-//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
+//===- DCE.cpp - Code to perform dead code elimination --------------------===//
//
-// 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.
+// The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
-
-#include "llvm/Transforms/Scalar.h"
-#include "llvm/Transforms/Utils/Local.h"
-#include "llvm/Transforms/Utils/BasicBlockUtils.h"
-#include "llvm/Type.h"
-#include "llvm/Analysis/Dominators.h"
-#include "llvm/iTerminators.h"
-#include "llvm/iPHINode.h"
-#include "llvm/Constant.h"
-#include "llvm/Support/CFG.h"
-#include "Support/STLExtras.h"
-#include "Support/DepthFirstIterator.h"
-#include "Support/StatisticReporter.h"
-#include <algorithm>
-#include <iostream>
-using std::cerr;
-using std::vector;
-
-static Statistic<> NumBlockRemoved("adce\t\t- Number of basic blocks removed");
-static Statistic<> NumInstRemoved ("adce\t\t- Number of instructions removed");
-
-namespace {
-
-//===----------------------------------------------------------------------===//
-// 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.
+// 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.
//
-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 {
- 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);
-
- inline void markInstructionLive(Instruction *I) {
- if (LiveSet.count(I)) return;
- DEBUG(cerr << "Insn Live: " << I);
- LiveSet.insert(I);
- WorkList.push_back(I);
- }
-
- inline void markTerminatorLive(const BasicBlock *BB) {
- DEBUG(cerr << "Terminat Live: " << BB->getTerminator());
- markInstructionLive((Instruction*)BB->getTerminator());
- }
-};
-
- RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
-} // End of anonymous namespace
+//===----------------------------------------------------------------------===//
-Pass *createAggressiveDCEPass() { return new ADCE(); }
+#define DEBUG_TYPE "adce"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Instructions.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/Support/InstIterator.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/SmallPtrSet.h"
-void ADCE::markBlockAlive(BasicBlock *BB) {
- // Mark the basic block as being newly ALIVE... and mark all branches that
- // this block is control dependant on as being alive also...
- //
- PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
+using namespace llvm;
- PostDominanceFrontier::const_iterator It = CDG.find(BB);
- if (It != CDG.end()) {
- // Get the blocks that this node is control dependant 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, then the terminator must be as well!
- markTerminatorLive(BB);
-}
+STATISTIC(NumRemoved, "Number of instructions removed");
-// 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(I);
- Changed = true;
- } else {
- ++I;
- }
- } else {
- ++I;
+namespace {
+ struct VISIBILITY_HIDDEN ADCE : public FunctionPass {
+ static char ID; // Pass identification, replacement for typeid
+ ADCE() : FunctionPass((intptr_t)&ID) {}
+
+ virtual bool runOnFunction(Function& F);
+
+ virtual void getAnalysisUsage(AnalysisUsage& AU) const {
+ AU.setPreservesCFG();
}
- return Changed;
+
+ };
}
+char ADCE::ID = 0;
+static RegisterPass<ADCE> X("adce", "Aggressive Dead Code Elimination");
-// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
-// true if the function was modified.
-//
-bool ADCE::doADCE() {
- bool MadeChanges = false;
-
- // 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.
- //
- for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
- BBI != BBE; ++BBI) {
- BasicBlock *BB = *BBI;
- for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
- if (II->hasSideEffects() || II->getOpcode() == Instruction::Ret) {
- markInstructionLive(II);
- ++II; // Increment the inst iterator if the inst wasn't deleted
- } else if (isInstructionTriviallyDead(II)) {
- // Remove the instruction from it's basic block...
- II = BB->getInstList().erase(II);
- ++NumInstRemoved;
- MadeChanges = true;
- } else {
- ++II; // Increment the inst iterator if the inst wasn't deleted
- }
- }
- }
-
- DEBUG(cerr << "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 neccesary as
- // well... make them live as well.
- //
- while (!WorkList.empty()) {
- Instruction *I = WorkList.back(); // Get an instruction that became live...
- WorkList.pop_back();
-
- BasicBlock *BB = I->getParent();
- if (!AliveBlocks.count(BB)) { // Basic block not alive yet...
- AliveBlocks.insert(BB); // Block is now ALIVE!
- markBlockAlive(BB); // Make it so now!
+bool ADCE::runOnFunction(Function& F) {
+ SmallPtrSet<Instruction*, 32> alive;
+ std::vector<Instruction*> worklist;
+
+ // 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()) ||
+ I->mayWriteToMemory()) {
+ 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);
- }
-
- if (DebugFlag) {
- cerr << "Current Function: X = Live\n";
- for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
- for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
- if (LiveSet.count(BI)) cerr << "X ";
- cerr << *BI;
- }
- }
-
- // Find the first postdominator of the entry node that is alive. Make it the
- // new entry node...
- //
- PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
-
-
- 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 refering to them.
- //
- dropReferencesOfDeadInstructionsInLiveBlock(I);
-
- } else { // If there are some blocks dead...
- // Insert a new entry node to eliminate the entry node as a special case.
- BasicBlock *NewEntry = new BasicBlock();
- NewEntry->getInstList().push_back(new BranchInst(&Func->front()));
- Func->getBasicBlockList().push_front(NewEntry);
- AliveBlocks.insert(NewEntry); // This block is always alive!
+
+ // Propagate liveness backwards to operands.
+ while (!worklist.empty()) {
+ Instruction* curr = worklist.back();
+ worklist.pop_back();
- // 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();
-
- // 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 fundementally changes (ie 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->getNode())) {
- LastNode = NextNode;
- NextNode = NextNode->getIDom();
- }
-
- // Get the basic blocks that we need...
- BasicBlock *LastDead = LastNode->getNode();
- BasicBlock *NextAlive = NextNode->getNode();
-
- // 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();
- PHINode *PN = dyn_cast<PHINode>(&*II); ++II) {
- // 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 refering to them.
- //
- dropReferencesOfDeadInstructionsInLiveBlock(BB);
- }
+ 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))
+ worklist.push_back(Inst);
}
-
- // Loop over all of the basic blocks in the function, dropping references of
- // the dead basic 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.
- 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...
- BB->getInstList().pop_back();
- const Type *RetTy = Func->getReturnType();
- Instruction *New = new ReturnInst(RetTy != Type::VoidTy ?
- Constant::getNullValue(RetTy) : 0);
- BB->getInstList().push_back(New);
- }
-
- BB->dropAllReferences();
- ++NumBlockRemoved;
- MadeChanges = true;
+
+ // 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.
+ SmallPtrSet<Instruction*, 32> dead;
+ for (inst_iterator I = inst_begin(F), E = inst_end(F); I != E; ++I)
+ if (!alive.count(I.getInstructionIterator())) {
+ dead.insert(I.getInstructionIterator());
+ I->dropAllReferences();
}
+
+ for (SmallPtrSet<Instruction*, 32>::iterator I = dead.begin(),
+ E = dead.end(); I != E; ++I) {
+ NumRemoved++;
+ (*I)->eraseFromParent();
}
-
- // 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...
- II = BI->getInstList().erase(II);
- ++NumInstRemoved;
- MadeChanges = true;
- } else {
- ++II;
- }
-
- ++BI; // Increment iterator...
- }
-
- return MadeChanges;
+
+ return !dead.empty();
}
+
+FunctionPass *llvm::createAggressiveDCEPass() {
+ return new ADCE();
+}
\ No newline at end of file
projects / oota-llvm.git / blobdiff