diff options
Diffstat (limited to 'llvm/lib/Transforms/Scalar')
| -rw-r--r-- | llvm/lib/Transforms/Scalar/DCE.cpp | 415 |
1 files changed, 85 insertions, 330 deletions
diff --git a/llvm/lib/Transforms/Scalar/DCE.cpp b/llvm/lib/Transforms/Scalar/DCE.cpp index af00102cd8d..5983910c182 100644 --- a/llvm/lib/Transforms/Scalar/DCE.cpp +++ b/llvm/lib/Transforms/Scalar/DCE.cpp @@ -1,38 +1,24 @@ //===- DCE.cpp - Code to perform dead code elimination --------------------===// // -// This file implements dead code elimination and basic block merging. +// This file implements dead inst elimination and dead code elimination. // -// Specifically, this: -// * removes definitions with no uses -// * 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 function 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. :( +// Dead Inst Elimination performs a single pass over the function removing +// instructions that are obviously dead. Dead Code Elimination is similar, but +// it rechecks instructions that were used by removed instructions to see if +// they are newly dead. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar/DCE.h" -#include "llvm/Module.h" -#include "llvm/GlobalVariable.h" -#include "llvm/iTerminators.h" -#include "llvm/iPHINode.h" -#include "llvm/Constant.h" -#include "llvm/Support/CFG.h" #include "llvm/Pass.h" -#include "Support/STLExtras.h" -#include <algorithm> +#include "llvm/InstrTypes.h" +#include "llvm/Function.h" +#include "llvm/Support/InstIterator.h" +#include <set> + +static inline bool isInstDead(Instruction *I) { + return I->use_empty() && !I->hasSideEffects() && !isa<TerminatorInst>(I); +} // dceInstruction - Inspect the instruction at *BBI and figure out if it's // [trivially] dead. If so, remove the instruction and update the iterator @@ -42,339 +28,108 @@ bool dceInstruction(BasicBlock::InstListType &BBIL, BasicBlock::iterator &BBI) { // Look for un"used" definitions... - if ((*BBI)->use_empty() && !(*BBI)->hasSideEffects() && - !isa<TerminatorInst>(*BBI)) { + if (isInstDead(*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 (dceInstruction(Vals, DI)) - Changed = true; - else - ++DI; - return Changed; -} +//===----------------------------------------------------------------------===// +// DeadInstElimination pass implementation +// -struct DeadInstElimination : public BasicBlockPass { - const char *getPassName() const { return "Dead Instruction Elimination"; } +namespace { + struct DeadInstElimination : public BasicBlockPass { + const char *getPassName() const { return "Dead Instruction Elimination"; } + + virtual bool runOnBasicBlock(BasicBlock *BB) { + BasicBlock::InstListType &Vals = BB->getInstList(); + bool Changed = false; + for (BasicBlock::iterator DI = Vals.begin(); DI != Vals.end(); ) + if (dceInstruction(Vals, DI)) + Changed = true; + else + ++DI; + return Changed; + } - virtual bool runOnBasicBlock(BasicBlock *BB) { - return RemoveUnusedDefs(BB->getInstList()); - } -}; + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.preservesCFG(); + } + }; +} Pass *createDeadInstEliminationPass() { return new DeadInstElimination(); } -// 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) { - pred_iterator PI(pred_begin(BB)); - if (PI == pred_end(BB) || ++PI != pred_end(BB)) - 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 = " << *pred_begin(BB); - - //cerr << "Function == " << 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) { - // Make all users of this instruction reference the constant instead - I->replaceAllUsesWith(Constant::getNullValue(I->getType())); -} - -// 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. +//===----------------------------------------------------------------------===// +// DeadCodeElimination pass implementation // -static bool PropogatePredecessorsForPHIs(BasicBlock *BB, BasicBlock *Succ) { - assert(*succ_begin(BB) == 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 std::vector<BasicBlock*> BBPreds(pred_begin(BB), pred_end(BB)); - - // Check to see if one of the predecessors of BB is already a predecessor of - // Succ. If so, we cannot do the transformation! - // - for (pred_iterator PI = pred_begin(Succ), PE = pred_end(Succ); - PI != PE; ++PI) { - if (find(BBPreds.begin(), BBPreds.end(), *PI) != BBPreds.end()) - return true; - } +namespace { + struct DCE : public FunctionPass { + const char *getPassName() const { return "Dead Code Elimination"; } - 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!"); + virtual bool runOnFunction(Function *F); - for (std::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); + virtual void getAnalysisUsage(AnalysisUsage &AU) const { + AU.preservesCFG(); } - - ++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 function may not be simplified. -// -bool SimplifyCFG(Function::iterator &BBIt) { - BasicBlock *BB = *BBIt; - Function *M = BB->getParent(); - - assert(BB && BB->getParent() && "Block not embedded in function!"); - 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 (pred_begin(BB) == pred_end(BB) && - !BB->hasConstantReferences()) { - //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(succ_begin(BB), succ_end(BB), - 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. - succ_iterator SI(succ_begin(BB)); - if (SI != succ_end(BB) && ++SI == succ_end(BB)) { // One succ? - if (BB->front()->isTerminator()) { // Terminator is the only instruction! - BasicBlock *Succ = *succ_begin(BB); // 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 << "Function after removal: \n" << M; - return true; - } - } - } - } - - // Merge basic blocks into their predecessor if there is only one distinct - // pred, and if there is only one distinct successor of the predecessor, and - // if there are no PHI nodes. - // - if (!isa<PHINode>(BB->front()) && !BB->hasConstantReferences()) { - pred_iterator PI(pred_begin(BB)), PE(pred_end(BB)); - BasicBlock *OnlyPred = *PI++; - for (; PI != PE; ++PI) // Search all predecessors, see if they are all same - if (*PI != OnlyPred) { - OnlyPred = 0; // There are multiple different predecessors... - break; - } +bool DCE::runOnFunction(Function *F) { + // Start out with all of the instructions in the worklist... + std::vector<Instruction*> WorkList(inst_begin(F), inst_end(F)); + std::set<Instruction*> DeadInsts; - BasicBlock *OnlySucc = 0; - if (OnlyPred && OnlyPred != BB) { // Don't break self loops - // Check to see if there is only one distinct successor... - succ_iterator SI(succ_begin(OnlyPred)), SE(succ_end(OnlyPred)); - OnlySucc = BB; - for (; SI != SE; ++SI) - if (*SI != OnlySucc) { - OnlySucc = 0; // There are multiple distinct successors! - break; - } - } - - if (OnlySucc) { - //cerr << "Merging: " << BB << "into: " << Pred; - TerminatorInst *Term = OnlyPred->getTerminator(); - - // Delete the unconditional branch from the predecessor... - BasicBlock::iterator DI = OnlyPred->end(); - delete OnlyPred->getInstList().remove(--DI); // Destroy branch - - // Move all definitions in the succecessor to the predecessor... - std::vector<Instruction*> Insts(BB->begin(), BB->end()); - BB->getInstList().remove(BB->begin(), BB->end()); - OnlyPred->getInstList().insert(OnlyPred->end(), - Insts.begin(), Insts.end()); - - // Remove basic block from the function... and advance iterator to the - // next valid block... - M->getBasicBlocks().remove(BBIt); - - // Make all PHI nodes that refered to BB now refer to Pred as their - // source... - BB->replaceAllUsesWith(OnlyPred); - - // Inherit predecessors name if it exists... - if (BB->hasName() && !OnlyPred->hasName()) - OnlyPred->setName(BB->getName()); - - delete BB; // You ARE the weakest link... goodbye - return true; - } - } - - return false; -} - -static bool DoDCEPass(Function *F) { - Function::iterator BBIt, BBEnd = F->end(); - if (F->begin() == BBEnd) return false; // Nothing to do - bool Changed = false; - - // Loop through now and remove instructions that have no uses... - for (BBIt = F->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... + // Loop over the worklist finding instructions that are dead. If they are + // dead make them drop all of their uses, making other instructions + // potentially dead, and work until the worklist is empty. // - for (BBIt = F->begin(), ++BBIt; BBIt != F->end(); ) { - if (SimplifyCFG(BBIt)) { - Changed = true; - } else { - ++BBIt; - } - } - - return Changed; -} - -// Remove unused global values - This removes unused global values of no -// possible value. This currently includes unused function prototypes and -// unitialized global variables. -// -static bool RemoveUnusedGlobalValues(Module *Mod) { - bool Changed = false; - - for (Module::iterator MI = Mod->begin(); MI != Mod->end(); ) { - Function *Meth = *MI; - if (Meth->isExternal() && Meth->use_size() == 0) { - // No references to prototype? - //cerr << "Removing function proto: " << Meth->getName() << endl; - delete Mod->getFunctionList().remove(MI); // Remove prototype - // Remove moves iterator to point to the next one automatically - Changed = true; - } else { - ++MI; // Skip prototype in use. + while (!WorkList.empty()) { + Instruction *I = WorkList.back(); + WorkList.pop_back(); + + if (isInstDead(I)) { // If the instruction is dead... + // Loop over all of the values that the instruction uses, if there are + // instructions being used, add them to the worklist, because they might + // go dead after this one is removed. + // + for (User::use_iterator UI = I->use_begin(), UE = I->use_end(); + UI != UE; ++UI) + if (Instruction *Used = dyn_cast<Instruction>(*UI)) + WorkList.push_back(Used); + + // Tell the instruction to let go of all of the values it uses... + I->dropAllReferences(); + + // Keep track of this instruction, because we are going to delete it later + DeadInsts.insert(I); } } - 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; - } + // If we found no dead instructions, we haven't changed the function... + if (DeadInsts.empty()) return false; + + // Otherwise, loop over the program, removing and deleting the instructions... + for (Function::iterator I = F->begin(), E = F->end(); I != E; ++I) { + BasicBlock::InstListType &BBIL = (*I)->getInstList(); + for (BasicBlock::iterator BI = BBIL.begin(); BI != BBIL.end(); ) + if (DeadInsts.count(*BI)) { // Is this instruction dead? + delete BBIL.remove(BI); // Yup, remove and delete inst + } else { // This instruction is not dead + ++BI; // Continue on to the next one... + } } - return Changed; -} - -namespace { - struct DeadCodeElimination : public FunctionPass { - const char *getPassName() const { return "Dead Code Elimination"; } - - // Pass Interface... - virtual bool doInitialization(Module *M) { - return RemoveUnusedGlobalValues(M); - } - - // It is possible that we may require multiple passes over the code to fully - // eliminate dead code. Iterate until we are done. - // - virtual bool runOnFunction(Function *F) { - bool Changed = false; - while (DoDCEPass(F)) Changed = true; - return Changed; - } - - virtual bool doFinalization(Module *M) { - return RemoveUnusedGlobalValues(M); - } - }; + return true; } Pass *createDeadCodeEliminationPass() { - return new DeadCodeElimination(); + return new DCE(); } |
