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/Analysis/LazyValueInfo.h"
21 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
22 #include "llvm/Transforms/Utils/Local.h"
23 #include "llvm/Transforms/Utils/SSAUpdater.h"
24 #include "llvm/Target/TargetData.h"
25 #include "llvm/ADT/DenseMap.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/ADT/STLExtras.h"
28 #include "llvm/ADT/SmallPtrSet.h"
29 #include "llvm/ADT/SmallSet.h"
30 #include "llvm/Support/CommandLine.h"
31 #include "llvm/Support/Debug.h"
32 #include "llvm/Support/ValueHandle.h"
33 #include "llvm/Support/raw_ostream.h"
36 STATISTIC(NumThreads, "Number of jumps threaded");
37 STATISTIC(NumFolds, "Number of terminators folded");
38 STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
40 static cl::opt<unsigned>
41 Threshold("jump-threading-threshold",
42 cl::desc("Max block size to duplicate for jump threading"),
43 cl::init(6), cl::Hidden);
45 // Turn on use of LazyValueInfo.
47 EnableLVI("enable-jump-threading-lvi", cl::ReallyHidden);
52 /// This pass performs 'jump threading', which looks at blocks that have
53 /// multiple predecessors and multiple successors. If one or more of the
54 /// predecessors of the block can be proven to always jump to one of the
55 /// successors, we forward the edge from the predecessor to the successor by
56 /// duplicating the contents of this block.
58 /// An example of when this can occur is code like this:
65 /// In this case, the unconditional branch at the end of the first if can be
66 /// revectored to the false side of the second if.
68 class JumpThreading : public FunctionPass {
72 SmallPtrSet<BasicBlock*, 16> LoopHeaders;
74 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
77 static char ID; // Pass identification
78 JumpThreading() : FunctionPass(&ID) {}
80 bool runOnFunction(Function &F);
82 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
84 AU.addRequired<LazyValueInfo>();
87 void FindLoopHeaders(Function &F);
88 bool ProcessBlock(BasicBlock *BB);
89 bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
91 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
92 const SmallVectorImpl<BasicBlock *> &PredBBs);
94 typedef SmallVectorImpl<std::pair<ConstantInt*,
95 BasicBlock*> > PredValueInfo;
97 bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
98 PredValueInfo &Result);
99 bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB);
102 bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
103 bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
105 bool ProcessBranchOnPHI(PHINode *PN);
106 bool ProcessBranchOnXOR(BinaryOperator *BO);
108 bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
112 char JumpThreading::ID = 0;
113 static RegisterPass<JumpThreading>
114 X("jump-threading", "Jump Threading");
116 // Public interface to the Jump Threading pass
117 FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
119 /// runOnFunction - Top level algorithm.
121 bool JumpThreading::runOnFunction(Function &F) {
122 DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
123 TD = getAnalysisIfAvailable<TargetData>();
124 LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0;
128 bool Changed, EverChanged = false;
131 for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
133 // Thread all of the branches we can over this block.
134 while (ProcessBlock(BB))
139 // If the block is trivially dead, zap it. This eliminates the successor
140 // edges which simplifies the CFG.
141 if (pred_begin(BB) == pred_end(BB) &&
142 BB != &BB->getParent()->getEntryBlock()) {
143 DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName()
144 << "' with terminator: " << *BB->getTerminator() << '\n');
145 LoopHeaders.erase(BB);
148 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
149 // Can't thread an unconditional jump, but if the block is "almost
150 // empty", we can replace uses of it with uses of the successor and make
152 if (BI->isUnconditional() &&
153 BB != &BB->getParent()->getEntryBlock()) {
154 BasicBlock::iterator BBI = BB->getFirstNonPHI();
155 // Ignore dbg intrinsics.
156 while (isa<DbgInfoIntrinsic>(BBI))
158 // If the terminator is the only non-phi instruction, try to nuke it.
159 if (BBI->isTerminator()) {
160 // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
161 // block, we have to make sure it isn't in the LoopHeaders set. We
162 // reinsert afterward if needed.
163 bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
164 BasicBlock *Succ = BI->getSuccessor(0);
166 if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
168 // If we deleted BB and BB was the header of a loop, then the
169 // successor is now the header of the loop.
173 if (ErasedFromLoopHeaders)
174 LoopHeaders.insert(BB);
179 EverChanged |= Changed;
186 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
187 /// thread across it.
188 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
189 /// Ignore PHI nodes, these will be flattened when duplication happens.
190 BasicBlock::const_iterator I = BB->getFirstNonPHI();
192 // FIXME: THREADING will delete values that are just used to compute the
193 // branch, so they shouldn't count against the duplication cost.
196 // Sum up the cost of each instruction until we get to the terminator. Don't
197 // include the terminator because the copy won't include it.
199 for (; !isa<TerminatorInst>(I); ++I) {
200 // Debugger intrinsics don't incur code size.
201 if (isa<DbgInfoIntrinsic>(I)) continue;
203 // If this is a pointer->pointer bitcast, it is free.
204 if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
207 // All other instructions count for at least one unit.
210 // Calls are more expensive. If they are non-intrinsic calls, we model them
211 // as having cost of 4. If they are a non-vector intrinsic, we model them
212 // as having cost of 2 total, and if they are a vector intrinsic, we model
213 // them as having cost 1.
214 if (const CallInst *CI = dyn_cast<CallInst>(I)) {
215 if (!isa<IntrinsicInst>(CI))
217 else if (!CI->getType()->isVectorTy())
222 // Threading through a switch statement is particularly profitable. If this
223 // block ends in a switch, decrease its cost to make it more likely to happen.
224 if (isa<SwitchInst>(I))
225 Size = Size > 6 ? Size-6 : 0;
230 /// FindLoopHeaders - We do not want jump threading to turn proper loop
231 /// structures into irreducible loops. Doing this breaks up the loop nesting
232 /// hierarchy and pessimizes later transformations. To prevent this from
233 /// happening, we first have to find the loop headers. Here we approximate this
234 /// by finding targets of backedges in the CFG.
236 /// Note that there definitely are cases when we want to allow threading of
237 /// edges across a loop header. For example, threading a jump from outside the
238 /// loop (the preheader) to an exit block of the loop is definitely profitable.
239 /// It is also almost always profitable to thread backedges from within the loop
240 /// to exit blocks, and is often profitable to thread backedges to other blocks
241 /// within the loop (forming a nested loop). This simple analysis is not rich
242 /// enough to track all of these properties and keep it up-to-date as the CFG
243 /// mutates, so we don't allow any of these transformations.
245 void JumpThreading::FindLoopHeaders(Function &F) {
246 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
247 FindFunctionBackedges(F, Edges);
249 for (unsigned i = 0, e = Edges.size(); i != e; ++i)
250 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
253 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
254 /// if we can infer that the value is a known ConstantInt in any of our
255 /// predecessors. If so, return the known list of value and pred BB in the
256 /// result vector. If a value is known to be undef, it is returned as null.
258 /// This returns true if there were any known values.
261 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
262 // If V is a constantint, then it is known in all predecessors.
263 if (isa<ConstantInt>(V) || isa<UndefValue>(V)) {
264 ConstantInt *CI = dyn_cast<ConstantInt>(V);
266 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
267 Result.push_back(std::make_pair(CI, *PI));
271 // If V is a non-instruction value, or an instruction in a different block,
272 // then it can't be derived from a PHI.
273 Instruction *I = dyn_cast<Instruction>(V);
274 if (I == 0 || I->getParent() != BB) {
276 // Okay, if this is a live-in value, see if it has a known value at the end
277 // of any of our predecessors.
279 // FIXME: This should be an edge property, not a block end property.
280 /// TODO: Per PR2563, we could infer value range information about a
281 /// predecessor based on its terminator.
284 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
285 // "I" is a non-local compare-with-a-constant instruction. This would be
286 // able to handle value inequalities better, for example if the compare is
287 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
288 // Perhaps getConstantOnEdge should be smart enough to do this?
290 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
291 // If the value is known by LazyValueInfo to be a constant in a
292 // predecessor, use that information to try to thread this block.
293 Constant *PredCst = LVI->getConstantOnEdge(V, *PI, BB);
295 (!isa<ConstantInt>(PredCst) && !isa<UndefValue>(PredCst)))
298 Result.push_back(std::make_pair(dyn_cast<ConstantInt>(PredCst), *PI));
301 return !Result.empty();
307 /// If I is a PHI node, then we know the incoming values for any constants.
308 if (PHINode *PN = dyn_cast<PHINode>(I)) {
309 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
310 Value *InVal = PN->getIncomingValue(i);
311 if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) {
312 ConstantInt *CI = dyn_cast<ConstantInt>(InVal);
313 Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i)));
316 return !Result.empty();
319 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals;
321 // Handle some boolean conditions.
322 if (I->getType()->getPrimitiveSizeInBits() == 1) {
324 // X & false -> false
325 if (I->getOpcode() == Instruction::Or ||
326 I->getOpcode() == Instruction::And) {
327 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals);
328 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals);
330 if (LHSVals.empty() && RHSVals.empty())
333 ConstantInt *InterestingVal;
334 if (I->getOpcode() == Instruction::Or)
335 InterestingVal = ConstantInt::getTrue(I->getContext());
337 InterestingVal = ConstantInt::getFalse(I->getContext());
339 // Scan for the sentinel. If we find an undef, force it to the
340 // interesting value: x|undef -> true and x&undef -> false.
341 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
342 if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0) {
343 Result.push_back(LHSVals[i]);
344 Result.back().first = InterestingVal;
346 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
347 if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0) {
348 Result.push_back(RHSVals[i]);
349 Result.back().first = InterestingVal;
351 return !Result.empty();
354 // Handle the NOT form of XOR.
355 if (I->getOpcode() == Instruction::Xor &&
356 isa<ConstantInt>(I->getOperand(1)) &&
357 cast<ConstantInt>(I->getOperand(1))->isOne()) {
358 ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result);
362 // Invert the known values.
363 for (unsigned i = 0, e = Result.size(); i != e; ++i)
366 cast<ConstantInt>(ConstantExpr::getNot(Result[i].first));
371 // Handle compare with phi operand, where the PHI is defined in this block.
372 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
373 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
374 if (PN && PN->getParent() == BB) {
375 // We can do this simplification if any comparisons fold to true or false.
377 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
378 BasicBlock *PredBB = PN->getIncomingBlock(i);
379 Value *LHS = PN->getIncomingValue(i);
380 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
382 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD);
384 if (!LVI || !isa<Constant>(RHS))
387 LazyValueInfo::Tristate
388 ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
389 cast<Constant>(RHS), PredBB, BB);
390 if (ResT == LazyValueInfo::Unknown)
392 Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
395 if (isa<UndefValue>(Res))
396 Result.push_back(std::make_pair((ConstantInt*)0, PredBB));
397 else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res))
398 Result.push_back(std::make_pair(CI, PredBB));
401 return !Result.empty();
405 // If comparing a live-in value against a constant, see if we know the
406 // live-in value on any predecessors.
407 if (LVI && isa<Constant>(Cmp->getOperand(1)) &&
408 Cmp->getType()->isIntegerTy() && // Not vector compare.
409 (!isa<Instruction>(Cmp->getOperand(0)) ||
410 cast<Instruction>(Cmp->getOperand(0))->getParent() != BB)) {
411 Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
413 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
414 // If the value is known by LazyValueInfo to be a constant in a
415 // predecessor, use that information to try to thread this block.
416 LazyValueInfo::Tristate
417 Res = LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
419 if (Res == LazyValueInfo::Unknown)
422 Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
423 Result.push_back(std::make_pair(cast<ConstantInt>(ResC), *PI));
426 return !Result.empty();
434 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
435 /// in an undefined jump, decide which block is best to revector to.
437 /// Since we can pick an arbitrary destination, we pick the successor with the
438 /// fewest predecessors. This should reduce the in-degree of the others.
440 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
441 TerminatorInst *BBTerm = BB->getTerminator();
442 unsigned MinSucc = 0;
443 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
444 // Compute the successor with the minimum number of predecessors.
445 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
446 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
447 TestBB = BBTerm->getSuccessor(i);
448 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
449 if (NumPreds < MinNumPreds)
456 /// ProcessBlock - If there are any predecessors whose control can be threaded
457 /// through to a successor, transform them now.
458 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
459 // If the block is trivially dead, just return and let the caller nuke it.
460 // This simplifies other transformations.
461 if (pred_begin(BB) == pred_end(BB) &&
462 BB != &BB->getParent()->getEntryBlock())
465 // If this block has a single predecessor, and if that pred has a single
466 // successor, merge the blocks. This encourages recursive jump threading
467 // because now the condition in this block can be threaded through
468 // predecessors of our predecessor block.
469 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
470 if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
472 // If SinglePred was a loop header, BB becomes one.
473 if (LoopHeaders.erase(SinglePred))
474 LoopHeaders.insert(BB);
476 // Remember if SinglePred was the entry block of the function. If so, we
477 // will need to move BB back to the entry position.
478 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
479 MergeBasicBlockIntoOnlyPred(BB);
481 if (isEntry && BB != &BB->getParent()->getEntryBlock())
482 BB->moveBefore(&BB->getParent()->getEntryBlock());
487 // Look to see if the terminator is a branch of switch, if not we can't thread
490 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
491 // Can't thread an unconditional jump.
492 if (BI->isUnconditional()) return false;
493 Condition = BI->getCondition();
494 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
495 Condition = SI->getCondition();
497 return false; // Must be an invoke.
499 // If the terminator of this block is branching on a constant, simplify the
500 // terminator to an unconditional branch. This can occur due to threading in
502 if (isa<ConstantInt>(Condition)) {
503 DEBUG(dbgs() << " In block '" << BB->getName()
504 << "' folding terminator: " << *BB->getTerminator() << '\n');
506 ConstantFoldTerminator(BB);
510 // If the terminator is branching on an undef, we can pick any of the
511 // successors to branch to. Let GetBestDestForJumpOnUndef decide.
512 if (isa<UndefValue>(Condition)) {
513 unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
515 // Fold the branch/switch.
516 TerminatorInst *BBTerm = BB->getTerminator();
517 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
518 if (i == BestSucc) continue;
519 RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD);
522 DEBUG(dbgs() << " In block '" << BB->getName()
523 << "' folding undef terminator: " << *BBTerm << '\n');
524 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
525 BBTerm->eraseFromParent();
529 Instruction *CondInst = dyn_cast<Instruction>(Condition);
531 // If the condition is an instruction defined in another block, see if a
532 // predecessor has the same condition:
537 !Condition->hasOneUse() && // Multiple uses.
538 (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
539 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
540 if (isa<BranchInst>(BB->getTerminator())) {
541 for (; PI != E; ++PI)
542 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
543 if (PBI->isConditional() && PBI->getCondition() == Condition &&
544 ProcessBranchOnDuplicateCond(*PI, BB))
547 assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
548 for (; PI != E; ++PI)
549 if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator()))
550 if (PSI->getCondition() == Condition &&
551 ProcessSwitchOnDuplicateCond(*PI, BB))
556 // All the rest of our checks depend on the condition being an instruction.
558 // FIXME: Unify this with code below.
559 if (LVI && ProcessThreadableEdges(Condition, BB))
565 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
567 (!isa<PHINode>(CondCmp->getOperand(0)) ||
568 cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB)) {
569 // If we have a comparison, loop over the predecessors to see if there is
570 // a condition with a lexically identical value.
571 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
572 for (; PI != E; ++PI)
573 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
574 if (PBI->isConditional() && *PI != BB) {
575 if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) {
576 if (CI->getOperand(0) == CondCmp->getOperand(0) &&
577 CI->getOperand(1) == CondCmp->getOperand(1) &&
578 CI->getPredicate() == CondCmp->getPredicate()) {
579 // TODO: Could handle things like (x != 4) --> (x == 17)
580 if (ProcessBranchOnDuplicateCond(*PI, BB))
588 // Check for some cases that are worth simplifying. Right now we want to look
589 // for loads that are used by a switch or by the condition for the branch. If
590 // we see one, check to see if it's partially redundant. If so, insert a PHI
591 // which can then be used to thread the values.
593 Value *SimplifyValue = CondInst;
594 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
595 if (isa<Constant>(CondCmp->getOperand(1)))
596 SimplifyValue = CondCmp->getOperand(0);
598 // TODO: There are other places where load PRE would be profitable, such as
599 // more complex comparisons.
600 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
601 if (SimplifyPartiallyRedundantLoad(LI))
605 // Handle a variety of cases where we are branching on something derived from
606 // a PHI node in the current block. If we can prove that any predecessors
607 // compute a predictable value based on a PHI node, thread those predecessors.
609 if (ProcessThreadableEdges(CondInst, BB))
612 // If this is an otherwise-unfoldable branch on a phi node in the current
613 // block, see if we can simplify.
614 if (PHINode *PN = dyn_cast<PHINode>(CondInst))
615 if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
616 return ProcessBranchOnPHI(PN);
619 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
620 if (CondInst->getOpcode() == Instruction::Xor &&
621 CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
622 return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
625 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know
626 // "(X == 4)", thread through this block.
631 /// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
632 /// block that jump on exactly the same condition. This means that we almost
633 /// always know the direction of the edge in the DESTBB:
635 /// br COND, DESTBB, BBY
637 /// br COND, BBZ, BBW
639 /// If DESTBB has multiple predecessors, we can't just constant fold the branch
640 /// in DESTBB, we have to thread over it.
641 bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
643 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
645 // If both successors of PredBB go to DESTBB, we don't know anything. We can
646 // fold the branch to an unconditional one, which allows other recursive
649 if (PredBI->getSuccessor(1) != BB)
651 else if (PredBI->getSuccessor(0) != BB)
654 DEBUG(dbgs() << " In block '" << PredBB->getName()
655 << "' folding terminator: " << *PredBB->getTerminator() << '\n');
657 ConstantFoldTerminator(PredBB);
661 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
663 // If the dest block has one predecessor, just fix the branch condition to a
664 // constant and fold it.
665 if (BB->getSinglePredecessor()) {
666 DEBUG(dbgs() << " In block '" << BB->getName()
667 << "' folding condition to '" << BranchDir << "': "
668 << *BB->getTerminator() << '\n');
670 Value *OldCond = DestBI->getCondition();
671 DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
673 ConstantFoldTerminator(BB);
674 RecursivelyDeleteTriviallyDeadInstructions(OldCond);
679 // Next, figure out which successor we are threading to.
680 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
682 SmallVector<BasicBlock*, 2> Preds;
683 Preds.push_back(PredBB);
685 // Ok, try to thread it!
686 return ThreadEdge(BB, Preds, SuccBB);
689 /// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
690 /// block that switch on exactly the same condition. This means that we almost
691 /// always know the direction of the edge in the DESTBB:
693 /// switch COND [... DESTBB, BBY ... ]
695 /// switch COND [... BBZ, BBW ]
697 /// Optimizing switches like this is very important, because simplifycfg builds
698 /// switches out of repeated 'if' conditions.
699 bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
700 BasicBlock *DestBB) {
701 // Can't thread edge to self.
702 if (PredBB == DestBB)
705 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
706 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
708 // There are a variety of optimizations that we can potentially do on these
709 // blocks: we order them from most to least preferable.
711 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB
712 // directly to their destination. This does not introduce *any* code size
713 // growth. Skip debug info first.
714 BasicBlock::iterator BBI = DestBB->begin();
715 while (isa<DbgInfoIntrinsic>(BBI))
718 // FIXME: Thread if it just contains a PHI.
719 if (isa<SwitchInst>(BBI)) {
720 bool MadeChange = false;
721 // Ignore the default edge for now.
722 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
723 ConstantInt *DestVal = DestSI->getCaseValue(i);
724 BasicBlock *DestSucc = DestSI->getSuccessor(i);
726 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if
727 // PredSI has an explicit case for it. If so, forward. If it is covered
728 // by the default case, we can't update PredSI.
729 unsigned PredCase = PredSI->findCaseValue(DestVal);
730 if (PredCase == 0) continue;
732 // If PredSI doesn't go to DestBB on this value, then it won't reach the
733 // case on this condition.
734 if (PredSI->getSuccessor(PredCase) != DestBB &&
735 DestSI->getSuccessor(i) != DestBB)
738 // Do not forward this if it already goes to this destination, this would
739 // be an infinite loop.
740 if (PredSI->getSuccessor(PredCase) == DestSucc)
743 // Otherwise, we're safe to make the change. Make sure that the edge from
744 // DestSI to DestSucc is not critical and has no PHI nodes.
745 DEBUG(dbgs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI);
746 DEBUG(dbgs() << "THROUGH: " << *DestSI);
748 // If the destination has PHI nodes, just split the edge for updating
750 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
751 SplitCriticalEdge(DestSI, i, this);
752 DestSucc = DestSI->getSuccessor(i);
754 FoldSingleEntryPHINodes(DestSucc);
755 PredSI->setSuccessor(PredCase, DestSucc);
767 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
768 /// load instruction, eliminate it by replacing it with a PHI node. This is an
769 /// important optimization that encourages jump threading, and needs to be run
770 /// interlaced with other jump threading tasks.
771 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
772 // Don't hack volatile loads.
773 if (LI->isVolatile()) return false;
775 // If the load is defined in a block with exactly one predecessor, it can't be
776 // partially redundant.
777 BasicBlock *LoadBB = LI->getParent();
778 if (LoadBB->getSinglePredecessor())
781 Value *LoadedPtr = LI->getOperand(0);
783 // If the loaded operand is defined in the LoadBB, it can't be available.
784 // TODO: Could do simple PHI translation, that would be fun :)
785 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
786 if (PtrOp->getParent() == LoadBB)
789 // Scan a few instructions up from the load, to see if it is obviously live at
790 // the entry to its block.
791 BasicBlock::iterator BBIt = LI;
793 if (Value *AvailableVal =
794 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
795 // If the value if the load is locally available within the block, just use
796 // it. This frequently occurs for reg2mem'd allocas.
797 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
799 // If the returned value is the load itself, replace with an undef. This can
800 // only happen in dead loops.
801 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
802 LI->replaceAllUsesWith(AvailableVal);
803 LI->eraseFromParent();
807 // Otherwise, if we scanned the whole block and got to the top of the block,
808 // we know the block is locally transparent to the load. If not, something
809 // might clobber its value.
810 if (BBIt != LoadBB->begin())
814 SmallPtrSet<BasicBlock*, 8> PredsScanned;
815 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
816 AvailablePredsTy AvailablePreds;
817 BasicBlock *OneUnavailablePred = 0;
819 // If we got here, the loaded value is transparent through to the start of the
820 // block. Check to see if it is available in any of the predecessor blocks.
821 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
823 BasicBlock *PredBB = *PI;
825 // If we already scanned this predecessor, skip it.
826 if (!PredsScanned.insert(PredBB))
829 // Scan the predecessor to see if the value is available in the pred.
830 BBIt = PredBB->end();
831 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6);
832 if (!PredAvailable) {
833 OneUnavailablePred = PredBB;
837 // If so, this load is partially redundant. Remember this info so that we
838 // can create a PHI node.
839 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
842 // If the loaded value isn't available in any predecessor, it isn't partially
844 if (AvailablePreds.empty()) return false;
846 // Okay, the loaded value is available in at least one (and maybe all!)
847 // predecessors. If the value is unavailable in more than one unique
848 // predecessor, we want to insert a merge block for those common predecessors.
849 // This ensures that we only have to insert one reload, thus not increasing
851 BasicBlock *UnavailablePred = 0;
853 // If there is exactly one predecessor where the value is unavailable, the
854 // already computed 'OneUnavailablePred' block is it. If it ends in an
855 // unconditional branch, we know that it isn't a critical edge.
856 if (PredsScanned.size() == AvailablePreds.size()+1 &&
857 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
858 UnavailablePred = OneUnavailablePred;
859 } else if (PredsScanned.size() != AvailablePreds.size()) {
860 // Otherwise, we had multiple unavailable predecessors or we had a critical
861 // edge from the one.
862 SmallVector<BasicBlock*, 8> PredsToSplit;
863 SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
865 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
866 AvailablePredSet.insert(AvailablePreds[i].first);
868 // Add all the unavailable predecessors to the PredsToSplit list.
869 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
871 if (!AvailablePredSet.count(*PI))
872 PredsToSplit.push_back(*PI);
874 // Split them out to their own block.
876 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
877 "thread-pre-split", this);
880 // If the value isn't available in all predecessors, then there will be
881 // exactly one where it isn't available. Insert a load on that edge and add
882 // it to the AvailablePreds list.
883 if (UnavailablePred) {
884 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
885 "Can't handle critical edge here!");
886 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
888 UnavailablePred->getTerminator());
889 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
892 // Now we know that each predecessor of this block has a value in
893 // AvailablePreds, sort them for efficient access as we're walking the preds.
894 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
896 // Create a PHI node at the start of the block for the PRE'd load value.
897 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
900 // Insert new entries into the PHI for each predecessor. A single block may
901 // have multiple entries here.
902 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
904 AvailablePredsTy::iterator I =
905 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
906 std::make_pair(*PI, (Value*)0));
908 assert(I != AvailablePreds.end() && I->first == *PI &&
909 "Didn't find entry for predecessor!");
911 PN->addIncoming(I->second, I->first);
914 //cerr << "PRE: " << *LI << *PN << "\n";
916 LI->replaceAllUsesWith(PN);
917 LI->eraseFromParent();
922 /// FindMostPopularDest - The specified list contains multiple possible
923 /// threadable destinations. Pick the one that occurs the most frequently in
926 FindMostPopularDest(BasicBlock *BB,
927 const SmallVectorImpl<std::pair<BasicBlock*,
928 BasicBlock*> > &PredToDestList) {
929 assert(!PredToDestList.empty());
931 // Determine popularity. If there are multiple possible destinations, we
932 // explicitly choose to ignore 'undef' destinations. We prefer to thread
933 // blocks with known and real destinations to threading undef. We'll handle
934 // them later if interesting.
935 DenseMap<BasicBlock*, unsigned> DestPopularity;
936 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
937 if (PredToDestList[i].second)
938 DestPopularity[PredToDestList[i].second]++;
940 // Find the most popular dest.
941 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
942 BasicBlock *MostPopularDest = DPI->first;
943 unsigned Popularity = DPI->second;
944 SmallVector<BasicBlock*, 4> SamePopularity;
946 for (++DPI; DPI != DestPopularity.end(); ++DPI) {
947 // If the popularity of this entry isn't higher than the popularity we've
948 // seen so far, ignore it.
949 if (DPI->second < Popularity)
951 else if (DPI->second == Popularity) {
952 // If it is the same as what we've seen so far, keep track of it.
953 SamePopularity.push_back(DPI->first);
955 // If it is more popular, remember it.
956 SamePopularity.clear();
957 MostPopularDest = DPI->first;
958 Popularity = DPI->second;
962 // Okay, now we know the most popular destination. If there is more than
963 // destination, we need to determine one. This is arbitrary, but we need
964 // to make a deterministic decision. Pick the first one that appears in the
966 if (!SamePopularity.empty()) {
967 SamePopularity.push_back(MostPopularDest);
968 TerminatorInst *TI = BB->getTerminator();
969 for (unsigned i = 0; ; ++i) {
970 assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
972 if (std::find(SamePopularity.begin(), SamePopularity.end(),
973 TI->getSuccessor(i)) == SamePopularity.end())
976 MostPopularDest = TI->getSuccessor(i);
981 // Okay, we have finally picked the most popular destination.
982 return MostPopularDest;
985 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) {
986 // If threading this would thread across a loop header, don't even try to
988 if (LoopHeaders.count(BB))
991 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues;
992 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues))
994 assert(!PredValues.empty() &&
995 "ComputeValueKnownInPredecessors returned true with no values");
997 DEBUG(dbgs() << "IN BB: " << *BB;
998 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
999 dbgs() << " BB '" << BB->getName() << "': FOUND condition = ";
1000 if (PredValues[i].first)
1001 dbgs() << *PredValues[i].first;
1004 dbgs() << " for pred '" << PredValues[i].second->getName()
1008 // Decide what we want to thread through. Convert our list of known values to
1009 // a list of known destinations for each pred. This also discards duplicate
1010 // predecessors and keeps track of the undefined inputs (which are represented
1011 // as a null dest in the PredToDestList).
1012 SmallPtrSet<BasicBlock*, 16> SeenPreds;
1013 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
1015 BasicBlock *OnlyDest = 0;
1016 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1018 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1019 BasicBlock *Pred = PredValues[i].second;
1020 if (!SeenPreds.insert(Pred))
1021 continue; // Duplicate predecessor entry.
1023 // If the predecessor ends with an indirect goto, we can't change its
1025 if (isa<IndirectBrInst>(Pred->getTerminator()))
1028 ConstantInt *Val = PredValues[i].first;
1031 if (Val == 0) // Undef.
1033 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1034 DestBB = BI->getSuccessor(Val->isZero());
1036 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
1037 DestBB = SI->getSuccessor(SI->findCaseValue(Val));
1040 // If we have exactly one destination, remember it for efficiency below.
1043 else if (OnlyDest != DestBB)
1044 OnlyDest = MultipleDestSentinel;
1046 PredToDestList.push_back(std::make_pair(Pred, DestBB));
1049 // If all edges were unthreadable, we fail.
1050 if (PredToDestList.empty())
1053 // Determine which is the most common successor. If we have many inputs and
1054 // this block is a switch, we want to start by threading the batch that goes
1055 // to the most popular destination first. If we only know about one
1056 // threadable destination (the common case) we can avoid this.
1057 BasicBlock *MostPopularDest = OnlyDest;
1059 if (MostPopularDest == MultipleDestSentinel)
1060 MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1062 // Now that we know what the most popular destination is, factor all
1063 // predecessors that will jump to it into a single predecessor.
1064 SmallVector<BasicBlock*, 16> PredsToFactor;
1065 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1066 if (PredToDestList[i].second == MostPopularDest) {
1067 BasicBlock *Pred = PredToDestList[i].first;
1069 // This predecessor may be a switch or something else that has multiple
1070 // edges to the block. Factor each of these edges by listing them
1071 // according to # occurrences in PredsToFactor.
1072 TerminatorInst *PredTI = Pred->getTerminator();
1073 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
1074 if (PredTI->getSuccessor(i) == BB)
1075 PredsToFactor.push_back(Pred);
1078 // If the threadable edges are branching on an undefined value, we get to pick
1079 // the destination that these predecessors should get to.
1080 if (MostPopularDest == 0)
1081 MostPopularDest = BB->getTerminator()->
1082 getSuccessor(GetBestDestForJumpOnUndef(BB));
1084 // Ok, try to thread it!
1085 return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1088 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1089 /// a PHI node in the current block. See if there are any simplifications we
1090 /// can do based on inputs to the phi node.
1092 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
1093 BasicBlock *BB = PN->getParent();
1095 // TODO: We could make use of this to do it once for blocks with common PHI
1097 SmallVector<BasicBlock*, 1> PredBBs;
1100 // If any of the predecessor blocks end in an unconditional branch, we can
1101 // *duplicate* the conditional branch into that block in order to further
1102 // encourage jump threading and to eliminate cases where we have branch on a
1103 // phi of an icmp (branch on icmp is much better).
1104 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1105 BasicBlock *PredBB = PN->getIncomingBlock(i);
1106 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1107 if (PredBr->isUnconditional()) {
1108 PredBBs[0] = PredBB;
1109 // Try to duplicate BB into PredBB.
1110 if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
1118 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
1119 /// a xor instruction in the current block. See if there are any
1120 /// simplifications we can do based on inputs to the xor.
1122 bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
1123 BasicBlock *BB = BO->getParent();
1125 // If either the LHS or RHS of the xor is a constant, don't do this
1127 if (isa<ConstantInt>(BO->getOperand(0)) ||
1128 isa<ConstantInt>(BO->getOperand(1)))
1131 // If the first instruction in BB isn't a phi, we won't be able to infer
1132 // anything special about any particular predecessor.
1133 if (!isa<PHINode>(BB->front()))
1136 // If we have a xor as the branch input to this block, and we know that the
1137 // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1138 // the condition into the predecessor and fix that value to true, saving some
1139 // logical ops on that path and encouraging other paths to simplify.
1141 // This copies something like this:
1144 // %X = phi i1 [1], [%X']
1145 // %Y = icmp eq i32 %A, %B
1146 // %Z = xor i1 %X, %Y
1151 // %Y = icmp ne i32 %A, %B
1154 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> XorOpValues;
1156 if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) {
1157 assert(XorOpValues.empty());
1158 if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues))
1163 assert(!XorOpValues.empty() &&
1164 "ComputeValueKnownInPredecessors returned true with no values");
1166 // Scan the information to see which is most popular: true or false. The
1167 // predecessors can be of the set true, false, or undef.
1168 unsigned NumTrue = 0, NumFalse = 0;
1169 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1170 if (!XorOpValues[i].first) continue; // Ignore undefs for the count.
1171 if (XorOpValues[i].first->isZero())
1177 // Determine which value to split on, true, false, or undef if neither.
1178 ConstantInt *SplitVal = 0;
1179 if (NumTrue > NumFalse)
1180 SplitVal = ConstantInt::getTrue(BB->getContext());
1181 else if (NumTrue != 0 || NumFalse != 0)
1182 SplitVal = ConstantInt::getFalse(BB->getContext());
1184 // Collect all of the blocks that this can be folded into so that we can
1185 // factor this once and clone it once.
1186 SmallVector<BasicBlock*, 8> BlocksToFoldInto;
1187 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1188 if (XorOpValues[i].first != SplitVal && XorOpValues[i].first != 0) continue;
1190 BlocksToFoldInto.push_back(XorOpValues[i].second);
1193 // If we inferred a value for all of the predecessors, then duplication won't
1194 // help us. However, we can just replace the LHS or RHS with the constant.
1195 if (BlocksToFoldInto.size() ==
1196 cast<PHINode>(BB->front()).getNumIncomingValues()) {
1197 if (SplitVal == 0) {
1198 // If all preds provide undef, just nuke the xor, because it is undef too.
1199 BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
1200 BO->eraseFromParent();
1201 } else if (SplitVal->isZero()) {
1202 // If all preds provide 0, replace the xor with the other input.
1203 BO->replaceAllUsesWith(BO->getOperand(isLHS));
1204 BO->eraseFromParent();
1206 // If all preds provide 1, set the computed value to 1.
1207 BO->setOperand(!isLHS, SplitVal);
1213 // Try to duplicate BB into PredBB.
1214 return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
1218 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1219 /// predecessor to the PHIBB block. If it has PHI nodes, add entries for
1220 /// NewPred using the entries from OldPred (suitably mapped).
1221 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1222 BasicBlock *OldPred,
1223 BasicBlock *NewPred,
1224 DenseMap<Instruction*, Value*> &ValueMap) {
1225 for (BasicBlock::iterator PNI = PHIBB->begin();
1226 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1227 // Ok, we have a PHI node. Figure out what the incoming value was for the
1229 Value *IV = PN->getIncomingValueForBlock(OldPred);
1231 // Remap the value if necessary.
1232 if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1233 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1234 if (I != ValueMap.end())
1238 PN->addIncoming(IV, NewPred);
1242 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1243 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1244 /// across BB. Transform the IR to reflect this change.
1245 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1246 const SmallVectorImpl<BasicBlock*> &PredBBs,
1247 BasicBlock *SuccBB) {
1248 // If threading to the same block as we come from, we would infinite loop.
1250 DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
1251 << "' - would thread to self!\n");
1255 // If threading this would thread across a loop header, don't thread the edge.
1256 // See the comments above FindLoopHeaders for justifications and caveats.
1257 if (LoopHeaders.count(BB)) {
1258 DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName()
1259 << "' to dest BB '" << SuccBB->getName()
1260 << "' - it might create an irreducible loop!\n");
1264 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
1265 if (JumpThreadCost > Threshold) {
1266 DEBUG(dbgs() << " Not threading BB '" << BB->getName()
1267 << "' - Cost is too high: " << JumpThreadCost << "\n");
1271 // And finally, do it! Start by factoring the predecessors is needed.
1273 if (PredBBs.size() == 1)
1274 PredBB = PredBBs[0];
1276 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1277 << " common predecessors.\n");
1278 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1282 // And finally, do it!
1283 DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '"
1284 << SuccBB->getName() << "' with cost: " << JumpThreadCost
1285 << ", across block:\n "
1288 // We are going to have to map operands from the original BB block to the new
1289 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
1290 // account for entry from PredBB.
1291 DenseMap<Instruction*, Value*> ValueMapping;
1293 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1294 BB->getName()+".thread",
1295 BB->getParent(), BB);
1296 NewBB->moveAfter(PredBB);
1298 BasicBlock::iterator BI = BB->begin();
1299 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1300 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1302 // Clone the non-phi instructions of BB into NewBB, keeping track of the
1303 // mapping and using it to remap operands in the cloned instructions.
1304 for (; !isa<TerminatorInst>(BI); ++BI) {
1305 Instruction *New = BI->clone();
1306 New->setName(BI->getName());
1307 NewBB->getInstList().push_back(New);
1308 ValueMapping[BI] = New;
1310 // Remap operands to patch up intra-block references.
1311 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1312 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1313 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1314 if (I != ValueMapping.end())
1315 New->setOperand(i, I->second);
1319 // We didn't copy the terminator from BB over to NewBB, because there is now
1320 // an unconditional jump to SuccBB. Insert the unconditional jump.
1321 BranchInst::Create(SuccBB, NewBB);
1323 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1324 // PHI nodes for NewBB now.
1325 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1327 // If there were values defined in BB that are used outside the block, then we
1328 // now have to update all uses of the value to use either the original value,
1329 // the cloned value, or some PHI derived value. This can require arbitrary
1330 // PHI insertion, of which we are prepared to do, clean these up now.
1331 SSAUpdater SSAUpdate;
1332 SmallVector<Use*, 16> UsesToRename;
1333 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1334 // Scan all uses of this instruction to see if it is used outside of its
1335 // block, and if so, record them in UsesToRename.
1336 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1338 Instruction *User = cast<Instruction>(*UI);
1339 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1340 if (UserPN->getIncomingBlock(UI) == BB)
1342 } else if (User->getParent() == BB)
1345 UsesToRename.push_back(&UI.getUse());
1348 // If there are no uses outside the block, we're done with this instruction.
1349 if (UsesToRename.empty())
1352 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1354 // We found a use of I outside of BB. Rename all uses of I that are outside
1355 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1356 // with the two values we know.
1357 SSAUpdate.Initialize(I);
1358 SSAUpdate.AddAvailableValue(BB, I);
1359 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1361 while (!UsesToRename.empty())
1362 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1363 DEBUG(dbgs() << "\n");
1367 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to
1368 // NewBB instead of BB. This eliminates predecessors from BB, which requires
1369 // us to simplify any PHI nodes in BB.
1370 TerminatorInst *PredTerm = PredBB->getTerminator();
1371 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1372 if (PredTerm->getSuccessor(i) == BB) {
1373 RemovePredecessorAndSimplify(BB, PredBB, TD);
1374 PredTerm->setSuccessor(i, NewBB);
1377 // At this point, the IR is fully up to date and consistent. Do a quick scan
1378 // over the new instructions and zap any that are constants or dead. This
1379 // frequently happens because of phi translation.
1380 SimplifyInstructionsInBlock(NewBB, TD);
1382 // Threaded an edge!
1387 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1388 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1389 /// If we can duplicate the contents of BB up into PredBB do so now, this
1390 /// improves the odds that the branch will be on an analyzable instruction like
1392 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1393 const SmallVectorImpl<BasicBlock *> &PredBBs) {
1394 assert(!PredBBs.empty() && "Can't handle an empty set");
1396 // If BB is a loop header, then duplicating this block outside the loop would
1397 // cause us to transform this into an irreducible loop, don't do this.
1398 // See the comments above FindLoopHeaders for justifications and caveats.
1399 if (LoopHeaders.count(BB)) {
1400 DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
1401 << "' into predecessor block '" << PredBBs[0]->getName()
1402 << "' - it might create an irreducible loop!\n");
1406 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
1407 if (DuplicationCost > Threshold) {
1408 DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
1409 << "' - Cost is too high: " << DuplicationCost << "\n");
1413 // And finally, do it! Start by factoring the predecessors is needed.
1415 if (PredBBs.size() == 1)
1416 PredBB = PredBBs[0];
1418 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1419 << " common predecessors.\n");
1420 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1424 // Okay, we decided to do this! Clone all the instructions in BB onto the end
1426 DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '"
1427 << PredBB->getName() << "' to eliminate branch on phi. Cost: "
1428 << DuplicationCost << " block is:" << *BB << "\n");
1430 // Unless PredBB ends with an unconditional branch, split the edge so that we
1431 // can just clone the bits from BB into the end of the new PredBB.
1432 BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
1434 if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) {
1435 PredBB = SplitEdge(PredBB, BB, this);
1436 OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1439 // We are going to have to map operands from the original BB block into the
1440 // PredBB block. Evaluate PHI nodes in BB.
1441 DenseMap<Instruction*, Value*> ValueMapping;
1443 BasicBlock::iterator BI = BB->begin();
1444 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1445 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1447 // Clone the non-phi instructions of BB into PredBB, keeping track of the
1448 // mapping and using it to remap operands in the cloned instructions.
1449 for (; BI != BB->end(); ++BI) {
1450 Instruction *New = BI->clone();
1452 // Remap operands to patch up intra-block references.
1453 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1454 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1455 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1456 if (I != ValueMapping.end())
1457 New->setOperand(i, I->second);
1460 // If this instruction can be simplified after the operands are updated,
1461 // just use the simplified value instead. This frequently happens due to
1463 if (Value *IV = SimplifyInstruction(New, TD)) {
1465 ValueMapping[BI] = IV;
1467 // Otherwise, insert the new instruction into the block.
1468 New->setName(BI->getName());
1469 PredBB->getInstList().insert(OldPredBranch, New);
1470 ValueMapping[BI] = New;
1474 // Check to see if the targets of the branch had PHI nodes. If so, we need to
1475 // add entries to the PHI nodes for branch from PredBB now.
1476 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1477 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1479 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1482 // If there were values defined in BB that are used outside the block, then we
1483 // now have to update all uses of the value to use either the original value,
1484 // the cloned value, or some PHI derived value. This can require arbitrary
1485 // PHI insertion, of which we are prepared to do, clean these up now.
1486 SSAUpdater SSAUpdate;
1487 SmallVector<Use*, 16> UsesToRename;
1488 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1489 // Scan all uses of this instruction to see if it is used outside of its
1490 // block, and if so, record them in UsesToRename.
1491 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1493 Instruction *User = cast<Instruction>(*UI);
1494 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1495 if (UserPN->getIncomingBlock(UI) == BB)
1497 } else if (User->getParent() == BB)
1500 UsesToRename.push_back(&UI.getUse());
1503 // If there are no uses outside the block, we're done with this instruction.
1504 if (UsesToRename.empty())
1507 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1509 // We found a use of I outside of BB. Rename all uses of I that are outside
1510 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1511 // with the two values we know.
1512 SSAUpdate.Initialize(I);
1513 SSAUpdate.AddAvailableValue(BB, I);
1514 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1516 while (!UsesToRename.empty())
1517 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1518 DEBUG(dbgs() << "\n");
1521 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1523 RemovePredecessorAndSimplify(BB, PredBB, TD);
1525 // Remove the unconditional branch at the end of the PredBB block.
1526 OldPredBranch->eraseFromParent();