bool runOnFunction(Function &F);
private:
inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI);
- inline bool canEliminateUnconditionalBranch(TerminatorInst *TI);
inline void eliminateUnconditionalBranch(BranchInst *BI);
- inline void InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
- BasicBlock *NewBlock);
- inline Value *GetValueInBlock(BasicBlock *BB, Value *OrigVal,
- std::map<BasicBlock*, ValueHolder> &ValueMap,
- std::map<BasicBlock*, ValueHolder> &OutValueMap);
- inline Value *GetValueOutBlock(BasicBlock *BB, Value *OrigVal,
- std::map<BasicBlock*, ValueHolder> &ValueMap,
- std::map<BasicBlock*, ValueHolder> &OutValueMap);
};
RegisterOpt<TailDup> X("tailduplicate", "Tail Duplication");
}
bool TailDup::runOnFunction(Function &F) {
bool Changed = false;
for (Function::iterator I = F.begin(), E = F.end(); I != E; )
- if (shouldEliminateUnconditionalBranch(I->getTerminator()) &&
- canEliminateUnconditionalBranch(I->getTerminator())) {
+ if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
Changed = true;
} else {
if (DBI->isUnconditional() && DBI->getSuccessor(0) == Dest)
return false; // Do not loop infinitely!
+ // FIXME: DemoteRegToStack cannot yet demote invoke instructions to the stack,
+ // because doing so would require breaking critical edges. This should be
+ // fixed eventually.
+ if (!DTI->use_empty())
+ return false;
+
// Do not bother working on dead blocks...
pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest);
if (PI == PE && Dest != Dest->getParent()->begin())
return true;
}
-/// canEliminateUnconditionalBranch - Unfortunately, the general form of tail
-/// duplication can do very bad things to SSA form, by destroying arbitrary
-/// relationships between dominators and dominator frontiers as it processes the
-/// program. The right solution for this is to have an incrementally updating
-/// dominator data structure, which can gracefully react to arbitrary
-/// "addEdge/removeEdge" changes to the CFG. Implementing this is nontrivial,
-/// however, so we just disable the transformation in cases where it is not
-/// currently safe.
-///
-bool TailDup::canEliminateUnconditionalBranch(TerminatorInst *TI) {
- // Basically, we refuse to make the transformation if any of the values
- // computed in the 'tail' are used in any other basic blocks.
- BasicBlock *BB = TI->getParent();
- BasicBlock *Tail = TI->getSuccessor(0);
- assert(isa<BranchInst>(TI) && cast<BranchInst>(TI)->isUnconditional());
-
- for (BasicBlock::iterator I = Tail->begin(), E = Tail->end(); I != E; ++I)
- for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
- ++UI) {
- Instruction *User = cast<Instruction>(*UI);
- if (User->getParent() != Tail && User->getParent() != BB)
- return false;
-
- // The 'swap' problem foils the tail duplication rewriting code.
- if (isa<PHINode>(User) && User->getParent() == Tail)
- return false;
- }
- return true;
-}
-
/// eliminateUnconditionalBranch - Clone the instructions from the destination
/// block into the source block, eliminating the specified unconditional branch.
DEBUG(std::cerr << "TailDuplication[" << SourceBlock->getParent()->getName()
<< "]: Eliminating branch: " << *Branch);
+ // Tail duplication can not update SSA properties correctly if the values
+ // defined in the duplicated tail are used outside of the tail itself. For
+ // this reason, we spill all values that are used outside of the tail to the
+ // stack.
+ for (BasicBlock::iterator I = DestBlock->begin(); I != DestBlock->end(); ++I)
+ for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
+ ++UI) {
+ bool ShouldDemote = false;
+ if (cast<Instruction>(*UI)->getParent() != DestBlock) {
+ // We must allow our successors to use tail values in their PHI nodes
+ // (if the incoming value corresponds to the tail block).
+ if (PHINode *PN = dyn_cast<PHINode>(*UI)) {
+ for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+ if (PN->getIncomingValue(i) == I &&
+ PN->getIncomingBlock(i) != DestBlock) {
+ ShouldDemote = true;
+ break;
+ }
+
+ } else {
+ ShouldDemote = true;
+ }
+ } else if (PHINode *PN = dyn_cast<PHINode>(cast<Instruction>(*UI))) {
+ // If the user of this instruction is a PHI node in the current block,
+ // spill the value.
+ ShouldDemote = true;
+ }
+
+ if (ShouldDemote) {
+ // We found a use outside of the tail. Create a new stack slot to
+ // break this inter-block usage pattern.
+ DemoteRegToStack(*I);
+ break;
+ }
+ }
+
// We are going to have to map operands from the original block B to the new
// copy of the block B'. If there are PHI nodes in the DestBlock, these PHI
// nodes also define part of this mapping. Loop over these PHI nodes, adding
PN->addIncoming(IV, SourceBlock);
}
}
-
- // Now that all of the instructions are correctly copied into the SourceBlock,
- // we have one more minor problem: the successors of the original DestBB may
- // use the values computed in DestBB either directly (if DestBB dominated the
- // block), or through a PHI node. In either case, we need to insert PHI nodes
- // into any successors of DestBB (which are now our successors) for each value
- // that is computed in DestBB, but is used outside of it. All of these uses
- // we have to rewrite with the new PHI node.
- //
- if (succ_begin(SourceBlock) != succ_end(SourceBlock)) // Avoid wasting time...
- for (BI = DestBlock->begin(); BI != DestBlock->end(); ++BI)
- if (BI->getType() != Type::VoidTy)
- InsertPHINodesIfNecessary(BI, ValueMapping[BI], SourceBlock);
+
+ // Next, remove the old branch instruction, and any PHI node entries that we
+ // had.
+ BI = Branch; ++BI; // Get an iterator to the first new instruction
+ DestBlock->removePredecessor(SourceBlock); // Remove entries in PHI nodes...
+ SourceBlock->getInstList().erase(Branch); // Destroy the uncond branch...
// Final step: now that we have finished everything up, walk the cloned
// instructions one last time, constant propagating and DCE'ing them, because
// they may not be needed anymore.
//
- BI = Branch; ++BI; // Get an iterator to the first new instruction
if (HadPHINodes)
while (BI != SourceBlock->end())
if (!dceInstruction(BI) && !doConstantPropagation(BI))
++BI;
- DestBlock->removePredecessor(SourceBlock); // Remove entries in PHI nodes...
- SourceBlock->getInstList().erase(Branch); // Destroy the uncond branch...
-
++NumEliminated; // We just killed a branch!
}
-
-/// InsertPHINodesIfNecessary - So at this point, we cloned the OrigInst
-/// instruction into the NewBlock with the value of NewInst. If OrigInst was
-/// used outside of its defining basic block, we need to insert a PHI nodes into
-/// the successors.
-///
-void TailDup::InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
- BasicBlock *NewBlock) {
- // Loop over all of the uses of OrigInst, rewriting them to be newly inserted
- // PHI nodes, unless they are in the same basic block as OrigInst.
- BasicBlock *OrigBlock = OrigInst->getParent();
- std::vector<Instruction*> Users;
- Users.reserve(OrigInst->use_size());
- for (Value::use_iterator I = OrigInst->use_begin(), E = OrigInst->use_end();
- I != E; ++I) {
- Instruction *In = cast<Instruction>(*I);
- if (In->getParent() != OrigBlock || // Don't modify uses in the orig block!
- isa<PHINode>(In))
- Users.push_back(In);
- }
-
- // The common case is that the instruction is only used within the block that
- // defines it. If we have this case, quick exit.
- //
- if (Users.empty()) return;
-
- // Otherwise, we have a more complex case, handle it now. This requires the
- // construction of a mapping between a basic block and the value to use when
- // in the scope of that basic block. This map will map to the original and
- // new values when in the original or new block, but will map to inserted PHI
- // nodes when in other blocks.
- //
- std::map<BasicBlock*, ValueHolder> ValueMap;
- std::map<BasicBlock*, ValueHolder> OutValueMap; // The outgoing value map
- OutValueMap[OrigBlock] = OrigInst;
- OutValueMap[NewBlock ] = NewInst; // Seed the initial values...
-
- DEBUG(std::cerr << " ** Inserting PHI nodes for " << OrigInst);
- while (!Users.empty()) {
- Instruction *User = Users.back(); Users.pop_back();
-
- if (PHINode *PN = dyn_cast<PHINode>(User)) {
- // PHI nodes must be handled specially here, because their operands are
- // actually defined in predecessor basic blocks, NOT in the block that the
- // PHI node lives in. Note that we have already added entries to PHI nods
- // which are in blocks that are immediate successors of OrigBlock, so
- // don't modify them again.
- for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
- if (PN->getIncomingValue(i) == OrigInst &&
- PN->getIncomingBlock(i) != OrigBlock) {
- Value *V = GetValueOutBlock(PN->getIncomingBlock(i), OrigInst,
- ValueMap, OutValueMap);
- PN->setIncomingValue(i, V);
- }
-
- } else {
- // Any other user of the instruction can just replace any uses with the
- // new value defined in the block it resides in.
- Value *V = GetValueInBlock(User->getParent(), OrigInst, ValueMap,
- OutValueMap);
- User->replaceUsesOfWith(OrigInst, V);
- }
- }
-}
-
-/// GetValueInBlock - This is a recursive method which inserts PHI nodes into
-/// the function until there is a value available in basic block BB.
-///
-Value *TailDup::GetValueInBlock(BasicBlock *BB, Value *OrigVal,
- std::map<BasicBlock*, ValueHolder> &ValueMap,
- std::map<BasicBlock*,ValueHolder> &OutValueMap){
- ValueHolder &BBVal = ValueMap[BB];
- if (BBVal) return BBVal; // Value already computed for this block?
-
- // If this block has no predecessors, then it must be unreachable, thus, it
- // doesn't matter which value we use.
- if (pred_begin(BB) == pred_end(BB))
- return BBVal = Constant::getNullValue(OrigVal->getType());
-
- // If there is no value already available in this basic block, we need to
- // either reuse a value from an incoming, dominating, basic block, or we need
- // to create a new PHI node to merge in different incoming values. Because we
- // don't know if we're part of a loop at this point or not, we create a PHI
- // node, even if we will ultimately eliminate it.
- PHINode *PN = new PHINode(OrigVal->getType(), OrigVal->getName()+".pn",
- BB->begin());
- BBVal = PN; // Insert this into the BBVal slot in case of cycles...
-
- ValueHolder &BBOutVal = OutValueMap[BB];
- if (BBOutVal == 0) BBOutVal = PN;
-
- // Now that we have created the PHI node, loop over all of the predecessors of
- // this block, computing an incoming value for the predecessor.
- std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
- for (unsigned i = 0, e = Preds.size(); i != e; ++i)
- PN->addIncoming(GetValueOutBlock(Preds[i], OrigVal, ValueMap, OutValueMap),
- Preds[i]);
-
- // The PHI node is complete. In many cases, however the PHI node was
- // ultimately unnecessary: we could have just reused a dominating incoming
- // value. If this is the case, nuke the PHI node and replace the map entry
- // with the dominating value.
- //
- assert(PN->getNumIncomingValues() > 0 && "No predecessors?");
-
- // Check to see if all of the elements in the PHI node are either the PHI node
- // itself or ONE particular value.
- unsigned i = 0;
- Value *ReplVal = PN->getIncomingValue(i);
- for (; ReplVal == PN && i != PN->getNumIncomingValues(); ++i)
- ReplVal = PN->getIncomingValue(i); // Skip values equal to the PN
-
- for (; i != PN->getNumIncomingValues(); ++i)
- if (PN->getIncomingValue(i) != PN && PN->getIncomingValue(i) != ReplVal) {
- ReplVal = 0;
- break;
- }
-
- // Found a value to replace the PHI node with?
- if (ReplVal && ReplVal != PN) {
- PN->replaceAllUsesWith(ReplVal);
- BB->getInstList().erase(PN); // Erase the PHI node...
- } else {
- ++NumPHINodes;
- }
-
- return BBVal;
-}
-
-Value *TailDup::GetValueOutBlock(BasicBlock *BB, Value *OrigVal,
- std::map<BasicBlock*, ValueHolder> &ValueMap,
- std::map<BasicBlock*, ValueHolder> &OutValueMap) {
- ValueHolder &BBVal = OutValueMap[BB];
- if (BBVal) return BBVal; // Value already computed for this block?
-
- return GetValueInBlock(BB, OrigVal, ValueMap, OutValueMap);
-}