llvm-6502/lib/Transforms/Scalar/TailDuplication.cpp
2003-08-31 21:17:44 +00:00

335 lines
14 KiB
C++

//===- TailDuplication.cpp - Simplify CFG through tail duplication --------===//
//
// This pass performs a limited form of tail duplication, intended to simplify
// CFGs by removing some unconditional branches. This pass is necessary to
// straighten out loops created by the C front-end, but also is capable of
// making other code nicer. After this pass is run, the CFG simplify pass
// should be run to clean up the mess.
//
// This pass could be enhanced in the future to use profile information to be
// more aggressive.
//
//===----------------------------------------------------------------------===//
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constant.h"
#include "llvm/Function.h"
#include "llvm/iPHINode.h"
#include "llvm/iTerminators.h"
#include "llvm/Pass.h"
#include "llvm/Type.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/ValueHolder.h"
#include "llvm/Transforms/Utils/Local.h"
#include "Support/Debug.h"
#include "Support/Statistic.h"
namespace {
Statistic<> NumEliminated("tailduplicate",
"Number of unconditional branches eliminated");
Statistic<> NumPHINodes("tailduplicate", "Number of phi nodes inserted");
class TailDup : public FunctionPass {
bool runOnFunction(Function &F);
private:
inline bool shouldEliminateUnconditionalBranch(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");
}
Pass *createTailDuplicationPass() { return new TailDup(); }
/// runOnFunction - Top level algorithm - Loop over each unconditional branch in
/// the function, eliminating it if it looks attractive enough.
///
bool TailDup::runOnFunction(Function &F) {
bool Changed = false;
for (Function::iterator I = F.begin(), E = F.end(); I != E; )
if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
Changed = true;
} else {
++I;
}
return Changed;
}
/// shouldEliminateUnconditionalBranch - Return true if this branch looks
/// attractive to eliminate. We eliminate the branch if the destination basic
/// block has <= 5 instructions in it, not counting PHI nodes. In practice,
/// since one of these is a terminator instruction, this means that we will add
/// up to 4 instructions to the new block.
///
/// We don't count PHI nodes in the count since they will be removed when the
/// contents of the block are copied over.
///
bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
BranchInst *BI = dyn_cast<BranchInst>(TI);
if (!BI || !BI->isUnconditional()) return false; // Not an uncond branch!
BasicBlock *Dest = BI->getSuccessor(0);
if (Dest == BI->getParent()) return false; // Do not loop infinitely!
// Do not inline a block if we will just get another branch to the same block!
if (BranchInst *DBI = dyn_cast<BranchInst>(Dest->getTerminator()))
if (DBI->isUnconditional() && DBI->getSuccessor(0) == Dest)
return false; // Do not loop infinitely!
// 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 false; // It's just a dead block, ignore it...
// Also, do not bother with blocks with only a single predecessor: simplify
// CFG will fold these two blocks together!
++PI;
if (PI == PE) return false; // Exactly one predecessor!
BasicBlock::iterator I = Dest->begin();
while (isa<PHINode>(*I)) ++I;
for (unsigned Size = 0; I != Dest->end(); ++Size, ++I)
if (Size == 6) return false; // The block is too large...
return true;
}
/// eliminateUnconditionalBranch - Clone the instructions from the destination
/// block into the source block, eliminating the specified unconditional branch.
/// If the destination block defines values used by successors of the dest
/// block, we may need to insert PHI nodes.
///
void TailDup::eliminateUnconditionalBranch(BranchInst *Branch) {
BasicBlock *SourceBlock = Branch->getParent();
BasicBlock *DestBlock = Branch->getSuccessor(0);
assert(SourceBlock != DestBlock && "Our predicate is broken!");
DEBUG(std::cerr << "TailDuplication[" << SourceBlock->getParent()->getName()
<< "]: Eliminating branch: " << *Branch);
// 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
// them to our mapping.
//
std::map<Value*, Value*> ValueMapping;
BasicBlock::iterator BI = DestBlock->begin();
bool HadPHINodes = isa<PHINode>(BI);
for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
ValueMapping[PN] = PN->getIncomingValueForBlock(SourceBlock);
// Clone the non-phi instructions of the dest block into the source block,
// keeping track of the mapping...
//
for (; BI != DestBlock->end(); ++BI) {
Instruction *New = BI->clone();
New->setName(BI->getName());
SourceBlock->getInstList().push_back(New);
ValueMapping[BI] = New;
}
// Now that we have built the mapping information and cloned all of the
// instructions (giving us a new terminator, among other things), walk the new
// instructions, rewriting references of old instructions to use new
// instructions.
//
BI = Branch; ++BI; // Get an iterator to the first new instruction
for (; BI != SourceBlock->end(); ++BI)
for (unsigned i = 0, e = BI->getNumOperands(); i != e; ++i)
if (Value *Remapped = ValueMapping[BI->getOperand(i)])
BI->setOperand(i, Remapped);
// Next we check to see if any of the successors of DestBlock had PHI nodes.
// If so, we need to add entries to the PHI nodes for SourceBlock now.
for (succ_iterator SI = succ_begin(DestBlock), SE = succ_end(DestBlock);
SI != SE; ++SI) {
BasicBlock *Succ = *SI;
for (BasicBlock::iterator PNI = Succ->begin();
PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
// Ok, we have a PHI node. Figure out what the incoming value was for the
// DestBlock.
Value *IV = PN->getIncomingValueForBlock(DestBlock);
// Remap the value if necessary...
if (Value *MappedIV = ValueMapping[IV])
IV = MappedIV;
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);
// 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);
}