X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FDCE.cpp;h=8dbcc23d7ec85677a75d2863323f85488f08bb41;hb=827454e6e28cfed93db990b03b720ef7c23e6917;hp=5bc3c5b0895f7c6941ab3dde43759c984d012d49;hpb=7fc9fe34390c66ca58646d09a87f7dbaacb6c1f8;p=oota-llvm.git diff --git a/lib/Transforms/Scalar/DCE.cpp b/lib/Transforms/Scalar/DCE.cpp index 5bc3c5b0895..8dbcc23d7ec 100644 --- a/lib/Transforms/Scalar/DCE.cpp +++ b/lib/Transforms/Scalar/DCE.cpp @@ -1,349 +1,135 @@ //===- DCE.cpp - Code to perform dead code elimination --------------------===// // -// This file implements dead code elimination and basic block merging. +// The LLVM Compiler Infrastructure // -// Specifically, this: -// * removes definitions with no uses (including unused constants) -// * removes basic blocks with no predecessors -// * merges a basic block into its predecessor if there is only one and the -// predecessor only has one successor. -// * Eliminates PHI nodes for basic blocks with a single predecessor -// * Eliminates a basic block that only contains an unconditional branch +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. // -// TODO: This should REALLY be recursive instead of iterative. Right now, we -// scan linearly through values, removing unused ones as we go. The problem is -// that this may cause other earlier values to become unused. To make sure that -// we get them all, we iterate until things stop changing. Instead, when -// removing a value, recheck all of its operands to see if they are now unused. -// Piece of cake, and more efficient as well. +//===----------------------------------------------------------------------===// +// +// This file implements dead inst elimination and dead code elimination. // -// Note, this is not trivial, because we have to worry about invalidating -// iterators. :( +// Dead Inst Elimination performs a single pass over the function removing +// instructions that are obviously dead. Dead Code Elimination is similar, but +// it rechecks instructions that were used by removed instructions to see if +// they are newly dead. // //===----------------------------------------------------------------------===// -#include "llvm/Module.h" -#include "llvm/Method.h" -#include "llvm/BasicBlock.h" -#include "llvm/iTerminators.h" -#include "llvm/iOther.h" -#include "llvm/Opt/AllOpts.h" -#include "llvm/Assembly/Writer.h" -#include "llvm/CFG.h" - -using namespace cfg; - -struct ConstPoolDCE { - enum { EndOffs = 0 }; - static bool isDCEable(const Value *) { return true; } -}; - -struct BasicBlockDCE { - enum { EndOffs = 1 }; - static bool isDCEable(const Instruction *I) { - return !I->hasSideEffects(); - } -}; - - -template -static bool RemoveUnusedDefs(ValueHolder &Vals, - DCEController DCEControl) { - bool Changed = false; - typedef ValueHolder Container; - - int Offset = DCEController::EndOffs; - for (Container::iterator DI = Vals.begin(); DI != Vals.end()-Offset; ) { - // Look for un"used" definitions... - if ((*DI)->use_empty() && DCEController::isDCEable(*DI)) { - // Bye bye - //cerr << "Removing: " << *DI; - delete Vals.remove(DI); - Changed = true; - } else { - ++DI; +#define DEBUG_TYPE "dce" +#include "llvm/Transforms/Scalar.h" +#include "llvm/Transforms/Utils/Local.h" +#include "llvm/Instruction.h" +#include "llvm/Pass.h" +#include "llvm/Support/InstIterator.h" +#include "llvm/ADT/Statistic.h" +using namespace llvm; + +STATISTIC(DIEEliminated, "Number of insts removed by DIE pass"); +STATISTIC(DCEEliminated, "Number of insts removed"); + +namespace { + //===--------------------------------------------------------------------===// + // DeadInstElimination pass implementation + // + struct DeadInstElimination : public BasicBlockPass { + static char ID; // Pass identification, replacement for typeid + DeadInstElimination() : BasicBlockPass(ID) { + initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry()); + } + virtual bool runOnBasicBlock(BasicBlock &BB) { + bool Changed = false; + for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) { + Instruction *Inst = DI++; + if (isInstructionTriviallyDead(Inst)) { + Inst->eraseFromParent(); + Changed = true; + ++DIEEliminated; + } + } + return Changed; } - } - return Changed; -} - -// RemoveSingularPHIs - This removes PHI nodes from basic blocks that have only -// a single predecessor. This means that the PHI node must only have a single -// RHS value and can be eliminated. -// -// This routine is very simple because we know that PHI nodes must be the first -// things in a basic block, if they are present. -// -static bool RemoveSingularPHIs(BasicBlock *BB) { - pred_iterator PI(pred_begin(BB)); - if (PI == pred_end(BB) || ++PI != pred_end(BB)) - return false; // More than one predecessor... - - Instruction *I = BB->front(); - if (!I->isPHINode()) return false; // No PHI nodes - - //cerr << "Killing PHIs from " << BB; - //cerr << "Pred #0 = " << *pred_begin(BB); - - //cerr << "Method == " << BB->getParent(); - - do { - PHINode *PN = (PHINode*)I; - assert(PN->getOperand(2) == 0 && "PHI node should only have one value!"); - Value *V = PN->getOperand(0); - - PN->replaceAllUsesWith(V); // Replace PHI node with its single value. - delete BB->getInstList().remove(BB->begin()); - - I = BB->front(); - } while (I->isPHINode()); - - return true; // Yes, we nuked at least one phi node -} - -bool DoRemoveUnusedConstants(SymTabValue *S) { - bool Changed = false; - ConstantPool &CP = S->getConstantPool(); - for (ConstantPool::plane_iterator PI = CP.begin(); PI != CP.end(); ++PI) - Changed |= RemoveUnusedDefs(**PI, ConstPoolDCE()); - return Changed; -} - -static void ReplaceUsesWithConstant(Instruction *I) { - // Get the method level constant pool - ConstantPool &CP = I->getParent()->getParent()->getConstantPool(); - - ConstPoolVal *CPV = 0; - ConstantPool::PlaneType *P; - if (!CP.getPlane(I->getType(), P)) { // Does plane exist? - // Yes, is it empty? - if (!P->empty()) CPV = P->front(); - } - - if (CPV == 0) { // We don't have an existing constant to reuse. Just add one. - CPV = ConstPoolVal::getNullConstant(I->getType()); // Create a new constant - // Add the new value to the constant pool... - CP.insert(CPV); - } - - // Make all users of this instruction reference the constant instead - I->replaceAllUsesWith(CPV); + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.setPreservesCFG(); + } + }; } -// RemovePredecessorFromBlock - This function is called when we are about -// to remove a predecessor from a basic block. This function takes care of -// removing the predecessor from the PHI nodes in BB so that after the pred -// is removed, the number of PHI slots per bb is equal to the number of -// predecessors. -// -static void RemovePredecessorFromBlock(BasicBlock *BB, BasicBlock *Pred) { - pred_iterator PI(pred_begin(BB)), EI(pred_end(BB)); - unsigned max_idx; - - //cerr << "RPFB: " << Pred << "From Block: " << BB; - - // Loop over the rest of the predecssors until we run out, or until we find - // out that there are more than 2 predecessors. - for (max_idx = 0; PI != EI && max_idx < 3; ++PI, ++max_idx) /*empty*/; - - // If there are exactly two predecessors, then we want to nuke the PHI nodes - // altogether. - bool NukePHIs = max_idx == 2; - assert(max_idx != 0 && "PHI Node in block with 0 predecessors!?!?!"); - - // Okay, now we know that we need to remove predecessor #pred_idx from all - // PHI nodes. Iterate over each PHI node fixing them up - BasicBlock::iterator II(BB->begin()); - for (; (*II)->isPHINode(); ++II) { - PHINode *PN = (PHINode*)*II; - PN->removeIncomingValue(BB); - - if (NukePHIs) { // Destroy the PHI altogether?? - assert(PN->getOperand(1) == 0 && "PHI node should only have one value!"); - Value *V = PN->getOperand(0); +char DeadInstElimination::ID = 0; +INITIALIZE_PASS(DeadInstElimination, "die", + "Dead Instruction Elimination", false, false) - PN->replaceAllUsesWith(V); // Replace PHI node with its single value. - delete BB->getInstList().remove(II); - } - } +Pass *llvm::createDeadInstEliminationPass() { + return new DeadInstElimination(); } -// PropogatePredecessors - This gets "Succ" ready to have the predecessors from -// "BB". This is a little tricky because "Succ" has PHI nodes, which need to -// have extra slots added to them to hold the merge edges from BB's -// predecessors. -// -// Assumption: BB is the single predecessor of Succ. -// -static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { - assert(Succ->front()->isPHINode() && "Only works on PHId BBs!"); - // If there is more than one predecessor, and there are PHI nodes in - // the successor, then we need to add incoming edges for the PHI nodes +namespace { + //===--------------------------------------------------------------------===// + // DeadCodeElimination pass implementation // - const vector BBPreds(pred_begin(BB), pred_end(BB)); + struct DCE : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + DCE() : FunctionPass(ID) { + initializeDCEPass(*PassRegistry::getPassRegistry()); + } - BasicBlock::iterator I = Succ->begin(); - do { // Loop over all of the PHI nodes in the successor BB - PHINode *PN = (PHINode*)*I; - Value *OldVal = PN->removeIncomingValue(BB); - assert(OldVal && "No entry in PHI for Pred BB!"); + virtual bool runOnFunction(Function &F); - for (vector::const_iterator PredI = BBPreds.begin(), - End = BBPreds.end(); PredI != End; ++PredI) { - // Add an incoming value for each of the new incoming values... - PN->addIncoming(OldVal, *PredI); + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.setPreservesCFG(); } - - ++I; - } while ((*I)->isPHINode()); + }; } -static bool DoDCEPass(Method *M) { - Method::iterator BBIt, BBEnd = M->end(); - if (M->begin() == BBEnd) return false; // Nothing to do - bool Changed = false; +char DCE::ID = 0; +INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false) - // Loop through now and remove instructions that have no uses... - for (BBIt = M->begin(); BBIt != BBEnd; ++BBIt) { - Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE()); - Changed |= RemoveSingularPHIs(*BBIt); - } +bool DCE::runOnFunction(Function &F) { + // Start out with all of the instructions in the worklist... + std::vector WorkList; + for (inst_iterator i = inst_begin(F), e = inst_end(F); i != e; ++i) + WorkList.push_back(&*i); - // Loop over all of the basic blocks (except the first one) and remove them - // if they are unneeded... + // Loop over the worklist finding instructions that are dead. If they are + // dead make them drop all of their uses, making other instructions + // potentially dead, and work until the worklist is empty. // - for (BBIt = M->begin(), ++BBIt; BBIt != M->end(); ++BBIt) { - BasicBlock *BB = *BBIt; - assert(BB->getTerminator() && "Degenerate basic block encountered!"); - -#if 0 // This is know to basically work? - // Remove basic blocks that have no predecessors... which are unreachable. - if (pred_begin(BB) == pred_end(BB) && - !BB->hasConstantPoolReferences() && 0) { - cerr << "Removing BB: \n" << BB; - - // Loop through all of our successors and make sure they know that one - // of their predecessors is going away. - for_each(succ_begin(BB), succ_end(BB), - bind_2nd(RemovePredecessorFromBlock, BB)); - - while (!BB->empty()) { - Instruction *I = BB->front(); - // If this instruction is used, replace uses with an arbitrary - // constant value. Because control flow can't get here, we don't care - // what we replace the value with. - if (!I->use_empty()) ReplaceUsesWithConstant(I); - - // Remove the instruction from the basic block - delete BB->getInstList().remove(BB->begin()); - } - delete M->getBasicBlocks().remove(BBIt); - --BBIt; // remove puts use on the next block, we want the previous one - Changed = true; - continue; - } -#endif - -#if 0 // This has problems - // Check to see if this block has no instructions and only a single - // successor. If so, replace block references with successor. - succ_iterator SI(succ_begin(BB)); - if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ? - Instruction *I = BB->front(); - if (I->isTerminator()) { // Terminator is the only instruction! - BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor - cerr << "Killing Trivial BB: \n" << BB; - - if (Succ->front()->isPHINode()) { - // If our successor has PHI nodes, then we need to update them to - // include entries for BB's predecessors, not for BB itself. - // - PropogatePredecessorsForPHIs(BB, Succ); - } - - BB->replaceAllUsesWith(Succ); - - BB = M->getBasicBlocks().remove(BBIt); - --BBIt; // remove puts use on the next block, we want the previous one - - if (BB->hasName() && !Succ->hasName()) // Transfer name if we can - Succ->setName(BB->getName()); - delete BB; // Delete basic block - - cerr << "Method after removal: \n" << M; - Changed = true; - continue; + bool MadeChange = false; + while (!WorkList.empty()) { + Instruction *I = WorkList.back(); + WorkList.pop_back(); + + if (isInstructionTriviallyDead(I)) { // If the instruction is dead. + // Loop over all of the values that the instruction uses, if there are + // instructions being used, add them to the worklist, because they might + // go dead after this one is removed. + // + for (User::op_iterator OI = I->op_begin(), E = I->op_end(); OI != E; ++OI) + if (Instruction *Used = dyn_cast(*OI)) + WorkList.push_back(Used); + + // Remove the instruction. + I->eraseFromParent(); + + // Remove the instruction from the worklist if it still exists in it. + for (std::vector::iterator WI = WorkList.begin(); + WI != WorkList.end(); ) { + if (*WI == I) + WI = WorkList.erase(WI); + else + ++WI; } - } -#endif - - // Merge basic blocks into their predecessor if there is only one pred, - // and if there is only one successor of the predecessor. - pred_iterator PI(pred_begin(BB)); - if (PI != pred_end(BB) && *PI != BB && // Not empty? Not same BB? - ++PI == pred_end(BB) && !BB->hasConstantPoolReferences()) { - BasicBlock *Pred = *pred_begin(BB); - TerminatorInst *Term = Pred->getTerminator(); - assert(Term != 0 && "malformed basic block without terminator!"); - - // Does the predecessor block only have a single successor? - succ_iterator SI(succ_begin(Pred)); - if (++SI == succ_end(Pred)) { - //cerr << "Merging: " << BB << "into: " << Pred; - // Delete the unconditianal branch from the predecessor... - BasicBlock::iterator DI = Pred->end(); - assert(Pred->getTerminator() && - "Degenerate basic block encountered!"); // Empty bb??? - delete Pred->getInstList().remove(--DI); // Destroy uncond branch - - // Move all definitions in the succecessor to the predecessor... - while (!BB->empty()) { - DI = BB->begin(); - Instruction *Def = BB->getInstList().remove(DI); // Remove from front - Pred->getInstList().push_back(Def); // Add to end... - } - - // Remove basic block from the method... and advance iterator to the - // next valid block... - BB = M->getBasicBlocks().remove(BBIt); - --BBIt; // remove puts us on the NEXT bb. We want the prev BB - Changed = true; - - // Make all PHI nodes that refered to BB now refer to Pred as their - // source... - BB->replaceAllUsesWith(Pred); - - // Inherit predecessors name if it exists... - if (BB->hasName() && !Pred->hasName()) Pred->setName(BB->getName()); - - // You ARE the weakest link... goodbye - delete BB; - - //WriteToVCG(M, "MergedInto"); - } + MadeChange = true; + ++DCEEliminated; } } - - // Remove unused constants - Changed |= DoRemoveUnusedConstants(M); - return Changed; + return MadeChange; } - -// It is possible that we may require multiple passes over the code to fully -// eliminate dead code. Iterate until we are done. -// -bool DoDeadCodeElimination(Method *M) { - bool Changed = false; - while (DoDCEPass(M)) Changed = true; - return Changed; +FunctionPass *llvm::createDeadCodeEliminationPass() { + return new DCE(); } -bool DoDeadCodeElimination(Module *C) { - bool Val = ApplyOptToAllMethods(C, DoDeadCodeElimination); - while (DoRemoveUnusedConstants(C)) Val = true; - return Val; -}