X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FADCE.cpp;h=bdf8029b18ef4694a8f8dd952ccd945497ec08da;hb=36d3e326dfcc3f2ff002e5e76178effb785d006d;hp=ffd16e1f1e5597745d0c4c9e42822bf15a1be0d3;hpb=d9036a1aad813b419edcec8e17d24bcaedd72065;p=oota-llvm.git diff --git a/lib/Transforms/Scalar/ADCE.cpp b/lib/Transforms/Scalar/ADCE.cpp index ffd16e1f1e5..bdf8029b18e 100644 --- a/lib/Transforms/Scalar/ADCE.cpp +++ b/lib/Transforms/Scalar/ADCE.cpp @@ -1,320 +1,96 @@ -//===- 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/Type.h" -#include "llvm/Analysis/Dominators.h" -#include "llvm/Analysis/Writer.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 -#include -using std::cerr; - -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 WorkList; // Instructions that just became live - std::set LiveSet; // The set of live instructions - - //===--------------------------------------------------------------------===// - // The public interface for this class - // -public: - const char *getPassName() const { return "Aggressive Dead Code Elimination"; } - - // 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(DominatorTree::PostDomID); - AU.addRequired(DominanceFrontier::PostDomID); - } - - - //===--------------------------------------------------------------------===// - // 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); - - 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()); - } -}; - -} // 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... - // - DominanceFrontier &CDG = - getAnalysis(DominanceFrontier::PostDomID); - - DominanceFrontier::const_iterator It = CDG.find(BB); - if (It != CDG.end()) { - // Get the blocks that this node is control dependant on... - const DominanceFrontier::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); -} +//===----------------------------------------------------------------------===// +#define DEBUG_TYPE "adce" +#include "llvm/Transforms/Scalar.h" +#include "llvm/BasicBlock.h" +#include "llvm/Instructions.h" +#include "llvm/Pass.h" +#include "llvm/Support/CFG.h" +#include "llvm/Support/Compiler.h" +#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" -// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning -// true if the function was modified. -// -bool ADCE::doADCE() { - bool MadeChanges = false; +using namespace llvm; - // 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; ) { - Instruction *I = *II; +STATISTIC(NumRemoved, "Number of instructions removed"); - if (I->hasSideEffects() || I->getOpcode() == Instruction::Ret) { - markInstructionLive(I); - ++II; // Increment the inst iterator if the inst wasn't deleted - } else if (isInstructionTriviallyDead(I)) { - // Remove the instruction from it's basic block... - delete BB->getInstList().remove(II); - ++NumInstRemoved; - MadeChanges = true; - } else { - ++II; // Increment the inst iterator if the inst wasn't deleted - } +namespace { + struct VISIBILITY_HIDDEN ADCE : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + ADCE() : FunctionPass(&ID) {} + + virtual bool runOnFunction(Function& F); + + 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(); +char ADCE::ID = 0; +static RegisterPass X("adce", "Aggressive Dead Code Elimination"); - 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 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()) || + 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(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; - } - } - - // Find the first postdominator of the entry node that is alive. Make it the - // new entry node... - // - DominatorTree &DT = getAnalysis(DominatorTree::PostDomID); - - // If there are some blocks dead... - if (AliveBlocks.size() != Func->size()) { - // 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->getBasicBlocks().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 - for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++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... - // - DominatorTree::Node *LastNode = DT[TI->getSuccessor(i)]; - DominatorTree::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. - // - for (BasicBlock::iterator I = BB->begin(), E = BB->end()-1; I != E; ++I) - if (!LiveSet.count(*I)) // Is this instruction alive? - (*I)->dropAllReferences(); // Nope, drop references... - } + 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); } - - // Loop over all of the basic blocks in the function, removing dead - // instructions from alive blocks, and dropping references of the dead blocks - // - for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) { - BasicBlock *BB = *I; - if (AliveBlocks.count(BB)) { - for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; ) - if (!LiveSet.count(*II)) { // Is this instruction alive? - // Nope... remove the instruction from it's basic block... - delete BB->getInstList().remove(II); - ++NumInstRemoved; - MadeChanges = true; - } else { - ++II; - } - } else { - // 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... - delete BB->getInstList().remove(BB->end()-1); - 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. + // 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(); } + + for (SmallVector::iterator I = worklist.begin(), + E = worklist.end(); I != E; ++I) { + NumRemoved++; + (*I)->eraseFromParent(); } - // Now loop through all of the blocks and delete them. 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 (Function::iterator BI = Func->begin(); BI != Func->end(); ) - if (!AliveBlocks.count(*BI)) - delete Func->getBasicBlocks().remove(BI); - else - ++BI; // Increment iterator... + return !worklist.empty(); +} - return MadeChanges; +FunctionPass *llvm::createAggressiveDCEPass() { + return new ADCE(); }