1 //===- JumpThreading.cpp - Thread control through conditional blocks ------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements the Jump Threading pass.
12 //===----------------------------------------------------------------------===//
14 #define DEBUG_TYPE "jump-threading"
15 #include "llvm/Transforms/Scalar.h"
16 #include "llvm/IntrinsicInst.h"
17 #include "llvm/LLVMContext.h"
18 #include "llvm/Pass.h"
19 #include "llvm/Analysis/InstructionSimplify.h"
20 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
21 #include "llvm/Transforms/Utils/Local.h"
22 #include "llvm/Transforms/Utils/SSAUpdater.h"
23 #include "llvm/Target/TargetData.h"
24 #include "llvm/ADT/DenseMap.h"
25 #include "llvm/ADT/Statistic.h"
26 #include "llvm/ADT/STLExtras.h"
27 #include "llvm/ADT/SmallPtrSet.h"
28 #include "llvm/ADT/SmallSet.h"
29 #include "llvm/Support/CommandLine.h"
30 #include "llvm/Support/Debug.h"
31 #include "llvm/Support/raw_ostream.h"
34 STATISTIC(NumThreads, "Number of jumps threaded");
35 STATISTIC(NumFolds, "Number of terminators folded");
36 STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
38 static cl::opt<unsigned>
39 Threshold("jump-threading-threshold",
40 cl::desc("Max block size to duplicate for jump threading"),
41 cl::init(6), cl::Hidden);
44 /// This pass performs 'jump threading', which looks at blocks that have
45 /// multiple predecessors and multiple successors. If one or more of the
46 /// predecessors of the block can be proven to always jump to one of the
47 /// successors, we forward the edge from the predecessor to the successor by
48 /// duplicating the contents of this block.
50 /// An example of when this can occur is code like this:
57 /// In this case, the unconditional branch at the end of the first if can be
58 /// revectored to the false side of the second if.
60 class JumpThreading : public FunctionPass {
63 SmallPtrSet<BasicBlock*, 16> LoopHeaders;
65 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
68 static char ID; // Pass identification
69 JumpThreading() : FunctionPass(&ID) {}
71 bool runOnFunction(Function &F);
72 void FindLoopHeaders(Function &F);
74 bool ProcessBlock(BasicBlock *BB);
75 bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
77 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
80 typedef SmallVectorImpl<std::pair<ConstantInt*,
81 BasicBlock*> > PredValueInfo;
83 bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
84 PredValueInfo &Result);
85 bool ProcessThreadableEdges(Instruction *CondInst, BasicBlock *BB);
88 bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
89 bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
91 bool ProcessJumpOnPHI(PHINode *PN);
93 bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
97 char JumpThreading::ID = 0;
98 static RegisterPass<JumpThreading>
99 X("jump-threading", "Jump Threading");
101 // Public interface to the Jump Threading pass
102 FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
104 /// runOnFunction - Top level algorithm.
106 bool JumpThreading::runOnFunction(Function &F) {
107 DEBUG(errs() << "Jump threading on function '" << F.getName() << "'\n");
108 TD = getAnalysisIfAvailable<TargetData>();
112 bool AnotherIteration = true, EverChanged = false;
113 while (AnotherIteration) {
114 AnotherIteration = false;
115 bool Changed = false;
116 for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
118 while (ProcessBlock(BB))
123 // If the block is trivially dead, zap it. This eliminates the successor
124 // edges which simplifies the CFG.
125 if (pred_begin(BB) == pred_end(BB) &&
126 BB != &BB->getParent()->getEntryBlock()) {
127 DEBUG(errs() << " JT: Deleting dead block '" << BB->getName()
128 << "' with terminator: " << *BB->getTerminator() << '\n');
129 LoopHeaders.erase(BB);
134 AnotherIteration = Changed;
135 EverChanged |= Changed;
142 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
143 /// thread across it.
144 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
145 /// Ignore PHI nodes, these will be flattened when duplication happens.
146 BasicBlock::const_iterator I = BB->getFirstNonPHI();
148 // Sum up the cost of each instruction until we get to the terminator. Don't
149 // include the terminator because the copy won't include it.
151 for (; !isa<TerminatorInst>(I); ++I) {
152 // Debugger intrinsics don't incur code size.
153 if (isa<DbgInfoIntrinsic>(I)) continue;
155 // If this is a pointer->pointer bitcast, it is free.
156 if (isa<BitCastInst>(I) && isa<PointerType>(I->getType()))
159 // All other instructions count for at least one unit.
162 // Calls are more expensive. If they are non-intrinsic calls, we model them
163 // as having cost of 4. If they are a non-vector intrinsic, we model them
164 // as having cost of 2 total, and if they are a vector intrinsic, we model
165 // them as having cost 1.
166 if (const CallInst *CI = dyn_cast<CallInst>(I)) {
167 if (!isa<IntrinsicInst>(CI))
169 else if (!isa<VectorType>(CI->getType()))
174 // Threading through a switch statement is particularly profitable. If this
175 // block ends in a switch, decrease its cost to make it more likely to happen.
176 if (isa<SwitchInst>(I))
177 Size = Size > 6 ? Size-6 : 0;
183 //===----------------------------------------------------------------------===//
186 /// RemovePredecessorAndSimplify - Like BasicBlock::removePredecessor, this
187 /// method is called when we're about to delete Pred as a predecessor of BB. If
188 /// BB contains any PHI nodes, this drops the entries in the PHI nodes for Pred.
190 /// Unlike the removePredecessor method, this attempts to simplify uses of PHI
191 /// nodes that collapse into identity values. For example, if we have:
192 /// x = phi(1, 0, 0, 0)
195 /// .. and delete the predecessor corresponding to the '1', this will attempt to
196 /// recursively fold the and to 0.
197 static void RemovePredecessorAndSimplify(BasicBlock *BB, BasicBlock *Pred,
199 // This only adjusts blocks with PHI nodes.
200 if (!isa<PHINode>(BB->begin()))
203 // Remove the entries for Pred from the PHI nodes in BB, but do not simplify
204 // them down. This will leave us with single entry phi nodes and other phis
205 // that can be removed.
206 BB->removePredecessor(Pred, true);
208 WeakVH PhiIt = &BB->front();
209 while (PHINode *PN = dyn_cast<PHINode>(PhiIt)) {
210 PhiIt = &*++BasicBlock::iterator(cast<Instruction>(PhiIt));
212 Value *PNV = PN->hasConstantValue();
213 if (PNV == 0) continue;
215 assert(PNV != PN && "hasConstantValue broken");
217 // If we're able to simplify the phi to a constant, simplify it into its
219 while (!PN->use_empty()) {
220 // Update the instruction to use the new value.
221 Use &U = PN->use_begin().getUse();
222 Instruction *User = cast<Instruction>(U.getUser());
225 // See if we can simplify it.
226 if (Value *V = SimplifyInstruction(User, TD)) {
227 User->replaceAllUsesWith(V);
228 User->eraseFromParent();
232 PN->replaceAllUsesWith(PNV);
233 PN->eraseFromParent();
235 // If recursive simplification ended up deleting the next PHI node we would
236 // iterate to, then our iterator is invalid, restart scanning from the top
238 if (PhiIt == 0) PhiIt = &BB->front();
242 //===----------------------------------------------------------------------===//
245 /// FindLoopHeaders - We do not want jump threading to turn proper loop
246 /// structures into irreducible loops. Doing this breaks up the loop nesting
247 /// hierarchy and pessimizes later transformations. To prevent this from
248 /// happening, we first have to find the loop headers. Here we approximate this
249 /// by finding targets of backedges in the CFG.
251 /// Note that there definitely are cases when we want to allow threading of
252 /// edges across a loop header. For example, threading a jump from outside the
253 /// loop (the preheader) to an exit block of the loop is definitely profitable.
254 /// It is also almost always profitable to thread backedges from within the loop
255 /// to exit blocks, and is often profitable to thread backedges to other blocks
256 /// within the loop (forming a nested loop). This simple analysis is not rich
257 /// enough to track all of these properties and keep it up-to-date as the CFG
258 /// mutates, so we don't allow any of these transformations.
260 void JumpThreading::FindLoopHeaders(Function &F) {
261 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
262 FindFunctionBackedges(F, Edges);
264 for (unsigned i = 0, e = Edges.size(); i != e; ++i)
265 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
268 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
269 /// if we can infer that the value is a known ConstantInt in any of our
270 /// predecessors. If so, return the known list of value and pred BB in the
271 /// result vector. If a value is known to be undef, it is returned as null.
273 /// The BB basic block is known to start with a PHI node.
275 /// This returns true if there were any known values.
278 /// TODO: Per PR2563, we could infer value range information about a predecessor
279 /// based on its terminator.
281 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
282 PHINode *TheFirstPHI = cast<PHINode>(BB->begin());
284 // If V is a constantint, then it is known in all predecessors.
285 if (isa<ConstantInt>(V) || isa<UndefValue>(V)) {
286 ConstantInt *CI = dyn_cast<ConstantInt>(V);
287 Result.resize(TheFirstPHI->getNumIncomingValues());
288 for (unsigned i = 0, e = Result.size(); i != e; ++i)
289 Result[i] = std::make_pair(CI, TheFirstPHI->getIncomingBlock(i));
293 // If V is a non-instruction value, or an instruction in a different block,
294 // then it can't be derived from a PHI.
295 Instruction *I = dyn_cast<Instruction>(V);
296 if (I == 0 || I->getParent() != BB)
299 /// If I is a PHI node, then we know the incoming values for any constants.
300 if (PHINode *PN = dyn_cast<PHINode>(I)) {
301 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
302 Value *InVal = PN->getIncomingValue(i);
303 if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) {
304 ConstantInt *CI = dyn_cast<ConstantInt>(InVal);
305 Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i)));
308 return !Result.empty();
311 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals;
313 // Handle some boolean conditions.
314 if (I->getType()->getPrimitiveSizeInBits() == 1) {
316 // X & false -> false
317 if (I->getOpcode() == Instruction::Or ||
318 I->getOpcode() == Instruction::And) {
319 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals);
320 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals);
322 if (LHSVals.empty() && RHSVals.empty())
325 ConstantInt *InterestingVal;
326 if (I->getOpcode() == Instruction::Or)
327 InterestingVal = ConstantInt::getTrue(I->getContext());
329 InterestingVal = ConstantInt::getFalse(I->getContext());
331 // Scan for the sentinel.
332 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
333 if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0)
334 Result.push_back(LHSVals[i]);
335 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
336 if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0)
337 Result.push_back(RHSVals[i]);
338 return !Result.empty();
341 // TODO: Should handle the NOT form of XOR.
345 // Handle compare with phi operand, where the PHI is defined in this block.
346 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
347 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
348 if (PN && PN->getParent() == BB) {
349 // We can do this simplification if any comparisons fold to true or false.
351 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
352 BasicBlock *PredBB = PN->getIncomingBlock(i);
353 Value *LHS = PN->getIncomingValue(i);
354 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
356 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS);
357 if (Res == 0) continue;
359 if (isa<UndefValue>(Res))
360 Result.push_back(std::make_pair((ConstantInt*)0, PredBB));
361 else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res))
362 Result.push_back(std::make_pair(CI, PredBB));
365 return !Result.empty();
368 // TODO: We could also recurse to see if we can determine constants another
376 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
377 /// in an undefined jump, decide which block is best to revector to.
379 /// Since we can pick an arbitrary destination, we pick the successor with the
380 /// fewest predecessors. This should reduce the in-degree of the others.
382 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
383 TerminatorInst *BBTerm = BB->getTerminator();
384 unsigned MinSucc = 0;
385 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
386 // Compute the successor with the minimum number of predecessors.
387 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
388 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
389 TestBB = BBTerm->getSuccessor(i);
390 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
391 if (NumPreds < MinNumPreds)
398 /// ProcessBlock - If there are any predecessors whose control can be threaded
399 /// through to a successor, transform them now.
400 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
401 // If this block has a single predecessor, and if that pred has a single
402 // successor, merge the blocks. This encourages recursive jump threading
403 // because now the condition in this block can be threaded through
404 // predecessors of our predecessor block.
405 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
406 if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
408 // If SinglePred was a loop header, BB becomes one.
409 if (LoopHeaders.erase(SinglePred))
410 LoopHeaders.insert(BB);
412 // Remember if SinglePred was the entry block of the function. If so, we
413 // will need to move BB back to the entry position.
414 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
415 MergeBasicBlockIntoOnlyPred(BB);
417 if (isEntry && BB != &BB->getParent()->getEntryBlock())
418 BB->moveBefore(&BB->getParent()->getEntryBlock());
423 // Look to see if the terminator is a branch of switch, if not we can't thread
426 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
427 // Can't thread an unconditional jump.
428 if (BI->isUnconditional()) return false;
429 Condition = BI->getCondition();
430 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
431 Condition = SI->getCondition();
433 return false; // Must be an invoke.
435 // If the terminator of this block is branching on a constant, simplify the
436 // terminator to an unconditional branch. This can occur due to threading in
438 if (isa<ConstantInt>(Condition)) {
439 DEBUG(errs() << " In block '" << BB->getName()
440 << "' folding terminator: " << *BB->getTerminator() << '\n');
442 ConstantFoldTerminator(BB);
446 // If the terminator is branching on an undef, we can pick any of the
447 // successors to branch to. Let GetBestDestForJumpOnUndef decide.
448 if (isa<UndefValue>(Condition)) {
449 unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
451 // Fold the branch/switch.
452 TerminatorInst *BBTerm = BB->getTerminator();
453 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
454 if (i == BestSucc) continue;
455 RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD);
458 DEBUG(errs() << " In block '" << BB->getName()
459 << "' folding undef terminator: " << *BBTerm << '\n');
460 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
461 BBTerm->eraseFromParent();
465 Instruction *CondInst = dyn_cast<Instruction>(Condition);
467 // If the condition is an instruction defined in another block, see if a
468 // predecessor has the same condition:
472 if (!Condition->hasOneUse() && // Multiple uses.
473 (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
474 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
475 if (isa<BranchInst>(BB->getTerminator())) {
476 for (; PI != E; ++PI)
477 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
478 if (PBI->isConditional() && PBI->getCondition() == Condition &&
479 ProcessBranchOnDuplicateCond(*PI, BB))
482 assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
483 for (; PI != E; ++PI)
484 if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator()))
485 if (PSI->getCondition() == Condition &&
486 ProcessSwitchOnDuplicateCond(*PI, BB))
491 // All the rest of our checks depend on the condition being an instruction.
495 // See if this is a phi node in the current block.
496 if (PHINode *PN = dyn_cast<PHINode>(CondInst))
497 if (PN->getParent() == BB)
498 return ProcessJumpOnPHI(PN);
500 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
501 if (!isa<PHINode>(CondCmp->getOperand(0)) ||
502 cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB) {
503 // If we have a comparison, loop over the predecessors to see if there is
504 // a condition with a lexically identical value.
505 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
506 for (; PI != E; ++PI)
507 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
508 if (PBI->isConditional() && *PI != BB) {
509 if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) {
510 if (CI->getOperand(0) == CondCmp->getOperand(0) &&
511 CI->getOperand(1) == CondCmp->getOperand(1) &&
512 CI->getPredicate() == CondCmp->getPredicate()) {
513 // TODO: Could handle things like (x != 4) --> (x == 17)
514 if (ProcessBranchOnDuplicateCond(*PI, BB))
522 // Check for some cases that are worth simplifying. Right now we want to look
523 // for loads that are used by a switch or by the condition for the branch. If
524 // we see one, check to see if it's partially redundant. If so, insert a PHI
525 // which can then be used to thread the values.
527 // This is particularly important because reg2mem inserts loads and stores all
528 // over the place, and this blocks jump threading if we don't zap them.
529 Value *SimplifyValue = CondInst;
530 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
531 if (isa<Constant>(CondCmp->getOperand(1)))
532 SimplifyValue = CondCmp->getOperand(0);
534 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
535 if (SimplifyPartiallyRedundantLoad(LI))
539 // Handle a variety of cases where we are branching on something derived from
540 // a PHI node in the current block. If we can prove that any predecessors
541 // compute a predictable value based on a PHI node, thread those predecessors.
543 // We only bother doing this if the current block has a PHI node and if the
544 // conditional instruction lives in the current block. If either condition
545 // fails, this won't be a computable value anyway.
546 if (CondInst->getParent() == BB && isa<PHINode>(BB->front()))
547 if (ProcessThreadableEdges(CondInst, BB))
551 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know
552 // "(X == 4)" thread through this block.
557 /// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
558 /// block that jump on exactly the same condition. This means that we almost
559 /// always know the direction of the edge in the DESTBB:
561 /// br COND, DESTBB, BBY
563 /// br COND, BBZ, BBW
565 /// If DESTBB has multiple predecessors, we can't just constant fold the branch
566 /// in DESTBB, we have to thread over it.
567 bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
569 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
571 // If both successors of PredBB go to DESTBB, we don't know anything. We can
572 // fold the branch to an unconditional one, which allows other recursive
575 if (PredBI->getSuccessor(1) != BB)
577 else if (PredBI->getSuccessor(0) != BB)
580 DEBUG(errs() << " In block '" << PredBB->getName()
581 << "' folding terminator: " << *PredBB->getTerminator() << '\n');
583 ConstantFoldTerminator(PredBB);
587 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
589 // If the dest block has one predecessor, just fix the branch condition to a
590 // constant and fold it.
591 if (BB->getSinglePredecessor()) {
592 DEBUG(errs() << " In block '" << BB->getName()
593 << "' folding condition to '" << BranchDir << "': "
594 << *BB->getTerminator() << '\n');
596 Value *OldCond = DestBI->getCondition();
597 DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
599 ConstantFoldTerminator(BB);
600 RecursivelyDeleteTriviallyDeadInstructions(OldCond);
605 // Next, figure out which successor we are threading to.
606 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
608 SmallVector<BasicBlock*, 2> Preds;
609 Preds.push_back(PredBB);
611 // Ok, try to thread it!
612 return ThreadEdge(BB, Preds, SuccBB);
615 /// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
616 /// block that switch on exactly the same condition. This means that we almost
617 /// always know the direction of the edge in the DESTBB:
619 /// switch COND [... DESTBB, BBY ... ]
621 /// switch COND [... BBZ, BBW ]
623 /// Optimizing switches like this is very important, because simplifycfg builds
624 /// switches out of repeated 'if' conditions.
625 bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
626 BasicBlock *DestBB) {
627 // Can't thread edge to self.
628 if (PredBB == DestBB)
631 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
632 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
634 // There are a variety of optimizations that we can potentially do on these
635 // blocks: we order them from most to least preferable.
637 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB
638 // directly to their destination. This does not introduce *any* code size
639 // growth. Skip debug info first.
640 BasicBlock::iterator BBI = DestBB->begin();
641 while (isa<DbgInfoIntrinsic>(BBI))
644 // FIXME: Thread if it just contains a PHI.
645 if (isa<SwitchInst>(BBI)) {
646 bool MadeChange = false;
647 // Ignore the default edge for now.
648 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
649 ConstantInt *DestVal = DestSI->getCaseValue(i);
650 BasicBlock *DestSucc = DestSI->getSuccessor(i);
652 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if
653 // PredSI has an explicit case for it. If so, forward. If it is covered
654 // by the default case, we can't update PredSI.
655 unsigned PredCase = PredSI->findCaseValue(DestVal);
656 if (PredCase == 0) continue;
658 // If PredSI doesn't go to DestBB on this value, then it won't reach the
659 // case on this condition.
660 if (PredSI->getSuccessor(PredCase) != DestBB &&
661 DestSI->getSuccessor(i) != DestBB)
664 // Otherwise, we're safe to make the change. Make sure that the edge from
665 // DestSI to DestSucc is not critical and has no PHI nodes.
666 DEBUG(errs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI);
667 DEBUG(errs() << "THROUGH: " << *DestSI);
669 // If the destination has PHI nodes, just split the edge for updating
671 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
672 SplitCriticalEdge(DestSI, i, this);
673 DestSucc = DestSI->getSuccessor(i);
675 FoldSingleEntryPHINodes(DestSucc);
676 PredSI->setSuccessor(PredCase, DestSucc);
688 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
689 /// load instruction, eliminate it by replacing it with a PHI node. This is an
690 /// important optimization that encourages jump threading, and needs to be run
691 /// interlaced with other jump threading tasks.
692 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
693 // Don't hack volatile loads.
694 if (LI->isVolatile()) return false;
696 // If the load is defined in a block with exactly one predecessor, it can't be
697 // partially redundant.
698 BasicBlock *LoadBB = LI->getParent();
699 if (LoadBB->getSinglePredecessor())
702 Value *LoadedPtr = LI->getOperand(0);
704 // If the loaded operand is defined in the LoadBB, it can't be available.
705 // FIXME: Could do PHI translation, that would be fun :)
706 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
707 if (PtrOp->getParent() == LoadBB)
710 // Scan a few instructions up from the load, to see if it is obviously live at
711 // the entry to its block.
712 BasicBlock::iterator BBIt = LI;
714 if (Value *AvailableVal = FindAvailableLoadedValue(LoadedPtr, LoadBB,
716 // If the value if the load is locally available within the block, just use
717 // it. This frequently occurs for reg2mem'd allocas.
718 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
720 // If the returned value is the load itself, replace with an undef. This can
721 // only happen in dead loops.
722 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
723 LI->replaceAllUsesWith(AvailableVal);
724 LI->eraseFromParent();
728 // Otherwise, if we scanned the whole block and got to the top of the block,
729 // we know the block is locally transparent to the load. If not, something
730 // might clobber its value.
731 if (BBIt != LoadBB->begin())
735 SmallPtrSet<BasicBlock*, 8> PredsScanned;
736 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
737 AvailablePredsTy AvailablePreds;
738 BasicBlock *OneUnavailablePred = 0;
740 // If we got here, the loaded value is transparent through to the start of the
741 // block. Check to see if it is available in any of the predecessor blocks.
742 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
744 BasicBlock *PredBB = *PI;
746 // If we already scanned this predecessor, skip it.
747 if (!PredsScanned.insert(PredBB))
750 // Scan the predecessor to see if the value is available in the pred.
751 BBIt = PredBB->end();
752 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6);
753 if (!PredAvailable) {
754 OneUnavailablePred = PredBB;
758 // If so, this load is partially redundant. Remember this info so that we
759 // can create a PHI node.
760 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
763 // If the loaded value isn't available in any predecessor, it isn't partially
765 if (AvailablePreds.empty()) return false;
767 // Okay, the loaded value is available in at least one (and maybe all!)
768 // predecessors. If the value is unavailable in more than one unique
769 // predecessor, we want to insert a merge block for those common predecessors.
770 // This ensures that we only have to insert one reload, thus not increasing
772 BasicBlock *UnavailablePred = 0;
774 // If there is exactly one predecessor where the value is unavailable, the
775 // already computed 'OneUnavailablePred' block is it. If it ends in an
776 // unconditional branch, we know that it isn't a critical edge.
777 if (PredsScanned.size() == AvailablePreds.size()+1 &&
778 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
779 UnavailablePred = OneUnavailablePred;
780 } else if (PredsScanned.size() != AvailablePreds.size()) {
781 // Otherwise, we had multiple unavailable predecessors or we had a critical
782 // edge from the one.
783 SmallVector<BasicBlock*, 8> PredsToSplit;
784 SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
786 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
787 AvailablePredSet.insert(AvailablePreds[i].first);
789 // Add all the unavailable predecessors to the PredsToSplit list.
790 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
792 if (!AvailablePredSet.count(*PI))
793 PredsToSplit.push_back(*PI);
795 // Split them out to their own block.
797 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
798 "thread-split", this);
801 // If the value isn't available in all predecessors, then there will be
802 // exactly one where it isn't available. Insert a load on that edge and add
803 // it to the AvailablePreds list.
804 if (UnavailablePred) {
805 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
806 "Can't handle critical edge here!");
807 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr",
808 UnavailablePred->getTerminator());
809 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
812 // Now we know that each predecessor of this block has a value in
813 // AvailablePreds, sort them for efficient access as we're walking the preds.
814 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
816 // Create a PHI node at the start of the block for the PRE'd load value.
817 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
820 // Insert new entries into the PHI for each predecessor. A single block may
821 // have multiple entries here.
822 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
824 AvailablePredsTy::iterator I =
825 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
826 std::make_pair(*PI, (Value*)0));
828 assert(I != AvailablePreds.end() && I->first == *PI &&
829 "Didn't find entry for predecessor!");
831 PN->addIncoming(I->second, I->first);
834 //cerr << "PRE: " << *LI << *PN << "\n";
836 LI->replaceAllUsesWith(PN);
837 LI->eraseFromParent();
842 /// FindMostPopularDest - The specified list contains multiple possible
843 /// threadable destinations. Pick the one that occurs the most frequently in
846 FindMostPopularDest(BasicBlock *BB,
847 const SmallVectorImpl<std::pair<BasicBlock*,
848 BasicBlock*> > &PredToDestList) {
849 assert(!PredToDestList.empty());
851 // Determine popularity. If there are multiple possible destinations, we
852 // explicitly choose to ignore 'undef' destinations. We prefer to thread
853 // blocks with known and real destinations to threading undef. We'll handle
854 // them later if interesting.
855 DenseMap<BasicBlock*, unsigned> DestPopularity;
856 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
857 if (PredToDestList[i].second)
858 DestPopularity[PredToDestList[i].second]++;
860 // Find the most popular dest.
861 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
862 BasicBlock *MostPopularDest = DPI->first;
863 unsigned Popularity = DPI->second;
864 SmallVector<BasicBlock*, 4> SamePopularity;
866 for (++DPI; DPI != DestPopularity.end(); ++DPI) {
867 // If the popularity of this entry isn't higher than the popularity we've
868 // seen so far, ignore it.
869 if (DPI->second < Popularity)
871 else if (DPI->second == Popularity) {
872 // If it is the same as what we've seen so far, keep track of it.
873 SamePopularity.push_back(DPI->first);
875 // If it is more popular, remember it.
876 SamePopularity.clear();
877 MostPopularDest = DPI->first;
878 Popularity = DPI->second;
882 // Okay, now we know the most popular destination. If there is more than
883 // destination, we need to determine one. This is arbitrary, but we need
884 // to make a deterministic decision. Pick the first one that appears in the
886 if (!SamePopularity.empty()) {
887 SamePopularity.push_back(MostPopularDest);
888 TerminatorInst *TI = BB->getTerminator();
889 for (unsigned i = 0; ; ++i) {
890 assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
892 if (std::find(SamePopularity.begin(), SamePopularity.end(),
893 TI->getSuccessor(i)) == SamePopularity.end())
896 MostPopularDest = TI->getSuccessor(i);
901 // Okay, we have finally picked the most popular destination.
902 return MostPopularDest;
905 bool JumpThreading::ProcessThreadableEdges(Instruction *CondInst,
907 // If threading this would thread across a loop header, don't even try to
909 if (LoopHeaders.count(BB))
912 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues;
913 if (!ComputeValueKnownInPredecessors(CondInst, BB, PredValues))
915 assert(!PredValues.empty() &&
916 "ComputeValueKnownInPredecessors returned true with no values");
918 DEBUG(errs() << "IN BB: " << *BB;
919 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
920 errs() << " BB '" << BB->getName() << "': FOUND condition = ";
921 if (PredValues[i].first)
922 errs() << *PredValues[i].first;
925 errs() << " for pred '" << PredValues[i].second->getName()
929 // Decide what we want to thread through. Convert our list of known values to
930 // a list of known destinations for each pred. This also discards duplicate
931 // predecessors and keeps track of the undefined inputs (which are represented
932 // as a null dest in the PredToDestList).
933 SmallPtrSet<BasicBlock*, 16> SeenPreds;
934 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
936 BasicBlock *OnlyDest = 0;
937 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
939 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
940 BasicBlock *Pred = PredValues[i].second;
941 if (!SeenPreds.insert(Pred))
942 continue; // Duplicate predecessor entry.
944 // If the predecessor ends with an indirect goto, we can't change its
946 if (isa<IndirectBrInst>(Pred->getTerminator()))
949 ConstantInt *Val = PredValues[i].first;
952 if (Val == 0) // Undef.
954 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
955 DestBB = BI->getSuccessor(Val->isZero());
957 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
958 DestBB = SI->getSuccessor(SI->findCaseValue(Val));
961 // If we have exactly one destination, remember it for efficiency below.
964 else if (OnlyDest != DestBB)
965 OnlyDest = MultipleDestSentinel;
967 PredToDestList.push_back(std::make_pair(Pred, DestBB));
970 // If all edges were unthreadable, we fail.
971 if (PredToDestList.empty())
974 // Determine which is the most common successor. If we have many inputs and
975 // this block is a switch, we want to start by threading the batch that goes
976 // to the most popular destination first. If we only know about one
977 // threadable destination (the common case) we can avoid this.
978 BasicBlock *MostPopularDest = OnlyDest;
980 if (MostPopularDest == MultipleDestSentinel)
981 MostPopularDest = FindMostPopularDest(BB, PredToDestList);
983 // Now that we know what the most popular destination is, factor all
984 // predecessors that will jump to it into a single predecessor.
985 SmallVector<BasicBlock*, 16> PredsToFactor;
986 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
987 if (PredToDestList[i].second == MostPopularDest) {
988 BasicBlock *Pred = PredToDestList[i].first;
990 // This predecessor may be a switch or something else that has multiple
991 // edges to the block. Factor each of these edges by listing them
992 // according to # occurrences in PredsToFactor.
993 TerminatorInst *PredTI = Pred->getTerminator();
994 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
995 if (PredTI->getSuccessor(i) == BB)
996 PredsToFactor.push_back(Pred);
999 // If the threadable edges are branching on an undefined value, we get to pick
1000 // the destination that these predecessors should get to.
1001 if (MostPopularDest == 0)
1002 MostPopularDest = BB->getTerminator()->
1003 getSuccessor(GetBestDestForJumpOnUndef(BB));
1005 // Ok, try to thread it!
1006 return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1009 /// ProcessJumpOnPHI - We have a conditional branch or switch on a PHI node in
1010 /// the current block. See if there are any simplifications we can do based on
1011 /// inputs to the phi node.
1013 bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) {
1014 BasicBlock *BB = PN->getParent();
1016 // If any of the predecessor blocks end in an unconditional branch, we can
1017 // *duplicate* the jump into that block in order to further encourage jump
1018 // threading and to eliminate cases where we have branch on a phi of an icmp
1019 // (branch on icmp is much better).
1021 // We don't want to do this tranformation for switches, because we don't
1022 // really want to duplicate a switch.
1023 if (isa<SwitchInst>(BB->getTerminator()))
1026 // Look for unconditional branch predecessors.
1027 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1028 BasicBlock *PredBB = PN->getIncomingBlock(i);
1029 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1030 if (PredBr->isUnconditional() &&
1031 // Try to duplicate BB into PredBB.
1032 DuplicateCondBranchOnPHIIntoPred(BB, PredBB))
1040 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1041 /// predecessor to the PHIBB block. If it has PHI nodes, add entries for
1042 /// NewPred using the entries from OldPred (suitably mapped).
1043 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1044 BasicBlock *OldPred,
1045 BasicBlock *NewPred,
1046 DenseMap<Instruction*, Value*> &ValueMap) {
1047 for (BasicBlock::iterator PNI = PHIBB->begin();
1048 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1049 // Ok, we have a PHI node. Figure out what the incoming value was for the
1051 Value *IV = PN->getIncomingValueForBlock(OldPred);
1053 // Remap the value if necessary.
1054 if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1055 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1056 if (I != ValueMap.end())
1060 PN->addIncoming(IV, NewPred);
1064 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1065 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1066 /// across BB. Transform the IR to reflect this change.
1067 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1068 const SmallVectorImpl<BasicBlock*> &PredBBs,
1069 BasicBlock *SuccBB) {
1070 // If threading to the same block as we come from, we would infinite loop.
1072 DEBUG(errs() << " Not threading across BB '" << BB->getName()
1073 << "' - would thread to self!\n");
1077 // If threading this would thread across a loop header, don't thread the edge.
1078 // See the comments above FindLoopHeaders for justifications and caveats.
1079 if (LoopHeaders.count(BB)) {
1080 DEBUG(errs() << " Not threading across loop header BB '" << BB->getName()
1081 << "' to dest BB '" << SuccBB->getName()
1082 << "' - it might create an irreducible loop!\n");
1086 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
1087 if (JumpThreadCost > Threshold) {
1088 DEBUG(errs() << " Not threading BB '" << BB->getName()
1089 << "' - Cost is too high: " << JumpThreadCost << "\n");
1093 // And finally, do it! Start by factoring the predecessors is needed.
1095 if (PredBBs.size() == 1)
1096 PredBB = PredBBs[0];
1098 DEBUG(errs() << " Factoring out " << PredBBs.size()
1099 << " common predecessors.\n");
1100 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1104 // And finally, do it!
1105 DEBUG(errs() << " Threading edge from '" << PredBB->getName() << "' to '"
1106 << SuccBB->getName() << "' with cost: " << JumpThreadCost
1107 << ", across block:\n "
1110 // We are going to have to map operands from the original BB block to the new
1111 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
1112 // account for entry from PredBB.
1113 DenseMap<Instruction*, Value*> ValueMapping;
1115 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1116 BB->getName()+".thread",
1117 BB->getParent(), BB);
1118 NewBB->moveAfter(PredBB);
1120 BasicBlock::iterator BI = BB->begin();
1121 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1122 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1124 // Clone the non-phi instructions of BB into NewBB, keeping track of the
1125 // mapping and using it to remap operands in the cloned instructions.
1126 for (; !isa<TerminatorInst>(BI); ++BI) {
1127 Instruction *New = BI->clone();
1128 New->setName(BI->getName());
1129 NewBB->getInstList().push_back(New);
1130 ValueMapping[BI] = New;
1132 // Remap operands to patch up intra-block references.
1133 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1134 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1135 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1136 if (I != ValueMapping.end())
1137 New->setOperand(i, I->second);
1141 // We didn't copy the terminator from BB over to NewBB, because there is now
1142 // an unconditional jump to SuccBB. Insert the unconditional jump.
1143 BranchInst::Create(SuccBB, NewBB);
1145 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1146 // PHI nodes for NewBB now.
1147 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1149 // If there were values defined in BB that are used outside the block, then we
1150 // now have to update all uses of the value to use either the original value,
1151 // the cloned value, or some PHI derived value. This can require arbitrary
1152 // PHI insertion, of which we are prepared to do, clean these up now.
1153 SSAUpdater SSAUpdate;
1154 SmallVector<Use*, 16> UsesToRename;
1155 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1156 // Scan all uses of this instruction to see if it is used outside of its
1157 // block, and if so, record them in UsesToRename.
1158 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1160 Instruction *User = cast<Instruction>(*UI);
1161 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1162 if (UserPN->getIncomingBlock(UI) == BB)
1164 } else if (User->getParent() == BB)
1167 UsesToRename.push_back(&UI.getUse());
1170 // If there are no uses outside the block, we're done with this instruction.
1171 if (UsesToRename.empty())
1174 DEBUG(errs() << "JT: Renaming non-local uses of: " << *I << "\n");
1176 // We found a use of I outside of BB. Rename all uses of I that are outside
1177 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1178 // with the two values we know.
1179 SSAUpdate.Initialize(I);
1180 SSAUpdate.AddAvailableValue(BB, I);
1181 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1183 while (!UsesToRename.empty())
1184 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1185 DEBUG(errs() << "\n");
1189 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to
1190 // NewBB instead of BB. This eliminates predecessors from BB, which requires
1191 // us to simplify any PHI nodes in BB.
1192 TerminatorInst *PredTerm = PredBB->getTerminator();
1193 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1194 if (PredTerm->getSuccessor(i) == BB) {
1195 RemovePredecessorAndSimplify(BB, PredBB, TD);
1196 PredTerm->setSuccessor(i, NewBB);
1199 // At this point, the IR is fully up to date and consistent. Do a quick scan
1200 // over the new instructions and zap any that are constants or dead. This
1201 // frequently happens because of phi translation.
1202 BI = NewBB->begin();
1203 for (BasicBlock::iterator E = NewBB->end(); BI != E; ) {
1204 Instruction *Inst = BI++;
1205 if (Value *V = SimplifyInstruction(Inst, TD)) {
1206 Inst->replaceAllUsesWith(V);
1207 Inst->eraseFromParent();
1211 RecursivelyDeleteTriviallyDeadInstructions(Inst);
1214 // Threaded an edge!
1219 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1220 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1221 /// If we can duplicate the contents of BB up into PredBB do so now, this
1222 /// improves the odds that the branch will be on an analyzable instruction like
1224 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1225 BasicBlock *PredBB) {
1226 // If BB is a loop header, then duplicating this block outside the loop would
1227 // cause us to transform this into an irreducible loop, don't do this.
1228 // See the comments above FindLoopHeaders for justifications and caveats.
1229 if (LoopHeaders.count(BB)) {
1230 DEBUG(errs() << " Not duplicating loop header '" << BB->getName()
1231 << "' into predecessor block '" << PredBB->getName()
1232 << "' - it might create an irreducible loop!\n");
1236 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
1237 if (DuplicationCost > Threshold) {
1238 DEBUG(errs() << " Not duplicating BB '" << BB->getName()
1239 << "' - Cost is too high: " << DuplicationCost << "\n");
1243 // Okay, we decided to do this! Clone all the instructions in BB onto the end
1245 DEBUG(errs() << " Duplicating block '" << BB->getName() << "' into end of '"
1246 << PredBB->getName() << "' to eliminate branch on phi. Cost: "
1247 << DuplicationCost << " block is:" << *BB << "\n");
1249 // We are going to have to map operands from the original BB block into the
1250 // PredBB block. Evaluate PHI nodes in BB.
1251 DenseMap<Instruction*, Value*> ValueMapping;
1253 BasicBlock::iterator BI = BB->begin();
1254 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1255 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1257 BranchInst *OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1259 // Clone the non-phi instructions of BB into PredBB, keeping track of the
1260 // mapping and using it to remap operands in the cloned instructions.
1261 for (; BI != BB->end(); ++BI) {
1262 Instruction *New = BI->clone();
1263 New->setName(BI->getName());
1264 PredBB->getInstList().insert(OldPredBranch, New);
1265 ValueMapping[BI] = New;
1267 // Remap operands to patch up intra-block references.
1268 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1269 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1270 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1271 if (I != ValueMapping.end())
1272 New->setOperand(i, I->second);
1276 // Check to see if the targets of the branch had PHI nodes. If so, we need to
1277 // add entries to the PHI nodes for branch from PredBB now.
1278 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1279 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1281 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1284 // If there were values defined in BB that are used outside the block, then we
1285 // now have to update all uses of the value to use either the original value,
1286 // the cloned value, or some PHI derived value. This can require arbitrary
1287 // PHI insertion, of which we are prepared to do, clean these up now.
1288 SSAUpdater SSAUpdate;
1289 SmallVector<Use*, 16> UsesToRename;
1290 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1291 // Scan all uses of this instruction to see if it is used outside of its
1292 // block, and if so, record them in UsesToRename.
1293 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1295 Instruction *User = cast<Instruction>(*UI);
1296 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1297 if (UserPN->getIncomingBlock(UI) == BB)
1299 } else if (User->getParent() == BB)
1302 UsesToRename.push_back(&UI.getUse());
1305 // If there are no uses outside the block, we're done with this instruction.
1306 if (UsesToRename.empty())
1309 DEBUG(errs() << "JT: Renaming non-local uses of: " << *I << "\n");
1311 // We found a use of I outside of BB. Rename all uses of I that are outside
1312 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1313 // with the two values we know.
1314 SSAUpdate.Initialize(I);
1315 SSAUpdate.AddAvailableValue(BB, I);
1316 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1318 while (!UsesToRename.empty())
1319 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1320 DEBUG(errs() << "\n");
1323 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1325 RemovePredecessorAndSimplify(BB, PredBB, TD);
1327 // Remove the unconditional branch at the end of the PredBB block.
1328 OldPredBranch->eraseFromParent();