| //===- DCE.cpp - Code to perform dead code elimination --------------------===// |
| // |
| // This file implements dead code elimination and basic block merging. |
| // |
| // 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 |
| // * Eliminates method prototypes that are not referenced |
| // |
| // TODO: This should REALLY be worklist driven 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. |
| // |
| // Note, this is not trivial, because we have to worry about invalidating |
| // iterators. :( |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Optimizations/DCE.h" |
| #include "llvm/Module.h" |
| #include "llvm/GlobalVariable.h" |
| #include "llvm/Method.h" |
| #include "llvm/BasicBlock.h" |
| #include "llvm/iTerminators.h" |
| #include "llvm/iPHINode.h" |
| #include "llvm/Assembly/Writer.h" |
| #include "Support/STLExtras.h" |
| #include <algorithm> |
| |
| // dceInstruction - Inspect the instruction at *BBI and figure out if it's |
| // [trivially] dead. If so, remove the instruction and update the iterator |
| // to point to the instruction that immediately succeeded the original |
| // instruction. |
| // |
| bool opt::DeadCodeElimination::dceInstruction(BasicBlock::InstListType &BBIL, |
| BasicBlock::iterator &BBI) { |
| // Look for un"used" definitions... |
| if ((*BBI)->use_empty() && !(*BBI)->hasSideEffects() && |
| !isa<TerminatorInst>(*BBI)) { |
| delete BBIL.remove(BBI); // Bye bye |
| return true; |
| } |
| return false; |
| } |
| |
| static inline bool RemoveUnusedDefs(BasicBlock::InstListType &Vals) { |
| bool Changed = false; |
| for (BasicBlock::InstListType::iterator DI = Vals.begin(); |
| DI != Vals.end(); ) |
| if (opt::DeadCodeElimination::dceInstruction(Vals, DI)) |
| Changed = true; |
| else |
| ++DI; |
| 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) { |
| BasicBlock::pred_iterator PI(BB->pred_begin()); |
| if (PI == BB->pred_end() || ++PI != BB->pred_end()) |
| return false; // More than one predecessor... |
| |
| Instruction *I = BB->front(); |
| if (!isa<PHINode>(I)) return false; // No PHI nodes |
| |
| //cerr << "Killing PHIs from " << BB; |
| //cerr << "Pred #0 = " << *BB->pred_begin(); |
| |
| //cerr << "Method == " << BB->getParent(); |
| |
| do { |
| PHINode *PN = cast<PHINode>(I); |
| assert(PN->getNumOperands() == 2 && "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->begin()); |
| |
| I = BB->front(); |
| } while (isa<PHINode>(I)); |
| |
| return true; // Yes, we nuked at least one phi node |
| } |
| |
| static void ReplaceUsesWithConstant(Instruction *I) { |
| ConstPoolVal *CPV = ConstPoolVal::getNullConstant(I->getType()); |
| |
| // Make all users of this instruction reference the constant instead |
| I->replaceAllUsesWith(CPV); |
| } |
| |
| // 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. This function returns true (failure) if the Succ BB already |
| // has a predecessor that is a predecessor of BB. |
| // |
| // Assumption: Succ is the single successor for BB. |
| // |
| static bool PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { |
| assert(*BB->succ_begin() == Succ && "Succ is not successor of BB!"); |
| assert(isa<PHINode>(Succ->front()) && "Only works on PHId BBs!"); |
| |
| // 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 |
| // |
| const vector<BasicBlock*> BBPreds(BB->pred_begin(), BB->pred_end()); |
| |
| // Check to see if one of the predecessors of BB is already a predecessor of |
| // Succ. If so, we cannot do the transformation! |
| // |
| for (BasicBlock::pred_iterator PI = Succ->pred_begin(), PE = Succ->pred_end(); |
| PI != PE; ++PI) { |
| if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) |
| return true; |
| } |
| |
| BasicBlock::iterator I = Succ->begin(); |
| do { // Loop over all of the PHI nodes in the successor BB |
| PHINode *PN = cast<PHINode>(*I); |
| Value *OldVal = PN->removeIncomingValue(BB); |
| assert(OldVal && "No entry in PHI for Pred BB!"); |
| |
| for (vector<BasicBlock*>::const_iterator PredI = BBPreds.begin(), |
| End = BBPreds.end(); PredI != End; ++PredI) { |
| // Add an incoming value for each of the new incoming values... |
| PN->addIncoming(OldVal, *PredI); |
| } |
| |
| ++I; |
| } while (isa<PHINode>(*I)); |
| return false; |
| } |
| |
| |
| // SimplifyCFG - This function is used to do simplification of a CFG. For |
| // example, it adjusts branches to branches to eliminate the extra hop, it |
| // eliminates unreachable basic blocks, and does other "peephole" optimization |
| // of the CFG. It returns true if a modification was made, and returns an |
| // iterator that designates the first element remaining after the block that |
| // was deleted. |
| // |
| // WARNING: The entry node of a method may not be simplified. |
| // |
| bool opt::SimplifyCFG(Method::iterator &BBIt) { |
| BasicBlock *BB = *BBIt; |
| Method *M = BB->getParent(); |
| |
| assert(BB && BB->getParent() && "Block not embedded in method!"); |
| assert(BB->getTerminator() && "Degenerate basic block encountered!"); |
| assert(BB->getParent()->front() != BB && "Can't Simplify entry block!"); |
| |
| |
| // Remove basic blocks that have no predecessors... which are unreachable. |
| if (BB->pred_begin() == BB->pred_end() && |
| !BB->hasConstantPoolReferences()) { |
| //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_each(BB->succ_begin(), BB->succ_end(), |
| std::bind2nd(std::mem_fun(&BasicBlock::removePredecessor), BB)); |
| |
| while (!BB->empty()) { |
| Instruction *I = BB->back(); |
| // 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. Note that since this block is |
| // unreachable, and all values contained within it must dominate their |
| // uses, that all uses will eventually be removed. |
| if (!I->use_empty()) ReplaceUsesWithConstant(I); |
| |
| // Remove the instruction from the basic block |
| delete BB->getInstList().pop_back(); |
| } |
| delete M->getBasicBlocks().remove(BBIt); |
| return true; |
| } |
| |
| // Check to see if this block has no instructions and only a single |
| // successor. If so, replace block references with successor. |
| BasicBlock::succ_iterator SI(BB->succ_begin()); |
| if (SI != BB->succ_end() && ++SI == BB->succ_end()) { // One succ? |
| if (BB->front()->isTerminator()) { // Terminator is the only instruction! |
| BasicBlock *Succ = *BB->succ_begin(); // There is exactly one successor |
| //cerr << "Killing Trivial BB: \n" << BB; |
| |
| if (Succ != BB) { // Arg, don't hurt infinite loops! |
| // If our successor has PHI nodes, then we need to update them to |
| // include entries for BB's predecessors, not for BB itself. |
| // Be careful though, if this transformation fails (returns true) then |
| // we cannot do this transformation! |
| // |
| if (!isa<PHINode>(Succ->front()) || |
| !PropogatePredecessorsForPHIs(BB, Succ)) { |
| |
| BB->replaceAllUsesWith(Succ); |
| BB = M->getBasicBlocks().remove(BBIt); |
| |
| 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; |
| return true; |
| } |
| } |
| } |
| } |
| |
| // Merge basic blocks into their predecessor if there is only one pred, |
| // and if there is only one successor of the predecessor. |
| BasicBlock::pred_iterator PI(BB->pred_begin()); |
| if (PI != BB->pred_end() && *PI != BB && // Not empty? Not same BB? |
| ++PI == BB->pred_end() && !BB->hasConstantPoolReferences()) { |
| BasicBlock *Pred = *BB->pred_begin(); |
| TerminatorInst *Term = Pred->getTerminator(); |
| assert(Term != 0 && "malformed basic block without terminator!"); |
| |
| // Does the predecessor block only have a single successor? |
| BasicBlock::succ_iterator SI(Pred->succ_begin()); |
| if (++SI == Pred->succ_end()) { |
| //cerr << "Merging: " << BB << "into: " << Pred; |
| |
| // Delete the unconditianal branch from the predecessor... |
| BasicBlock::iterator DI = Pred->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->empty()) { |
| DI = BB->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 = M->getBasicBlocks().remove(BBIt); |
| |
| // 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()); |
| |
| delete BB; // You ARE the weakest link... goodbye |
| return true; |
| } |
| } |
| |
| return false; |
| } |
| |
| static bool DoDCEPass(Method *M) { |
| Method::iterator BBIt, BBEnd = M->end(); |
| if (M->begin() == BBEnd) return false; // Nothing to do |
| bool Changed = false; |
| |
| // Loop through now and remove instructions that have no uses... |
| for (BBIt = M->begin(); BBIt != BBEnd; ++BBIt) { |
| Changed |= RemoveUnusedDefs((*BBIt)->getInstList()); |
| Changed |= RemoveSingularPHIs(*BBIt); |
| } |
| |
| // Loop over all of the basic blocks (except the first one) and remove them |
| // if they are unneeded... |
| // |
| for (BBIt = M->begin(), ++BBIt; BBIt != M->end(); ) { |
| if (opt::SimplifyCFG(BBIt)) { |
| Changed = true; |
| } else { |
| ++BBIt; |
| } |
| } |
| |
| return Changed; |
| } |
| |
| |
| // It is possible that we may require multiple passes over the code to fully |
| // eliminate dead code. Iterate until we are done. |
| // |
| bool opt::DeadCodeElimination::doDCE(Method *M) { |
| bool Changed = false; |
| while (DoDCEPass(M)) Changed = true; |
| return Changed; |
| } |
| |
| bool opt::DeadCodeElimination::RemoveUnusedGlobalValues(Module *Mod) { |
| bool Changed = false; |
| |
| for (Module::iterator MI = Mod->begin(); MI != Mod->end(); ) { |
| Method *Meth = *MI; |
| if (Meth->isExternal() && Meth->use_size() == 0) { |
| // No references to prototype? |
| //cerr << "Removing method proto: " << Meth->getName() << endl; |
| delete Mod->getMethodList().remove(MI); // Remove prototype |
| // Remove moves iterator to point to the next one automatically |
| Changed = true; |
| } else { |
| ++MI; // Skip prototype in use. |
| } |
| } |
| |
| for (Module::giterator GI = Mod->gbegin(); GI != Mod->gend(); ) { |
| GlobalVariable *GV = *GI; |
| if (!GV->hasInitializer() && GV->use_size() == 0) { |
| // No references to uninitialized global variable? |
| //cerr << "Removing global var: " << GV->getName() << endl; |
| delete Mod->getGlobalList().remove(GI); |
| // Remove moves iterator to point to the next one automatically |
| Changed = true; |
| } else { |
| ++GI; |
| } |
| } |
| |
| return Changed; |
| } |