#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/Local.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Type.h"
-#include "llvm/Analysis/Dominators.h"
-#include "llvm/Analysis/Writer.h"
+#include "llvm/Analysis/PostDominators.h"
#include "llvm/iTerminators.h"
#include "llvm/iPHINode.h"
#include "llvm/Constant.h"
#include <algorithm>
#include <iostream>
using std::cerr;
+using std::vector;
static Statistic<> NumBlockRemoved("adce\t\t- Number of basic blocks removed");
static Statistic<> NumInstRemoved ("adce\t\t- Number of instructions removed");
// The public interface for this class
//
public:
- const char *getPassName() const { return "Aggressive Dead Code Elimination"; }
-
// Execute the Aggressive Dead Code Elimination Algorithm
//
- virtual bool runOnFunction(Function *F) {
- Func = F;
+ virtual bool runOnFunction(Function &F) {
+ Func = &F;
bool Changed = doADCE();
assert(WorkList.empty());
LiveSet.clear();
// getAnalysisUsage - We require post dominance frontiers (aka Control
// Dependence Graph)
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
- AU.addRequired(DominatorTree::PostDomID);
- AU.addRequired(DominanceFrontier::PostDomID);
+ AU.addRequired<PostDominatorTree>();
+ AU.addRequired<PostDominanceFrontier>();
}
void markBlockAlive(BasicBlock *BB);
+
+ // dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
+ // instructions in the specified basic block, dropping references on
+ // instructions that are dead according to LiveSet.
+ bool dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB);
+
inline void markInstructionLive(Instruction *I) {
if (LiveSet.count(I)) return;
DEBUG(cerr << "Insn Live: " << I);
}
};
+ RegisterOpt<ADCE> X("adce", "Aggressive Dead Code Elimination");
} // End of anonymous namespace
Pass *createAggressiveDCEPass() { return new ADCE(); }
-
void ADCE::markBlockAlive(BasicBlock *BB) {
// Mark the basic block as being newly ALIVE... and mark all branches that
// this block is control dependant on as being alive also...
//
- DominanceFrontier &CDG =
- getAnalysis<DominanceFrontier>(DominanceFrontier::PostDomID);
+ PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
- DominanceFrontier::const_iterator It = CDG.find(BB);
+ PostDominanceFrontier::const_iterator It = CDG.find(BB);
if (It != CDG.end()) {
// Get the blocks that this node is control dependant on...
- const DominanceFrontier::DomSetType &CDB = It->second;
+ const PostDominanceFrontier::DomSetType &CDB = It->second;
for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
bind_obj(this, &ADCE::markTerminatorLive));
}
markTerminatorLive(BB);
}
+// dropReferencesOfDeadInstructionsInLiveBlock - Loop over all of the
+// instructions in the specified basic block, dropping references on
+// instructions that are dead according to LiveSet.
+bool ADCE::dropReferencesOfDeadInstructionsInLiveBlock(BasicBlock *BB) {
+ bool Changed = false;
+ for (BasicBlock::iterator I = BB->begin(), E = --BB->end(); I != E; )
+ if (!LiveSet.count(I)) { // Is this instruction alive?
+ I->dropAllReferences(); // Nope, drop references...
+ if (PHINode *PN = dyn_cast<PHINode>(&*I)) {
+ // We don't want to leave PHI nodes in the program that have
+ // #arguments != #predecessors, so we remove them now.
+ //
+ PN->replaceAllUsesWith(Constant::getNullValue(PN->getType()));
+
+ // Delete the instruction...
+ I = BB->getInstList().erase(I);
+ Changed = true;
+ } else {
+ ++I;
+ }
+ } else {
+ ++I;
+ }
+ return Changed;
+}
+
// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
// true if the function was modified.
BBI != BBE; ++BBI) {
BasicBlock *BB = *BBI;
for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
- Instruction *I = *II;
-
- if (I->hasSideEffects() || I->getOpcode() == Instruction::Ret) {
- markInstructionLive(I);
+ if (II->hasSideEffects() || II->getOpcode() == Instruction::Ret) {
+ markInstructionLive(II);
++II; // Increment the inst iterator if the inst wasn't deleted
- } else if (isInstructionTriviallyDead(I)) {
+ } else if (isInstructionTriviallyDead(II)) {
// Remove the instruction from it's basic block...
- delete BB->getInstList().remove(II);
+ II = BB->getInstList().erase(II);
++NumInstRemoved;
MadeChanges = true;
} else {
if (DebugFlag) {
cerr << "Current Function: X = Live\n";
for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
- for (BasicBlock::iterator BI = (*I)->begin(), BE = (*I)->end();
- BI != BE; ++BI) {
- if (LiveSet.count(*BI)) cerr << "X ";
+ for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
+ if (LiveSet.count(BI)) cerr << "X ";
cerr << *BI;
}
}
// Find the first postdominator of the entry node that is alive. Make it the
// new entry node...
//
- DominatorTree &DT = getAnalysis<DominatorTree>(DominatorTree::PostDomID);
-
- // If there are some blocks dead...
- if (AliveBlocks.size() != Func->size()) {
- // Insert a new entry node to eliminate the entry node as a special case.
- BasicBlock *NewEntry = new BasicBlock();
- NewEntry->getInstList().push_back(new BranchInst(Func->front()));
- Func->getBasicBlocks().push_front(NewEntry);
- AliveBlocks.insert(NewEntry); // This block is always alive!
+ PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
+
+
+ if (AliveBlocks.size() == Func->size()) { // No dead blocks?
+ for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
+ // Loop over all of the instructions in the function, telling dead
+ // instructions to drop their references. This is so that the next sweep
+ // over the program can safely delete dead instructions without other dead
+ // instructions still refering to them.
+ //
+ dropReferencesOfDeadInstructionsInLiveBlock(I);
+
+ } else { // If there are some blocks dead...
+ // If the entry node is dead, insert a new entry node to eliminate the entry
+ // node as a special case.
+ //
+ if (!AliveBlocks.count(&Func->front())) {
+ BasicBlock *NewEntry = new BasicBlock();
+ new BranchInst(&Func->front(), NewEntry->end());
+ Func->getBasicBlockList().push_front(NewEntry);
+ AliveBlocks.insert(NewEntry); // This block is always alive!
+ }
// Loop over all of the alive blocks in the function. If any successor
// blocks are not alive, we adjust the outgoing branches to branch to the
// the block to reflect this.
//
for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
- if (AliveBlocks.count(*I)) {
- BasicBlock *BB = *I;
+ if (AliveBlocks.count(I)) {
+ BasicBlock *BB = I;
TerminatorInst *TI = BB->getTerminator();
- // Loop over all of the successors, looking for ones that are not alive
- for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
+ // Loop over all of the successors, looking for ones that are not alive.
+ // We cannot save the number of successors in the terminator instruction
+ // here because we may remove them if we don't have a postdominator...
+ //
+ for (unsigned i = 0; i != TI->getNumSuccessors(); ++i)
if (!AliveBlocks.count(TI->getSuccessor(i))) {
// Scan up the postdominator tree, looking for the first
// postdominator that is alive, and the last postdominator that is
// dead...
//
- DominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
- DominatorTree::Node *NextNode = LastNode->getIDom();
- while (!AliveBlocks.count(NextNode->getNode())) {
- LastNode = NextNode;
- NextNode = NextNode->getIDom();
- }
-
- // Get the basic blocks that we need...
- BasicBlock *LastDead = LastNode->getNode();
- BasicBlock *NextAlive = NextNode->getNode();
-
- // Make the conditional branch now go to the next alive block...
- TI->getSuccessor(i)->removePredecessor(BB);
- TI->setSuccessor(i, NextAlive);
-
- // If there are PHI nodes in NextAlive, we need to add entries to
- // the PHI nodes for the new incoming edge. The incoming values
- // should be identical to the incoming values for LastDead.
+ PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
+
+ // There is a special case here... if there IS no post-dominator for
+ // the block we have no owhere to point our branch to. Instead,
+ // convert it to a return. This can only happen if the code
+ // branched into an infinite loop. Note that this may not be
+ // desirable, because we _are_ altering the behavior of the code.
+ // This is a well known drawback of ADCE, so in the future if we
+ // choose to revisit the decision, this is where it should be.
//
- for (BasicBlock::iterator II = NextAlive->begin();
- PHINode *PN = dyn_cast<PHINode>(*II); ++II) {
- // Get the incoming value for LastDead...
- int OldIdx = PN->getBasicBlockIndex(LastDead);
- assert(OldIdx != -1 && "LastDead is not a pred of NextAlive!");
- Value *InVal = PN->getIncomingValue(OldIdx);
-
- // Add an incoming value for BB now...
- PN->addIncoming(InVal, BB);
+ if (LastNode == 0) { // No postdominator!
+ // Call RemoveSuccessor to transmogrify the terminator instruction
+ // to not contain the outgoing branch, or to create a new
+ // terminator if the form fundementally changes (ie unconditional
+ // branch to return). Note that this will change a branch into an
+ // infinite loop into a return instruction!
+ //
+ RemoveSuccessor(TI, i);
+
+ // RemoveSuccessor may replace TI... make sure we have a fresh
+ // pointer... and e variable.
+ //
+ TI = BB->getTerminator();
+
+ // Rescan this successor...
+ --i;
+ } else {
+ PostDominatorTree::Node *NextNode = LastNode->getIDom();
+
+ while (!AliveBlocks.count(NextNode->getNode())) {
+ LastNode = NextNode;
+ NextNode = NextNode->getIDom();
+ }
+
+ // Get the basic blocks that we need...
+ BasicBlock *LastDead = LastNode->getNode();
+ BasicBlock *NextAlive = NextNode->getNode();
+
+ // Make the conditional branch now go to the next alive block...
+ TI->getSuccessor(i)->removePredecessor(BB);
+ TI->setSuccessor(i, NextAlive);
+
+ // If there are PHI nodes in NextAlive, we need to add entries to
+ // the PHI nodes for the new incoming edge. The incoming values
+ // should be identical to the incoming values for LastDead.
+ //
+ for (BasicBlock::iterator II = NextAlive->begin();
+ PHINode *PN = dyn_cast<PHINode>(&*II); ++II) {
+ // Get the incoming value for LastDead...
+ int OldIdx = PN->getBasicBlockIndex(LastDead);
+ assert(OldIdx != -1 && "LastDead is not a pred of NextAlive!");
+ Value *InVal = PN->getIncomingValue(OldIdx);
+
+ // Add an incoming value for BB now...
+ PN->addIncoming(InVal, BB);
+ }
}
}
// sweep over the program can safely delete dead instructions without
// other dead instructions still refering to them.
//
- for (BasicBlock::iterator I = BB->begin(), E = BB->end()-1; I != E; ++I)
- if (!LiveSet.count(*I)) // Is this instruction alive?
- (*I)->dropAllReferences(); // Nope, drop references...
+ dropReferencesOfDeadInstructionsInLiveBlock(BB);
}
}
// Loop over all of the basic blocks in the function, dropping references of
// the dead basic blocks
//
- for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I) {
- BasicBlock *BB = *I;
+ for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) {
if (!AliveBlocks.count(BB)) {
// Remove all outgoing edges from this basic block and convert the
// terminator into a return instruction.
}
// Delete the old terminator instruction...
- delete BB->getInstList().remove(BB->end()-1);
+ BB->getInstList().pop_back();
const Type *RetTy = Func->getReturnType();
- Instruction *New = new ReturnInst(RetTy != Type::VoidTy ?
- Constant::getNullValue(RetTy) : 0);
- BB->getInstList().push_back(New);
+ new ReturnInst(RetTy != Type::VoidTy ? Constant::getNullValue(RetTy) :0,
+ BB->end());
}
BB->dropAllReferences();
// instructions from alive blocks.
//
for (Function::iterator BI = Func->begin(); BI != Func->end(); )
- if (!AliveBlocks.count(*BI))
- delete Func->getBasicBlocks().remove(BI);
- else {
- BasicBlock *BB = *BI;
- for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; )
- if (!LiveSet.count(*II)) { // Is this instruction alive?
+ if (!AliveBlocks.count(BI)) { // Delete dead blocks...
+ BI = Func->getBasicBlockList().erase(BI);
+ } else { // Scan alive blocks...
+ for (BasicBlock::iterator II = BI->begin(); II != --BI->end(); )
+ if (!LiveSet.count(II)) { // Is this instruction alive?
// Nope... remove the instruction from it's basic block...
- delete BB->getInstList().remove(II);
+ II = BI->getInstList().erase(II);
++NumInstRemoved;
MadeChanges = true;
} else {
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