X-Git-Url: http://plrg.eecs.uci.edu/git/?p=oota-llvm.git;a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FADCE.cpp;h=590a52da6b192665f4d1574cf80063aa5312027b;hp=76a09f80e48984d9f9f94c5b31c2e8710230667c;hb=2027fcbfdaea79a5486db80a4da7407f50f7f4ec;hpb=b7427031372337e6d67f9573ec6c722ab5ea913e diff --git a/lib/Transforms/Scalar/ADCE.cpp b/lib/Transforms/Scalar/ADCE.cpp index 76a09f80e48..590a52da6b1 100644 --- a/lib/Transforms/Scalar/ADCE.cpp +++ b/lib/Transforms/Scalar/ADCE.cpp @@ -1,490 +1,108 @@ -//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===// +//===- ADCE.cpp - Code to perform dead code elimination -------------------===// // // The LLVM Compiler Infrastructure // -// This file was developed by the LLVM research group and is distributed under -// the University of Illinois Open Source License. See LICENSE.TXT for details. +// This file is distributed under the University of Illinois Open Source +// License. See LICENSE.TXT for details. // //===----------------------------------------------------------------------===// // -// 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. +// 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. // //===----------------------------------------------------------------------===// -#include "llvm/Transforms/Scalar.h" -#include "llvm/Constants.h" -#include "llvm/Instructions.h" -#include "llvm/Analysis/AliasAnalysis.h" -#include "llvm/Analysis/PostDominators.h" -#include "llvm/Support/CFG.h" -#include "llvm/Transforms/Utils/BasicBlockUtils.h" -#include "llvm/Transforms/Utils/Local.h" -#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h" -#include "llvm/Support/Debug.h" +#include "llvm/Transforms/Scalar/ADCE.h" #include "llvm/ADT/DepthFirstIterator.h" +#include "llvm/ADT/SmallPtrSet.h" +#include "llvm/ADT/SmallVector.h" #include "llvm/ADT/Statistic.h" -#include "llvm/ADT/STLExtras.h" -#include +#include "llvm/Analysis/GlobalsModRef.h" +#include "llvm/IR/BasicBlock.h" +#include "llvm/IR/CFG.h" +#include "llvm/IR/InstIterator.h" +#include "llvm/IR/Instructions.h" +#include "llvm/IR/IntrinsicInst.h" +#include "llvm/Pass.h" +#include "llvm/Transforms/Scalar.h" using namespace llvm; -namespace { - Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed"); - Statistic<> NumInstRemoved ("adce", "Number of instructions removed"); - Statistic<> NumCallRemoved ("adce", "Number of calls and invokes removed"); - -//===----------------------------------------------------------------------===// -// 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 { - // We require that all function nodes are unified, because otherwise code - // can be marked live that wouldn't necessarily be otherwise. - AU.addRequired(); - AU.addRequired(); - 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); - - - // deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in - // the specified basic block, deleting ones that are dead according to - // LiveSet. - bool deleteDeadInstructionsInLiveBlock(BasicBlock *BB); - - TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI); - - inline void markInstructionLive(Instruction *I) { - if (!LiveSet.insert(I).second) return; - DOUT << "Insn Live: " << *I; - WorkList.push_back(I); - } - - inline void markTerminatorLive(const BasicBlock *BB) { - DOUT << "Terminator Live: " << *BB->getTerminator(); - markInstructionLive(const_cast(BB->getTerminator())); - } -}; +#define DEBUG_TYPE "adce" - RegisterPass X("adce", "Aggressive Dead Code Elimination"); -} // End of anonymous namespace +STATISTIC(NumRemoved, "Number of instructions removed"); -FunctionPass *llvm::createAggressiveDCEPass() { return new ADCE(); } +static bool aggressiveDCE(Function& F) { + SmallPtrSet Alive; + SmallVector Worklist; -void ADCE::markBlockAlive(BasicBlock *BB) { - // Mark the basic block as being newly ALIVE... and mark all branches that - // this block is control dependent 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 dependent on... - const PostDominanceFrontier::DomSetType &CDB = It->second; - for (PostDominanceFrontier::DomSetType::const_iterator I = - CDB.begin(), E = CDB.end(); I != E; ++I) - markTerminatorLive(*I); // Mark all their terminators as live + // Collect the set of "root" instructions that are known live. + for (Instruction &I : instructions(F)) { + if (isa(I) || isa(I) || I.isEHPad() || + I.mayHaveSideEffects()) { + Alive.insert(&I); + Worklist.push_back(&I); + } } - // If this basic block is live, and it ends in an unconditional branch, then - // the branch is alive as well... - if (BranchInst *BI = dyn_cast(BB->getTerminator())) - if (BI->isUnconditional()) - markTerminatorLive(BB); -} - -// deleteDeadInstructionsInLiveBlock - Loop over all of the instructions in the -// specified basic block, deleting ones that are dead according to LiveSet. -bool ADCE::deleteDeadInstructionsInLiveBlock(BasicBlock *BB) { - bool Changed = false; - for (BasicBlock::iterator II = BB->begin(), E = --BB->end(); II != E; ) { - Instruction *I = II++; - if (!LiveSet.count(I)) { // Is this instruction alive? - if (!I->use_empty()) - I->replaceAllUsesWith(UndefValue::get(I->getType())); - - // Nope... remove the instruction from it's basic block... - if (isa(I)) - ++NumCallRemoved; - else - ++NumInstRemoved; - BB->getInstList().erase(I); - Changed = true; + // Propagate liveness backwards to operands. + while (!Worklist.empty()) { + Instruction *Curr = Worklist.pop_back_val(); + for (Use &OI : Curr->operands()) { + if (Instruction *Inst = dyn_cast(OI)) + if (Alive.insert(Inst).second) + Worklist.push_back(Inst); } } - return Changed; -} - - -/// convertToUnconditionalBranch - Transform this conditional terminator -/// instruction into an unconditional branch because we don't care which of the -/// successors it goes to. This eliminate a use of the condition as well. -/// -TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) { - BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI); - BasicBlock *BB = TI->getParent(); - - // Remove entries from PHI nodes to avoid confusing ourself later... - for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i) - TI->getSuccessor(i)->removePredecessor(BB); - - // Delete the old branch itself... - BB->getInstList().erase(TI); - return NB; -} - - -// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning -// true if the function was modified. -// -bool ADCE::doADCE() { - bool MadeChanges = false; - - AliasAnalysis &AA = getAnalysis(); - - // Iterate over all invokes in the function, turning invokes into calls if - // they cannot throw. - for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) - if (InvokeInst *II = dyn_cast(BB->getTerminator())) - if (Function *F = II->getCalledFunction()) - if (AA.onlyReadsMemory(F)) { - // The function cannot unwind. Convert it to a call with a branch - // after it to the normal destination. - std::vector Args(II->op_begin()+3, II->op_end()); - std::string Name = II->getName(); II->setName(""); - CallInst *NewCall = new CallInst(F, Args, Name, II); - NewCall->setCallingConv(II->getCallingConv()); - II->replaceAllUsesWith(NewCall); - new BranchInst(II->getNormalDest(), II); - - // Update PHI nodes in the unwind destination - II->getUnwindDest()->removePredecessor(BB); - BB->getInstList().erase(II); - - if (NewCall->use_empty()) { - BB->getInstList().erase(NewCall); - ++NumCallRemoved; - } - } - - // 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. - // - std::set ReachableBBs; - for (df_ext_iterator - BBI = df_ext_begin(&Func->front(), ReachableBBs), - BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) { - BasicBlock *BB = *BBI; - for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) { - Instruction *I = II++; - if (CallInst *CI = dyn_cast(I)) { - Function *F = CI->getCalledFunction(); - if (F && AA.onlyReadsMemory(F)) { - if (CI->use_empty()) { - BB->getInstList().erase(CI); - ++NumCallRemoved; - } - } else { - markInstructionLive(I); - } - } else if (I->mayWriteToMemory() || isa(I) || - isa(I) || isa(I)) { - // FIXME: Unreachable instructions should not be marked intrinsically - // live here. - markInstructionLive(I); - } else if (isInstructionTriviallyDead(I)) { - // Remove the instruction from it's basic block... - BB->getInstList().erase(I); - ++NumInstRemoved; - } + // 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 (Instruction &I : instructions(F)) { + if (!Alive.count(&I)) { + Worklist.push_back(&I); + I.dropAllReferences(); } } - // Check to ensure we have an exit node for this CFG. If we don't, we won't - // have any post-dominance information, thus we cannot perform our - // transformations safely. - // - PostDominatorTree &DT = getAnalysis(); - if (DT[&Func->getEntryBlock()] == 0) { - WorkList.clear(); - return MadeChanges; + for (Instruction *&I : Worklist) { + ++NumRemoved; + I->eraseFromParent(); } - // Scan the function marking blocks without post-dominance information as - // live. Blocks without post-dominance information occur when there is an - // infinite loop in the program. Because the infinite loop could contain a - // function which unwinds, exits or has side-effects, we don't want to delete - // the infinite loop or those blocks leading up to it. - for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) - if (DT[I] == 0 && ReachableBBs.count(I)) - for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI) - markInstructionLive((*PI)->getTerminator()); - - DOUT << "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 necessary as - // well... make them live as well. - // - while (!WorkList.empty()) { - Instruction *I = WorkList.back(); // Get an instruction that became live... - WorkList.pop_back(); + return !Worklist.empty(); +} - BasicBlock *BB = I->getParent(); - if (!ReachableBBs.count(BB)) continue; - if (AliveBlocks.insert(BB).second) // Basic block not alive yet. - markBlockAlive(BB); // Make it so now! +PreservedAnalyses ADCEPass::run(Function &F) { + if (aggressiveDCE(F)) + return PreservedAnalyses::none(); + return PreservedAnalyses::all(); +} - // 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 (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) { - // If the incoming edge is clearly dead, it won't have control - // dependence information. Do not mark it live. - BasicBlock *PredBB = PN->getIncomingBlock(i); - if (ReachableBBs.count(PredBB)) { - // FIXME: This should mark the control dependent edge as live, not - // necessarily the predecessor itself! - if (AliveBlocks.insert(PredBB).second) - markBlockAlive(PN->getIncomingBlock(i)); // Block is newly ALIVE! - if (Instruction *Op = dyn_cast(PN->getIncomingValue(i))) - markInstructionLive(Op); - } - } - } else { - // 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); - } +namespace { +struct ADCELegacyPass : public FunctionPass { + static char ID; // Pass identification, replacement for typeid + ADCELegacyPass() : FunctionPass(ID) { + initializeADCELegacyPassPass(*PassRegistry::getPassRegistry()); } - DEBUG( - DOUT << "Current Function: X = Live\n"; - for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){ - DOUT << I->getName() << ":\t" - << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n"); - for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){ - if (LiveSet.count(BI)) DOUT << "X "; - DOUT << *BI; - } - }); - - // All blocks being live is a common case, handle it specially. - 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 deleting instructions - // to drop their references. - deleteDeadInstructionsInLiveBlock(I); - - // Check to make sure the terminator instruction is live. If it isn't, - // this means that the condition that it branches on (we know it is not an - // unconditional branch), is not needed to make the decision of where to - // go to, because all outgoing edges go to the same place. We must remove - // the use of the condition (because it's probably dead), so we convert - // the terminator to an unconditional branch. - // - TerminatorInst *TI = I->getTerminator(); - if (!LiveSet.count(TI)) - convertToUnconditionalBranch(TI); - } - - return MadeChanges; + bool runOnFunction(Function& F) override { + if (skipOptnoneFunction(F)) + return false; + return aggressiveDCE(F); } - - // 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(); - new BranchInst(&Func->front(), NewEntry); - Func->getBasicBlockList().push_front(NewEntry); - AliveBlocks.insert(NewEntry); // This block is always alive! - LiveSet.insert(NewEntry->getTerminator()); // The branch is live + void getAnalysisUsage(AnalysisUsage& AU) const override { + AU.setPreservesCFG(); + AU.addPreserved(); } +}; +} - // 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(); - - // If the terminator instruction is alive, but the block it is contained - // in IS alive, this means that this terminator is a conditional branch on - // a condition that doesn't matter. Make it an unconditional branch to - // ONE of the successors. This has the side effect of dropping a use of - // the conditional value, which may also be dead. - if (!LiveSet.count(TI)) - TI = convertToUnconditionalBranch(TI); - - // 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)]; - PostDominatorTree::Node *NextNode = 0; - - if (LastNode) { - NextNode = LastNode->getIDom(); - while (!AliveBlocks.count(NextNode->getBlock())) { - LastNode = NextNode; - NextNode = NextNode->getIDom(); - if (NextNode == 0) { - LastNode = 0; - break; - } - } - } - - // There is a special case here... if there IS no post-dominator for - // the block we have nowhere 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! - if (!isa(TI)) { - // Call RemoveSuccessor to transmogrify the terminator instruction - // to not contain the outgoing branch, or to create a new - // terminator if the form fundamentally changes (i.e., - // 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. - // - TI = BB->getTerminator(); - - // Rescan this successor... - --i; - } else { - - } - } else { - // Get the basic blocks that we need... - BasicBlock *LastDead = LastNode->getBlock(); - BasicBlock *NextAlive = NextNode->getBlock(); - - // 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(); - isa(II); ++II) { - PHINode *PN = cast(II); - if (LiveSet.count(PN)) { // Only modify live phi nodes - // 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, deleting - // dead instructions. This is so that the next sweep over the program - // can safely delete dead instructions without other dead instructions - // still referring to them. - // - deleteDeadInstructionsInLiveBlock(BB); - } - - // Loop over all of the basic blocks in the function, dropping references of - // the dead basic blocks. We must do this after the previous step to avoid - // dropping references to PHIs which still have entries... - // - std::vector DeadBlocks; - for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) - if (!AliveBlocks.count(BB)) { - // Remove PHI node entries for this block in live successor blocks. - for (succ_iterator SI = succ_begin(BB), E = succ_end(BB); SI != E; ++SI) - if (!SI->empty() && isa(SI->front()) && AliveBlocks.count(*SI)) - (*SI)->removePredecessor(BB); - - BB->dropAllReferences(); - MadeChanges = true; - DeadBlocks.push_back(BB); - } - - NumBlockRemoved += DeadBlocks.size(); - - // 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). - for (std::vector::iterator I = DeadBlocks.begin(), - E = DeadBlocks.end(); I != E; ++I) - Func->getBasicBlockList().erase(*I); +char ADCELegacyPass::ID = 0; +INITIALIZE_PASS(ADCELegacyPass, "adce", "Aggressive Dead Code Elimination", + false, false) - return MadeChanges; -} +FunctionPass *llvm::createAggressiveDCEPass() { return new ADCELegacyPass(); }