X-Git-Url: http://plrg.eecs.uci.edu/git/?a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FDCE.cpp;h=b67c3c7742fd7d37bbf53b9c6e603c7880f0cd91;hb=4188784a044d5328619dc7e690d5005372c65e9a;hp=29e3ba1a7196c6db390ab4b76f0bc2c9c726e7c6;hpb=d36c91c9c6f87d28230c3ae0e4381473eac7944a;p=oota-llvm.git diff --git a/lib/Transforms/Scalar/DCE.cpp b/lib/Transforms/Scalar/DCE.cpp index 29e3ba1a719..b67c3c7742f 100644 --- a/lib/Transforms/Scalar/DCE.cpp +++ b/lib/Transforms/Scalar/DCE.cpp @@ -1,335 +1,156 @@ //===- 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(); - } -}; +#include "llvm/Transforms/Scalar.h" +#include "llvm/ADT/SetVector.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/IR/InstIterator.h" +#include "llvm/IR/Instruction.h" +#include "llvm/Pass.h" +#include "llvm/Analysis/TargetLibraryInfo.h" +#include "llvm/Transforms/Utils/Local.h" +using namespace llvm; +#define DEBUG_TYPE "dce" -template -static bool RemoveUnusedDefs(ValueHolder &Vals, - DCEController DCEControl) { - bool Changed = false; - typedef ValueHolder Container; +STATISTIC(DIEEliminated, "Number of insts removed by DIE pass"); +STATISTIC(DCEEliminated, "Number of insts removed"); - 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++; +namespace { + //===--------------------------------------------------------------------===// + // DeadInstElimination pass implementation + // + struct DeadInstElimination : public BasicBlockPass { + static char ID; // Pass identification, replacement for typeid + DeadInstElimination() : BasicBlockPass(ID) { + initializeDeadInstEliminationPass(*PassRegistry::getPassRegistry()); + } + bool runOnBasicBlock(BasicBlock &BB) override { + if (skipOptnoneFunction(BB)) + return false; + auto *TLIP = getAnalysisIfAvailable(); + TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr; + 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; } - } - 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->getInstList().front(); - if (I->getInstType() != Instruction::PHINode) 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->getInstList().begin()); - - I = BB->getInstList().front(); - } while (I->getInstType() == Instruction::PHINode); - - 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; + void getAnalysisUsage(AnalysisUsage &AU) const override { + AU.setPreservesCFG(); + } + }; } -static void ReplaceUsesWithConstant(Instruction *I) { - // Get the method level constant pool - ConstantPool &CP = I->getParent()->getParent()->getConstantPool(); +char DeadInstElimination::ID = 0; +INITIALIZE_PASS(DeadInstElimination, "die", + "Dead Instruction Elimination", false, false) - 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); +Pass *llvm::createDeadInstEliminationPass() { + return new DeadInstElimination(); } -// 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::InstListType::iterator II(BB->getInstList().begin()); - for (; (*II)->getInstType() == Instruction::PHINode; ++II) { - PHINode *PN = (PHINode*)*II; - PN->removeIncomingValue(BB); +namespace { + //===--------------------------------------------------------------------===// + // DeadCodeElimination pass implementation + // + struct DCE : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + DCE() : FunctionPass(ID) { + initializeDCEPass(*PassRegistry::getPassRegistry()); + } - if (NukePHIs) { // Destroy the PHI altogether?? - assert(PN->getOperand(1) == 0 && "PHI node should only have one value!"); - Value *V = PN->getOperand(0); + bool runOnFunction(Function &F) override; - PN->replaceAllUsesWith(V); // Replace PHI node with its single value. - delete BB->getInstList().remove(II); + void getAnalysisUsage(AnalysisUsage &AU) const override { + AU.setPreservesCFG(); } - } + }; } -// 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(BB && Succ && *pred_begin(Succ) == BB && "BB is only pred of Succ" && - ++pred_begin(Succ) == pred_end(Succ)); +char DCE::ID = 0; +INITIALIZE_PASS(DCE, "dce", "Dead Code Elimination", false, false) + +static bool DCEInstruction(Instruction *I, + SmallSetVector &WorkList, + const TargetLibraryInfo *TLI) { + if (isInstructionTriviallyDead(I, TLI)) { + // Null out all of the instruction's operands to see if any operand becomes + // dead as we go. + for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) { + Value *OpV = I->getOperand(i); + I->setOperand(i, nullptr); + + if (!OpV->use_empty() || I == OpV) + continue; + + // If the operand is an instruction that became dead as we nulled out the + // operand, and if it is 'trivially' dead, delete it in a future loop + // iteration. + if (Instruction *OpI = dyn_cast(OpV)) + if (isInstructionTriviallyDead(OpI, TLI)) + WorkList.insert(OpI); + } - // 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 - pred_iterator PI(pred_begin(BB)); - for (; PI != pred_end(BB); ++PI) { - // TODO: + I->eraseFromParent(); + ++DCEEliminated; + return true; } + return false; } -static bool DoDCEPass(Method *M) { - Method::BasicBlocksType &BBs = M->getBasicBlocks(); - Method::BasicBlocksType::iterator BBIt, BBEnd = BBs.end(); - if (BBs.begin() == BBEnd) return false; // Nothing to do - bool Changed = false; - - // Loop through now and remove instructions that have no uses... - for (BBIt = BBs.begin(); BBIt != BBEnd; BBIt++) { - Changed |= RemoveUnusedDefs((*BBIt)->getInstList(), BasicBlockDCE()); - Changed |= RemoveSingularPHIs(*BBIt); +bool DCE::runOnFunction(Function &F) { + if (skipOptnoneFunction(F)) + return false; + + auto *TLIP = getAnalysisIfAvailable(); + TargetLibraryInfo *TLI = TLIP ? &TLIP->getTLI() : nullptr; + + bool MadeChange = false; + SmallSetVector WorkList; + // Iterate over the original function, only adding insts to the worklist + // if they actually need to be revisited. This avoids having to pre-init + // the worklist with the entire function's worth of instructions. + for (inst_iterator FI = inst_begin(F), FE = inst_end(F); FI != FE;) { + Instruction *I = &*FI; + ++FI; + + // We're visiting this instruction now, so make sure it's not in the + // worklist from an earlier visit. + if (!WorkList.count(I)) + MadeChange |= DCEInstruction(I, WorkList, TLI); } - // Loop over all of the basic blocks (except the first one) and remove them - // if they are unneeded... - // - for (BBIt = BBs.begin(), ++BBIt; BBIt != BBs.end(); ++BBIt) { - BasicBlock *BB = *BBIt; - assert(BB->getTerminator() && "Degenerate basic block encountered!"); - -#if 0 - // 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 (succ_iterator SI = succ_begin(BB), EI = succ_end(BB); SI != EI; ++SI) - RemovePredecessorFromBlock(*SI, BB); - - while (!BB->getInstList().empty()) { - Instruction *I = BB->getInstList().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->getInstList().begin()); - } - delete BBs.remove(BBIt); - --BBIt; // remove puts use on the next block, we want the previous one - Changed = true; - continue; - } - - // 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->getInstList().front(); - if (I->isTerminator()) { // Terminator is the only instruction! - - if (Succ->getInstList().front()->getInstType() == Instruction::PHINode){ - // Add entries to the PHI nodes so that the PHI nodes have the right - // number of entries... - PropogatePredecessorsForPHIs(BB, Succ); - } - - BasicBlock *Succ = *succ_begin(BB); // There is exactly one successor - BB->replaceAllUsesWith(Succ); - cerr << "Killing Trivial BB: \n" << BB; - - BB = BBs.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; - } - } -#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::InstListType::iterator DI = Pred->getInstList().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->getInstList().empty()) { - DI = BB->getInstList().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 = BBs.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"); - } - } + while (!WorkList.empty()) { + Instruction *I = WorkList.pop_back_val(); + MadeChange |= DCEInstruction(I, WorkList, TLI); } - - // 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; -}