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/Analysis/Loads.h"
22 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
23 #include "llvm/Transforms/Utils/Local.h"
24 #include "llvm/Transforms/Utils/SSAUpdater.h"
25 #include "llvm/Target/TargetData.h"
26 #include "llvm/ADT/DenseMap.h"
27 #include "llvm/ADT/Statistic.h"
28 #include "llvm/ADT/STLExtras.h"
29 #include "llvm/ADT/SmallPtrSet.h"
30 #include "llvm/ADT/SmallSet.h"
31 #include "llvm/Support/CommandLine.h"
32 #include "llvm/Support/Debug.h"
33 #include "llvm/Support/ValueHandle.h"
34 #include "llvm/Support/raw_ostream.h"
37 STATISTIC(NumThreads, "Number of jumps threaded");
38 STATISTIC(NumFolds, "Number of terminators folded");
39 STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
41 static cl::opt<unsigned>
42 Threshold("jump-threading-threshold",
43 cl::desc("Max block size to duplicate for jump threading"),
44 cl::init(6), cl::Hidden);
46 // Turn on use of LazyValueInfo.
48 EnableLVI("enable-jump-threading-lvi",
49 cl::desc("Use LVI for jump threading"),
56 /// This pass performs 'jump threading', which looks at blocks that have
57 /// multiple predecessors and multiple successors. If one or more of the
58 /// predecessors of the block can be proven to always jump to one of the
59 /// successors, we forward the edge from the predecessor to the successor by
60 /// duplicating the contents of this block.
62 /// An example of when this can occur is code like this:
69 /// In this case, the unconditional branch at the end of the first if can be
70 /// revectored to the false side of the second if.
72 class JumpThreading : public FunctionPass {
76 SmallPtrSet<BasicBlock*, 16> LoopHeaders;
78 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
81 static char ID; // Pass identification
82 JumpThreading() : FunctionPass(ID) {}
84 bool runOnFunction(Function &F);
86 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
88 AU.addRequired<LazyValueInfo>();
91 void FindLoopHeaders(Function &F);
92 bool ProcessBlock(BasicBlock *BB);
93 bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
95 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
96 const SmallVectorImpl<BasicBlock *> &PredBBs);
98 typedef SmallVectorImpl<std::pair<ConstantInt*,
99 BasicBlock*> > PredValueInfo;
101 bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
102 PredValueInfo &Result);
103 bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB);
106 bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
107 bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
109 bool ProcessBranchOnPHI(PHINode *PN);
110 bool ProcessBranchOnXOR(BinaryOperator *BO);
112 bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
116 char JumpThreading::ID = 0;
117 INITIALIZE_PASS(JumpThreading, "jump-threading",
118 "Jump Threading", false, false);
120 // Public interface to the Jump Threading pass
121 FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
123 /// runOnFunction - Top level algorithm.
125 bool JumpThreading::runOnFunction(Function &F) {
126 DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
127 TD = getAnalysisIfAvailable<TargetData>();
128 LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0;
132 bool Changed, EverChanged = false;
135 for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
137 // Thread all of the branches we can over this block.
138 while (ProcessBlock(BB))
143 // If the block is trivially dead, zap it. This eliminates the successor
144 // edges which simplifies the CFG.
145 if (pred_begin(BB) == pred_end(BB) &&
146 BB != &BB->getParent()->getEntryBlock()) {
147 DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName()
148 << "' with terminator: " << *BB->getTerminator() << '\n');
149 LoopHeaders.erase(BB);
152 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
153 // Can't thread an unconditional jump, but if the block is "almost
154 // empty", we can replace uses of it with uses of the successor and make
156 if (BI->isUnconditional() &&
157 BB != &BB->getParent()->getEntryBlock()) {
158 BasicBlock::iterator BBI = BB->getFirstNonPHI();
159 // Ignore dbg intrinsics.
160 while (isa<DbgInfoIntrinsic>(BBI))
162 // If the terminator is the only non-phi instruction, try to nuke it.
163 if (BBI->isTerminator()) {
164 // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
165 // block, we have to make sure it isn't in the LoopHeaders set. We
166 // reinsert afterward if needed.
167 bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
168 BasicBlock *Succ = BI->getSuccessor(0);
170 if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
172 // If we deleted BB and BB was the header of a loop, then the
173 // successor is now the header of the loop.
177 if (ErasedFromLoopHeaders)
178 LoopHeaders.insert(BB);
183 EverChanged |= Changed;
190 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
191 /// thread across it.
192 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
193 /// Ignore PHI nodes, these will be flattened when duplication happens.
194 BasicBlock::const_iterator I = BB->getFirstNonPHI();
196 // FIXME: THREADING will delete values that are just used to compute the
197 // branch, so they shouldn't count against the duplication cost.
200 // Sum up the cost of each instruction until we get to the terminator. Don't
201 // include the terminator because the copy won't include it.
203 for (; !isa<TerminatorInst>(I); ++I) {
204 // Debugger intrinsics don't incur code size.
205 if (isa<DbgInfoIntrinsic>(I)) continue;
207 // If this is a pointer->pointer bitcast, it is free.
208 if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
211 // All other instructions count for at least one unit.
214 // Calls are more expensive. If they are non-intrinsic calls, we model them
215 // as having cost of 4. If they are a non-vector intrinsic, we model them
216 // as having cost of 2 total, and if they are a vector intrinsic, we model
217 // them as having cost 1.
218 if (const CallInst *CI = dyn_cast<CallInst>(I)) {
219 if (!isa<IntrinsicInst>(CI))
221 else if (!CI->getType()->isVectorTy())
226 // Threading through a switch statement is particularly profitable. If this
227 // block ends in a switch, decrease its cost to make it more likely to happen.
228 if (isa<SwitchInst>(I))
229 Size = Size > 6 ? Size-6 : 0;
234 /// FindLoopHeaders - We do not want jump threading to turn proper loop
235 /// structures into irreducible loops. Doing this breaks up the loop nesting
236 /// hierarchy and pessimizes later transformations. To prevent this from
237 /// happening, we first have to find the loop headers. Here we approximate this
238 /// by finding targets of backedges in the CFG.
240 /// Note that there definitely are cases when we want to allow threading of
241 /// edges across a loop header. For example, threading a jump from outside the
242 /// loop (the preheader) to an exit block of the loop is definitely profitable.
243 /// It is also almost always profitable to thread backedges from within the loop
244 /// to exit blocks, and is often profitable to thread backedges to other blocks
245 /// within the loop (forming a nested loop). This simple analysis is not rich
246 /// enough to track all of these properties and keep it up-to-date as the CFG
247 /// mutates, so we don't allow any of these transformations.
249 void JumpThreading::FindLoopHeaders(Function &F) {
250 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
251 FindFunctionBackedges(F, Edges);
253 for (unsigned i = 0, e = Edges.size(); i != e; ++i)
254 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
257 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
258 /// if we can infer that the value is a known ConstantInt in any of our
259 /// predecessors. If so, return the known list of value and pred BB in the
260 /// result vector. If a value is known to be undef, it is returned as null.
262 /// This returns true if there were any known values.
265 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
266 // If V is a constantint, then it is known in all predecessors.
267 if (isa<ConstantInt>(V) || isa<UndefValue>(V)) {
268 ConstantInt *CI = dyn_cast<ConstantInt>(V);
270 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
271 Result.push_back(std::make_pair(CI, *PI));
275 // If V is a non-instruction value, or an instruction in a different block,
276 // then it can't be derived from a PHI.
277 Instruction *I = dyn_cast<Instruction>(V);
278 if (I == 0 || I->getParent() != BB) {
280 // Okay, if this is a live-in value, see if it has a known value at the end
281 // of any of our predecessors.
283 // FIXME: This should be an edge property, not a block end property.
284 /// TODO: Per PR2563, we could infer value range information about a
285 /// predecessor based on its terminator.
288 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
289 // "I" is a non-local compare-with-a-constant instruction. This would be
290 // able to handle value inequalities better, for example if the compare is
291 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
292 // Perhaps getConstantOnEdge should be smart enough to do this?
294 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
296 // If the value is known by LazyValueInfo to be a constant in a
297 // predecessor, use that information to try to thread this block.
298 Constant *PredCst = LVI->getConstantOnEdge(V, P, BB);
300 (!isa<ConstantInt>(PredCst) && !isa<UndefValue>(PredCst)))
303 Result.push_back(std::make_pair(dyn_cast<ConstantInt>(PredCst), P));
306 return !Result.empty();
312 /// If I is a PHI node, then we know the incoming values for any constants.
313 if (PHINode *PN = dyn_cast<PHINode>(I)) {
314 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
315 Value *InVal = PN->getIncomingValue(i);
316 if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) {
317 ConstantInt *CI = dyn_cast<ConstantInt>(InVal);
318 Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i)));
321 return !Result.empty();
324 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals;
326 // Handle some boolean conditions.
327 if (I->getType()->getPrimitiveSizeInBits() == 1) {
329 // X & false -> false
330 if (I->getOpcode() == Instruction::Or ||
331 I->getOpcode() == Instruction::And) {
332 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals);
333 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals);
335 if (LHSVals.empty() && RHSVals.empty())
338 ConstantInt *InterestingVal;
339 if (I->getOpcode() == Instruction::Or)
340 InterestingVal = ConstantInt::getTrue(I->getContext());
342 InterestingVal = ConstantInt::getFalse(I->getContext());
344 // Scan for the sentinel. If we find an undef, force it to the
345 // interesting value: x|undef -> true and x&undef -> false.
346 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
347 if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0) {
348 Result.push_back(LHSVals[i]);
349 Result.back().first = InterestingVal;
351 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
352 if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0) {
353 // If we already inferred a value for this block on the LHS, don't
355 bool HasValue = false;
356 for (unsigned r = 0, e = Result.size(); r != e; ++r)
357 if (Result[r].second == RHSVals[i].second) {
363 Result.push_back(RHSVals[i]);
364 Result.back().first = InterestingVal;
367 return !Result.empty();
370 // Handle the NOT form of XOR.
371 if (I->getOpcode() == Instruction::Xor &&
372 isa<ConstantInt>(I->getOperand(1)) &&
373 cast<ConstantInt>(I->getOperand(1))->isOne()) {
374 ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result);
378 // Invert the known values.
379 for (unsigned i = 0, e = Result.size(); i != e; ++i)
382 cast<ConstantInt>(ConstantExpr::getNot(Result[i].first));
387 // Handle compare with phi operand, where the PHI is defined in this block.
388 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
389 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
390 if (PN && PN->getParent() == BB) {
391 // We can do this simplification if any comparisons fold to true or false.
393 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
394 BasicBlock *PredBB = PN->getIncomingBlock(i);
395 Value *LHS = PN->getIncomingValue(i);
396 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
398 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD);
400 if (!LVI || !isa<Constant>(RHS))
403 LazyValueInfo::Tristate
404 ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
405 cast<Constant>(RHS), PredBB, BB);
406 if (ResT == LazyValueInfo::Unknown)
408 Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
411 if (isa<UndefValue>(Res))
412 Result.push_back(std::make_pair((ConstantInt*)0, PredBB));
413 else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res))
414 Result.push_back(std::make_pair(CI, PredBB));
417 return !Result.empty();
421 // If comparing a live-in value against a constant, see if we know the
422 // live-in value on any predecessors.
423 if (LVI && isa<Constant>(Cmp->getOperand(1)) &&
424 Cmp->getType()->isIntegerTy() && // Not vector compare.
425 (!isa<Instruction>(Cmp->getOperand(0)) ||
426 cast<Instruction>(Cmp->getOperand(0))->getParent() != BB)) {
427 Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
429 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
431 // If the value is known by LazyValueInfo to be a constant in a
432 // predecessor, use that information to try to thread this block.
433 LazyValueInfo::Tristate
434 Res = LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
436 if (Res == LazyValueInfo::Unknown)
439 Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
440 Result.push_back(std::make_pair(cast<ConstantInt>(ResC), P));
443 return !Result.empty();
451 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
452 /// in an undefined jump, decide which block is best to revector to.
454 /// Since we can pick an arbitrary destination, we pick the successor with the
455 /// fewest predecessors. This should reduce the in-degree of the others.
457 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
458 TerminatorInst *BBTerm = BB->getTerminator();
459 unsigned MinSucc = 0;
460 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
461 // Compute the successor with the minimum number of predecessors.
462 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
463 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
464 TestBB = BBTerm->getSuccessor(i);
465 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
466 if (NumPreds < MinNumPreds)
473 /// ProcessBlock - If there are any predecessors whose control can be threaded
474 /// through to a successor, transform them now.
475 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
476 // If the block is trivially dead, just return and let the caller nuke it.
477 // This simplifies other transformations.
478 if (pred_begin(BB) == pred_end(BB) &&
479 BB != &BB->getParent()->getEntryBlock())
482 // If this block has a single predecessor, and if that pred has a single
483 // successor, merge the blocks. This encourages recursive jump threading
484 // because now the condition in this block can be threaded through
485 // predecessors of our predecessor block.
486 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
487 if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
489 // If SinglePred was a loop header, BB becomes one.
490 if (LoopHeaders.erase(SinglePred))
491 LoopHeaders.insert(BB);
493 // Remember if SinglePred was the entry block of the function. If so, we
494 // will need to move BB back to the entry position.
495 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
496 MergeBasicBlockIntoOnlyPred(BB);
498 if (isEntry && BB != &BB->getParent()->getEntryBlock())
499 BB->moveBefore(&BB->getParent()->getEntryBlock());
504 // Look to see if the terminator is a branch of switch, if not we can't thread
507 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
508 // Can't thread an unconditional jump.
509 if (BI->isUnconditional()) return false;
510 Condition = BI->getCondition();
511 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
512 Condition = SI->getCondition();
514 return false; // Must be an invoke.
516 // If the terminator of this block is branching on a constant, simplify the
517 // terminator to an unconditional branch. This can occur due to threading in
519 if (isa<ConstantInt>(Condition)) {
520 DEBUG(dbgs() << " In block '" << BB->getName()
521 << "' folding terminator: " << *BB->getTerminator() << '\n');
523 ConstantFoldTerminator(BB);
527 // If the terminator is branching on an undef, we can pick any of the
528 // successors to branch to. Let GetBestDestForJumpOnUndef decide.
529 if (isa<UndefValue>(Condition)) {
530 unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
532 // Fold the branch/switch.
533 TerminatorInst *BBTerm = BB->getTerminator();
534 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
535 if (i == BestSucc) continue;
536 RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD);
539 DEBUG(dbgs() << " In block '" << BB->getName()
540 << "' folding undef terminator: " << *BBTerm << '\n');
541 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
542 BBTerm->eraseFromParent();
546 Instruction *CondInst = dyn_cast<Instruction>(Condition);
548 // If the condition is an instruction defined in another block, see if a
549 // predecessor has the same condition:
554 !Condition->hasOneUse() && // Multiple uses.
555 (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
556 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
557 if (isa<BranchInst>(BB->getTerminator())) {
558 for (; PI != E; ++PI) {
560 if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator()))
561 if (PBI->isConditional() && PBI->getCondition() == Condition &&
562 ProcessBranchOnDuplicateCond(P, BB))
566 assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
567 for (; PI != E; ++PI) {
569 if (SwitchInst *PSI = dyn_cast<SwitchInst>(P->getTerminator()))
570 if (PSI->getCondition() == Condition &&
571 ProcessSwitchOnDuplicateCond(P, BB))
577 // All the rest of our checks depend on the condition being an instruction.
579 // FIXME: Unify this with code below.
580 if (LVI && ProcessThreadableEdges(Condition, BB))
586 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
588 (!isa<PHINode>(CondCmp->getOperand(0)) ||
589 cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB)) {
590 // If we have a comparison, loop over the predecessors to see if there is
591 // a condition with a lexically identical value.
592 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
593 for (; PI != E; ++PI) {
595 if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator()))
596 if (PBI->isConditional() && P != BB) {
597 if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) {
598 if (CI->getOperand(0) == CondCmp->getOperand(0) &&
599 CI->getOperand(1) == CondCmp->getOperand(1) &&
600 CI->getPredicate() == CondCmp->getPredicate()) {
601 // TODO: Could handle things like (x != 4) --> (x == 17)
602 if (ProcessBranchOnDuplicateCond(P, BB))
611 // Check for some cases that are worth simplifying. Right now we want to look
612 // for loads that are used by a switch or by the condition for the branch. If
613 // we see one, check to see if it's partially redundant. If so, insert a PHI
614 // which can then be used to thread the values.
616 Value *SimplifyValue = CondInst;
617 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
618 if (isa<Constant>(CondCmp->getOperand(1)))
619 SimplifyValue = CondCmp->getOperand(0);
621 // TODO: There are other places where load PRE would be profitable, such as
622 // more complex comparisons.
623 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
624 if (SimplifyPartiallyRedundantLoad(LI))
628 // Handle a variety of cases where we are branching on something derived from
629 // a PHI node in the current block. If we can prove that any predecessors
630 // compute a predictable value based on a PHI node, thread those predecessors.
632 if (ProcessThreadableEdges(CondInst, BB))
635 // If this is an otherwise-unfoldable branch on a phi node in the current
636 // block, see if we can simplify.
637 if (PHINode *PN = dyn_cast<PHINode>(CondInst))
638 if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
639 return ProcessBranchOnPHI(PN);
642 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
643 if (CondInst->getOpcode() == Instruction::Xor &&
644 CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
645 return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
648 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know
649 // "(X == 4)", thread through this block.
654 /// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
655 /// block that jump on exactly the same condition. This means that we almost
656 /// always know the direction of the edge in the DESTBB:
658 /// br COND, DESTBB, BBY
660 /// br COND, BBZ, BBW
662 /// If DESTBB has multiple predecessors, we can't just constant fold the branch
663 /// in DESTBB, we have to thread over it.
664 bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
666 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
668 // If both successors of PredBB go to DESTBB, we don't know anything. We can
669 // fold the branch to an unconditional one, which allows other recursive
672 if (PredBI->getSuccessor(1) != BB)
674 else if (PredBI->getSuccessor(0) != BB)
677 DEBUG(dbgs() << " In block '" << PredBB->getName()
678 << "' folding terminator: " << *PredBB->getTerminator() << '\n');
680 ConstantFoldTerminator(PredBB);
684 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
686 // If the dest block has one predecessor, just fix the branch condition to a
687 // constant and fold it.
688 if (BB->getSinglePredecessor()) {
689 DEBUG(dbgs() << " In block '" << BB->getName()
690 << "' folding condition to '" << BranchDir << "': "
691 << *BB->getTerminator() << '\n');
693 Value *OldCond = DestBI->getCondition();
694 DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
696 // Delete dead instructions before we fold the branch. Folding the branch
697 // can eliminate edges from the CFG which can end up deleting OldCond.
698 RecursivelyDeleteTriviallyDeadInstructions(OldCond);
699 ConstantFoldTerminator(BB);
704 // Next, figure out which successor we are threading to.
705 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
707 SmallVector<BasicBlock*, 2> Preds;
708 Preds.push_back(PredBB);
710 // Ok, try to thread it!
711 return ThreadEdge(BB, Preds, SuccBB);
714 /// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
715 /// block that switch on exactly the same condition. This means that we almost
716 /// always know the direction of the edge in the DESTBB:
718 /// switch COND [... DESTBB, BBY ... ]
720 /// switch COND [... BBZ, BBW ]
722 /// Optimizing switches like this is very important, because simplifycfg builds
723 /// switches out of repeated 'if' conditions.
724 bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
725 BasicBlock *DestBB) {
726 // Can't thread edge to self.
727 if (PredBB == DestBB)
730 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
731 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
733 // There are a variety of optimizations that we can potentially do on these
734 // blocks: we order them from most to least preferable.
736 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB
737 // directly to their destination. This does not introduce *any* code size
738 // growth. Skip debug info first.
739 BasicBlock::iterator BBI = DestBB->begin();
740 while (isa<DbgInfoIntrinsic>(BBI))
743 // FIXME: Thread if it just contains a PHI.
744 if (isa<SwitchInst>(BBI)) {
745 bool MadeChange = false;
746 // Ignore the default edge for now.
747 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
748 ConstantInt *DestVal = DestSI->getCaseValue(i);
749 BasicBlock *DestSucc = DestSI->getSuccessor(i);
751 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if
752 // PredSI has an explicit case for it. If so, forward. If it is covered
753 // by the default case, we can't update PredSI.
754 unsigned PredCase = PredSI->findCaseValue(DestVal);
755 if (PredCase == 0) continue;
757 // If PredSI doesn't go to DestBB on this value, then it won't reach the
758 // case on this condition.
759 if (PredSI->getSuccessor(PredCase) != DestBB &&
760 DestSI->getSuccessor(i) != DestBB)
763 // Do not forward this if it already goes to this destination, this would
764 // be an infinite loop.
765 if (PredSI->getSuccessor(PredCase) == DestSucc)
768 // Otherwise, we're safe to make the change. Make sure that the edge from
769 // DestSI to DestSucc is not critical and has no PHI nodes.
770 DEBUG(dbgs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI);
771 DEBUG(dbgs() << "THROUGH: " << *DestSI);
773 // If the destination has PHI nodes, just split the edge for updating
775 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
776 SplitCriticalEdge(DestSI, i, this);
777 DestSucc = DestSI->getSuccessor(i);
779 FoldSingleEntryPHINodes(DestSucc);
780 PredSI->setSuccessor(PredCase, DestSucc);
792 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
793 /// load instruction, eliminate it by replacing it with a PHI node. This is an
794 /// important optimization that encourages jump threading, and needs to be run
795 /// interlaced with other jump threading tasks.
796 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
797 // Don't hack volatile loads.
798 if (LI->isVolatile()) return false;
800 // If the load is defined in a block with exactly one predecessor, it can't be
801 // partially redundant.
802 BasicBlock *LoadBB = LI->getParent();
803 if (LoadBB->getSinglePredecessor())
806 Value *LoadedPtr = LI->getOperand(0);
808 // If the loaded operand is defined in the LoadBB, it can't be available.
809 // TODO: Could do simple PHI translation, that would be fun :)
810 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
811 if (PtrOp->getParent() == LoadBB)
814 // Scan a few instructions up from the load, to see if it is obviously live at
815 // the entry to its block.
816 BasicBlock::iterator BBIt = LI;
818 if (Value *AvailableVal =
819 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
820 // If the value if the load is locally available within the block, just use
821 // it. This frequently occurs for reg2mem'd allocas.
822 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
824 // If the returned value is the load itself, replace with an undef. This can
825 // only happen in dead loops.
826 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
827 LI->replaceAllUsesWith(AvailableVal);
828 LI->eraseFromParent();
832 // Otherwise, if we scanned the whole block and got to the top of the block,
833 // we know the block is locally transparent to the load. If not, something
834 // might clobber its value.
835 if (BBIt != LoadBB->begin())
839 SmallPtrSet<BasicBlock*, 8> PredsScanned;
840 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
841 AvailablePredsTy AvailablePreds;
842 BasicBlock *OneUnavailablePred = 0;
844 // If we got here, the loaded value is transparent through to the start of the
845 // block. Check to see if it is available in any of the predecessor blocks.
846 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
848 BasicBlock *PredBB = *PI;
850 // If we already scanned this predecessor, skip it.
851 if (!PredsScanned.insert(PredBB))
854 // Scan the predecessor to see if the value is available in the pred.
855 BBIt = PredBB->end();
856 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6);
857 if (!PredAvailable) {
858 OneUnavailablePred = PredBB;
862 // If so, this load is partially redundant. Remember this info so that we
863 // can create a PHI node.
864 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
867 // If the loaded value isn't available in any predecessor, it isn't partially
869 if (AvailablePreds.empty()) return false;
871 // Okay, the loaded value is available in at least one (and maybe all!)
872 // predecessors. If the value is unavailable in more than one unique
873 // predecessor, we want to insert a merge block for those common predecessors.
874 // This ensures that we only have to insert one reload, thus not increasing
876 BasicBlock *UnavailablePred = 0;
878 // If there is exactly one predecessor where the value is unavailable, the
879 // already computed 'OneUnavailablePred' block is it. If it ends in an
880 // unconditional branch, we know that it isn't a critical edge.
881 if (PredsScanned.size() == AvailablePreds.size()+1 &&
882 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
883 UnavailablePred = OneUnavailablePred;
884 } else if (PredsScanned.size() != AvailablePreds.size()) {
885 // Otherwise, we had multiple unavailable predecessors or we had a critical
886 // edge from the one.
887 SmallVector<BasicBlock*, 8> PredsToSplit;
888 SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
890 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
891 AvailablePredSet.insert(AvailablePreds[i].first);
893 // Add all the unavailable predecessors to the PredsToSplit list.
894 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
897 // If the predecessor is an indirect goto, we can't split the edge.
898 if (isa<IndirectBrInst>(P->getTerminator()))
901 if (!AvailablePredSet.count(P))
902 PredsToSplit.push_back(P);
905 // Split them out to their own block.
907 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
908 "thread-pre-split", this);
911 // If the value isn't available in all predecessors, then there will be
912 // exactly one where it isn't available. Insert a load on that edge and add
913 // it to the AvailablePreds list.
914 if (UnavailablePred) {
915 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
916 "Can't handle critical edge here!");
917 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
919 UnavailablePred->getTerminator());
920 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
923 // Now we know that each predecessor of this block has a value in
924 // AvailablePreds, sort them for efficient access as we're walking the preds.
925 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
927 // Create a PHI node at the start of the block for the PRE'd load value.
928 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
931 // Insert new entries into the PHI for each predecessor. A single block may
932 // have multiple entries here.
933 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
936 AvailablePredsTy::iterator I =
937 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
938 std::make_pair(P, (Value*)0));
940 assert(I != AvailablePreds.end() && I->first == P &&
941 "Didn't find entry for predecessor!");
943 PN->addIncoming(I->second, I->first);
946 //cerr << "PRE: " << *LI << *PN << "\n";
948 LI->replaceAllUsesWith(PN);
949 LI->eraseFromParent();
954 /// FindMostPopularDest - The specified list contains multiple possible
955 /// threadable destinations. Pick the one that occurs the most frequently in
958 FindMostPopularDest(BasicBlock *BB,
959 const SmallVectorImpl<std::pair<BasicBlock*,
960 BasicBlock*> > &PredToDestList) {
961 assert(!PredToDestList.empty());
963 // Determine popularity. If there are multiple possible destinations, we
964 // explicitly choose to ignore 'undef' destinations. We prefer to thread
965 // blocks with known and real destinations to threading undef. We'll handle
966 // them later if interesting.
967 DenseMap<BasicBlock*, unsigned> DestPopularity;
968 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
969 if (PredToDestList[i].second)
970 DestPopularity[PredToDestList[i].second]++;
972 // Find the most popular dest.
973 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
974 BasicBlock *MostPopularDest = DPI->first;
975 unsigned Popularity = DPI->second;
976 SmallVector<BasicBlock*, 4> SamePopularity;
978 for (++DPI; DPI != DestPopularity.end(); ++DPI) {
979 // If the popularity of this entry isn't higher than the popularity we've
980 // seen so far, ignore it.
981 if (DPI->second < Popularity)
983 else if (DPI->second == Popularity) {
984 // If it is the same as what we've seen so far, keep track of it.
985 SamePopularity.push_back(DPI->first);
987 // If it is more popular, remember it.
988 SamePopularity.clear();
989 MostPopularDest = DPI->first;
990 Popularity = DPI->second;
994 // Okay, now we know the most popular destination. If there is more than
995 // destination, we need to determine one. This is arbitrary, but we need
996 // to make a deterministic decision. Pick the first one that appears in the
998 if (!SamePopularity.empty()) {
999 SamePopularity.push_back(MostPopularDest);
1000 TerminatorInst *TI = BB->getTerminator();
1001 for (unsigned i = 0; ; ++i) {
1002 assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
1004 if (std::find(SamePopularity.begin(), SamePopularity.end(),
1005 TI->getSuccessor(i)) == SamePopularity.end())
1008 MostPopularDest = TI->getSuccessor(i);
1013 // Okay, we have finally picked the most popular destination.
1014 return MostPopularDest;
1017 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) {
1018 // If threading this would thread across a loop header, don't even try to
1020 if (LoopHeaders.count(BB))
1023 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues;
1024 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues))
1026 assert(!PredValues.empty() &&
1027 "ComputeValueKnownInPredecessors returned true with no values");
1029 DEBUG(dbgs() << "IN BB: " << *BB;
1030 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1031 dbgs() << " BB '" << BB->getName() << "': FOUND condition = ";
1032 if (PredValues[i].first)
1033 dbgs() << *PredValues[i].first;
1036 dbgs() << " for pred '" << PredValues[i].second->getName()
1040 // Decide what we want to thread through. Convert our list of known values to
1041 // a list of known destinations for each pred. This also discards duplicate
1042 // predecessors and keeps track of the undefined inputs (which are represented
1043 // as a null dest in the PredToDestList).
1044 SmallPtrSet<BasicBlock*, 16> SeenPreds;
1045 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
1047 BasicBlock *OnlyDest = 0;
1048 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1050 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1051 BasicBlock *Pred = PredValues[i].second;
1052 if (!SeenPreds.insert(Pred))
1053 continue; // Duplicate predecessor entry.
1055 // If the predecessor ends with an indirect goto, we can't change its
1057 if (isa<IndirectBrInst>(Pred->getTerminator()))
1060 ConstantInt *Val = PredValues[i].first;
1063 if (Val == 0) // Undef.
1065 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1066 DestBB = BI->getSuccessor(Val->isZero());
1068 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
1069 DestBB = SI->getSuccessor(SI->findCaseValue(Val));
1072 // If we have exactly one destination, remember it for efficiency below.
1075 else if (OnlyDest != DestBB)
1076 OnlyDest = MultipleDestSentinel;
1078 PredToDestList.push_back(std::make_pair(Pred, DestBB));
1081 // If all edges were unthreadable, we fail.
1082 if (PredToDestList.empty())
1085 // Determine which is the most common successor. If we have many inputs and
1086 // this block is a switch, we want to start by threading the batch that goes
1087 // to the most popular destination first. If we only know about one
1088 // threadable destination (the common case) we can avoid this.
1089 BasicBlock *MostPopularDest = OnlyDest;
1091 if (MostPopularDest == MultipleDestSentinel)
1092 MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1094 // Now that we know what the most popular destination is, factor all
1095 // predecessors that will jump to it into a single predecessor.
1096 SmallVector<BasicBlock*, 16> PredsToFactor;
1097 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1098 if (PredToDestList[i].second == MostPopularDest) {
1099 BasicBlock *Pred = PredToDestList[i].first;
1101 // This predecessor may be a switch or something else that has multiple
1102 // edges to the block. Factor each of these edges by listing them
1103 // according to # occurrences in PredsToFactor.
1104 TerminatorInst *PredTI = Pred->getTerminator();
1105 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
1106 if (PredTI->getSuccessor(i) == BB)
1107 PredsToFactor.push_back(Pred);
1110 // If the threadable edges are branching on an undefined value, we get to pick
1111 // the destination that these predecessors should get to.
1112 if (MostPopularDest == 0)
1113 MostPopularDest = BB->getTerminator()->
1114 getSuccessor(GetBestDestForJumpOnUndef(BB));
1116 // Ok, try to thread it!
1117 return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1120 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1121 /// a PHI node in the current block. See if there are any simplifications we
1122 /// can do based on inputs to the phi node.
1124 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
1125 BasicBlock *BB = PN->getParent();
1127 // TODO: We could make use of this to do it once for blocks with common PHI
1129 SmallVector<BasicBlock*, 1> PredBBs;
1132 // If any of the predecessor blocks end in an unconditional branch, we can
1133 // *duplicate* the conditional branch into that block in order to further
1134 // encourage jump threading and to eliminate cases where we have branch on a
1135 // phi of an icmp (branch on icmp is much better).
1136 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1137 BasicBlock *PredBB = PN->getIncomingBlock(i);
1138 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1139 if (PredBr->isUnconditional()) {
1140 PredBBs[0] = PredBB;
1141 // Try to duplicate BB into PredBB.
1142 if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
1150 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
1151 /// a xor instruction in the current block. See if there are any
1152 /// simplifications we can do based on inputs to the xor.
1154 bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
1155 BasicBlock *BB = BO->getParent();
1157 // If either the LHS or RHS of the xor is a constant, don't do this
1159 if (isa<ConstantInt>(BO->getOperand(0)) ||
1160 isa<ConstantInt>(BO->getOperand(1)))
1163 // If the first instruction in BB isn't a phi, we won't be able to infer
1164 // anything special about any particular predecessor.
1165 if (!isa<PHINode>(BB->front()))
1168 // If we have a xor as the branch input to this block, and we know that the
1169 // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1170 // the condition into the predecessor and fix that value to true, saving some
1171 // logical ops on that path and encouraging other paths to simplify.
1173 // This copies something like this:
1176 // %X = phi i1 [1], [%X']
1177 // %Y = icmp eq i32 %A, %B
1178 // %Z = xor i1 %X, %Y
1183 // %Y = icmp ne i32 %A, %B
1186 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> XorOpValues;
1188 if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) {
1189 assert(XorOpValues.empty());
1190 if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues))
1195 assert(!XorOpValues.empty() &&
1196 "ComputeValueKnownInPredecessors returned true with no values");
1198 // Scan the information to see which is most popular: true or false. The
1199 // predecessors can be of the set true, false, or undef.
1200 unsigned NumTrue = 0, NumFalse = 0;
1201 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1202 if (!XorOpValues[i].first) continue; // Ignore undefs for the count.
1203 if (XorOpValues[i].first->isZero())
1209 // Determine which value to split on, true, false, or undef if neither.
1210 ConstantInt *SplitVal = 0;
1211 if (NumTrue > NumFalse)
1212 SplitVal = ConstantInt::getTrue(BB->getContext());
1213 else if (NumTrue != 0 || NumFalse != 0)
1214 SplitVal = ConstantInt::getFalse(BB->getContext());
1216 // Collect all of the blocks that this can be folded into so that we can
1217 // factor this once and clone it once.
1218 SmallVector<BasicBlock*, 8> BlocksToFoldInto;
1219 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1220 if (XorOpValues[i].first != SplitVal && XorOpValues[i].first != 0) continue;
1222 BlocksToFoldInto.push_back(XorOpValues[i].second);
1225 // If we inferred a value for all of the predecessors, then duplication won't
1226 // help us. However, we can just replace the LHS or RHS with the constant.
1227 if (BlocksToFoldInto.size() ==
1228 cast<PHINode>(BB->front()).getNumIncomingValues()) {
1229 if (SplitVal == 0) {
1230 // If all preds provide undef, just nuke the xor, because it is undef too.
1231 BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
1232 BO->eraseFromParent();
1233 } else if (SplitVal->isZero()) {
1234 // If all preds provide 0, replace the xor with the other input.
1235 BO->replaceAllUsesWith(BO->getOperand(isLHS));
1236 BO->eraseFromParent();
1238 // If all preds provide 1, set the computed value to 1.
1239 BO->setOperand(!isLHS, SplitVal);
1245 // Try to duplicate BB into PredBB.
1246 return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
1250 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1251 /// predecessor to the PHIBB block. If it has PHI nodes, add entries for
1252 /// NewPred using the entries from OldPred (suitably mapped).
1253 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1254 BasicBlock *OldPred,
1255 BasicBlock *NewPred,
1256 DenseMap<Instruction*, Value*> &ValueMap) {
1257 for (BasicBlock::iterator PNI = PHIBB->begin();
1258 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1259 // Ok, we have a PHI node. Figure out what the incoming value was for the
1261 Value *IV = PN->getIncomingValueForBlock(OldPred);
1263 // Remap the value if necessary.
1264 if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1265 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1266 if (I != ValueMap.end())
1270 PN->addIncoming(IV, NewPred);
1274 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1275 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1276 /// across BB. Transform the IR to reflect this change.
1277 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1278 const SmallVectorImpl<BasicBlock*> &PredBBs,
1279 BasicBlock *SuccBB) {
1280 // If threading to the same block as we come from, we would infinite loop.
1282 DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
1283 << "' - would thread to self!\n");
1287 // If threading this would thread across a loop header, don't thread the edge.
1288 // See the comments above FindLoopHeaders for justifications and caveats.
1289 if (LoopHeaders.count(BB)) {
1290 DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName()
1291 << "' to dest BB '" << SuccBB->getName()
1292 << "' - it might create an irreducible loop!\n");
1296 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
1297 if (JumpThreadCost > Threshold) {
1298 DEBUG(dbgs() << " Not threading BB '" << BB->getName()
1299 << "' - Cost is too high: " << JumpThreadCost << "\n");
1303 // And finally, do it! Start by factoring the predecessors is needed.
1305 if (PredBBs.size() == 1)
1306 PredBB = PredBBs[0];
1308 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1309 << " common predecessors.\n");
1310 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1314 // And finally, do it!
1315 DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '"
1316 << SuccBB->getName() << "' with cost: " << JumpThreadCost
1317 << ", across block:\n "
1321 LVI->threadEdge(PredBB, BB, SuccBB);
1323 // We are going to have to map operands from the original BB block to the new
1324 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
1325 // account for entry from PredBB.
1326 DenseMap<Instruction*, Value*> ValueMapping;
1328 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1329 BB->getName()+".thread",
1330 BB->getParent(), BB);
1331 NewBB->moveAfter(PredBB);
1333 BasicBlock::iterator BI = BB->begin();
1334 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1335 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1337 // Clone the non-phi instructions of BB into NewBB, keeping track of the
1338 // mapping and using it to remap operands in the cloned instructions.
1339 for (; !isa<TerminatorInst>(BI); ++BI) {
1340 Instruction *New = BI->clone();
1341 New->setName(BI->getName());
1342 NewBB->getInstList().push_back(New);
1343 ValueMapping[BI] = New;
1345 // Remap operands to patch up intra-block references.
1346 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1347 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1348 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1349 if (I != ValueMapping.end())
1350 New->setOperand(i, I->second);
1354 // We didn't copy the terminator from BB over to NewBB, because there is now
1355 // an unconditional jump to SuccBB. Insert the unconditional jump.
1356 BranchInst::Create(SuccBB, NewBB);
1358 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1359 // PHI nodes for NewBB now.
1360 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1362 // If there were values defined in BB that are used outside the block, then we
1363 // now have to update all uses of the value to use either the original value,
1364 // the cloned value, or some PHI derived value. This can require arbitrary
1365 // PHI insertion, of which we are prepared to do, clean these up now.
1366 SSAUpdater SSAUpdate;
1367 SmallVector<Use*, 16> UsesToRename;
1368 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1369 // Scan all uses of this instruction to see if it is used outside of its
1370 // block, and if so, record them in UsesToRename.
1371 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1373 Instruction *User = cast<Instruction>(*UI);
1374 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1375 if (UserPN->getIncomingBlock(UI) == BB)
1377 } else if (User->getParent() == BB)
1380 UsesToRename.push_back(&UI.getUse());
1383 // If there are no uses outside the block, we're done with this instruction.
1384 if (UsesToRename.empty())
1387 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1389 // We found a use of I outside of BB. Rename all uses of I that are outside
1390 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1391 // with the two values we know.
1392 SSAUpdate.Initialize(I);
1393 SSAUpdate.AddAvailableValue(BB, I);
1394 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1396 while (!UsesToRename.empty())
1397 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1398 DEBUG(dbgs() << "\n");
1402 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to
1403 // NewBB instead of BB. This eliminates predecessors from BB, which requires
1404 // us to simplify any PHI nodes in BB.
1405 TerminatorInst *PredTerm = PredBB->getTerminator();
1406 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1407 if (PredTerm->getSuccessor(i) == BB) {
1408 RemovePredecessorAndSimplify(BB, PredBB, TD);
1409 PredTerm->setSuccessor(i, NewBB);
1412 // At this point, the IR is fully up to date and consistent. Do a quick scan
1413 // over the new instructions and zap any that are constants or dead. This
1414 // frequently happens because of phi translation.
1415 SimplifyInstructionsInBlock(NewBB, TD);
1417 // Threaded an edge!
1422 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1423 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1424 /// If we can duplicate the contents of BB up into PredBB do so now, this
1425 /// improves the odds that the branch will be on an analyzable instruction like
1427 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1428 const SmallVectorImpl<BasicBlock *> &PredBBs) {
1429 assert(!PredBBs.empty() && "Can't handle an empty set");
1431 // If BB is a loop header, then duplicating this block outside the loop would
1432 // cause us to transform this into an irreducible loop, don't do this.
1433 // See the comments above FindLoopHeaders for justifications and caveats.
1434 if (LoopHeaders.count(BB)) {
1435 DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
1436 << "' into predecessor block '" << PredBBs[0]->getName()
1437 << "' - it might create an irreducible loop!\n");
1441 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
1442 if (DuplicationCost > Threshold) {
1443 DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
1444 << "' - Cost is too high: " << DuplicationCost << "\n");
1448 // And finally, do it! Start by factoring the predecessors is needed.
1450 if (PredBBs.size() == 1)
1451 PredBB = PredBBs[0];
1453 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1454 << " common predecessors.\n");
1455 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1459 // Okay, we decided to do this! Clone all the instructions in BB onto the end
1461 DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '"
1462 << PredBB->getName() << "' to eliminate branch on phi. Cost: "
1463 << DuplicationCost << " block is:" << *BB << "\n");
1465 // Unless PredBB ends with an unconditional branch, split the edge so that we
1466 // can just clone the bits from BB into the end of the new PredBB.
1467 BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
1469 if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) {
1470 PredBB = SplitEdge(PredBB, BB, this);
1471 OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1474 // We are going to have to map operands from the original BB block into the
1475 // PredBB block. Evaluate PHI nodes in BB.
1476 DenseMap<Instruction*, Value*> ValueMapping;
1478 BasicBlock::iterator BI = BB->begin();
1479 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1480 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1482 // Clone the non-phi instructions of BB into PredBB, keeping track of the
1483 // mapping and using it to remap operands in the cloned instructions.
1484 for (; BI != BB->end(); ++BI) {
1485 Instruction *New = BI->clone();
1487 // Remap operands to patch up intra-block references.
1488 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1489 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1490 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1491 if (I != ValueMapping.end())
1492 New->setOperand(i, I->second);
1495 // If this instruction can be simplified after the operands are updated,
1496 // just use the simplified value instead. This frequently happens due to
1498 if (Value *IV = SimplifyInstruction(New, TD)) {
1500 ValueMapping[BI] = IV;
1502 // Otherwise, insert the new instruction into the block.
1503 New->setName(BI->getName());
1504 PredBB->getInstList().insert(OldPredBranch, New);
1505 ValueMapping[BI] = New;
1509 // Check to see if the targets of the branch had PHI nodes. If so, we need to
1510 // add entries to the PHI nodes for branch from PredBB now.
1511 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1512 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1514 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1517 // If there were values defined in BB that are used outside the block, then we
1518 // now have to update all uses of the value to use either the original value,
1519 // the cloned value, or some PHI derived value. This can require arbitrary
1520 // PHI insertion, of which we are prepared to do, clean these up now.
1521 SSAUpdater SSAUpdate;
1522 SmallVector<Use*, 16> UsesToRename;
1523 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1524 // Scan all uses of this instruction to see if it is used outside of its
1525 // block, and if so, record them in UsesToRename.
1526 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1528 Instruction *User = cast<Instruction>(*UI);
1529 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1530 if (UserPN->getIncomingBlock(UI) == BB)
1532 } else if (User->getParent() == BB)
1535 UsesToRename.push_back(&UI.getUse());
1538 // If there are no uses outside the block, we're done with this instruction.
1539 if (UsesToRename.empty())
1542 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1544 // We found a use of I outside of BB. Rename all uses of I that are outside
1545 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1546 // with the two values we know.
1547 SSAUpdate.Initialize(I);
1548 SSAUpdate.AddAvailableValue(BB, I);
1549 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1551 while (!UsesToRename.empty())
1552 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1553 DEBUG(dbgs() << "\n");
1556 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1558 RemovePredecessorAndSimplify(BB, PredBB, TD);
1560 // Remove the unconditional branch at the end of the PredBB block.
1561 OldPredBranch->eraseFromParent();