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//===- ADCE.cpp - Code to perform dead code elimination -------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file implements the Aggressive Dead Code Elimination pass. This pass
// optimistically assumes that all instructions are dead until proven otherwise,
// allowing it to eliminate dead computations that other DCE passes do not
// catch, particularly involving loop computations.
#include "llvm/Transforms/Scalar/ADCE.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/GraphTraits.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/IteratedDominanceFrontier.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/ProfileData/InstrProf.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include <cassert>
#include <cstddef>
#include <utility>
using namespace llvm;
#define DEBUG_TYPE "adce"
STATISTIC(NumRemoved, "Number of instructions removed");
STATISTIC(NumBranchesRemoved, "Number of branch instructions removed");
// This is a temporary option until we change the interface to this pass based
// on optimization level.
static cl::opt<bool> RemoveControlFlowFlag("adce-remove-control-flow",
cl::init(true), cl::Hidden);
// This option enables removing of may-be-infinite loops which have no other
// effect.
static cl::opt<bool> RemoveLoops("adce-remove-loops", cl::init(false),
namespace {
/// Information about Instructions
struct InstInfoType {
/// True if the associated instruction is live.
bool Live = false;
/// Quick access to information for block containing associated Instruction.
struct BlockInfoType *Block = nullptr;
/// Information about basic blocks relevant to dead code elimination.
struct BlockInfoType {
/// True when this block contains a live instructions.
bool Live = false;
/// True when this block ends in an unconditional branch.
bool UnconditionalBranch = false;
/// True when this block is known to have live PHI nodes.
bool HasLivePhiNodes = false;
/// Control dependence sources need to be live for this block.
bool CFLive = false;
/// Quick access to the LiveInfo for the terminator,
/// holds the value &InstInfo[Terminator]
InstInfoType *TerminatorLiveInfo = nullptr;
/// Corresponding BasicBlock.
BasicBlock *BB = nullptr;
/// Cache of BB->getTerminator().
Instruction *Terminator = nullptr;
/// Post-order numbering of reverse control flow graph.
unsigned PostOrder;
bool terminatorIsLive() const { return TerminatorLiveInfo->Live; }
class AggressiveDeadCodeElimination {
Function &F;
// ADCE does not use DominatorTree per se, but it updates it to preserve the
// analysis.
DominatorTree *DT;
PostDominatorTree &PDT;
/// Mapping of blocks to associated information, an element in BlockInfoVec.
/// Use MapVector to get deterministic iteration order.
MapVector<BasicBlock *, BlockInfoType> BlockInfo;
bool isLive(BasicBlock *BB) { return BlockInfo[BB].Live; }
/// Mapping of instructions to associated information.
DenseMap<Instruction *, InstInfoType> InstInfo;
bool isLive(Instruction *I) { return InstInfo[I].Live; }
/// Instructions known to be live where we need to mark
/// reaching definitions as live.
SmallVector<Instruction *, 128> Worklist;
/// Debug info scopes around a live instruction.
SmallPtrSet<const Metadata *, 32> AliveScopes;
/// Set of blocks with not known to have live terminators.
SmallSetVector<BasicBlock *, 16> BlocksWithDeadTerminators;
/// The set of blocks which we have determined whose control
/// dependence sources must be live and which have not had
/// those dependences analyzed.
SmallPtrSet<BasicBlock *, 16> NewLiveBlocks;
/// Set up auxiliary data structures for Instructions and BasicBlocks and
/// initialize the Worklist to the set of must-be-live Instruscions.
void initialize();
/// Return true for operations which are always treated as live.
bool isAlwaysLive(Instruction &I);
/// Return true for instrumentation instructions for value profiling.
bool isInstrumentsConstant(Instruction &I);
/// Propagate liveness to reaching definitions.
void markLiveInstructions();
/// Mark an instruction as live.
void markLive(Instruction *I);
/// Mark a block as live.
void markLive(BlockInfoType &BB);
void markLive(BasicBlock *BB) { markLive(BlockInfo[BB]); }
/// Mark terminators of control predecessors of a PHI node live.
void markPhiLive(PHINode *PN);
/// Record the Debug Scopes which surround live debug information.
void collectLiveScopes(const DILocalScope &LS);
void collectLiveScopes(const DILocation &DL);
/// Analyze dead branches to find those whose branches are the sources
/// of control dependences impacting a live block. Those branches are
/// marked live.
void markLiveBranchesFromControlDependences();
/// Remove instructions not marked live, return if any instruction was
/// removed.
bool removeDeadInstructions();
/// Identify connected sections of the control flow graph which have
/// dead terminators and rewrite the control flow graph to remove them.
bool updateDeadRegions();
/// Set the BlockInfo::PostOrder field based on a post-order
/// numbering of the reverse control flow graph.
void computeReversePostOrder();
/// Make the terminator of this block an unconditional branch to \p Target.
void makeUnconditional(BasicBlock *BB, BasicBlock *Target);
AggressiveDeadCodeElimination(Function &F, DominatorTree *DT,
PostDominatorTree &PDT)
: F(F), DT(DT), PDT(PDT) {}
bool performDeadCodeElimination();
} // end anonymous namespace
bool AggressiveDeadCodeElimination::performDeadCodeElimination() {
return removeDeadInstructions();
static bool isUnconditionalBranch(Instruction *Term) {
auto *BR = dyn_cast<BranchInst>(Term);
return BR && BR->isUnconditional();
void AggressiveDeadCodeElimination::initialize() {
auto NumBlocks = F.size();
// We will have an entry in the map for each block so we grow the
// structure to twice that size to keep the load factor low in the hash table.
size_t NumInsts = 0;
// Iterate over blocks and initialize BlockInfoVec entries, count
// instructions to size the InstInfo hash table.
for (auto &BB : F) {
NumInsts += BB.size();
auto &Info = BlockInfo[&BB];
Info.BB = &BB;
Info.Terminator = BB.getTerminator();
Info.UnconditionalBranch = isUnconditionalBranch(Info.Terminator);
// Initialize instruction map and set pointers to block info.
for (auto &BBInfo : BlockInfo)
for (Instruction &I : *BBInfo.second.BB)
InstInfo[&I].Block = &BBInfo.second;
// Since BlockInfoVec holds pointers into InstInfo and vice-versa, we may not
// add any more elements to either after this point.
for (auto &BBInfo : BlockInfo)
BBInfo.second.TerminatorLiveInfo = &InstInfo[BBInfo.second.Terminator];
// Collect the set of "root" instructions that are known live.
for (Instruction &I : instructions(F))
if (isAlwaysLive(I))
if (!RemoveControlFlowFlag)
if (!RemoveLoops) {
// This stores state for the depth-first iterator. In addition
// to recording which nodes have been visited we also record whether
// a node is currently on the "stack" of active ancestors of the current
// node.
using StatusMap = DenseMap<BasicBlock *, bool>;
class DFState : public StatusMap {
std::pair<StatusMap::iterator, bool> insert(BasicBlock *BB) {
return StatusMap::insert(std::make_pair(BB, true));
// Invoked after we have visited all children of a node.
void completed(BasicBlock *BB) { (*this)[BB] = false; }
// Return true if \p BB is currently on the active stack
// of ancestors.
bool onStack(BasicBlock *BB) {
auto Iter = find(BB);
return Iter != end() && Iter->second;
} State;
// Iterate over blocks in depth-first pre-order and
// treat all edges to a block already seen as loop back edges
// and mark the branch live it if there is a back edge.
for (auto *BB: depth_first_ext(&F.getEntryBlock(), State)) {
Instruction *Term = BB->getTerminator();
if (isLive(Term))
for (auto *Succ : successors(BB))
if (State.onStack(Succ)) {
// back edge....
// Mark blocks live if there is no path from the block to a
// return of the function.
// We do this by seeing which of the postdomtree root children exit the
// program, and for all others, mark the subtree live.
for (auto &PDTChild : children<DomTreeNode *>(PDT.getRootNode())) {
auto *BB = PDTChild->getBlock();
auto &Info = BlockInfo[BB];
// Real function return
if (isa<ReturnInst>(Info.Terminator)) {
LLVM_DEBUG(dbgs() << "post-dom root child is a return: " << BB->getName()
<< '\n';);
// This child is something else, like an infinite loop.
for (auto DFNode : depth_first(PDTChild))
// Treat the entry block as always live
auto *BB = &F.getEntryBlock();
auto &EntryInfo = BlockInfo[BB];
EntryInfo.Live = true;
if (EntryInfo.UnconditionalBranch)
// Build initial collection of blocks with dead terminators
for (auto &BBInfo : BlockInfo)
if (!BBInfo.second.terminatorIsLive())
bool AggressiveDeadCodeElimination::isAlwaysLive(Instruction &I) {
// TODO -- use llvm::isInstructionTriviallyDead
if (I.isEHPad() || I.mayHaveSideEffects() || !I.willReturn()) {
// Skip any value profile instrumentation calls if they are
// instrumenting constants.
if (isInstrumentsConstant(I))
return false;
return true;
if (!I.isTerminator())
return false;
if (RemoveControlFlowFlag && (isa<BranchInst>(I) || isa<SwitchInst>(I)))
return false;
return true;
// Check if this instruction is a runtime call for value profiling and
// if it's instrumenting a constant.
bool AggressiveDeadCodeElimination::isInstrumentsConstant(Instruction &I) {
// TODO -- move this test into llvm::isInstructionTriviallyDead
if (CallInst *CI = dyn_cast<CallInst>(&I))
if (Function *Callee = CI->getCalledFunction())
if (Callee->getName().equals(getInstrProfValueProfFuncName()))
if (isa<Constant>(CI->getArgOperand(0)))
return true;
return false;
void AggressiveDeadCodeElimination::markLiveInstructions() {
// Propagate liveness backwards to operands.
do {
// Worklist holds newly discovered live instructions
// where we need to mark the inputs as live.
while (!Worklist.empty()) {
Instruction *LiveInst = Worklist.pop_back_val();
LLVM_DEBUG(dbgs() << "work live: "; LiveInst->dump(););
for (Use &OI : LiveInst->operands())
if (Instruction *Inst = dyn_cast<Instruction>(OI))
if (auto *PN = dyn_cast<PHINode>(LiveInst))
// After data flow liveness has been identified, examine which branch
// decisions are required to determine live instructions are executed.
} while (!Worklist.empty());
void AggressiveDeadCodeElimination::markLive(Instruction *I) {
auto &Info = InstInfo[I];
if (Info.Live)
LLVM_DEBUG(dbgs() << "mark live: "; I->dump());
Info.Live = true;
// Collect the live debug info scopes attached to this instruction.
if (const DILocation *DL = I->getDebugLoc())
// Mark the containing block live
auto &BBInfo = *Info.Block;
if (BBInfo.Terminator == I) {
// For live terminators, mark destination blocks
// live to preserve this control flow edges.
if (!BBInfo.UnconditionalBranch)
for (auto *BB : successors(I->getParent()))
void AggressiveDeadCodeElimination::markLive(BlockInfoType &BBInfo) {
if (BBInfo.Live)
LLVM_DEBUG(dbgs() << "mark block live: " << BBInfo.BB->getName() << '\n');
BBInfo.Live = true;
if (!BBInfo.CFLive) {
BBInfo.CFLive = true;
// Mark unconditional branches at the end of live
// blocks as live since there is no work to do for them later
if (BBInfo.UnconditionalBranch)
void AggressiveDeadCodeElimination::collectLiveScopes(const DILocalScope &LS) {
if (!AliveScopes.insert(&LS).second)
if (isa<DISubprogram>(LS))
// Tail-recurse through the scope chain.
void AggressiveDeadCodeElimination::collectLiveScopes(const DILocation &DL) {
// Even though DILocations are not scopes, shove them into AliveScopes so we
// don't revisit them.
if (!AliveScopes.insert(&DL).second)
// Collect live scopes from the scope chain.
// Tail-recurse through the inlined-at chain.
if (const DILocation *IA = DL.getInlinedAt())
void AggressiveDeadCodeElimination::markPhiLive(PHINode *PN) {
auto &Info = BlockInfo[PN->getParent()];
// Only need to check this once per block.
if (Info.HasLivePhiNodes)
Info.HasLivePhiNodes = true;
// If a predecessor block is not live, mark it as control-flow live
// which will trigger marking live branches upon which
// that block is control dependent.
for (auto *PredBB : predecessors(Info.BB)) {
auto &Info = BlockInfo[PredBB];
if (!Info.CFLive) {
Info.CFLive = true;
void AggressiveDeadCodeElimination::markLiveBranchesFromControlDependences() {
if (BlocksWithDeadTerminators.empty())
dbgs() << "new live blocks:\n";
for (auto *BB : NewLiveBlocks)
dbgs() << "\t" << BB->getName() << '\n';
dbgs() << "dead terminator blocks:\n";
for (auto *BB : BlocksWithDeadTerminators)
dbgs() << "\t" << BB->getName() << '\n';
// The dominance frontier of a live block X in the reverse
// control graph is the set of blocks upon which X is control
// dependent. The following sequence computes the set of blocks
// which currently have dead terminators that are control
// dependence sources of a block which is in NewLiveBlocks.
const SmallPtrSet<BasicBlock *, 16> BWDT{
SmallVector<BasicBlock *, 32> IDFBlocks;
ReverseIDFCalculator IDFs(PDT);
// Dead terminators which control live blocks are now marked live.
for (auto *BB : IDFBlocks) {
LLVM_DEBUG(dbgs() << "live control in: " << BB->getName() << '\n');
// Routines to update the CFG and SSA information before removing dead code.
bool AggressiveDeadCodeElimination::removeDeadInstructions() {
// Updates control and dataflow around dead blocks
bool RegionsUpdated = updateDeadRegions();
for (Instruction &I : instructions(F)) {
// Check if the instruction is alive.
if (isLive(&I))
if (auto *DII = dyn_cast<DbgVariableIntrinsic>(&I)) {
// Check if the scope of this variable location is alive.
if (AliveScopes.count(DII->getDebugLoc()->getScope()))
// If intrinsic is pointing at a live SSA value, there may be an
// earlier optimization bug: if we know the location of the variable,
// why isn't the scope of the location alive?
for (Value *V : DII->location_ops()) {
if (Instruction *II = dyn_cast<Instruction>(V)) {
if (isLive(II)) {
dbgs() << "Dropping debug info for " << *DII << "\n";
// The inverse of the live set is the dead set. These are those instructions
// that have no side effects and do not influence the control flow or return
// value of the function, and may therefore be deleted safely.
// NOTE: We reuse the Worklist vector here for memory efficiency.
for (Instruction &I : instructions(F)) {
// Check if the instruction is alive.
if (isLive(&I))
if (auto *DII = dyn_cast<DbgInfoIntrinsic>(&I)) {
// Check if the scope of this variable location is alive.
if (AliveScopes.count(DII->getDebugLoc()->getScope()))
// Fallthrough and drop the intrinsic.
// Prepare to delete.
for (Instruction *&I : Worklist) {
return !Worklist.empty() || RegionsUpdated;
// A dead region is the set of dead blocks with a common live post-dominator.
bool AggressiveDeadCodeElimination::updateDeadRegions() {
dbgs() << "final dead terminator blocks: " << '\n';
for (auto *BB : BlocksWithDeadTerminators)
dbgs() << '\t' << BB->getName()
<< (BlockInfo[BB].Live ? " LIVE\n" : "\n");
// Don't compute the post ordering unless we needed it.
bool HavePostOrder = false;
bool Changed = false;
for (auto *BB : BlocksWithDeadTerminators) {
auto &Info = BlockInfo[BB];
if (Info.UnconditionalBranch) {
InstInfo[Info.Terminator].Live = true;
if (!HavePostOrder) {
HavePostOrder = true;
// Add an unconditional branch to the successor closest to the
// end of the function which insures a path to the exit for each
// live edge.
BlockInfoType *PreferredSucc = nullptr;
for (auto *Succ : successors(BB)) {
auto *Info = &BlockInfo[Succ];
if (!PreferredSucc || PreferredSucc->PostOrder < Info->PostOrder)
PreferredSucc = Info;
assert((PreferredSucc && PreferredSucc->PostOrder > 0) &&
"Failed to find safe successor for dead branch");
// Collect removed successors to update the (Post)DominatorTrees.
SmallPtrSet<BasicBlock *, 4> RemovedSuccessors;
bool First = true;
for (auto *Succ : successors(BB)) {
if (!First || Succ != PreferredSucc->BB) {
} else
First = false;
makeUnconditional(BB, PreferredSucc->BB);
// Inform the dominators about the deleted CFG edges.
SmallVector<DominatorTree::UpdateType, 4> DeletedEdges;
for (auto *Succ : RemovedSuccessors) {
// It might have happened that the same successor appeared multiple times
// and the CFG edge wasn't really removed.
if (Succ != PreferredSucc->BB) {
LLVM_DEBUG(dbgs() << "ADCE: (Post)DomTree edge enqueued for deletion"
<< BB->getName() << " -> " << Succ->getName()
<< "\n");
DeletedEdges.push_back({DominatorTree::Delete, BB, Succ});
DomTreeUpdater(DT, &PDT, DomTreeUpdater::UpdateStrategy::Eager)
NumBranchesRemoved += 1;
Changed = true;
return Changed;
// reverse top-sort order
void AggressiveDeadCodeElimination::computeReversePostOrder() {
// This provides a post-order numbering of the reverse control flow graph
// Note that it is incomplete in the presence of infinite loops but we don't
// need numbers blocks which don't reach the end of the functions since
// all branches in those blocks are forced live.
// For each block without successors, extend the DFS from the block
// backward through the graph
SmallPtrSet<BasicBlock*, 16> Visited;
unsigned PostOrder = 0;
for (auto &BB : F) {
if (!succ_empty(&BB))
for (BasicBlock *Block : inverse_post_order_ext(&BB,Visited))
BlockInfo[Block].PostOrder = PostOrder++;
void AggressiveDeadCodeElimination::makeUnconditional(BasicBlock *BB,
BasicBlock *Target) {
Instruction *PredTerm = BB->getTerminator();
// Collect the live debug info scopes attached to this instruction.
if (const DILocation *DL = PredTerm->getDebugLoc())
// Just mark live an existing unconditional branch
if (isUnconditionalBranch(PredTerm)) {
PredTerm->setSuccessor(0, Target);
InstInfo[PredTerm].Live = true;
LLVM_DEBUG(dbgs() << "making unconditional " << BB->getName() << '\n');
NumBranchesRemoved += 1;
IRBuilder<> Builder(PredTerm);
auto *NewTerm = Builder.CreateBr(Target);
InstInfo[NewTerm].Live = true;
if (const DILocation *DL = PredTerm->getDebugLoc())
// Pass Manager integration code
PreservedAnalyses ADCEPass::run(Function &F, FunctionAnalysisManager &FAM) {
// ADCE does not need DominatorTree, but require DominatorTree here
// to update analysis if it is already available.
auto *DT = FAM.getCachedResult<DominatorTreeAnalysis>(F);
auto &PDT = FAM.getResult<PostDominatorTreeAnalysis>(F);
if (!AggressiveDeadCodeElimination(F, DT, PDT).performDeadCodeElimination())
return PreservedAnalyses::all();
PreservedAnalyses PA;
// TODO: We could track if we have actually done CFG changes.
if (!RemoveControlFlowFlag)
else {
return PA;
namespace {
struct ADCELegacyPass : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
ADCELegacyPass() : FunctionPass(ID) {
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
// ADCE does not need DominatorTree, but require DominatorTree here
// to update analysis if it is already available.
auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
auto *DT = DTWP ? &DTWP->getDomTree() : nullptr;
auto &PDT = getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
return AggressiveDeadCodeElimination(F, DT, PDT)
void getAnalysisUsage(AnalysisUsage &AU) const override {
if (!RemoveControlFlowFlag)
else {
} // end anonymous namespace
char ADCELegacyPass::ID = 0;
"Aggressive Dead Code Elimination", false, false)
INITIALIZE_PASS_END(ADCELegacyPass, "adce", "Aggressive Dead Code Elimination",
false, false)
FunctionPass *llvm::createAggressiveDCEPass() { return new ADCELegacyPass(); }