-//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
-//
+//===- DCE.cpp - Code to perform dead code elimination --------------------===//
+//
// The LLVM Compiler Infrastructure
//
-// This file was developed by the LLVM research group and is distributed under
-// the University of Illinois Open Source License. See LICENSE.TXT for details.
-//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
//===----------------------------------------------------------------------===//
//
-// This file implements "aggressive" dead code elimination. ADCE is DCe where
-// values are assumed to be dead until proven otherwise. This is similar to
-// SCCP, except applied to the liveness of values.
+// 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.h"
-#include "llvm/Constant.h"
-#include "llvm/Instructions.h"
-#include "llvm/Type.h"
-#include "llvm/Analysis/AliasAnalysis.h"
-#include "llvm/Analysis/PostDominators.h"
-#include "llvm/Support/CFG.h"
-#include "llvm/Transforms/Utils/BasicBlockUtils.h"
-#include "llvm/Transforms/Utils/Local.h"
-#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
-#include "llvm/Support/Debug.h"
#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
-#include "llvm/ADT/STLExtras.h"
-#include <algorithm>
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/CFG.h"
+#include "llvm/IR/InstIterator.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/Pass.h"
using namespace llvm;
-namespace {
- Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed");
- Statistic<> NumInstRemoved ("adce", "Number of instructions removed");
- Statistic<> NumCallRemoved ("adce", "Number of calls and invokes removed");
+#define DEBUG_TYPE "adce"
-//===----------------------------------------------------------------------===//
-// ADCE Class
-//
-// This class does all of the work of Aggressive Dead Code Elimination.
-// It's public interface consists of a constructor and a doADCE() method.
-//
-class ADCE : public FunctionPass {
- Function *Func; // The function that we are working on
- std::vector<Instruction*> WorkList; // Instructions that just became live
- std::set<Instruction*> LiveSet; // The set of live instructions
-
- //===--------------------------------------------------------------------===//
- // The public interface for this class
- //
-public:
- // Execute the Aggressive Dead Code Elimination Algorithm
- //
- virtual bool runOnFunction(Function &F) {
- Func = &F;
- bool Changed = doADCE();
- assert(WorkList.empty());
- LiveSet.clear();
- return Changed;
- }
- // getAnalysisUsage - We require post dominance frontiers (aka Control
- // Dependence Graph)
- virtual void getAnalysisUsage(AnalysisUsage &AU) const {
- // We require that all function nodes are unified, because otherwise code
- // can be marked live that wouldn't necessarily be otherwise.
- AU.addRequired<UnifyFunctionExitNodes>();
- AU.addRequired<AliasAnalysis>();
- AU.addRequired<PostDominatorTree>();
- AU.addRequired<PostDominanceFrontier>();
- }
-
-
- //===--------------------------------------------------------------------===//
- // The implementation of this class
- //
-private:
- // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
- // true if the function was modified.
- //
- bool doADCE();
-
- void markBlockAlive(BasicBlock *BB);
-
-
- // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
- // instructions in the specified basic block, dropping references on
- // instructions that are dead according to LiveSet.
- bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB);
-
- TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
-
- inline void markInstructionLive(Instruction *I) {
- if (!LiveSet.insert(I).second) return;
- DEBUG(std::cerr << "Insn Live: " << *I);
- WorkList.push_back(I);
- }
+STATISTIC(NumRemoved, "Number of instructions removed");
- inline void markTerminatorLive(const BasicBlock *BB) {
- DEBUG(std::cerr << "Terminator Live: " << *BB->getTerminator());
- markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
- }
-};
-
- RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
-} // End of anonymous namespace
-
-FunctionPass *llvm::createAggressiveDCEPass() { return new ADCE(); }
-
-void ADCE::markBlockAlive(BasicBlock *BB) {
- // Mark the basic block as being newly ALIVE... and mark all branches that
- // this block is control dependent on as being alive also...
- //
- PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
-
- PostDominanceFrontier::const_iterator It = CDG.find(BB);
- if (It != CDG.end()) {
- // Get the blocks that this node is control dependent on...
- const PostDominanceFrontier::DomSetType &CDB = It->second;
- for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
- bind_obj(this, &ADCE::markTerminatorLive));
- }
-
- // If this basic block is live, and it ends in an unconditional branch, then
- // the branch is alive as well...
- if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
- if (BI->isUnconditional())
- markTerminatorLive(BB);
-}
-
-// dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
-// instructions in the specified basic block, dropping references on
-// instructions that are dead according to LiveSet.
-bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) {
- bool Changed = false;
- for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; )
- if (!LiveSet.count(I)) { // Is this instruction alive?
- I->dropAllReferences(); // Nope, drop references...
- if (PHINode *PN = dyn_cast<PHINode>(I)) {
- // We don't want to leave PHI nodes in the program that have
- // #arguments != #predecessors, so we remove them now.
- //
- PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
-
- // Delete the instruction...
- ++I;
- BB->getInstList().erase(PN);
- Changed = true;
- ++NumInstRemoved;
- } else {
- ++I;
- }
- } else {
- ++I;
+namespace {
+ struct ADCE : public FunctionPass {
+ static char ID; // Pass identification, replacement for typeid
+ ADCE() : FunctionPass(ID) {
+ initializeADCEPass(*PassRegistry::getPassRegistry());
}
- return Changed;
-}
-
-/// convertToUnconditionalBranch - Transform this conditional terminator
-/// instruction into an unconditional branch because we don't care which of the
-/// successors it goes to. This eliminate a use of the condition as well.
-///
-TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
- BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
- BasicBlock *BB = TI->getParent();
+ bool runOnFunction(Function& F) override;
- // Remove entries from PHI nodes to avoid confusing ourself later...
- for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
- TI->getSuccessor(i)->removePredecessor(BB);
-
- // Delete the old branch itself...
- BB->getInstList().erase(TI);
- return NB;
-}
-
-
-// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
-// true if the function was modified.
-//
-bool ADCE::doADCE() {
- bool MadeChanges = false;
-
- AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
-
-
- // Iterate over all invokes in the function, turning invokes into calls if
- // they cannot throw.
- for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
- if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
- if (Function *F = II->getCalledFunction())
- if (AA.onlyReadsMemory(F)) {
- // The function cannot unwind. Convert it to a call with a branch
- // after it to the normal destination.
- std::vector<Value*> Args(II->op_begin()+3, II->op_end());
- std::string Name = II->getName(); II->setName("");
- Instruction *NewCall = new CallInst(F, Args, Name, II);
- II->replaceAllUsesWith(NewCall);
- new BranchInst(II->getNormalDest(), II);
-
- // Update PHI nodes in the unwind destination
- II->getUnwindDest()->removePredecessor(BB);
- BB->getInstList().erase(II);
-
- if (NewCall->use_empty()) {
- BB->getInstList().erase(NewCall);
- ++NumCallRemoved;
- }
- }
-
- // Iterate over all of the instructions in the function, eliminating trivially
- // dead instructions, and marking instructions live that are known to be
- // needed. Perform the walk in depth first order so that we avoid marking any
- // instructions live in basic blocks that are unreachable. These blocks will
- // be eliminated later, along with the instructions inside.
- //
- std::set<BasicBlock*> ReachableBBs;
- for (df_ext_iterator<BasicBlock*>
- BBI = df_ext_begin(&Func->front(), ReachableBBs),
- BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) {
- BasicBlock *BB = *BBI;
- for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
- Instruction *I = II++;
- if (CallInst *CI = dyn_cast<CallInst>(I)) {
- Function *F = CI->getCalledFunction();
- if (F && AA.onlyReadsMemory(F)) {
- if (CI->use_empty()) {
- BB->getInstList().erase(CI);
- ++NumCallRemoved;
- }
- } else {
- markInstructionLive(I);
- }
- } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
- isa<UnwindInst>(I) || isa<UnreachableInst>(I)) {
- // FIXME: Unreachable instructions should not be marked intrinsically
- // live here.
- markInstructionLive(I);
- } else if (isInstructionTriviallyDead(I)) {
- // Remove the instruction from it's basic block...
- BB->getInstList().erase(I);
- ++NumInstRemoved;
- }
+ void getAnalysisUsage(AnalysisUsage& AU) const override {
+ AU.setPreservesCFG();
}
- }
-
- // Check to ensure we have an exit node for this CFG. If we don't, we won't
- // have any post-dominance information, thus we cannot perform our
- // transformations safely.
- //
- PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
- if (DT[&Func->getEntryBlock()] == 0) {
- WorkList.clear();
- return MadeChanges;
- }
-
- // Scan the function marking blocks without post-dominance information as
- // live. Blocks without post-dominance information occur when there is an
- // infinite loop in the program. Because the infinite loop could contain a
- // function which unwinds, exits or has side-effects, we don't want to delete
- // the infinite loop or those blocks leading up to it.
- for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
- if (DT[I] == 0)
- for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
- markInstructionLive((*PI)->getTerminator());
-
+ };
+}
- DEBUG(std::cerr << "Processing work list\n");
+char ADCE::ID = 0;
+INITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false)
- // AliveBlocks - Set of basic blocks that we know have instructions that are
- // alive in them...
- //
- std::set<BasicBlock*> AliveBlocks;
+bool ADCE::runOnFunction(Function& F) {
+ if (skipOptnoneFunction(F))
+ return false;
- // Process the work list of instructions that just became live... if they
- // became live, then that means that all of their operands are necessary as
- // well... make them live as well.
- //
- while (!WorkList.empty()) {
- Instruction *I = WorkList.back(); // Get an instruction that became live...
- WorkList.pop_back();
+ SmallPtrSet<Instruction*, 128> alive;
+ SmallVector<Instruction*, 128> worklist;
- BasicBlock *BB = I->getParent();
- if (!ReachableBBs.count(BB)) continue;
- if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
- AliveBlocks.insert(BB); // Block is now ALIVE!
- markBlockAlive(BB); // Make it so now!
+ // Collect the set of "root" instructions that are known live.
+ for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
+ if (isa<TerminatorInst>(I.getInstructionIterator()) ||
+ isa<DbgInfoIntrinsic>(I.getInstructionIterator()) ||
+ isa<LandingPadInst>(I.getInstructionIterator()) ||
+ I->mayHaveSideEffects()) {
+ alive.insert(I.getInstructionIterator());
+ worklist.push_back(I.getInstructionIterator());
}
- // PHI nodes are a special case, because the incoming values are actually
- // defined in the predecessor nodes of this block, meaning that the PHI
- // makes the predecessors alive.
- //
- if (PHINode *PN = dyn_cast<PHINode>(I))
- for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
- if (!AliveBlocks.count(*PI)) {
- AliveBlocks.insert(BB); // Block is now ALIVE!
- markBlockAlive(*PI);
- }
-
- // Loop over all of the operands of the live instruction, making sure that
- // they are known to be alive as well...
- //
- for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
- if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
- markInstructionLive(Operand);
+ // Propagate liveness backwards to operands.
+ while (!worklist.empty()) {
+ Instruction* curr = worklist.pop_back_val();
+ for (Instruction::op_iterator OI = curr->op_begin(), OE = curr->op_end();
+ OI != OE; ++OI)
+ if (Instruction* Inst = dyn_cast<Instruction>(OI))
+ if (alive.insert(Inst).second)
+ worklist.push_back(Inst);
}
- DEBUG(
- std::cerr << "Current Function: X = Live\n";
- for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
- std::cerr << I->getName() << ":\t"
- << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
- for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
- if (LiveSet.count(BI)) std::cerr << "X ";
- std::cerr << *BI;
- }
- });
-
- // Find the first postdominator of the entry node that is alive. Make it the
- // new entry node...
- //
- if (AliveBlocks.size() == Func->size()) { // No dead blocks?
- for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
- // Loop over all of the instructions in the function, telling dead
- // instructions to drop their references. This is so that the next sweep
- // over the program can safely delete dead instructions without other dead
- // instructions still referring to them.
- //
- dropReferencesOfDeadInstructionsInLiveBlock(I);
-
- // Check to make sure the terminator instruction is live. If it isn't,
- // this means that the condition that it branches on (we know it is not an
- // unconditional branch), is not needed to make the decision of where to
- // go to, because all outgoing edges go to the same place. We must remove
- // the use of the condition (because it's probably dead), so we convert
- // the terminator to a conditional branch.
- //
- TerminatorInst *TI = I->getTerminator();
- if (!LiveSet.count(TI))
- convertToUnconditionalBranch(TI);
- }
-
- } else { // If there are some blocks dead...
- // If the entry node is dead, insert a new entry node to eliminate the entry
- // node as a special case.
- //
- if (!AliveBlocks.count(&Func->front())) {
- BasicBlock *NewEntry = new BasicBlock();
- new BranchInst(&Func->front(), NewEntry);
- Func->getBasicBlockList().push_front(NewEntry);
- AliveBlocks.insert(NewEntry); // This block is always alive!
- LiveSet.insert(NewEntry->getTerminator()); // The branch is live
+ // The inverse of the live set is the dead set. These are those instructions
+ // which 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 (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
+ if (!alive.count(I.getInstructionIterator())) {
+ worklist.push_back(I.getInstructionIterator());
+ I->dropAllReferences();
}
-
- // Loop over all of the alive blocks in the function. If any successor
- // blocks are not alive, we adjust the outgoing branches to branch to the
- // first live postdominator of the live block, adjusting any PHI nodes in
- // the block to reflect this.
- //
- for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
- if (AliveBlocks.count(I)) {
- BasicBlock *BB = I;
- TerminatorInst *TI = BB->getTerminator();
-
- // If the terminator instruction is alive, but the block it is contained
- // in IS alive, this means that this terminator is a conditional branch
- // on a condition that doesn't matter. Make it an unconditional branch
- // to ONE of the successors. This has the side effect of dropping a use
- // of the conditional value, which may also be dead.
- if (!LiveSet.count(TI))
- TI = convertToUnconditionalBranch(TI);
- // Loop over all of the successors, looking for ones that are not alive.
- // We cannot save the number of successors in the terminator instruction
- // here because we may remove them if we don't have a postdominator...
- //
- for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
- if (!AliveBlocks.count(TI->getSuccessor(i))) {
- // Scan up the postdominator tree, looking for the first
- // postdominator that is alive, and the last postdominator that is
- // dead...
- //
- PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
-
- // There is a special case here... if there IS no post-dominator for
- // the block we have no owhere to point our branch to. Instead,
- // convert it to a return. This can only happen if the code
- // branched into an infinite loop. Note that this may not be
- // desirable, because we _are_ altering the behavior of the code.
- // This is a well known drawback of ADCE, so in the future if we
- // choose to revisit the decision, this is where it should be.
- //
- if (LastNode == 0) { // No postdominator!
- // Call RemoveSuccessor to transmogrify the terminator instruction
- // to not contain the outgoing branch, or to create a new
- // terminator if the form fundamentally changes (i.e.,
- // unconditional branch to return). Note that this will change a
- // branch into an infinite loop into a return instruction!
- //
- RemoveSuccessor(TI, i);
-
- // RemoveSuccessor may replace TI... make sure we have a fresh
- // pointer... and e variable.
- //
- TI = BB->getTerminator();
-
- // Rescan this successor...
- --i;
- } else {
- PostDominatorTree::Node *NextNode = LastNode->getIDom();
-
- while (!AliveBlocks.count(NextNode->getBlock())) {
- LastNode = NextNode;
- NextNode = NextNode->getIDom();
- }
-
- // Get the basic blocks that we need...
- BasicBlock *LastDead = LastNode->getBlock();
- BasicBlock *NextAlive = NextNode->getBlock();
-
- // Make the conditional branch now go to the next alive block...
- TI->getSuccessor(i)->removePredecessor(BB);
- TI->setSuccessor(i, NextAlive);
-
- // If there are PHI nodes in NextAlive, we need to add entries to
- // the PHI nodes for the new incoming edge. The incoming values
- // should be identical to the incoming values for LastDead.
- //
- for (BasicBlock::iterator II = NextAlive->begin();
- isa<PHINode>(II); ++II) {
- PHINode *PN = cast<PHINode>(II);
- if (LiveSet.count(PN)) { // Only modify live phi nodes
- // Get the incoming value for LastDead...
- int OldIdx = PN->getBasicBlockIndex(LastDead);
- assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
- Value *InVal = PN->getIncomingValue(OldIdx);
-
- // Add an incoming value for BB now...
- PN->addIncoming(InVal, BB);
- }
- }
- }
- }
-
- // Now loop over all of the instructions in the basic block, telling
- // dead instructions to drop their references. This is so that the next
- // sweep over the program can safely delete dead instructions without
- // other dead instructions still referring to them.
- //
- dropReferencesOfDeadInstructionsInLiveBlock(BB);
- }
- }
-
- // We make changes if there are any dead blocks in the function...
- if (unsigned NumDeadBlocks = Func->size() - AliveBlocks.size()) {
- MadeChanges = true;
- NumBlockRemoved += NumDeadBlocks;
+ for (SmallVectorImpl<Instruction *>::iterator I = worklist.begin(),
+ E = worklist.end(); I != E; ++I) {
+ ++NumRemoved;
+ (*I)->eraseFromParent();
}
- // Loop over all of the basic blocks in the function, removing control flow
- // edges to live blocks (also eliminating any entries in PHI functions in
- // referenced blocks).
- //
- for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
- if (!AliveBlocks.count(BB)) {
- // Remove all outgoing edges from this basic block and convert the
- // terminator into a return instruction.
- std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
-
- if (!Succs.empty()) {
- // Loop over all of the successors, removing this block from PHI node
- // entries that might be in the block...
- while (!Succs.empty()) {
- Succs.back()->removePredecessor(BB);
- Succs.pop_back();
- }
-
- // Delete the old terminator instruction...
- const Type *TermTy = BB->getTerminator()->getType();
- if (TermTy != Type::VoidTy)
- BB->getTerminator()->replaceAllUsesWith(
- Constant::getNullValue(TermTy));
- BB->getInstList().pop_back();
- const Type *RetTy = Func->getReturnType();
- new ReturnInst(RetTy != Type::VoidTy ?
- Constant::getNullValue(RetTy) : 0, BB);
- }
- }
-
-
- // Loop over all of the basic blocks in the function, dropping references of
- // the dead basic blocks. We must do this after the previous step to avoid
- // dropping references to PHIs which still have entries...
- //
- for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
- if (!AliveBlocks.count(BB))
- BB->dropAllReferences();
-
- // Now loop through all of the blocks and delete the dead ones. We can safely
- // do this now because we know that there are no references to dead blocks
- // (because they have dropped all of their references... we also remove dead
- // instructions from alive blocks.
- //
- for (Function::iterator BI = Func->begin(); BI != Func->end(); )
- if (!AliveBlocks.count(BI)) { // Delete dead blocks...
- BI = Func->getBasicBlockList().erase(BI);
- } else { // Scan alive blocks...
- for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
- if (!LiveSet.count(II)) { // Is this instruction alive?
- // Nope... remove the instruction from it's basic block...
- if (isa<CallInst>(II))
- ++NumCallRemoved;
- else
- ++NumInstRemoved;
- II = BI->getInstList().erase(II);
- MadeChanges = true;
- } else {
- ++II;
- }
-
- ++BI; // Increment iterator...
- }
+ return !worklist.empty();
+}
- return MadeChanges;
+FunctionPass *llvm::createAggressiveDCEPass() {
+ return new ADCE();
}
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