X-Git-Url: http://demsky.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FDCE.cpp;h=e8a090af40c316f1d3d09b53d00b926f67b633aa;hb=d93e8a06b2ca09ab18f390cd514b7443e2e571f7;hp=e089525eca4a597e7d804779a0138f9e07d5402d;hpb=5680ee6b28974c9cf6cab9f603d30e6eda2715bc;p=oota-llvm.git diff --git a/lib/Transforms/Scalar/DCE.cpp b/lib/Transforms/Scalar/DCE.cpp index e089525eca4..e8a090af40c 100644 --- a/lib/Transforms/Scalar/DCE.cpp +++ b/lib/Transforms/Scalar/DCE.cpp @@ -1,333 +1,134 @@ //===- 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 -// * Eliminates method prototypes that are not referenced -// -// TODO: This should REALLY be worklist driven 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. -// -// Note, this is not trivial, because we have to worry about invalidating -// iterators. :( +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// - -#include "llvm/Optimizations/DCE.h" -#include "llvm/Support/STLExtras.h" -#include "llvm/Module.h" -#include "llvm/GlobalVariable.h" -#include "llvm/Method.h" -#include "llvm/BasicBlock.h" -#include "llvm/iTerminators.h" -#include "llvm/iOther.h" -#include "llvm/Assembly/Writer.h" -#include - -struct ConstPoolDCE { - enum { EndOffs = 0 }; - static bool isDCEable(const ConstPoolVal *CPV) { - // TODO: The bytecode writer requires that all used types are in the - // constant pool for the current method. This is messy and is really - // irritating. FIXME - return CPV->getType() != Type::TypeTy; // Don't DCE Type plane constants! - } -}; - -struct BasicBlockDCE { - enum { EndOffs = 1 }; - static bool isDCEable(const Instruction *I) { - return !I->hasSideEffects(); - } -}; - - -template -static bool RemoveUnusedDefs(Container &Vals, DCEController DCEControl) { - bool Changed = false; - int Offset = DCEController::EndOffs; - - for (typename 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; - } - } - 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. +// This file implements dead inst elimination and dead code elimination. // -static bool RemoveSingularPHIs(BasicBlock *BB) { - BasicBlock::pred_iterator PI(BB->pred_begin()); - if (PI == BB->pred_end() || ++PI != BB->pred_end()) - return false; // More than one predecessor... - - Instruction *I = BB->front(); - if (!isa(I)) return false; // No PHI nodes - - //cerr << "Killing PHIs from " << BB; - //cerr << "Pred #0 = " << *BB->pred_begin(); - - //cerr << "Method == " << BB->getParent(); - - do { - PHINode *PN = cast(I); - assert(PN->getNumOperands() == 2 && "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 (isa(I)); - - return true; // Yes, we nuked at least one phi node -} - -static void ReplaceUsesWithConstant(Instruction *I) { - ConstPoolVal *CPV = ConstPoolVal::getNullConstant(I->getType()); - - // Make all users of this instruction reference the constant instead - I->replaceAllUsesWith(CPV); -} - -// 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. +// 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. // -// Assumption: BB is the single predecessor of Succ. -// -static void PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { - assert(isa(Succ->front()) && "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 +#define DEBUG_TYPE "dce" +#include "llvm/Transforms/Scalar.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/IR/Instruction.h" +#include "llvm/Pass.h" +#include "llvm/Support/InstIterator.h" +#include "llvm/Target/TargetLibraryInfo.h" +#include "llvm/Transforms/Utils/Local.h" +using namespace llvm; + +STATISTIC(DIEEliminated, "Number of insts removed by DIE pass"); +STATISTIC(DCEEliminated, "Number of insts removed"); + +namespace { + //===--------------------------------------------------------------------===// + // DeadInstElimination pass implementation // - const vector BBPreds(BB->pred_begin(), BB->pred_end()); - - BasicBlock::iterator I = Succ->begin(); - do { // Loop over all of the PHI nodes in the successor BB - PHINode *PN = cast(*I); - Value *OldVal = PN->removeIncomingValue(BB); - assert(OldVal && "No entry in PHI for Pred BB!"); - - 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); + struct DeadInstElimination : public BasicBlockPass { + static char ID; // Pass identification, replacement for typeid + DeadInstElimination() : BasicBlockPass(ID) { + initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry()); + } + virtual bool runOnBasicBlock(BasicBlock &BB) { + TargetLibraryInfo *TLI = getAnalysisIfAvailable(); + bool Changed = false; + for (BasicBlock::iterator DI = BB.begin(); DI != BB.end(); ) { + Instruction *Inst = DI++; + if (isInstructionTriviallyDead(Inst, TLI)) { + Inst->eraseFromParent(); + Changed = true; + ++DIEEliminated; + } + } + return Changed; } - ++I; - } while (isa(*I)); + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.setPreservesCFG(); + } + }; } +char DeadInstElimination::ID = 0; +INITIALIZE_PASS(DeadInstElimination, "die", + "Dead Instruction Elimination", false, false) -// SimplifyCFG - This function is used to do simplification of a CFG. For -// example, it adjusts branches to branches to eliminate the extra hop, it -// eliminates unreachable basic blocks, and does other "peephole" optimization -// of the CFG. It returns true if a modification was made, and returns an -// iterator that designates the first element remaining after the block that -// was deleted. -// -// WARNING: The entry node of a method may not be simplified. -// -bool opt::SimplifyCFG(Method::iterator &BBIt) { - BasicBlock *BB = *BBIt; - Method *M = BB->getParent(); - - assert(BB && BB->getParent() && "Block not embedded in method!"); - assert(BB->getTerminator() && "Degenerate basic block encountered!"); - assert(BB->getParent()->front() != BB && "Can't Simplify entry block!"); - - - // Remove basic blocks that have no predecessors... which are unreachable. - if (BB->pred_begin() == BB->pred_end() && - !BB->hasConstantPoolReferences()) { - //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(BB->succ_begin(), BB->succ_end(), - std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB)); +Pass *llvm::createDeadInstEliminationPass() { + return new DeadInstElimination(); +} - while (!BB->empty()) { - Instruction *I = BB->back(); - // 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. Note that since this block is - // unreachable, and all values contained within it must dominate their - // uses, that all uses will eventually be removed. - if (!I->use_empty()) ReplaceUsesWithConstant(I); - - // Remove the instruction from the basic block - delete BB->getInstList().pop_back(); - } - delete M->getBasicBlocks().remove(BBIt); - return true; - } - // Check to see if this block has no instructions and only a single - // successor. If so, replace block references with successor. - BasicBlock::succ_iterator SI(BB->succ_begin()); - if (SI != BB->succ_end() && ++SI == BB->succ_end()) { // One succ? - Instruction *I = BB->front(); - if (I->isTerminator()) { // Terminator is the only instruction! - BasicBlock *Succ = *BB->succ_begin(); // There is exactly one successor - //cerr << "Killing Trivial BB: \n" << BB; - - if (Succ != BB) { // Arg, don't hurt infinite loops! - if (isa(Succ->front())) { - // 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); - - 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; - return true; - } +namespace { + //===--------------------------------------------------------------------===// + // DeadCodeElimination pass implementation + // + struct DCE : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + DCE() : FunctionPass(ID) { + initializeDCEPass(*PassRegistry::getPassRegistry()); } - } - // Merge basic blocks into their predecessor if there is only one pred, - // and if there is only one successor of the predecessor. - BasicBlock::pred_iterator PI(BB->pred_begin()); - if (PI != BB->pred_end() && *PI != BB && // Not empty? Not same BB? - ++PI == BB->pred_end() && !BB->hasConstantPoolReferences()) { - BasicBlock *Pred = *BB->pred_begin(); - TerminatorInst *Term = Pred->getTerminator(); - assert(Term != 0 && "malformed basic block without terminator!"); - - // Does the predecessor block only have a single successor? - BasicBlock::succ_iterator SI(Pred->succ_begin()); - if (++SI == Pred->succ_end()) { - //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); + virtual bool runOnFunction(Function &F); - // 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()); - - delete BB; // You ARE the weakest link... goodbye - return true; + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.setPreservesCFG(); } - } - - return false; + }; } -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) { + TargetLibraryInfo *TLI = getAnalysisIfAvailable(); - // Loop over all of the basic blocks (except the first one) and remove them - // if they are unneeded... + // 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 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(); ) { - if (opt::SimplifyCFG(BBIt)) { - Changed = true; - } else { - ++BBIt; + bool MadeChange = false; + while (!WorkList.empty()) { + Instruction *I = WorkList.back(); + WorkList.pop_back(); + + if (isInstructionTriviallyDead(I, TLI)) { // 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. + WorkList.erase(std::remove(WorkList.begin(), WorkList.end(), I), + WorkList.end()); + + MadeChange = true; + ++DCEEliminated; } } - - 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 opt::DeadCodeElimination::doDCE(Method *M) { - bool Changed = false; - while (DoDCEPass(M)) Changed = true; - return Changed; +FunctionPass *llvm::createDeadCodeEliminationPass() { + return new DCE(); } -bool opt::DeadCodeElimination::RemoveUnusedGlobalValues(Module *Mod) { - bool Changed = false; - - for (Module::iterator MI = Mod->begin(); MI != Mod->end(); ) { - Method *Meth = *MI; - if (Meth->isExternal() && Meth->use_size() == 0) { - // No references to prototype? - //cerr << "Removing method proto: " << Meth->getName() << endl; - delete Mod->getMethodList().remove(MI); // Remove prototype - // Remove moves iterator to point to the next one automatically - Changed = true; - } else { - ++MI; // Skip prototype in use. - } - } - - for (Module::giterator GI = Mod->gbegin(); GI != Mod->gend(); ) { - GlobalVariable *GV = *GI; - if (!GV->hasInitializer() && GV->use_size() == 0) { - // No references to uninitialized global variable? - //cerr << "Removing global var: " << GV->getName() << endl; - delete Mod->getGlobalList().remove(GI); - // Remove moves iterator to point to the next one automatically - Changed = true; - } else { - ++GI; - } - } - - return Changed; -}