LLVM API Documentation

Main Page | Namespace List | Class Hierarchy | Alphabetical List | Class List | Directories | File List | Namespace Members | Class Members | File Members | Related Pages

ADCE.cpp

Go to the documentation of this file.
00001 //===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
00002 // 
00003 //                     The LLVM Compiler Infrastructure
00004 //
00005 // This file was developed by the LLVM research group and is distributed under
00006 // the University of Illinois Open Source License. See LICENSE.TXT for details.
00007 // 
00008 //===----------------------------------------------------------------------===//
00009 //
00010 // This file implements "aggressive" dead code elimination.  ADCE is DCe where
00011 // values are assumed to be dead until proven otherwise.  This is similar to 
00012 // SCCP, except applied to the liveness of values.
00013 //
00014 //===----------------------------------------------------------------------===//
00015 
00016 #include "llvm/Transforms/Scalar.h"
00017 #include "llvm/Constant.h"
00018 #include "llvm/Instructions.h"
00019 #include "llvm/Type.h"
00020 #include "llvm/Analysis/AliasAnalysis.h"
00021 #include "llvm/Analysis/PostDominators.h"
00022 #include "llvm/Support/CFG.h"
00023 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
00024 #include "llvm/Transforms/Utils/Local.h"
00025 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
00026 #include "llvm/Support/Debug.h"
00027 #include "llvm/ADT/DepthFirstIterator.h"
00028 #include "llvm/ADT/Statistic.h"
00029 #include "llvm/ADT/STLExtras.h"
00030 #include <algorithm>
00031 using namespace llvm;
00032 
00033 namespace {
00034   Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed");
00035   Statistic<> NumInstRemoved ("adce", "Number of instructions removed");
00036   Statistic<> NumCallRemoved ("adce", "Number of calls and invokes removed");
00037 
00038 //===----------------------------------------------------------------------===//
00039 // ADCE Class
00040 //
00041 // This class does all of the work of Aggressive Dead Code Elimination.
00042 // It's public interface consists of a constructor and a doADCE() method.
00043 //
00044 class ADCE : public FunctionPass {
00045   Function *Func;                       // The function that we are working on
00046   std::vector<Instruction*> WorkList;   // Instructions that just became live
00047   std::set<Instruction*>    LiveSet;    // The set of live instructions
00048 
00049   //===--------------------------------------------------------------------===//
00050   // The public interface for this class
00051   //
00052 public:
00053   // Execute the Aggressive Dead Code Elimination Algorithm
00054   //
00055   virtual bool runOnFunction(Function &F) {
00056     Func = &F;
00057     bool Changed = doADCE();
00058     assert(WorkList.empty());
00059     LiveSet.clear();
00060     return Changed;
00061   }
00062   // getAnalysisUsage - We require post dominance frontiers (aka Control
00063   // Dependence Graph)
00064   virtual void getAnalysisUsage(AnalysisUsage &AU) const {
00065     // We require that all function nodes are unified, because otherwise code
00066     // can be marked live that wouldn't necessarily be otherwise.
00067     AU.addRequired<UnifyFunctionExitNodes>();
00068     AU.addRequired<AliasAnalysis>();
00069     AU.addRequired<PostDominatorTree>();
00070     AU.addRequired<PostDominanceFrontier>();
00071   }
00072 
00073 
00074   //===--------------------------------------------------------------------===//
00075   // The implementation of this class
00076   //
00077 private:
00078   // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
00079   // true if the function was modified.
00080   //
00081   bool doADCE();
00082 
00083   void markBlockAlive(BasicBlock *BB);
00084 
00085 
00086   // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
00087   // instructions in the specified basic block, dropping references on
00088   // instructions that are dead according to LiveSet.
00089   bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB);
00090 
00091   TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
00092 
00093   inline void markInstructionLive(Instruction *I) {
00094     if (LiveSet.count(I)) return;
00095     DEBUG(std::cerr << "Insn Live: " << *I);
00096     LiveSet.insert(I);
00097     WorkList.push_back(I);
00098   }
00099 
00100   inline void markTerminatorLive(const BasicBlock *BB) {
00101     DEBUG(std::cerr << "Terminator Live: " << *BB->getTerminator());
00102     markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
00103   }
00104 };
00105 
00106   RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
00107 } // End of anonymous namespace
00108 
00109 FunctionPass *llvm::createAggressiveDCEPass() { return new ADCE(); }
00110 
00111 void ADCE::markBlockAlive(BasicBlock *BB) {
00112   // Mark the basic block as being newly ALIVE... and mark all branches that
00113   // this block is control dependent on as being alive also...
00114   //
00115   PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
00116 
00117   PostDominanceFrontier::const_iterator It = CDG.find(BB);
00118   if (It != CDG.end()) {
00119     // Get the blocks that this node is control dependent on...
00120     const PostDominanceFrontier::DomSetType &CDB = It->second;
00121     for_each(CDB.begin(), CDB.end(),   // Mark all their terminators as live
00122              bind_obj(this, &ADCE::markTerminatorLive));
00123   }
00124   
00125   // If this basic block is live, and it ends in an unconditional branch, then
00126   // the branch is alive as well...
00127   if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
00128     if (BI->isUnconditional())
00129       markTerminatorLive(BB);
00130 }
00131 
00132 // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
00133 // instructions in the specified basic block, dropping references on
00134 // instructions that are dead according to LiveSet.
00135 bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) {
00136   bool Changed = false;
00137   for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; )
00138     if (!LiveSet.count(I)) {              // Is this instruction alive?
00139       I->dropAllReferences();             // Nope, drop references... 
00140       if (PHINode *PN = dyn_cast<PHINode>(I)) {
00141         // We don't want to leave PHI nodes in the program that have
00142         // #arguments != #predecessors, so we remove them now.
00143         //
00144         PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
00145 
00146         // Delete the instruction...
00147         ++I;
00148         BB->getInstList().erase(PN);
00149         Changed = true;
00150         ++NumInstRemoved;
00151       } else {
00152         ++I;
00153       }
00154     } else {
00155       ++I;
00156     }
00157   return Changed;
00158 }
00159 
00160 
00161 /// convertToUnconditionalBranch - Transform this conditional terminator
00162 /// instruction into an unconditional branch because we don't care which of the
00163 /// successors it goes to.  This eliminate a use of the condition as well.
00164 ///
00165 TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
00166   BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
00167   BasicBlock *BB = TI->getParent();
00168 
00169   // Remove entries from PHI nodes to avoid confusing ourself later...
00170   for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
00171     TI->getSuccessor(i)->removePredecessor(BB);
00172   
00173   // Delete the old branch itself...
00174   BB->getInstList().erase(TI);
00175   return NB;
00176 }
00177 
00178 
00179 // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
00180 // true if the function was modified.
00181 //
00182 bool ADCE::doADCE() {
00183   bool MadeChanges = false;
00184 
00185   AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
00186 
00187 
00188   // Iterate over all invokes in the function, turning invokes into calls if
00189   // they cannot throw.
00190   for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
00191     if (InvokeInst *II = dyn_cast<InvokeInst>(BB->getTerminator()))
00192       if (Function *F = II->getCalledFunction())
00193         if (AA.onlyReadsMemory(F)) {
00194           // The function cannot unwind.  Convert it to a call with a branch
00195           // after it to the normal destination.
00196           std::vector<Value*> Args(II->op_begin()+3, II->op_end());
00197           std::string Name = II->getName(); II->setName("");
00198           Instruction *NewCall = new CallInst(F, Args, Name, II);
00199           II->replaceAllUsesWith(NewCall);
00200           new BranchInst(II->getNormalDest(), II);
00201 
00202           // Update PHI nodes in the unwind destination
00203           II->getUnwindDest()->removePredecessor(BB);
00204           BB->getInstList().erase(II);
00205 
00206           if (NewCall->use_empty()) {
00207             BB->getInstList().erase(NewCall);
00208             ++NumCallRemoved;
00209           }
00210         }
00211 
00212   // Iterate over all of the instructions in the function, eliminating trivially
00213   // dead instructions, and marking instructions live that are known to be 
00214   // needed.  Perform the walk in depth first order so that we avoid marking any
00215   // instructions live in basic blocks that are unreachable.  These blocks will
00216   // be eliminated later, along with the instructions inside.
00217   //
00218   std::set<BasicBlock*> ReachableBBs;
00219   for (df_ext_iterator<BasicBlock*>
00220          BBI = df_ext_begin(&Func->front(), ReachableBBs),
00221          BBE = df_ext_end(&Func->front(), ReachableBBs); BBI != BBE; ++BBI) {
00222     BasicBlock *BB = *BBI;
00223     for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
00224       Instruction *I = II++;
00225       if (CallInst *CI = dyn_cast<CallInst>(I)) {
00226         Function *F = CI->getCalledFunction();
00227         if (F && AA.onlyReadsMemory(F)) {
00228           if (CI->use_empty()) {
00229             BB->getInstList().erase(CI);
00230             ++NumCallRemoved;
00231           }
00232         } else {
00233           markInstructionLive(I);
00234         }
00235       } else if (I->mayWriteToMemory() || isa<ReturnInst>(I) ||
00236                  isa<UnwindInst>(I) || isa<UnreachableInst>(I)) {
00237         // FIXME: Unreachable instructions should not be marked intrinsically
00238         // live here.
00239   markInstructionLive(I);
00240       } else if (isInstructionTriviallyDead(I)) {
00241         // Remove the instruction from it's basic block...
00242         BB->getInstList().erase(I);
00243         ++NumInstRemoved;
00244       }
00245     }
00246   }
00247 
00248   // Check to ensure we have an exit node for this CFG.  If we don't, we won't
00249   // have any post-dominance information, thus we cannot perform our
00250   // transformations safely.
00251   //
00252   PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
00253   if (DT[&Func->getEntryBlock()] == 0) {
00254     WorkList.clear();
00255     return MadeChanges;
00256   }
00257 
00258   // Scan the function marking blocks without post-dominance information as
00259   // live.  Blocks without post-dominance information occur when there is an
00260   // infinite loop in the program.  Because the infinite loop could contain a
00261   // function which unwinds, exits or has side-effects, we don't want to delete
00262   // the infinite loop or those blocks leading up to it.
00263   for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
00264     if (DT[I] == 0)
00265       for (pred_iterator PI = pred_begin(I), E = pred_end(I); PI != E; ++PI)
00266         markInstructionLive((*PI)->getTerminator());
00267 
00268 
00269 
00270   DEBUG(std::cerr << "Processing work list\n");
00271 
00272   // AliveBlocks - Set of basic blocks that we know have instructions that are
00273   // alive in them...
00274   //
00275   std::set<BasicBlock*> AliveBlocks;
00276 
00277   // Process the work list of instructions that just became live... if they
00278   // became live, then that means that all of their operands are necessary as
00279   // well... make them live as well.
00280   //
00281   while (!WorkList.empty()) {
00282     Instruction *I = WorkList.back(); // Get an instruction that became live...
00283     WorkList.pop_back();
00284 
00285     BasicBlock *BB = I->getParent();
00286     if (!ReachableBBs.count(BB)) continue;
00287     if (!AliveBlocks.count(BB)) {     // Basic block not alive yet...
00288       AliveBlocks.insert(BB);         // Block is now ALIVE!
00289       markBlockAlive(BB);             // Make it so now!
00290     }
00291 
00292     // PHI nodes are a special case, because the incoming values are actually
00293     // defined in the predecessor nodes of this block, meaning that the PHI
00294     // makes the predecessors alive.
00295     //
00296     if (PHINode *PN = dyn_cast<PHINode>(I))
00297       for (pred_iterator PI = pred_begin(BB), PE = pred_end(BB); PI != PE; ++PI)
00298         if (!AliveBlocks.count(*PI)) {
00299           AliveBlocks.insert(BB);         // Block is now ALIVE!
00300           markBlockAlive(*PI);
00301         }
00302 
00303     // Loop over all of the operands of the live instruction, making sure that
00304     // they are known to be alive as well...
00305     //
00306     for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
00307       if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
00308   markInstructionLive(Operand);
00309   }
00310 
00311   DEBUG(
00312     std::cerr << "Current Function: X = Live\n";
00313     for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
00314       std::cerr << I->getName() << ":\t"
00315                 << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
00316       for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
00317         if (LiveSet.count(BI)) std::cerr << "X ";
00318         std::cerr << *BI;
00319       }
00320     });
00321 
00322   // Find the first postdominator of the entry node that is alive.  Make it the
00323   // new entry node...
00324   //
00325   if (AliveBlocks.size() == Func->size()) {  // No dead blocks?
00326     for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
00327       // Loop over all of the instructions in the function, telling dead
00328       // instructions to drop their references.  This is so that the next sweep
00329       // over the program can safely delete dead instructions without other dead
00330       // instructions still referring to them.
00331       //
00332       dropReferencesOfDeadInstructionsInLiveBlock(I);
00333 
00334       // Check to make sure the terminator instruction is live.  If it isn't,
00335       // this means that the condition that it branches on (we know it is not an
00336       // unconditional branch), is not needed to make the decision of where to
00337       // go to, because all outgoing edges go to the same place.  We must remove
00338       // the use of the condition (because it's probably dead), so we convert
00339       // the terminator to a conditional branch.
00340       //
00341       TerminatorInst *TI = I->getTerminator();
00342       if (!LiveSet.count(TI))
00343         convertToUnconditionalBranch(TI);
00344     }
00345     
00346   } else {                                   // If there are some blocks dead...
00347     // If the entry node is dead, insert a new entry node to eliminate the entry
00348     // node as a special case.
00349     //
00350     if (!AliveBlocks.count(&Func->front())) {
00351       BasicBlock *NewEntry = new BasicBlock();
00352       new BranchInst(&Func->front(), NewEntry);
00353       Func->getBasicBlockList().push_front(NewEntry);
00354       AliveBlocks.insert(NewEntry);    // This block is always alive!
00355       LiveSet.insert(NewEntry->getTerminator());  // The branch is live
00356     }
00357     
00358     // Loop over all of the alive blocks in the function.  If any successor
00359     // blocks are not alive, we adjust the outgoing branches to branch to the
00360     // first live postdominator of the live block, adjusting any PHI nodes in
00361     // the block to reflect this.
00362     //
00363     for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
00364       if (AliveBlocks.count(I)) {
00365         BasicBlock *BB = I;
00366         TerminatorInst *TI = BB->getTerminator();
00367       
00368         // If the terminator instruction is alive, but the block it is contained
00369         // in IS alive, this means that this terminator is a conditional branch
00370         // on a condition that doesn't matter.  Make it an unconditional branch
00371         // to ONE of the successors.  This has the side effect of dropping a use
00372         // of the conditional value, which may also be dead.
00373         if (!LiveSet.count(TI))
00374           TI = convertToUnconditionalBranch(TI);
00375 
00376         // Loop over all of the successors, looking for ones that are not alive.
00377         // We cannot save the number of successors in the terminator instruction
00378         // here because we may remove them if we don't have a postdominator...
00379         //
00380         for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
00381           if (!AliveBlocks.count(TI->getSuccessor(i))) {
00382             // Scan up the postdominator tree, looking for the first
00383             // postdominator that is alive, and the last postdominator that is
00384             // dead...
00385             //
00386             PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
00387 
00388             // There is a special case here... if there IS no post-dominator for
00389             // the block we have no owhere to point our branch to.  Instead,
00390             // convert it to a return.  This can only happen if the code
00391             // branched into an infinite loop.  Note that this may not be
00392             // desirable, because we _are_ altering the behavior of the code.
00393             // This is a well known drawback of ADCE, so in the future if we
00394             // choose to revisit the decision, this is where it should be.
00395             //
00396             if (LastNode == 0) {        // No postdominator!
00397               // Call RemoveSuccessor to transmogrify the terminator instruction
00398               // to not contain the outgoing branch, or to create a new
00399               // terminator if the form fundamentally changes (i.e.,
00400               // unconditional branch to return).  Note that this will change a
00401               // branch into an infinite loop into a return instruction!
00402               //
00403               RemoveSuccessor(TI, i);
00404 
00405               // RemoveSuccessor may replace TI... make sure we have a fresh
00406               // pointer... and e variable.
00407               //
00408               TI = BB->getTerminator();
00409 
00410               // Rescan this successor...
00411               --i;
00412             } else {
00413               PostDominatorTree::Node *NextNode = LastNode->getIDom();
00414 
00415               while (!AliveBlocks.count(NextNode->getBlock())) {
00416                 LastNode = NextNode;
00417                 NextNode = NextNode->getIDom();
00418               }
00419             
00420               // Get the basic blocks that we need...
00421               BasicBlock *LastDead = LastNode->getBlock();
00422               BasicBlock *NextAlive = NextNode->getBlock();
00423 
00424               // Make the conditional branch now go to the next alive block...
00425               TI->getSuccessor(i)->removePredecessor(BB);
00426               TI->setSuccessor(i, NextAlive);
00427 
00428               // If there are PHI nodes in NextAlive, we need to add entries to
00429               // the PHI nodes for the new incoming edge.  The incoming values
00430               // should be identical to the incoming values for LastDead.
00431               //
00432               for (BasicBlock::iterator II = NextAlive->begin();
00433                    isa<PHINode>(II); ++II) {
00434                 PHINode *PN = cast<PHINode>(II);
00435                 if (LiveSet.count(PN)) {  // Only modify live phi nodes
00436                   // Get the incoming value for LastDead...
00437                   int OldIdx = PN->getBasicBlockIndex(LastDead);
00438                   assert(OldIdx != -1 &&"LastDead is not a pred of NextAlive!");
00439                   Value *InVal = PN->getIncomingValue(OldIdx);
00440                   
00441                   // Add an incoming value for BB now...
00442                   PN->addIncoming(InVal, BB);
00443                 }
00444               }
00445             }
00446           }
00447 
00448         // Now loop over all of the instructions in the basic block, telling
00449         // dead instructions to drop their references.  This is so that the next
00450         // sweep over the program can safely delete dead instructions without
00451         // other dead instructions still referring to them.
00452         //
00453         dropReferencesOfDeadInstructionsInLiveBlock(BB);
00454       }
00455   }
00456 
00457   // We make changes if there are any dead blocks in the function...
00458   if (unsigned NumDeadBlocks = Func->size() - AliveBlocks.size()) {
00459     MadeChanges = true;
00460     NumBlockRemoved += NumDeadBlocks;
00461   }
00462 
00463   // Loop over all of the basic blocks in the function, removing control flow
00464   // edges to live blocks (also eliminating any entries in PHI functions in
00465   // referenced blocks).
00466   //
00467   for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
00468     if (!AliveBlocks.count(BB)) {
00469       // Remove all outgoing edges from this basic block and convert the
00470       // terminator into a return instruction.
00471       std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
00472       
00473       if (!Succs.empty()) {
00474         // Loop over all of the successors, removing this block from PHI node
00475         // entries that might be in the block...
00476         while (!Succs.empty()) {
00477           Succs.back()->removePredecessor(BB);
00478           Succs.pop_back();
00479         }
00480         
00481         // Delete the old terminator instruction...
00482         const Type *TermTy = BB->getTerminator()->getType();
00483         if (TermTy != Type::VoidTy)
00484           BB->getTerminator()->replaceAllUsesWith(
00485                                Constant::getNullValue(TermTy));
00486         BB->getInstList().pop_back();
00487         const Type *RetTy = Func->getReturnType();
00488         new ReturnInst(RetTy != Type::VoidTy ?
00489                        Constant::getNullValue(RetTy) : 0, BB);
00490       }
00491     }
00492 
00493 
00494   // Loop over all of the basic blocks in the function, dropping references of
00495   // the dead basic blocks.  We must do this after the previous step to avoid
00496   // dropping references to PHIs which still have entries...
00497   //
00498   for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
00499     if (!AliveBlocks.count(BB))
00500       BB->dropAllReferences();
00501 
00502   // Now loop through all of the blocks and delete the dead ones.  We can safely
00503   // do this now because we know that there are no references to dead blocks
00504   // (because they have dropped all of their references...  we also remove dead
00505   // instructions from alive blocks.
00506   //
00507   for (Function::iterator BI = Func->begin(); BI != Func->end(); )
00508     if (!AliveBlocks.count(BI)) {                // Delete dead blocks...
00509       BI = Func->getBasicBlockList().erase(BI);
00510     } else {                                     // Scan alive blocks...
00511       for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
00512         if (!LiveSet.count(II)) {             // Is this instruction alive?
00513           // Nope... remove the instruction from it's basic block...
00514           if (isa<CallInst>(II))
00515             ++NumCallRemoved;
00516           else
00517             ++NumInstRemoved;
00518           II = BI->getInstList().erase(II);
00519           MadeChanges = true;
00520         } else {
00521           ++II;
00522         }
00523 
00524       ++BI;                                           // Increment iterator...
00525     }
00526 
00527   return MadeChanges;
00528 }