//===- 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
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();
}
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