X-Git-Url: http://plrg.eecs.uci.edu/git/?p=oota-llvm.git;a=blobdiff_plain;f=lib%2FTransforms%2FScalar%2FADCE.cpp;h=5770bb692c23196095a9040e1ecfef85e3af95d3;hp=062b52ff50c0e090e688854084d937fc684596a4;hb=551ccae044b0ff658fe629dd67edd5ffe75d10e8;hpb=5f0eb8da62308126d5b61e3eee5bee75b9dc5194 diff --git a/lib/Transforms/Scalar/ADCE.cpp b/lib/Transforms/Scalar/ADCE.cpp index 062b52ff50c..5770bb692c2 100644 --- a/lib/Transforms/Scalar/ADCE.cpp +++ b/lib/Transforms/Scalar/ADCE.cpp @@ -1,4 +1,11 @@ //===- ADCE.cpp - Code to perform aggressive 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 implements "aggressive" dead code elimination. ADCE is DCe where // values are assumed to be dead until proven otherwise. This is similar to @@ -7,26 +14,26 @@ //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar.h" -#include "llvm/Transforms/Utils/Local.h" -#include "llvm/Transforms/Utils/BasicBlockUtils.h" -#include "llvm/Type.h" -#include "llvm/Analysis/Dominators.h" -#include "llvm/iTerminators.h" -#include "llvm/iPHINode.h" #include "llvm/Constant.h" +#include "llvm/Instructions.h" +#include "llvm/Type.h" +#include "llvm/Analysis/AliasAnalysis.h" +#include "llvm/Analysis/PostDominators.h" #include "llvm/Support/CFG.h" -#include "Support/STLExtras.h" -#include "Support/DepthFirstIterator.h" -#include "Support/StatisticReporter.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/ADT/DepthFirstIterator.h" +#include "llvm/ADT/Statistic.h" +#include "llvm/ADT/STLExtras.h" #include -#include -using std::cerr; -using std::vector; - -static Statistic<> NumBlockRemoved("adce\t\t- Number of basic blocks removed"); -static Statistic<> NumInstRemoved ("adce\t\t- Number of instructions removed"); +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 @@ -55,6 +62,10 @@ public: // 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(); } @@ -77,40 +88,45 @@ private: // instructions that are dead according to LiveSet. bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB); + TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI); + inline void markInstructionLive(Instruction *I) { if (LiveSet.count(I)) return; - DEBUG(cerr << "Insn Live: " << I); + DEBUG(std::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()); + DEBUG(std::cerr << "Terminator Live: " << *BB->getTerminator()); + markInstructionLive(const_cast(BB->getTerminator())); } }; RegisterOpt X("adce", "Aggressive Dead Code Elimination"); } // End of anonymous namespace -Pass *createAggressiveDCEPass() { return new ADCE(); } +Pass *llvm::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... + // 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 dependant on... + // Get the blocks that this node is control dependent on... const PostDominanceFrontier::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); + // 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); } // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the @@ -121,15 +137,17 @@ bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) { for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; ) if (!LiveSet.count(I)) { // Is this instruction alive? I->dropAllReferences(); // Nope, drop references... - if (PHINode *PN = dyn_cast(&*I)) { + if (PHINode *PN = dyn_cast(I)) { // We don't want to leave PHI nodes in the program that have // #arguments != #predecessors, so we remove them now. // PN->replaceAllUsesWith(Constant::getNullValue(PN->getType())); - + // Delete the instruction... - I = BB->getInstList().erase(I); + ++I; + BB->getInstList().erase(PN); Changed = true; + ++NumInstRemoved; } else { ++I; } @@ -140,37 +158,114 @@ bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) { } +/// 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(""); + Instruction *NewCall = new CallInst(F, Args, Name, II); + 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. // - for (df_iterator BBI = df_begin(Func), BBE = df_end(Func); - BBI != BBE; ++BBI) { + 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; ) { - if (II->hasSideEffects() || II->getOpcode() == Instruction::Ret) { - markInstructionLive(II); - ++II; // Increment the inst iterator if the inst wasn't deleted - } else if (isInstructionTriviallyDead(II)) { + 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)) { + markInstructionLive(I); + } else if (isInstructionTriviallyDead(I)) { // Remove the instruction from it's basic block... - II = BB->getInstList().erase(II); + BB->getInstList().erase(I); ++NumInstRemoved; - MadeChanges = true; - } else { - ++II; // Increment the inst iterator if the inst wasn't deleted } } } - DEBUG(cerr << "Processing work list\n"); + // 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; + } + + // 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) + for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI) + markInstructionLive((*PI)->getTerminator()); + + + + DEBUG(std::cerr << "Processing work list\n"); // AliveBlocks - Set of basic blocks that we know have instructions that are // alive in them... @@ -178,7 +273,7 @@ bool ADCE::doADCE() { 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 + // became live, then that means that all of their operands are necessary as // well... make them live as well. // while (!WorkList.empty()) { @@ -186,6 +281,7 @@ bool ADCE::doADCE() { WorkList.pop_back(); BasicBlock *BB = I->getParent(); + if (!ReachableBBs.count(BB)) continue; if (!AliveBlocks.count(BB)) { // Basic block not alive yet... AliveBlocks.insert(BB); // Block is now ALIVE! markBlockAlive(BB); // Make it so now! @@ -210,36 +306,52 @@ bool ADCE::doADCE() { markInstructionLive(Operand); } - if (DebugFlag) { - cerr << "Current Function: X = Live\n"; - for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) + DEBUG( + std::cerr << "Current Function: X = Live\n"; + for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){ + std::cerr << 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)) cerr << "X "; - cerr << *BI; + if (LiveSet.count(BI)) std::cerr << "X "; + std::cerr << *BI; } - } + }); // Find the first postdominator of the entry node that is alive. Make it the // new entry node... // - PostDominatorTree &DT = getAnalysis(); - - if (AliveBlocks.size() == Func->size()) { // No dead blocks? - for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) + for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) { // Loop over all of the instructions in the function, 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. + // instructions still referring to them. // dropReferencesOfDeadInstructionsInLiveBlock(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 a conditional branch. + // + TerminatorInst *TI = I->getTerminator(); + if (!LiveSet.count(TI)) + convertToUnconditionalBranch(TI); + } } else { // If there are some blocks dead... - // 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->getBasicBlockList().push_front(NewEntry); - AliveBlocks.insert(NewEntry); // This block is always alive! + // 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 + } // 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 @@ -251,6 +363,14 @@ bool ADCE::doADCE() { 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... @@ -274,9 +394,9 @@ bool ADCE::doADCE() { if (LastNode == 0) { // No postdominator! // Call RemoveSuccessor to transmogrify the terminator instruction // to not contain the outgoing branch, or to create a new - // terminator if the form fundementally changes (ie unconditional - // branch to return). Note that this will change a branch into an - // infinite loop into a return instruction! + // 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); @@ -290,14 +410,14 @@ bool ADCE::doADCE() { } else { PostDominatorTree::Node *NextNode = LastNode->getIDom(); - while (!AliveBlocks.count(NextNode->getNode())) { + while (!AliveBlocks.count(NextNode->getBlock())) { LastNode = NextNode; NextNode = NextNode->getIDom(); } // Get the basic blocks that we need... - BasicBlock *LastDead = LastNode->getNode(); - BasicBlock *NextAlive = NextNode->getNode(); + BasicBlock *LastDead = LastNode->getBlock(); + BasicBlock *NextAlive = NextNode->getBlock(); // Make the conditional branch now go to the next alive block... TI->getSuccessor(i)->removePredecessor(BB); @@ -308,35 +428,43 @@ bool ADCE::doADCE() { // 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); - } + PHINode *PN = dyn_cast(II); ++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, 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. + // other dead instructions still referring to them. // dropReferencesOfDeadInstructionsInLiveBlock(BB); } } - // Loop over all of the basic blocks in the function, dropping references of - // the dead basic blocks + // We make changes if there are any dead blocks in the function... + if (unsigned NumDeadBlocks = Func->size() - AliveBlocks.size()) { + MadeChanges = true; + NumBlockRemoved += NumDeadBlocks; + } + + // Loop over all of the basic blocks in the function, removing control flow + // edges to live blocks (also eliminating any entries in PHI functions in + // referenced blocks). // - for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) { + for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) if (!AliveBlocks.count(BB)) { // Remove all outgoing edges from this basic block and convert the // terminator into a return instruction. - vector Succs(succ_begin(BB), succ_end(BB)); + std::vector Succs(succ_begin(BB), succ_end(BB)); if (!Succs.empty()) { // Loop over all of the successors, removing this block from PHI node @@ -347,18 +475,25 @@ bool ADCE::doADCE() { } // Delete the old terminator instruction... + const Type *TermTy = BB->getTerminator()->getType(); + if (TermTy != Type::VoidTy) + BB->getTerminator()->replaceAllUsesWith( + Constant::getNullValue(TermTy)); BB->getInstList().pop_back(); const Type *RetTy = Func->getReturnType(); - Instruction *New = new ReturnInst(RetTy != Type::VoidTy ? - Constant::getNullValue(RetTy) : 0); - BB->getInstList().push_back(New); + new ReturnInst(RetTy != Type::VoidTy ? + Constant::getNullValue(RetTy) : 0, 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... + // + for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) + if (!AliveBlocks.count(BB)) BB->dropAllReferences(); - ++NumBlockRemoved; - MadeChanges = true; - } - } // 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 @@ -372,8 +507,11 @@ bool ADCE::doADCE() { for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); ) if (!LiveSet.count(II)) { // Is this instruction alive? // Nope... remove the instruction from it's basic block... + if (isa(II)) + ++NumCallRemoved; + else + ++NumInstRemoved; II = BI->getInstList().erase(II); - ++NumInstRemoved; MadeChanges = true; } else { ++II;