X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FADCE.cpp;h=b344952cc5fcaed42a60e05442db327d9bc65204;hb=ad41dcfd875e8d3421a5f0970b6a886b6e10b3b7;hp=15b71fdf55ff7310aab707c3d195ab8339f302bd;hpb=a69fd903585a665c031d5aa3fdfb8dc919b44bef;p=oota-llvm.git diff --git a/lib/Transforms/Scalar/ADCE.cpp b/lib/Transforms/Scalar/ADCE.cpp index 15b71fdf55f..b344952cc5f 100644 --- a/lib/Transforms/Scalar/ADCE.cpp +++ b/lib/Transforms/Scalar/ADCE.cpp @@ -1,390 +1,97 @@ -//===- 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. +// +//===----------------------------------------------------------------------===// +// +// 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. // //===----------------------------------------------------------------------===// +#define DEBUG_TYPE "adce" #include "llvm/Transforms/Scalar.h" -#include "llvm/Transforms/Utils/Local.h" -#include "llvm/Transforms/Utils/BasicBlockUtils.h" -#include "llvm/Type.h" -#include "llvm/Analysis/PostDominators.h" -#include "llvm/iTerminators.h" -#include "llvm/iPHINode.h" -#include "llvm/Constant.h" +#include "llvm/BasicBlock.h" +#include "llvm/Instructions.h" +#include "llvm/IntrinsicInst.h" +#include "llvm/Pass.h" #include "llvm/Support/CFG.h" -#include "Support/STLExtras.h" -#include "Support/DepthFirstIterator.h" -#include "Support/StatisticReporter.h" -#include -#include -using std::cerr; -using std::vector; +#include "llvm/Support/InstIterator.h" +#include "llvm/ADT/DepthFirstIterator.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/ADT/Statistic.h" +using namespace llvm; -static Statistic<> NumBlockRemoved("adce\t\t- Number of basic blocks removed"); -static Statistic<> NumInstRemoved ("adce\t\t- Number of instructions removed"); +STATISTIC(NumRemoved, "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. -// -class ADCE : public FunctionPass { - Function *Func; // The function that we are working on - std::vector WorkList; // Instructions that just became live - std::set 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(); - AU.addRequired(); - } - - - //===--------------------------------------------------------------------===// - // 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 X("adce", "Aggressive Dead Code Elimination"); -} // End of anonymous namespace - -Pass *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 dependant on as being alive also... - // - PostDominanceFrontier &CDG = getAnalysis(); - - 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); -} - -// 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(&*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; + struct ADCE : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + ADCE() : FunctionPass(ID) { + initializeADCEPass(*PassRegistry::getPassRegistry()); } - return Changed; -} + virtual bool runOnFunction(Function& F); -// 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 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 - } + virtual void getAnalysisUsage(AnalysisUsage& AU) const { + AU.setPreservesCFG(); } - } - - DEBUG(cerr << "Processing work list\n"); - - // AliveBlocks - Set of basic blocks that we know have instructions that are - // alive in them... - // - std::set 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! +char ADCE::ID = 0; +INITIALIZE_PASS(ADCE, "adce", "Aggressive Dead Code Elimination", false, false) + +bool ADCE::runOnFunction(Function& F) { + SmallPtrSet alive; + SmallVector 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(I.getInstructionIterator()) || + isa(I.getInstructionIterator()) || + isa(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(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(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; - } + // 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(OI)) + if (alive.insert(Inst)) + worklist.push_back(Inst); } - // Find the first postdominator of the entry node that is alive. Make it the - // new entry node... - // - PostDominatorTree &DT = getAnalysis(); - - - 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... - // 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(); - NewEntry->getInstList().push_back(new BranchInst(&Func->front())); - Func->getBasicBlockList().push_front(NewEntry); - AliveBlocks.insert(NewEntry); // This block is always alive! + // 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(); - - // 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(&*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 (SmallVector::iterator I = worklist.begin(), + E = worklist.end(); I != E; ++I) { + ++NumRemoved; + (*I)->eraseFromParent(); } - // 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 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; - } - } - - // 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 !worklist.empty(); +} - return MadeChanges; +FunctionPass *llvm::createAggressiveDCEPass() { + return new ADCE(); }