLive(Instruction &I) {
293 // TODO -- use llvm::isInstructionTriviallyDead
294 if (I.isEHPad() || I.mayHaveSideEffects()) {
295 // Skip any value profile instrumentation calls if they are
296 // instrumenting constants.
297 if (isInstrumentsConstant(I))
301 if (!isa<TerminatorInst>(I))
303 if (RemoveControlFlowFlag && (isa<BranchInst>(I) || isa<SwitchInst>(I)))
308 // Check if this instruction is a runtime call for value profiling and
309 // if it's instrumenting a constant.
310 bool AggressiveDeadCodeElimination::isInstrumentsConstant(Instruction &I) {
311 // TODO -- move this test into llvm::isInstructionTriviallyDead
312 if (CallInst *CI = dyn_cast<CallInst>(&I))
313 if (Function *Callee = CI->getCalledFunction())
314 if (Callee->getName().equals(getInstrProfValueProfFuncName()))
315 if (isa<Constant>(CI->getArgOperand(0)))
320 void AggressiveDeadCodeElimination::markLiveInstructions() {
322 // Propagate liveness backwards to operands.
324 // Worklist holds newly discovered live instructions
325 // where we need to mark the inputs as live.
326 while (!Worklist.empty()) {
327 Instruction *LiveInst = Worklist.pop_back_val();
328 DEBUG(dbgs() << "work live: "; LiveInst->dump(););
330 for (Use &OI : LiveInst->operands())
331 if (Instruction *Inst = dyn_cast<Instruction>(OI))
334 if (auto *PN = dyn_cast<PHINode>(LiveInst))
338 // After data flow liveness has been identified, examine which branch
339 // decisions are required to determine live instructions are executed.
340 markLiveBranchesFromControlDependences();
342 } while (!Worklist.empty());
345 void AggressiveDeadCodeElimination::markLive(Instruction *I) {
347 auto &Info = InstInfo[I];
351 DEBUG(dbgs() << "mark live: "; I->dump());
353 Worklist.push_back(I);
355 // Collect the live debug info scopes attached to this instruction.
356 if (const DILocation *DL = I->getDebugLoc())
357 collectLiveScopes(*DL);
359 // Mark the containing block live
360 auto &BBInfo = *Info.Block;
361 if (BBInfo.Terminator == I) {
362 BlocksWithDeadTerminators.erase(BBInfo.BB);
363 // For live terminators, mark destination blocks
364 // live to preserve this control flow edges.
365 if (!BBInfo.UnconditionalBranch)
366 for (auto *BB : successors(I->getParent()))
372 void AggressiveDeadCodeElimination::markLive(BlockInfoType &BBInfo) {
375 DEBUG(dbgs() << "mark block live: " << BBInfo.BB->getName() << '\n');
377 if (!BBInfo.CFLive) {
378 BBInfo.CFLive = true;
379 NewLiveBlocks.insert(BBInfo.BB);
382 // Mark unconditional branches at the end of live
383 // blocks as live since there is no work to do for them later
384 if (BBInfo.UnconditionalBranch)
385 markLive(BBInfo.Terminator);
388 void AggressiveDeadCodeElimination::collectLiveScopes(const DILocalScope &LS) {
389 if (!AliveScopes.insert(&LS).second)
392 if (isa<DISubprogram>(LS))
395 // Tail-recurse through the scope chain.
396 collectLiveScopes(cast<DILocalScope>(*LS.getScope()));
399 void AggressiveDeadCodeElimination::collectLiveScopes(const DILocation &DL) {
400 // Even though DILocations are not scopes, shove them into AliveScopes so we
401 // don't revisit them.
402 if (!AliveScopes.insert(&DL).second)
405 // Collect live scopes from the scope chain.
406 collectLiveScopes(*DL.getScope());
408 // Tail-recurse through the inlined-at chain.
409 if (const DILocation *IA = DL.getInlinedAt())
410 collectLiveScopes(*IA);
413 void AggressiveDeadCodeElimination::markPhiLive(PHINode *PN) {
414 auto &Info = BlockInfo[PN->getParent()];
415 // Only need to check this once per block.
416 if (Info.HasLivePhiNodes)
418 Info.HasLivePhiNodes = true;
420 // If a predecessor block is not live, mark it as control-flow live
421 // which will trigger marking live branches upon which
422 // that block is control dependent.
423 for (auto *PredBB : predecessors(Info.BB)) {
424 auto &Info = BlockInfo[PredBB];
427 NewLiveBlocks.insert(PredBB);
432 void AggressiveDeadCodeElimination::markLiveBranchesFromControlDependences() {
434 if (BlocksWithDeadTerminators.empty())
438 dbgs() << "new live blocks:\n";
439 for (auto *BB : NewLiveBlocks)
440 dbgs() << "\t" << BB->getName() << '\n';
441 dbgs() << "dead terminator blocks:\n";
442 for (auto *BB : BlocksWithDeadTerminators)
443 dbgs() << "\t" << BB->getName() << '\n';
446 // The dominance frontier of a live block X in the reverse
447 // control graph is the set of blocks upon which X is control
448 // dependent. The following sequence computes the set of blocks
449 // which currently have dead terminators that are control
450 // dependence sources of a block which is in NewLiveBlocks.
452 SmallVector<BasicBlock *, 32> IDFBlocks;
453 ReverseIDFCalculator IDFs(PDT);
454 IDFs.setDefiningBlocks(NewLiveBlocks);
455 IDFs.setLiveInBlocks(BlocksWithDeadTerminators);
456 IDFs.calculate(IDFBlocks);
457 NewLiveBlocks.clear();
459 // Dead terminators which control live blocks are now marked live.
460 for (auto *BB : IDFBlocks) {
461 DEBUG(dbgs() << "live control in: " << BB->getName() << '\n');
462 markLive(BB->getTerminator());
466 //===----------------------------------------------------------------------===//
468 // Routines to update the CFG and SSA information before removing dead code.
470 //===----------------------------------------------------------------------===//
471 bool AggressiveDeadCodeElimination::removeDeadInstructions() {
473 // Updates control and dataflow around dead blocks
477 for (Instruction &I : instructions(F)) {
478 // Check if the instruction is alive.
482 if (auto *DII = dyn_cast<DbgInfoIntrinsic>(&I)) {
483 // Check if the scope of this variable location is alive.
484 if (AliveScopes.count(DII->getDebugLoc()->getScope()))
487 // If intrinsic is pointing at a live SSA value, there may be an
488 // earlier optimization bug: if we know the location of the variable,
489 // why isn't the scope of the location alive?
490 if (Value *V = DII->getVariableLocation())
491 if (Instruction *II = dyn_cast<Instruction>(V))
493 dbgs() << "Dropping debug info for " << *DII << "\n";
498 // The inverse of the live set is the dead set. These are those instructions
499 // that have no side effects and do not influence the control flow or return
500 // value of the function, and may therefore be deleted safely.
501 // NOTE: We reuse the Worklist vector here for memory efficiency.
502 for (Instruction &I : instructions(F)) {
503 // Check if the instruction is alive.
507 if (auto *DII = dyn_cast<DbgInfoIntrinsic>(&I)) {
508 // Check if the scope of this variable location is alive.
509 if (AliveScopes.count(DII->getDebugLoc()->getScope()))
512 // Fallthrough and drop the intrinsic.
515 // Prepare to delete.
516 Worklist.push_back(&I);
517 I.dropAllReferences();
520 for (Instruction *&I : Worklist) {
522 I->eraseFromParent();
525 return !Worklist.empty();
528 // A dead region is the set of dead blocks with a common live post-dominator.
529 void AggressiveDeadCodeElimination::updateDeadRegions() {
532 dbgs() << "final dead terminator blocks: " << '\n';
533 for (auto *BB : BlocksWithDeadTerminators)
534 dbgs() << '\t' << BB->getName()
535 << (BlockInfo[BB].Live ? " LIVE\n" : "\n");
538 // Don't compute the post ordering unless we needed it.
539 bool HavePostOrder = false;
541 for (auto *BB : BlocksWithDeadTerminators) {
542 auto &Info = BlockInfo[BB];
543 if (Info.UnconditionalBranch) {
544 InstInfo[Info.Terminator].Live = true;
548 if (!HavePostOrder) {
549 computeReversePostOrder();
550 HavePostOrder = true;
553 // Add an unconditional branch to the successor closest to the
554 // end of the function which insures a path to the exit for each
556 BlockInfoType *PreferredSucc = nullptr;
557 for (auto *Succ : successors(BB)) {
558 auto *Info = &BlockInfo[Succ];
559 if (!PreferredSucc || PreferredSucc->PostOrder < Info->PostOrder)
560 PreferredSucc = Info;
562 assert((PreferredSucc && PreferredSucc->PostOrder > 0) &&
563 "Failed to find safe successor for dead branch");
565 for (auto *Succ : successors(BB)) {
566 if (!First || Succ != PreferredSucc->BB)
567 Succ->removePredecessor(BB);
571 makeUnconditional(BB, PreferredSucc->BB);
572 NumBranchesRemoved += 1;
576 // reverse top-sort order
577 void AggressiveDeadCodeElimination::computeReversePostOrder() {
579 // This provides a post-order numbering of the reverse control flow graph
580 // Note that it is incomplete in the presence of infinite loops but we don't
581 // need numbers blocks which don't reach the end of the functions since
582 // all branches in those blocks are forced live.
584 // For each block without successors, extend the DFS from the block
585 // backward through the graph
586 SmallPtrSet<BasicBlock*, 16> Visited;
587 unsigned PostOrder = 0;
589 if (succ_begin(&BB) != succ_end(&BB))
591 for (BasicBlock *Block : inverse_post_order_ext(&BB,Visited))
592 BlockInfo[Block].PostOrder = PostOrder++;
596 void AggressiveDeadCodeElimination::makeUnconditional(BasicBlock *BB,
597 BasicBlock *Target) {
598 TerminatorInst *PredTerm = BB->getTerminator();
599 // Collect the live debug info scopes attached to this instruction.
600 if (const DILocation *DL = PredTerm->getDebugLoc())
601 collectLiveScopes(*DL);
603 // Just mark live an existing unconditional branch
604 if (isUnconditionalBranch(PredTerm)) {
605 PredTerm->setSuccessor(0, Target);
606 InstInfo[PredTerm].Live = true;
609 DEBUG(dbgs() << "making unconditional " << BB->getName() << '\n');
610 NumBranchesRemoved += 1;
611 IRBuilder<> Builder(PredTerm);
612 auto *NewTerm = Builder.CreateBr(Target);
613 InstInfo[NewTerm].Live = true;
614 if (const DILocation *DL = PredTerm->getDebugLoc())
615 NewTerm->setDebugLoc(DL);
618 //===----------------------------------------------------------------------===//
620 // Pass Manager integration code
622 //===----------------------------------------------------------------------===//
623 PreservedAnalyses ADCEPass::run(Function &F, FunctionAnalysisManager &FAM) {
624 auto &PDT = FAM.getResult<PostDominatorTreeAnalysis>(F);
625 if (!AggressiveDeadCodeElimination(F, PDT).performDeadCodeElimination())
626 return PreservedAnalyses::all();
628 PreservedAnalyses PA;
629 PA.preserveSet<CFGAnalyses>();
630 PA.preserve<GlobalsAA>();
635 struct ADCELegacyPass : public FunctionPass {
636 static char ID; // Pass identification, replacement for typeid
637 ADCELegacyPass() : FunctionPass(ID) {
638 initializeADCELegacyPassPass(*PassRegistry::getPassRegistry());
641 bool runOnFunction(Function &F) override {
644 auto &PDT = getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
645 return AggressiveDeadCodeElimination(F, PDT).performDeadCodeElimination();
648 void getAnalysisUsage(AnalysisUsage &AU) const override {
649 AU.addRequired<PostDominatorTreeWrapperPass>();
650 if (!RemoveControlFlowFlag)
651 AU.setPreservesCFG();
652 AU.addPreserved<GlobalsAAWrapperPass>();
657 char ADCELegacyPass::ID = 0;
658 INITIALIZE_PASS_BEGIN(ADCELegacyPass, "adce",
659 "Aggressive Dead Code Elimination", false, false)
660 INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass)
661 INITIALIZE_PASS_END(ADCELegacyPass, "adce", "Aggressive Dead Code Elimination",
664 FunctionPass *llvm::createAggressiveDCEPass() { return new ADCELegacyPass(); }