-//===- ADCE.cpp - Code to perform agressive dead code elimination ---------===//
+//===- ADCE.cpp - Code to perform aggressive dead code elimination --------===//
//
-// This file implements "agressive" dead code elimination. ADCE is DCe where
+// This file implements "aggressive" dead code elimination. ADCE is DCe where
// values are assumed to be dead until proven otherwise. This is similar to
// SCCP, except applied to the liveness of values.
//
//===----------------------------------------------------------------------===//
-#include "llvm/Transforms/Scalar/DCE.h"
-#include "llvm/Instruction.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Type.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/Writer.h"
#define DEBUG_ADCE 1
+namespace {
+
//===----------------------------------------------------------------------===//
// ADCE Class
//
-// This class does all of the work of Agressive Dead Code Elimination.
+// This class does all of the work of Aggressive Dead Code Elimination.
// It's public interface consists of a constructor and a doADCE() method.
//
-class ADCE {
- Function *M; // The function that we are working on
+class ADCE : public FunctionPass {
+ Function *Func; // The function that we are working on
std::vector<Instruction*> WorkList; // Instructions that just became live
std::set<Instruction*> LiveSet; // The set of live instructions
bool MadeChanges;
// The public interface for this class
//
public:
- // ADCE Ctor - Save the function to operate on...
- inline ADCE(Function *f) : M(f), MadeChanges(false) {}
+ const char *getPassName() const { return "Aggressive Dead Code Elimination"; }
+
+ // doADCE - Execute the Aggressive Dead Code Elimination Algorithm
+ //
+ virtual bool runOnFunction(Function *F) {
+ Func = F; MadeChanges = false;
+ doADCE(getAnalysis<DominanceFrontier>(DominanceFrontier::PostDomID));
+ assert(WorkList.empty());
+ LiveSet.clear();
+ return MadeChanges;
+ }
+ // getAnalysisUsage - We require post dominance frontiers (aka Control
+ // Dependence Graph)
+ virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+ AU.addRequired(DominanceFrontier::PostDomID);
+ }
- // doADCE() - Run the Agressive Dead Code Elimination algorithm, returning
- // true if the function was modified.
- bool doADCE(cfg::DominanceFrontier &CDG);
//===--------------------------------------------------------------------===//
// The implementation of this class
//
private:
+ // doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
+ // true if the function was modified.
+ //
+ void doADCE(DominanceFrontier &CDG);
+
inline void markInstructionLive(Instruction *I) {
if (LiveSet.count(I)) return;
#ifdef DEBUG_ADCE
const std::set<BasicBlock*> &AliveBlocks);
};
+} // End of anonymous namespace
+
+Pass *createAggressiveDCEPass() {
+ return new ADCE();
+}
-// doADCE() - Run the Agressive Dead Code Elimination algorithm, returning
+// doADCE() - Run the Aggressive Dead Code Elimination algorithm, returning
// true if the function was modified.
//
-bool ADCE::doADCE(cfg::DominanceFrontier &CDG) {
+void ADCE::doADCE(DominanceFrontier &CDG) {
#ifdef DEBUG_ADCE
- cerr << "Function: " << M;
+ cerr << "Function: " << Func;
#endif
// Iterate over all of the instructions in the function, eliminating trivially
// instructions live in basic blocks that are unreachable. These blocks will
// be eliminated later, along with the instructions inside.
//
- for (df_iterator<Function*> BBI = df_begin(M),
- BBE = df_end(M);
+ for (df_iterator<Function*> BBI = df_begin(Func), BBE = df_end(Func);
BBI != BBE; ++BBI) {
BasicBlock *BB = *BBI;
for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
if (I->hasSideEffects() || I->getOpcode() == Instruction::Ret) {
markInstructionLive(I);
+ ++II; // Increment the inst iterator if the inst wasn't deleted
+ } else if (isInstructionTriviallyDead(I)) {
+ // Remove the instruction from it's basic block...
+ delete BB->getInstList().remove(II);
+ MadeChanges = true;
} else {
- // Check to see if anything is trivially dead
- if (I->use_size() == 0 && I->getType() != Type::VoidTy) {
- // Remove the instruction from it's basic block...
- delete BB->getInstList().remove(II);
- MadeChanges = true;
- continue; // Don't increment the iterator past the current slot
- }
+ ++II; // Increment the inst iterator if the inst wasn't deleted
}
-
- ++II; // Increment the inst iterator if the inst wasn't deleted
}
}
// this block is control dependant on as being alive also...
//
AliveBlocks.insert(BB); // Block is now ALIVE!
- cfg::DominanceFrontier::const_iterator It = CDG.find(BB);
+ DominanceFrontier::const_iterator It = CDG.find(BB);
if (It != CDG.end()) {
// Get the blocks that this node is control dependant on...
- const cfg::DominanceFrontier::DomSetType &CDB = It->second;
+ const DominanceFrontier::DomSetType &CDB = It->second;
for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
bind_obj(this, &ADCE::markTerminatorLive));
}
// Loop over all of the operands of the live instruction, making sure that
// they are known to be alive as well...
//
- for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op) {
+ for (unsigned op = 0, End = I->getNumOperands(); op != End; ++op)
if (Instruction *Operand = dyn_cast<Instruction>(I->getOperand(op)))
markInstructionLive(Operand);
- }
}
#ifdef DEBUG_ADCE
cerr << "Current Function: X = Live\n";
- for (Function::iterator I = M->begin(), E = M->end(); I != E; ++I)
+ 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 ";
// order, patching up references to dead blocks...
//
std::set<BasicBlock*> VisitedBlocks;
- BasicBlock *EntryBlock = fixupCFG(M->front(), VisitedBlocks, AliveBlocks);
- if (EntryBlock && EntryBlock != M->front()) {
- if (isa<PHINode>(EntryBlock->front())) {
- // Cannot make the first block be a block with a PHI node in it! Instead,
- // strip the first basic block of the function to contain no instructions,
- // then add a simple branch to the "real" entry node...
- //
- BasicBlock *E = M->front();
- if (!isa<TerminatorInst>(E->front()) || // Check for an actual change...
- cast<TerminatorInst>(E->front())->getNumSuccessors() != 1 ||
- cast<TerminatorInst>(E->front())->getSuccessor(0) != EntryBlock) {
- E->getInstList().delete_all(); // Delete all instructions in block
- E->getInstList().push_back(new BranchInst(EntryBlock));
- MadeChanges = true;
- }
- AliveBlocks.insert(E);
-
- // Next we need to change any PHI nodes in the entry block to refer to the
- // new predecessor node...
-
-
- } else {
- // We need to move the new entry block to be the first bb of the function
- Function::iterator EBI = find(M->begin(), M->end(), EntryBlock);
- std::swap(*EBI, *M->begin()); // Exchange old location with start of fn
- MadeChanges = true;
- }
- }
+ BasicBlock *EntryBlock = fixupCFG(Func->front(), VisitedBlocks, AliveBlocks);
// Now go through and tell dead blocks to drop all of their references so they
- // can be safely deleted.
+ // can be safely deleted. Also, as we are doing so, if the block has
+ // successors that are still live (and that have PHI nodes in them), remove
+ // the entry for this block from the phi nodes.
//
- for (Function::iterator BI = M->begin(), BE = M->end(); BI != BE; ++BI) {
+ for (Function::iterator BI = Func->begin(), BE = Func->end(); BI != BE; ++BI){
BasicBlock *BB = *BI;
if (!AliveBlocks.count(BB)) {
+ // Remove entries from successors PHI nodes if they are still alive...
+ for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI)
+ if (AliveBlocks.count(*SI)) { // Only if the successor is alive...
+ BasicBlock *Succ = *SI;
+ for (BasicBlock::iterator I = Succ->begin();// Loop over all PHI nodes
+ PHINode *PN = dyn_cast<PHINode>(*I); ++I)
+ PN->removeIncomingValue(BB); // Remove value for this block
+ }
+
BB->dropAllReferences();
}
}
+ cerr << "Before Deleting Blocks: " << Func;
+
// Now loop through all of the blocks and delete them. We can safely do this
// now because we know that there are no references to dead blocks (because
// they have dropped all of their references...
//
- for (Function::iterator BI = M->begin(); BI != M->end();) {
+ for (Function::iterator BI = Func->begin(); BI != Func->end();) {
if (!AliveBlocks.count(*BI)) {
- delete M->getBasicBlocks().remove(BI);
+ delete Func->getBasicBlocks().remove(BI);
MadeChanges = true;
continue; // Don't increment iterator
}
++BI; // Increment iterator...
}
- return MadeChanges;
+ if (EntryBlock && EntryBlock != Func->front()) {
+ // We need to move the new entry block to be the first bb of the function
+ Function::iterator EBI = find(Func->begin(), Func->end(), EntryBlock);
+ std::swap(*EBI, *Func->begin()); // Exchange old location with start of fn
+ }
+
+ while (PHINode *PN = dyn_cast<PHINode>(EntryBlock->front())) {
+ assert(PN->getNumIncomingValues() == 1 &&
+ "Can only have a single incoming value at this point...");
+ // The incoming value must be outside of the scope of the function, a
+ // global variable, constant or parameter maybe...
+ //
+ PN->replaceAllUsesWith(PN->getIncomingValue(0));
+
+ // Nuke the phi node...
+ delete EntryBlock->getInstList().remove(EntryBlock->begin());
+ }
}
if (AliveBlocks.count(BB)) { // Is the block alive?
// Yes it's alive: loop through and eliminate all dead instructions in block
- for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; ) {
- Instruction *I = *II;
- if (!LiveSet.count(I)) { // Is this instruction alive?
+ for (BasicBlock::iterator II = BB->begin(); II != BB->end()-1; )
+ if (!LiveSet.count(*II)) { // Is this instruction alive?
// Nope... remove the instruction from it's basic block...
delete BB->getInstList().remove(II);
MadeChanges = true;
- continue; // Don't increment II
+ } else {
+ ++II;
}
- ++II;
- }
// Recursively traverse successors of this basic block.
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI) {
for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE; ++SI) {
BasicBlock *RetBB = fixupCFG(*SI, VisitedBlocks, AliveBlocks);
if (RetBB) {
- assert(ReturnBB == 0 && "One one live child allowed!");
+ assert(ReturnBB == 0 && "At most one live child allowed!");
ReturnBB = RetBB;
}
}
}
}
-namespace {
- struct AgressiveDCE : public FunctionPass {
- // doADCE - Execute the Agressive Dead Code Elimination Algorithm
- //
- virtual bool runOnFunction(Function *F) {
- return ADCE(F).doADCE(
- getAnalysis<cfg::DominanceFrontier>(cfg::DominanceFrontier::PostDomID));
- }
- // getAnalysisUsage - We require post dominance frontiers (aka Control
- // Dependence Graph)
- virtual void getAnalysisUsage(AnalysisUsage &AU) const {
- AU.addRequired(cfg::DominanceFrontier::PostDomID);
- }
- };
-}
-
-Pass *createAgressiveDCEPass() {
- return new AgressiveDCE();
-}
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