-//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
+//===- ADCE.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/Constants.h"
-#include "llvm/Instructions.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/Transforms/Scalar/ADCE.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/Analysis/GlobalsModRef.h"
+#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"
+#include "llvm/Transforms/Scalar.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");
-
-//===----------------------------------------------------------------------===//
-// 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);
-
-
- // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in
- // the specified basic block, deleting ones that are dead according to
- // LiveSet.
- bool deleteDeadInstructionsInLiveBlock(BasicBlock *BB);
-
- TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
-
- inline void markInstructionLive(Instruction *I) {
- if (!LiveSet.insert(I).second) return;
- DOUT << "Insn Live: " << *I;
- WorkList.push_back(I);
- }
-
- inline void markTerminatorLive(const BasicBlock *BB) {
- DOUT << "Terminator Live: " << *BB->getTerminator();
- markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
- }
-};
+#define DEBUG_TYPE "adce"
- RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
-} // End of anonymous namespace
+STATISTIC(NumRemoved, "Number of instructions removed");
-FunctionPass *llvm::createAggressiveDCEPass() { return new ADCE(); }
+static bool aggressiveDCE(Function& F) {
+ SmallPtrSet<Instruction*, 128> Alive;
+ SmallVector<Instruction*, 128> Worklist;
-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 (PostDominanceFrontier::DomSetType::const_iterator I =
- CDB.begin(), E = CDB.end(); I != E; ++I)
- markTerminatorLive(*I); // Mark all their terminators as live
+ // Collect the set of "root" instructions that are known live.
+ for (Instruction &I : instructions(F)) {
+ if (isa<TerminatorInst>(I) || isa<DbgInfoIntrinsic>(I) || I.isEHPad() ||
+ I.mayHaveSideEffects()) {
+ Alive.insert(&I);
+ Worklist.push_back(&I);
+ }
}
- // 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);
-}
-
-// deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in the
-// specified basic block, deleting ones that are dead according to LiveSet.
-bool ADCE::deleteDeadInstructionsInLiveBlock(BasicBlock *BB) {
- bool Changed = false;
- for (BasicBlock::iterator II = BB->begin(), E = --BB->end(); II != E; ) {
- Instruction *I = II++;
- if (!LiveSet.count(I)) { // Is this instruction alive?
- if (!I->use_empty())
- I->replaceAllUsesWith(UndefValue::get(I->getType()));
-
- // Nope... remove the instruction from it's basic block...
- if (isa<CallInst>(I))
- ++NumCallRemoved;
- else
- ++NumInstRemoved;
- BB->getInstList().erase(I);
- Changed = true;
+ // Propagate liveness backwards to operands.
+ while (!Worklist.empty()) {
+ Instruction *Curr = Worklist.pop_back_val();
+ for (Use &OI : Curr->operands()) {
+ if (Instruction *Inst = dyn_cast<Instruction>(OI))
+ if (Alive.insert(Inst).second)
+ Worklist.push_back(Inst);
}
}
- 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();
-
- // 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("");
- CallInst *NewCall = new CallInst(F, Args, Name, II);
- NewCall->setCallingConv(II->getCallingConv());
- 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;
- }
+ // 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 (Instruction &I : instructions(F)) {
+ if (!Alive.count(&I)) {
+ Worklist.push_back(&I);
+ I.dropAllReferences();
}
}
- // 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;
+ for (Instruction *&I : Worklist) {
+ ++NumRemoved;
+ I->eraseFromParent();
}
- // 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 && ReachableBBs.count(I))
- for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
- markInstructionLive((*PI)->getTerminator());
-
- DOUT << "Processing work list\n";
-
- // AliveBlocks - Set of basic blocks that we know have instructions that are
- // alive in them...
- //
- std::set<BasicBlock*> AliveBlocks;
-
- // 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();
+ return !Worklist.empty();
+}
- BasicBlock *BB = I->getParent();
- if (!ReachableBBs.count(BB)) continue;
- if (AliveBlocks.insert(BB).second) // Basic block not alive yet.
- markBlockAlive(BB); // Make it so now!
+PreservedAnalyses ADCEPass::run(Function &F) {
+ if (aggressiveDCE(F))
+ return PreservedAnalyses::none();
+ return PreservedAnalyses::all();
+}
- // 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 (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
- // If the incoming edge is clearly dead, it won't have control
- // dependence information. Do not mark it live.
- BasicBlock *PredBB = PN->getIncomingBlock(i);
- if (ReachableBBs.count(PredBB)) {
- // FIXME: This should mark the control dependent edge as live, not
- // necessarily the predecessor itself!
- if (AliveBlocks.insert(PredBB).second)
- markBlockAlive(PN->getIncomingBlock(i)); // Block is newly ALIVE!
- if (Instruction *Op = dyn_cast<Instruction>(PN->getIncomingValue(i)))
- markInstructionLive(Op);
- }
- }
- } else {
- // 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);
- }
+namespace {
+struct ADCELegacyPass : public FunctionPass {
+ static char ID; // Pass identification, replacement for typeid
+ ADCELegacyPass() : FunctionPass(ID) {
+ initializeADCELegacyPassPass(*PassRegistry::getPassRegistry());
}
- DEBUG(
- DOUT << "Current Function: X = Live\n";
- for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
- DOUT << 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)) DOUT << "X ";
- DOUT << *BI;
- }
- });
-
- // All blocks being live is a common case, handle it specially.
- 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 deleting instructions
- // to drop their references.
- deleteDeadInstructionsInLiveBlock(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 an unconditional branch.
- //
- TerminatorInst *TI = I->getTerminator();
- if (!LiveSet.count(TI))
- convertToUnconditionalBranch(TI);
- }
-
- return MadeChanges;
+ bool runOnFunction(Function& F) override {
+ if (skipOptnoneFunction(F))
+ return false;
+ return aggressiveDCE(F);
}
-
- // 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
+ void getAnalysisUsage(AnalysisUsage& AU) const override {
+ AU.setPreservesCFG();
+ AU.addPreserved<GlobalsAAWrapperPass>();
}
+};
+}
- // 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)];
- PostDominatorTree::Node *NextNode = 0;
-
- if (LastNode) {
- NextNode = LastNode->getIDom();
- while (!AliveBlocks.count(NextNode->getBlock())) {
- LastNode = NextNode;
- NextNode = NextNode->getIDom();
- if (NextNode == 0) {
- LastNode = 0;
- break;
- }
- }
- }
-
- // There is a special case here... if there IS no post-dominator for
- // the block we have nowhere 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!
- if (!isa<InvokeInst>(TI)) {
- // 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.
- //
- TI = BB->getTerminator();
-
- // Rescan this successor...
- --i;
- } else {
-
- }
- } else {
- // 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, deleting
- // dead instructions. This is so that the next sweep over the program
- // can safely delete dead instructions without other dead instructions
- // still referring to them.
- //
- deleteDeadInstructionsInLiveBlock(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...
- //
- std::vector<BasicBlock*> DeadBlocks;
- for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
- if (!AliveBlocks.count(BB)) {
- // Remove PHI node entries for this block in live successor blocks.
- for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI)
- if (!SI->empty() && isa<PHINode>(SI->front()) && AliveBlocks.count(*SI))
- (*SI)->removePredecessor(BB);
-
- BB->dropAllReferences();
- MadeChanges = true;
- DeadBlocks.push_back(BB);
- }
-
- NumBlockRemoved += DeadBlocks.size();
-
- // 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).
- for (std::vector<BasicBlock*>::iterator I = DeadBlocks.begin(),
- E = DeadBlocks.end(); I != E; ++I)
- Func->getBasicBlockList().erase(*I);
+char ADCELegacyPass::ID = 0;
+INITIALIZE_PASS(ADCELegacyPass, "adce", "Aggressive Dead Code Elimination",
+ false, false)
- return MadeChanges;
-}
+FunctionPass *llvm::createAggressiveDCEPass() { return new ADCELegacyPass(); }
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