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ADCE.cpp

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