#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/PostDominators.h"
#include "llvm/iTerminators.h"
#include "llvm/iPHINode.h"
#include "llvm/Constant.h"
#include "llvm/Support/CFG.h"
-#include "Support/STLExtras.h"
+#include "Support/Debug.h"
#include "Support/DepthFirstIterator.h"
-#include "Support/StatisticReporter.h"
+#include "Support/Statistic.h"
+#include "Support/STLExtras.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");
namespace {
+ Statistic<> NumBlockRemoved("adce", "Number of basic blocks removed");
+ Statistic<> NumInstRemoved ("adce", "Number of instructions removed");
//===----------------------------------------------------------------------===//
// ADCE Class
// getAnalysisUsage - We require post dominance frontiers (aka Control
// Dependence Graph)
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
- AU.addRequired(PostDominatorTree::ID);
- AU.addRequired(PostDominanceFrontier::ID);
+ 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);
+
+ TerminatorInst *convertToUnconditionalBranch(TerminatorInst *TI);
+
inline void markInstructionLive(Instruction *I) {
if (LiveSet.count(I)) return;
- DEBUG(cerr << "Insn Live: " << I);
+ DEBUG(std::cerr << "Insn Live: " << I);
LiveSet.insert(I);
WorkList.push_back(I);
}
inline void markTerminatorLive(const BasicBlock *BB) {
- DEBUG(cerr << "Terminat Live: " << BB->getTerminator());
- markInstructionLive((Instruction*)BB->getTerminator());
+ DEBUG(std::cerr << "Terminator Live: " << BB->getTerminator());
+ markInstructionLive(const_cast<TerminatorInst*>(BB->getTerminator()));
}
};
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...
+ // this block is control dependent on as being alive also...
//
PostDominanceFrontier &CDG = getAnalysis<PostDominanceFrontier>();
PostDominanceFrontier::const_iterator It = CDG.find(BB);
if (It != CDG.end()) {
- // Get the blocks that this node is control dependant on...
+ // Get the blocks that this node is control dependent on...
const PostDominanceFrontier::DomSetType &CDB = It->second;
for_each(CDB.begin(), CDB.end(), // Mark all their terminators as live
bind_obj(this, &ADCE::markTerminatorLive));
}
- // If this basic block is live, then the terminator must be as well!
- markTerminatorLive(BB);
+ // If this basic block is live, and it ends in an unconditional branch, then
+ // the branch is alive as well...
+ if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
+ if (BI->isUnconditional())
+ 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;
+}
+
+
+/// convertToUnconditionalBranch - Transform this conditional terminator
+/// instruction into an unconditional branch because we don't care which of the
+/// successors it goes to. This eliminate a use of the condition as well.
+///
+TerminatorInst *ADCE::convertToUnconditionalBranch(TerminatorInst *TI) {
+ BranchInst *NB = new BranchInst(TI->getSuccessor(0), TI);
+ BasicBlock *BB = TI->getParent();
+
+ // Remove entries from PHI nodes to avoid confusing ourself later...
+ for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
+ TI->getSuccessor(i)->removePredecessor(BB);
+
+ // Delete the old branch itself...
+ BB->getInstList().erase(TI);
+ return NB;
}
BBI != BBE; ++BBI) {
BasicBlock *BB = *BBI;
for (BasicBlock::iterator II = BB->begin(), EI = BB->end(); II != EI; ) {
- if (II->hasSideEffects() || II->getOpcode() == Instruction::Ret) {
+ if (II->mayWriteToMemory() || isa<ReturnInst>(II) || isa<UnwindInst>(II)){
markInstructionLive(II);
++II; // Increment the inst iterator if the inst wasn't deleted
} else if (isInstructionTriviallyDead(II)) {
}
}
- DEBUG(cerr << "Processing work list\n");
+ // Check to ensure we have an exit node for this CFG. If we don't, we won't
+ // have any post-dominance information, thus we cannot perform our
+ // transformations safely.
+ //
+ PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
+ if (DT[&Func->getEntryBlock()] == 0) {
+ WorkList.clear();
+ return MadeChanges;
+ }
+
+ DEBUG(std::cerr << "Processing work list\n");
// AliveBlocks - Set of basic blocks that we know have instructions that are
// alive in them...
std::set<BasicBlock*> AliveBlocks;
// Process the work list of instructions that just became live... if they
- // became live, then that means that all of their operands are neccesary as
+ // became live, then that means that all of their operands are necessary as
// well... make them live as well.
//
while (!WorkList.empty()) {
markInstructionLive(Operand);
}
- if (DebugFlag) {
- cerr << "Current Function: X = Live\n";
- for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I)
+ DEBUG(
+ std::cerr << "Current Function: X = Live\n";
+ for (Function::iterator I = Func->begin(), E = Func->end(); I != E; ++I){
+ std::cerr << I->getName() << ":\t"
+ << (AliveBlocks.count(I) ? "LIVE\n" : "DEAD\n");
for (BasicBlock::iterator BI = I->begin(), BE = I->end(); BI != BE; ++BI){
- if (LiveSet.count(BI)) cerr << "X ";
- cerr << *BI;
+ if (LiveSet.count(BI)) std::cerr << "X ";
+ std::cerr << *BI;
}
- }
+ });
// Find the first postdominator of the entry node that is alive. Make it the
// new entry node...
//
- PostDominatorTree &DT = getAnalysis<PostDominatorTree>();
-
- // 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->getBasicBlockList().push_front(NewEntry);
- AliveBlocks.insert(NewEntry); // This block is always alive!
+ 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);
+
+ // Check to make sure the terminator instruction is live. If it isn't,
+ // this means that the condition that it branches on (we know it is not an
+ // unconditional branch), is not needed to make the decision of where to
+ // go to, because all outgoing edges go to the same place. We must remove
+ // the use of the condition (because it's probably dead), so we convert
+ // the terminator to a conditional branch.
+ //
+ TerminatorInst *TI = I->getTerminator();
+ if (!LiveSet.count(TI))
+ convertToUnconditionalBranch(TI);
+ }
+
+ } 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();
+ NewEntry->getInstList().push_back(new BranchInst(&Func->front()));
+ Func->getBasicBlockList().push_front(NewEntry);
+ AliveBlocks.insert(NewEntry); // This block is always alive!
+ LiveSet.insert(NewEntry->getTerminator()); // The branch is live
+ }
// 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
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)
+ // If the terminator instruction is alive, but the block it is contained
+ // in IS alive, this means that this terminator is a conditional branch
+ // on a condition that doesn't matter. Make it an unconditional branch
+ // to ONE of the successors. This has the side effect of dropping a use
+ // of the conditional value, which may also be dead.
+ if (!LiveSet.count(TI))
+ TI = convertToUnconditionalBranch(TI);
+
+ // 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...
//
PostDominatorTree::Node *LastNode = DT[TI->getSuccessor(i)];
- 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.
+
+ // 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->getBlock())) {
+ LastNode = NextNode;
+ NextNode = NextNode->getIDom();
+ }
+
+ // Get the basic blocks that we need...
+ BasicBlock *LastDead = LastNode->getBlock();
+ BasicBlock *NextAlive = NextNode->getBlock();
+
+ // 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)
+ if (LiveSet.count(PN)) { // Only modify live phi nodes
+ // 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(); 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
+ // We make changes if there are any dead blocks in the function...
+ if (unsigned NumDeadBlocks = Func->size() - AliveBlocks.size()) {
+ MadeChanges = true;
+ NumBlockRemoved += NumDeadBlocks;
+ }
+
+ // Loop over all of the basic blocks in the function, removing control flow
+ // edges to live blocks (also eliminating any entries in PHI functions in
+ // referenced blocks).
//
- for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB) {
+ 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.
- vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
+ std::vector<BasicBlock*> Succs(succ_begin(BB), succ_end(BB));
if (!Succs.empty()) {
// Loop over all of the successors, removing this block from PHI node
// Delete the old terminator instruction...
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);
+ BB->getInstList().push_back(new ReturnInst(RetTy != Type::VoidTy ?
+ Constant::getNullValue(RetTy) : 0));
}
+ }
+
+ // Loop over all of the basic blocks in the function, dropping references of
+ // the dead basic blocks. We must do this after the previous step to avoid
+ // dropping references to PHIs which still have entries...
+ //
+ for (Function::iterator BB = Func->begin(), E = Func->end(); BB != E; ++BB)
+ if (!AliveBlocks.count(BB))
BB->dropAllReferences();
- ++NumBlockRemoved;
- MadeChanges = true;
- }
- }
// Now loop through all of the blocks and delete the dead ones. We can safely
// do this now because we know that there are no references to dead blocks
// instructions from alive blocks.
//
for (Function::iterator BI = Func->begin(); BI != Func->end(); )
- if (!AliveBlocks.count(BI))
+ if (!AliveBlocks.count(BI)) { // Delete dead blocks...
BI = Func->getBasicBlockList().erase(BI);
- else {
+ } 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...