1 //===- TailDuplication.cpp - Simplify CFG through tail duplication --------===//
3 // This pass performs a limited form of tail duplication, intended to simplify
4 // CFGs by removing some unconditional branches. This pass is necessary to
5 // straighten out loops created by the C front-end, but also is capable of
6 // making other code nicer. After this pass is run, the CFG simplify pass
7 // should be run to clean up the mess.
9 // This pass could be enhanced in the future to use profile information to be
12 //===----------------------------------------------------------------------===//
14 #include "llvm/Transforms/Scalar.h"
15 #include "llvm/Function.h"
16 #include "llvm/iPHINode.h"
17 #include "llvm/iTerminators.h"
18 #include "llvm/Pass.h"
19 #include "llvm/Type.h"
20 #include "llvm/Support/CFG.h"
21 #include "llvm/Transforms/Utils/Local.h"
22 #include "Support/Debug.h"
23 #include "Support/Statistic.h"
26 Statistic<> NumEliminated("tailduplicate",
27 "Number of unconditional branches eliminated");
28 Statistic<> NumPHINodes("tailduplicate", "Number of phi nodes inserted");
30 class TailDup : public FunctionPass {
31 bool runOnFunction(Function &F);
33 inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI);
34 inline void eliminateUnconditionalBranch(BranchInst *BI);
35 inline void InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
36 BasicBlock *NewBlock);
37 inline Value *GetValueInBlock(BasicBlock *BB, Value *OrigVal,
38 std::map<BasicBlock*, Value*> &ValueMap,
39 std::map<BasicBlock*, Value*> &OutValueMap);
40 inline Value *GetValueOutBlock(BasicBlock *BB, Value *OrigVal,
41 std::map<BasicBlock*, Value*> &ValueMap,
42 std::map<BasicBlock*, Value*> &OutValueMap);
44 RegisterOpt<TailDup> X("tailduplicate", "Tail Duplication");
47 Pass *createTailDuplicationPass() { return new TailDup(); }
49 /// runOnFunction - Top level algorithm - Loop over each unconditional branch in
50 /// the function, eliminating it if it looks attractive enough.
52 bool TailDup::runOnFunction(Function &F) {
54 for (Function::iterator I = F.begin(), E = F.end(); I != E; )
55 if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
56 eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
64 /// shouldEliminateUnconditionalBranch - Return true if this branch looks
65 /// attractive to eliminate. We eliminate the branch if the destination basic
66 /// block has <= 5 instructions in it, not counting PHI nodes. In practice,
67 /// since one of these is a terminator instruction, this means that we will add
68 /// up to 4 instructions to the new block.
70 /// We don't count PHI nodes in the count since they will be removed when the
71 /// contents of the block are copied over.
73 bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
74 BranchInst *BI = dyn_cast<BranchInst>(TI);
75 if (!BI || !BI->isUnconditional()) return false; // Not an uncond branch!
77 BasicBlock *Dest = BI->getSuccessor(0);
78 if (Dest == BI->getParent()) return false; // Do not loop infinitely!
80 // Do not inline a block if we will just get another branch to the same block!
81 if (BranchInst *DBI = dyn_cast<BranchInst>(Dest->getTerminator()))
82 if (DBI->isUnconditional() && DBI->getSuccessor(0) == Dest)
83 return false; // Do not loop infinitely!
85 // Do not bother working on dead blocks...
86 pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest);
87 if (PI == PE && Dest != Dest->getParent()->begin())
88 return false; // It's just a dead block, ignore it...
90 // Also, do not bother with blocks with only a single predecessor: simplify
91 // CFG will fold these two blocks together!
93 if (PI == PE) return false; // Exactly one predecessor!
95 BasicBlock::iterator I = Dest->begin();
96 while (isa<PHINode>(*I)) ++I;
98 for (unsigned Size = 0; I != Dest->end(); ++Size, ++I)
99 if (Size == 6) return false; // The block is too large...
104 /// eliminateUnconditionalBranch - Clone the instructions from the destination
105 /// block into the source block, eliminating the specified unconditional branch.
106 /// If the destination block defines values used by successors of the dest
107 /// block, we may need to insert PHI nodes.
109 void TailDup::eliminateUnconditionalBranch(BranchInst *Branch) {
110 BasicBlock *SourceBlock = Branch->getParent();
111 BasicBlock *DestBlock = Branch->getSuccessor(0);
112 assert(SourceBlock != DestBlock && "Our predicate is broken!");
114 DEBUG(std::cerr << "TailDuplication[" << SourceBlock->getParent()->getName()
115 << "]: Eliminating branch: " << *Branch);
117 // We are going to have to map operands from the original block B to the new
118 // copy of the block B'. If there are PHI nodes in the DestBlock, these PHI
119 // nodes also define part of this mapping. Loop over these PHI nodes, adding
120 // them to our mapping.
122 std::map<Value*, Value*> ValueMapping;
124 BasicBlock::iterator BI = DestBlock->begin();
125 bool HadPHINodes = isa<PHINode>(BI);
126 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
127 ValueMapping[PN] = PN->getIncomingValueForBlock(SourceBlock);
129 // Clone the non-phi instructions of the dest block into the source block,
130 // keeping track of the mapping...
132 for (; BI != DestBlock->end(); ++BI) {
133 Instruction *New = BI->clone();
134 New->setName(BI->getName());
135 SourceBlock->getInstList().push_back(New);
136 ValueMapping[BI] = New;
139 // Now that we have built the mapping information and cloned all of the
140 // instructions (giving us a new terminator, among other things), walk the new
141 // instructions, rewriting references of old instructions to use new
144 BI = Branch; ++BI; // Get an iterator to the first new instruction
145 for (; BI != SourceBlock->end(); ++BI)
146 for (unsigned i = 0, e = BI->getNumOperands(); i != e; ++i)
147 if (Value *Remapped = ValueMapping[BI->getOperand(i)])
148 BI->setOperand(i, Remapped);
150 // Next we check to see if any of the successors of DestBlock had PHI nodes.
151 // If so, we need to add entries to the PHI nodes for SourceBlock now.
152 for (succ_iterator SI = succ_begin(DestBlock), SE = succ_end(DestBlock);
154 BasicBlock *Succ = *SI;
155 for (BasicBlock::iterator PNI = Succ->begin();
156 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
157 // Ok, we have a PHI node. Figure out what the incoming value was for the
159 Value *IV = PN->getIncomingValueForBlock(DestBlock);
161 // Remap the value if necessary...
162 if (Value *MappedIV = ValueMapping[IV])
164 PN->addIncoming(IV, SourceBlock);
168 // Now that all of the instructions are correctly copied into the SourceBlock,
169 // we have one more minor problem: the successors of the original DestBB may
170 // use the values computed in DestBB either directly (if DestBB dominated the
171 // block), or through a PHI node. In either case, we need to insert PHI nodes
172 // into any successors of DestBB (which are now our successors) for each value
173 // that is computed in DestBB, but is used outside of it. All of these uses
174 // we have to rewrite with the new PHI node.
176 if (succ_begin(SourceBlock) != succ_end(SourceBlock)) // Avoid wasting time...
177 for (BI = DestBlock->begin(); BI != DestBlock->end(); ++BI)
178 if (BI->getType() != Type::VoidTy)
179 InsertPHINodesIfNecessary(BI, ValueMapping[BI], SourceBlock);
181 // Final step: now that we have finished everything up, walk the cloned
182 // instructions one last time, constant propagating and DCE'ing them, because
183 // they may not be needed anymore.
185 BI = Branch; ++BI; // Get an iterator to the first new instruction
187 while (BI != SourceBlock->end())
188 if (!dceInstruction(BI) && !doConstantPropagation(BI))
191 DestBlock->removePredecessor(SourceBlock); // Remove entries in PHI nodes...
192 SourceBlock->getInstList().erase(Branch); // Destroy the uncond branch...
194 ++NumEliminated; // We just killed a branch!
197 /// InsertPHINodesIfNecessary - So at this point, we cloned the OrigInst
198 /// instruction into the NewBlock with the value of NewInst. If OrigInst was
199 /// used outside of its defining basic block, we need to insert a PHI nodes into
202 void TailDup::InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
203 BasicBlock *NewBlock) {
204 // Loop over all of the uses of OrigInst, rewriting them to be newly inserted
205 // PHI nodes, unless they are in the same basic block as OrigInst.
206 BasicBlock *OrigBlock = OrigInst->getParent();
207 std::vector<Instruction*> Users;
208 Users.reserve(OrigInst->use_size());
209 for (Value::use_iterator I = OrigInst->use_begin(), E = OrigInst->use_end();
211 Instruction *In = cast<Instruction>(*I);
212 if (In->getParent() != OrigBlock || // Don't modify uses in the orig block!
217 // The common case is that the instruction is only used within the block that
218 // defines it. If we have this case, quick exit.
220 if (Users.empty()) return;
222 // Otherwise, we have a more complex case, handle it now. This requires the
223 // construction of a mapping between a basic block and the value to use when
224 // in the scope of that basic block. This map will map to the original and
225 // new values when in the original or new block, but will map to inserted PHI
226 // nodes when in other blocks.
228 std::map<BasicBlock*, Value*> ValueMap;
229 std::map<BasicBlock*, Value*> OutValueMap; // The outgoing value map
230 OutValueMap[OrigBlock] = OrigInst;
231 OutValueMap[NewBlock ] = NewInst; // Seed the initial values...
233 DEBUG(std::cerr << " ** Inserting PHI nodes for " << OrigInst);
234 while (!Users.empty()) {
235 Instruction *User = Users.back(); Users.pop_back();
237 if (PHINode *PN = dyn_cast<PHINode>(User)) {
238 // PHI nodes must be handled specially here, because their operands are
239 // actually defined in predecessor basic blocks, NOT in the block that the
240 // PHI node lives in. Note that we have already added entries to PHI nods
241 // which are in blocks that are immediate successors of OrigBlock, so
242 // don't modify them again.
243 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
244 if (PN->getIncomingValue(i) == OrigInst &&
245 PN->getIncomingBlock(i) != OrigBlock) {
246 Value *V = GetValueOutBlock(PN->getIncomingBlock(i), OrigInst,
247 ValueMap, OutValueMap);
248 PN->setIncomingValue(i, V);
252 // Any other user of the instruction can just replace any uses with the
253 // new value defined in the block it resides in.
254 Value *V = GetValueInBlock(User->getParent(), OrigInst, ValueMap,
256 User->replaceUsesOfWith(OrigInst, V);
261 /// GetValueInBlock - This is a recursive method which inserts PHI nodes into
262 /// the function until there is a value available in basic block BB.
264 Value *TailDup::GetValueInBlock(BasicBlock *BB, Value *OrigVal,
265 std::map<BasicBlock*, Value*> &ValueMap,
266 std::map<BasicBlock*, Value*> &OutValueMap) {
267 Value*& BBVal = ValueMap[BB];
268 if (BBVal) return BBVal; // Value already computed for this block?
270 assert(pred_begin(BB) != pred_end(BB) &&
271 "Propagating PHI nodes to unreachable blocks?");
273 // If there is no value already available in this basic block, we need to
274 // either reuse a value from an incoming, dominating, basic block, or we need
275 // to create a new PHI node to merge in different incoming values. Because we
276 // don't know if we're part of a loop at this point or not, we create a PHI
277 // node, even if we will ultimately eliminate it.
278 PHINode *PN = new PHINode(OrigVal->getType(), OrigVal->getName()+".pn",
280 BBVal = PN; // Insert this into the BBVal slot in case of cycles...
282 Value*& BBOutVal = OutValueMap[BB];
283 if (BBOutVal == 0) BBOutVal = PN;
285 // Now that we have created the PHI node, loop over all of the predecessors of
286 // this block, computing an incoming value for the predecessor.
287 std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
288 for (unsigned i = 0, e = Preds.size(); i != e; ++i)
289 PN->addIncoming(GetValueOutBlock(Preds[i], OrigVal, ValueMap, OutValueMap),
292 // The PHI node is complete. In many cases, however the PHI node was
293 // ultimately unnecessary: we could have just reused a dominating incoming
294 // value. If this is the case, nuke the PHI node and replace the map entry
295 // with the dominating value.
297 assert(PN->getNumIncomingValues() > 0 && "No predecessors?");
299 // Check to see if all of the elements in the PHI node are either the PHI node
300 // itself or ONE particular value.
302 Value *ReplVal = PN->getIncomingValue(i);
303 for (; ReplVal == PN && i != PN->getNumIncomingValues(); ++i)
304 ReplVal = PN->getIncomingValue(i); // Skip values equal to the PN
306 for (; i != PN->getNumIncomingValues(); ++i)
307 if (PN->getIncomingValue(i) != PN && PN->getIncomingValue(i) != ReplVal) {
312 // Found a value to replace the PHI node with?
313 if (ReplVal && ReplVal != PN) {
314 PN->replaceAllUsesWith(ReplVal);
316 if (BBOutVal == PN) BBOutVal = ReplVal;
317 BB->getInstList().erase(PN); // Erase the PHI node...
325 Value *TailDup::GetValueOutBlock(BasicBlock *BB, Value *OrigVal,
326 std::map<BasicBlock*, Value*> &ValueMap,
327 std::map<BasicBlock*, Value*> &OutValueMap) {
328 Value*& BBVal = OutValueMap[BB];
329 if (BBVal) return BBVal; // Value already computed for this block?
331 return BBVal = GetValueInBlock(BB, OrigVal, ValueMap, OutValueMap);