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", cl::ReallyHidden);
53 /// This pass performs 'jump threading', which looks at blocks that have
54 /// multiple predecessors and multiple successors. If one or more of the
55 /// predecessors of the block can be proven to always jump to one of the
56 /// successors, we forward the edge from the predecessor to the successor by
57 /// duplicating the contents of this block.
59 /// An example of when this can occur is code like this:
66 /// In this case, the unconditional branch at the end of the first if can be
67 /// revectored to the false side of the second if.
69 class JumpThreading : public FunctionPass {
73 SmallPtrSet<BasicBlock*, 16> LoopHeaders;
75 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
78 static char ID; // Pass identification
79 JumpThreading() : FunctionPass(&ID) {}
81 bool runOnFunction(Function &F);
83 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
85 AU.addRequired<LazyValueInfo>();
88 void FindLoopHeaders(Function &F);
89 bool ProcessBlock(BasicBlock *BB);
90 bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
92 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
93 const SmallVectorImpl<BasicBlock *> &PredBBs);
95 typedef SmallVectorImpl<std::pair<ConstantInt*,
96 BasicBlock*> > PredValueInfo;
98 bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
99 PredValueInfo &Result);
100 bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB);
103 bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
104 bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
106 bool ProcessBranchOnPHI(PHINode *PN);
107 bool ProcessBranchOnXOR(BinaryOperator *BO);
109 bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
113 char JumpThreading::ID = 0;
114 static RegisterPass<JumpThreading>
115 X("jump-threading", "Jump Threading");
117 // Public interface to the Jump Threading pass
118 FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
120 /// runOnFunction - Top level algorithm.
122 bool JumpThreading::runOnFunction(Function &F) {
123 DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
124 TD = getAnalysisIfAvailable<TargetData>();
125 LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0;
129 bool Changed, EverChanged = false;
132 for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
134 // Thread all of the branches we can over this block.
135 while (ProcessBlock(BB))
140 // If the block is trivially dead, zap it. This eliminates the successor
141 // edges which simplifies the CFG.
142 if (pred_begin(BB) == pred_end(BB) &&
143 BB != &BB->getParent()->getEntryBlock()) {
144 DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName()
145 << "' with terminator: " << *BB->getTerminator() << '\n');
146 LoopHeaders.erase(BB);
149 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
150 // Can't thread an unconditional jump, but if the block is "almost
151 // empty", we can replace uses of it with uses of the successor and make
153 if (BI->isUnconditional() &&
154 BB != &BB->getParent()->getEntryBlock()) {
155 BasicBlock::iterator BBI = BB->getFirstNonPHI();
156 // Ignore dbg intrinsics.
157 while (isa<DbgInfoIntrinsic>(BBI))
159 // If the terminator is the only non-phi instruction, try to nuke it.
160 if (BBI->isTerminator()) {
161 // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
162 // block, we have to make sure it isn't in the LoopHeaders set. We
163 // reinsert afterward if needed.
164 bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
165 BasicBlock *Succ = BI->getSuccessor(0);
167 if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
169 // If we deleted BB and BB was the header of a loop, then the
170 // successor is now the header of the loop.
174 if (ErasedFromLoopHeaders)
175 LoopHeaders.insert(BB);
180 EverChanged |= Changed;
187 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
188 /// thread across it.
189 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
190 /// Ignore PHI nodes, these will be flattened when duplication happens.
191 BasicBlock::const_iterator I = BB->getFirstNonPHI();
193 // FIXME: THREADING will delete values that are just used to compute the
194 // branch, so they shouldn't count against the duplication cost.
197 // Sum up the cost of each instruction until we get to the terminator. Don't
198 // include the terminator because the copy won't include it.
200 for (; !isa<TerminatorInst>(I); ++I) {
201 // Debugger intrinsics don't incur code size.
202 if (isa<DbgInfoIntrinsic>(I)) continue;
204 // If this is a pointer->pointer bitcast, it is free.
205 if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
208 // All other instructions count for at least one unit.
211 // Calls are more expensive. If they are non-intrinsic calls, we model them
212 // as having cost of 4. If they are a non-vector intrinsic, we model them
213 // as having cost of 2 total, and if they are a vector intrinsic, we model
214 // them as having cost 1.
215 if (const CallInst *CI = dyn_cast<CallInst>(I)) {
216 if (!isa<IntrinsicInst>(CI))
218 else if (!CI->getType()->isVectorTy())
223 // Threading through a switch statement is particularly profitable. If this
224 // block ends in a switch, decrease its cost to make it more likely to happen.
225 if (isa<SwitchInst>(I))
226 Size = Size > 6 ? Size-6 : 0;
231 /// FindLoopHeaders - We do not want jump threading to turn proper loop
232 /// structures into irreducible loops. Doing this breaks up the loop nesting
233 /// hierarchy and pessimizes later transformations. To prevent this from
234 /// happening, we first have to find the loop headers. Here we approximate this
235 /// by finding targets of backedges in the CFG.
237 /// Note that there definitely are cases when we want to allow threading of
238 /// edges across a loop header. For example, threading a jump from outside the
239 /// loop (the preheader) to an exit block of the loop is definitely profitable.
240 /// It is also almost always profitable to thread backedges from within the loop
241 /// to exit blocks, and is often profitable to thread backedges to other blocks
242 /// within the loop (forming a nested loop). This simple analysis is not rich
243 /// enough to track all of these properties and keep it up-to-date as the CFG
244 /// mutates, so we don't allow any of these transformations.
246 void JumpThreading::FindLoopHeaders(Function &F) {
247 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
248 FindFunctionBackedges(F, Edges);
250 for (unsigned i = 0, e = Edges.size(); i != e; ++i)
251 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
254 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
255 /// if we can infer that the value is a known ConstantInt in any of our
256 /// predecessors. If so, return the known list of value and pred BB in the
257 /// result vector. If a value is known to be undef, it is returned as null.
259 /// This returns true if there were any known values.
262 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
263 // If V is a constantint, then it is known in all predecessors.
264 if (isa<ConstantInt>(V) || isa<UndefValue>(V)) {
265 ConstantInt *CI = dyn_cast<ConstantInt>(V);
267 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
268 Result.push_back(std::make_pair(CI, *PI));
272 // If V is a non-instruction value, or an instruction in a different block,
273 // then it can't be derived from a PHI.
274 Instruction *I = dyn_cast<Instruction>(V);
275 if (I == 0 || I->getParent() != BB) {
277 // Okay, if this is a live-in value, see if it has a known value at the end
278 // of any of our predecessors.
280 // FIXME: This should be an edge property, not a block end property.
281 /// TODO: Per PR2563, we could infer value range information about a
282 /// predecessor based on its terminator.
285 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
286 // "I" is a non-local compare-with-a-constant instruction. This would be
287 // able to handle value inequalities better, for example if the compare is
288 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
289 // Perhaps getConstantOnEdge should be smart enough to do this?
291 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
293 // If the value is known by LazyValueInfo to be a constant in a
294 // predecessor, use that information to try to thread this block.
295 Constant *PredCst = LVI->getConstantOnEdge(V, P, BB);
297 (!isa<ConstantInt>(PredCst) && !isa<UndefValue>(PredCst)))
300 Result.push_back(std::make_pair(dyn_cast<ConstantInt>(PredCst), P));
303 return !Result.empty();
309 /// If I is a PHI node, then we know the incoming values for any constants.
310 if (PHINode *PN = dyn_cast<PHINode>(I)) {
311 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
312 Value *InVal = PN->getIncomingValue(i);
313 if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) {
314 ConstantInt *CI = dyn_cast<ConstantInt>(InVal);
315 Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i)));
318 return !Result.empty();
321 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals;
323 // Handle some boolean conditions.
324 if (I->getType()->getPrimitiveSizeInBits() == 1) {
326 // X & false -> false
327 if (I->getOpcode() == Instruction::Or ||
328 I->getOpcode() == Instruction::And) {
329 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals);
330 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals);
332 if (LHSVals.empty() && RHSVals.empty())
335 ConstantInt *InterestingVal;
336 if (I->getOpcode() == Instruction::Or)
337 InterestingVal = ConstantInt::getTrue(I->getContext());
339 InterestingVal = ConstantInt::getFalse(I->getContext());
341 // Scan for the sentinel. If we find an undef, force it to the
342 // interesting value: x|undef -> true and x&undef -> false.
343 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
344 if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0) {
345 Result.push_back(LHSVals[i]);
346 Result.back().first = InterestingVal;
348 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
349 if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0) {
350 // If we already inferred a value for this block on the LHS, don't
352 bool HasValue = false;
353 for (unsigned r = 0, e = Result.size(); r != e; ++r)
354 if (Result[r].second == RHSVals[i].second) {
360 Result.push_back(RHSVals[i]);
361 Result.back().first = InterestingVal;
364 return !Result.empty();
367 // Handle the NOT form of XOR.
368 if (I->getOpcode() == Instruction::Xor &&
369 isa<ConstantInt>(I->getOperand(1)) &&
370 cast<ConstantInt>(I->getOperand(1))->isOne()) {
371 ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result);
375 // Invert the known values.
376 for (unsigned i = 0, e = Result.size(); i != e; ++i)
379 cast<ConstantInt>(ConstantExpr::getNot(Result[i].first));
384 // Handle compare with phi operand, where the PHI is defined in this block.
385 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
386 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
387 if (PN && PN->getParent() == BB) {
388 // We can do this simplification if any comparisons fold to true or false.
390 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
391 BasicBlock *PredBB = PN->getIncomingBlock(i);
392 Value *LHS = PN->getIncomingValue(i);
393 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
395 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD);
397 if (!LVI || !isa<Constant>(RHS))
400 LazyValueInfo::Tristate
401 ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
402 cast<Constant>(RHS), PredBB, BB);
403 if (ResT == LazyValueInfo::Unknown)
405 Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
408 if (isa<UndefValue>(Res))
409 Result.push_back(std::make_pair((ConstantInt*)0, PredBB));
410 else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res))
411 Result.push_back(std::make_pair(CI, PredBB));
414 return !Result.empty();
418 // If comparing a live-in value against a constant, see if we know the
419 // live-in value on any predecessors.
420 if (LVI && isa<Constant>(Cmp->getOperand(1)) &&
421 Cmp->getType()->isIntegerTy() && // Not vector compare.
422 (!isa<Instruction>(Cmp->getOperand(0)) ||
423 cast<Instruction>(Cmp->getOperand(0))->getParent() != BB)) {
424 Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
426 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
428 // If the value is known by LazyValueInfo to be a constant in a
429 // predecessor, use that information to try to thread this block.
430 LazyValueInfo::Tristate
431 Res = LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
433 if (Res == LazyValueInfo::Unknown)
436 Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
437 Result.push_back(std::make_pair(cast<ConstantInt>(ResC), P));
440 return !Result.empty();
448 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
449 /// in an undefined jump, decide which block is best to revector to.
451 /// Since we can pick an arbitrary destination, we pick the successor with the
452 /// fewest predecessors. This should reduce the in-degree of the others.
454 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
455 TerminatorInst *BBTerm = BB->getTerminator();
456 unsigned MinSucc = 0;
457 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
458 // Compute the successor with the minimum number of predecessors.
459 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
460 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
461 TestBB = BBTerm->getSuccessor(i);
462 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
463 if (NumPreds < MinNumPreds)
470 /// ProcessBlock - If there are any predecessors whose control can be threaded
471 /// through to a successor, transform them now.
472 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
473 // If the block is trivially dead, just return and let the caller nuke it.
474 // This simplifies other transformations.
475 if (pred_begin(BB) == pred_end(BB) &&
476 BB != &BB->getParent()->getEntryBlock())
479 // If this block has a single predecessor, and if that pred has a single
480 // successor, merge the blocks. This encourages recursive jump threading
481 // because now the condition in this block can be threaded through
482 // predecessors of our predecessor block.
483 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
484 if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
486 // If SinglePred was a loop header, BB becomes one.
487 if (LoopHeaders.erase(SinglePred))
488 LoopHeaders.insert(BB);
490 // Remember if SinglePred was the entry block of the function. If so, we
491 // will need to move BB back to the entry position.
492 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
493 MergeBasicBlockIntoOnlyPred(BB);
495 if (isEntry && BB != &BB->getParent()->getEntryBlock())
496 BB->moveBefore(&BB->getParent()->getEntryBlock());
501 // Look to see if the terminator is a branch of switch, if not we can't thread
504 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
505 // Can't thread an unconditional jump.
506 if (BI->isUnconditional()) return false;
507 Condition = BI->getCondition();
508 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
509 Condition = SI->getCondition();
511 return false; // Must be an invoke.
513 // If the terminator of this block is branching on a constant, simplify the
514 // terminator to an unconditional branch. This can occur due to threading in
516 if (isa<ConstantInt>(Condition)) {
517 DEBUG(dbgs() << " In block '" << BB->getName()
518 << "' folding terminator: " << *BB->getTerminator() << '\n');
520 ConstantFoldTerminator(BB);
524 // If the terminator is branching on an undef, we can pick any of the
525 // successors to branch to. Let GetBestDestForJumpOnUndef decide.
526 if (isa<UndefValue>(Condition)) {
527 unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
529 // Fold the branch/switch.
530 TerminatorInst *BBTerm = BB->getTerminator();
531 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
532 if (i == BestSucc) continue;
533 RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD);
536 DEBUG(dbgs() << " In block '" << BB->getName()
537 << "' folding undef terminator: " << *BBTerm << '\n');
538 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
539 BBTerm->eraseFromParent();
543 Instruction *CondInst = dyn_cast<Instruction>(Condition);
545 // If the condition is an instruction defined in another block, see if a
546 // predecessor has the same condition:
551 !Condition->hasOneUse() && // Multiple uses.
552 (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
553 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
554 if (isa<BranchInst>(BB->getTerminator())) {
555 for (; PI != E; ++PI) {
557 if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator()))
558 if (PBI->isConditional() && PBI->getCondition() == Condition &&
559 ProcessBranchOnDuplicateCond(P, BB))
563 assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
564 for (; PI != E; ++PI) {
566 if (SwitchInst *PSI = dyn_cast<SwitchInst>(P->getTerminator()))
567 if (PSI->getCondition() == Condition &&
568 ProcessSwitchOnDuplicateCond(P, BB))
574 // All the rest of our checks depend on the condition being an instruction.
576 // FIXME: Unify this with code below.
577 if (LVI && ProcessThreadableEdges(Condition, BB))
583 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
585 (!isa<PHINode>(CondCmp->getOperand(0)) ||
586 cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB)) {
587 // If we have a comparison, loop over the predecessors to see if there is
588 // a condition with a lexically identical value.
589 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
590 for (; PI != E; ++PI) {
592 if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator()))
593 if (PBI->isConditional() && P != BB) {
594 if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) {
595 if (CI->getOperand(0) == CondCmp->getOperand(0) &&
596 CI->getOperand(1) == CondCmp->getOperand(1) &&
597 CI->getPredicate() == CondCmp->getPredicate()) {
598 // TODO: Could handle things like (x != 4) --> (x == 17)
599 if (ProcessBranchOnDuplicateCond(P, BB))
608 // Check for some cases that are worth simplifying. Right now we want to look
609 // for loads that are used by a switch or by the condition for the branch. If
610 // we see one, check to see if it's partially redundant. If so, insert a PHI
611 // which can then be used to thread the values.
613 Value *SimplifyValue = CondInst;
614 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
615 if (isa<Constant>(CondCmp->getOperand(1)))
616 SimplifyValue = CondCmp->getOperand(0);
618 // TODO: There are other places where load PRE would be profitable, such as
619 // more complex comparisons.
620 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
621 if (SimplifyPartiallyRedundantLoad(LI))
625 // Handle a variety of cases where we are branching on something derived from
626 // a PHI node in the current block. If we can prove that any predecessors
627 // compute a predictable value based on a PHI node, thread those predecessors.
629 if (ProcessThreadableEdges(CondInst, BB))
632 // If this is an otherwise-unfoldable branch on a phi node in the current
633 // block, see if we can simplify.
634 if (PHINode *PN = dyn_cast<PHINode>(CondInst))
635 if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
636 return ProcessBranchOnPHI(PN);
639 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
640 if (CondInst->getOpcode() == Instruction::Xor &&
641 CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
642 return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
645 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know
646 // "(X == 4)", thread through this block.
651 /// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
652 /// block that jump on exactly the same condition. This means that we almost
653 /// always know the direction of the edge in the DESTBB:
655 /// br COND, DESTBB, BBY
657 /// br COND, BBZ, BBW
659 /// If DESTBB has multiple predecessors, we can't just constant fold the branch
660 /// in DESTBB, we have to thread over it.
661 bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
663 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
665 // If both successors of PredBB go to DESTBB, we don't know anything. We can
666 // fold the branch to an unconditional one, which allows other recursive
669 if (PredBI->getSuccessor(1) != BB)
671 else if (PredBI->getSuccessor(0) != BB)
674 DEBUG(dbgs() << " In block '" << PredBB->getName()
675 << "' folding terminator: " << *PredBB->getTerminator() << '\n');
677 ConstantFoldTerminator(PredBB);
681 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
683 // If the dest block has one predecessor, just fix the branch condition to a
684 // constant and fold it.
685 if (BB->getSinglePredecessor()) {
686 DEBUG(dbgs() << " In block '" << BB->getName()
687 << "' folding condition to '" << BranchDir << "': "
688 << *BB->getTerminator() << '\n');
690 Value *OldCond = DestBI->getCondition();
691 DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
693 // Delete dead instructions before we fold the branch. Folding the branch
694 // can eliminate edges from the CFG which can end up deleting OldCond.
695 RecursivelyDeleteTriviallyDeadInstructions(OldCond);
696 ConstantFoldTerminator(BB);
701 // Next, figure out which successor we are threading to.
702 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
704 SmallVector<BasicBlock*, 2> Preds;
705 Preds.push_back(PredBB);
707 // Ok, try to thread it!
708 return ThreadEdge(BB, Preds, SuccBB);
711 /// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
712 /// block that switch on exactly the same condition. This means that we almost
713 /// always know the direction of the edge in the DESTBB:
715 /// switch COND [... DESTBB, BBY ... ]
717 /// switch COND [... BBZ, BBW ]
719 /// Optimizing switches like this is very important, because simplifycfg builds
720 /// switches out of repeated 'if' conditions.
721 bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
722 BasicBlock *DestBB) {
723 // Can't thread edge to self.
724 if (PredBB == DestBB)
727 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
728 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
730 // There are a variety of optimizations that we can potentially do on these
731 // blocks: we order them from most to least preferable.
733 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB
734 // directly to their destination. This does not introduce *any* code size
735 // growth. Skip debug info first.
736 BasicBlock::iterator BBI = DestBB->begin();
737 while (isa<DbgInfoIntrinsic>(BBI))
740 // FIXME: Thread if it just contains a PHI.
741 if (isa<SwitchInst>(BBI)) {
742 bool MadeChange = false;
743 // Ignore the default edge for now.
744 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
745 ConstantInt *DestVal = DestSI->getCaseValue(i);
746 BasicBlock *DestSucc = DestSI->getSuccessor(i);
748 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if
749 // PredSI has an explicit case for it. If so, forward. If it is covered
750 // by the default case, we can't update PredSI.
751 unsigned PredCase = PredSI->findCaseValue(DestVal);
752 if (PredCase == 0) continue;
754 // If PredSI doesn't go to DestBB on this value, then it won't reach the
755 // case on this condition.
756 if (PredSI->getSuccessor(PredCase) != DestBB &&
757 DestSI->getSuccessor(i) != DestBB)
760 // Do not forward this if it already goes to this destination, this would
761 // be an infinite loop.
762 if (PredSI->getSuccessor(PredCase) == DestSucc)
765 // Otherwise, we're safe to make the change. Make sure that the edge from
766 // DestSI to DestSucc is not critical and has no PHI nodes.
767 DEBUG(dbgs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI);
768 DEBUG(dbgs() << "THROUGH: " << *DestSI);
770 // If the destination has PHI nodes, just split the edge for updating
772 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
773 SplitCriticalEdge(DestSI, i, this);
774 DestSucc = DestSI->getSuccessor(i);
776 FoldSingleEntryPHINodes(DestSucc);
777 PredSI->setSuccessor(PredCase, DestSucc);
789 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
790 /// load instruction, eliminate it by replacing it with a PHI node. This is an
791 /// important optimization that encourages jump threading, and needs to be run
792 /// interlaced with other jump threading tasks.
793 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
794 // Don't hack volatile loads.
795 if (LI->isVolatile()) return false;
797 // If the load is defined in a block with exactly one predecessor, it can't be
798 // partially redundant.
799 BasicBlock *LoadBB = LI->getParent();
800 if (LoadBB->getSinglePredecessor())
803 Value *LoadedPtr = LI->getOperand(0);
805 // If the loaded operand is defined in the LoadBB, it can't be available.
806 // TODO: Could do simple PHI translation, that would be fun :)
807 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
808 if (PtrOp->getParent() == LoadBB)
811 // Scan a few instructions up from the load, to see if it is obviously live at
812 // the entry to its block.
813 BasicBlock::iterator BBIt = LI;
815 if (Value *AvailableVal =
816 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
817 // If the value if the load is locally available within the block, just use
818 // it. This frequently occurs for reg2mem'd allocas.
819 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
821 // If the returned value is the load itself, replace with an undef. This can
822 // only happen in dead loops.
823 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
824 LI->replaceAllUsesWith(AvailableVal);
825 LI->eraseFromParent();
829 // Otherwise, if we scanned the whole block and got to the top of the block,
830 // we know the block is locally transparent to the load. If not, something
831 // might clobber its value.
832 if (BBIt != LoadBB->begin())
836 SmallPtrSet<BasicBlock*, 8> PredsScanned;
837 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
838 AvailablePredsTy AvailablePreds;
839 BasicBlock *OneUnavailablePred = 0;
841 // If we got here, the loaded value is transparent through to the start of the
842 // block. Check to see if it is available in any of the predecessor blocks.
843 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
845 BasicBlock *PredBB = *PI;
847 // If we already scanned this predecessor, skip it.
848 if (!PredsScanned.insert(PredBB))
851 // Scan the predecessor to see if the value is available in the pred.
852 BBIt = PredBB->end();
853 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6);
854 if (!PredAvailable) {
855 OneUnavailablePred = PredBB;
859 // If so, this load is partially redundant. Remember this info so that we
860 // can create a PHI node.
861 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
864 // If the loaded value isn't available in any predecessor, it isn't partially
866 if (AvailablePreds.empty()) return false;
868 // Okay, the loaded value is available in at least one (and maybe all!)
869 // predecessors. If the value is unavailable in more than one unique
870 // predecessor, we want to insert a merge block for those common predecessors.
871 // This ensures that we only have to insert one reload, thus not increasing
873 BasicBlock *UnavailablePred = 0;
875 // If there is exactly one predecessor where the value is unavailable, the
876 // already computed 'OneUnavailablePred' block is it. If it ends in an
877 // unconditional branch, we know that it isn't a critical edge.
878 if (PredsScanned.size() == AvailablePreds.size()+1 &&
879 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
880 UnavailablePred = OneUnavailablePred;
881 } else if (PredsScanned.size() != AvailablePreds.size()) {
882 // Otherwise, we had multiple unavailable predecessors or we had a critical
883 // edge from the one.
884 SmallVector<BasicBlock*, 8> PredsToSplit;
885 SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
887 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
888 AvailablePredSet.insert(AvailablePreds[i].first);
890 // Add all the unavailable predecessors to the PredsToSplit list.
891 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
894 // If the predecessor is an indirect goto, we can't split the edge.
895 if (isa<IndirectBrInst>(P->getTerminator()))
898 if (!AvailablePredSet.count(P))
899 PredsToSplit.push_back(P);
902 // Split them out to their own block.
904 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
905 "thread-pre-split", this);
908 // If the value isn't available in all predecessors, then there will be
909 // exactly one where it isn't available. Insert a load on that edge and add
910 // it to the AvailablePreds list.
911 if (UnavailablePred) {
912 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
913 "Can't handle critical edge here!");
914 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
916 UnavailablePred->getTerminator());
917 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
920 // Now we know that each predecessor of this block has a value in
921 // AvailablePreds, sort them for efficient access as we're walking the preds.
922 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
924 // Create a PHI node at the start of the block for the PRE'd load value.
925 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
928 // Insert new entries into the PHI for each predecessor. A single block may
929 // have multiple entries here.
930 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
933 AvailablePredsTy::iterator I =
934 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
935 std::make_pair(P, (Value*)0));
937 assert(I != AvailablePreds.end() && I->first == P &&
938 "Didn't find entry for predecessor!");
940 PN->addIncoming(I->second, I->first);
943 //cerr << "PRE: " << *LI << *PN << "\n";
945 LI->replaceAllUsesWith(PN);
946 LI->eraseFromParent();
951 /// FindMostPopularDest - The specified list contains multiple possible
952 /// threadable destinations. Pick the one that occurs the most frequently in
955 FindMostPopularDest(BasicBlock *BB,
956 const SmallVectorImpl<std::pair<BasicBlock*,
957 BasicBlock*> > &PredToDestList) {
958 assert(!PredToDestList.empty());
960 // Determine popularity. If there are multiple possible destinations, we
961 // explicitly choose to ignore 'undef' destinations. We prefer to thread
962 // blocks with known and real destinations to threading undef. We'll handle
963 // them later if interesting.
964 DenseMap<BasicBlock*, unsigned> DestPopularity;
965 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
966 if (PredToDestList[i].second)
967 DestPopularity[PredToDestList[i].second]++;
969 // Find the most popular dest.
970 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
971 BasicBlock *MostPopularDest = DPI->first;
972 unsigned Popularity = DPI->second;
973 SmallVector<BasicBlock*, 4> SamePopularity;
975 for (++DPI; DPI != DestPopularity.end(); ++DPI) {
976 // If the popularity of this entry isn't higher than the popularity we've
977 // seen so far, ignore it.
978 if (DPI->second < Popularity)
980 else if (DPI->second == Popularity) {
981 // If it is the same as what we've seen so far, keep track of it.
982 SamePopularity.push_back(DPI->first);
984 // If it is more popular, remember it.
985 SamePopularity.clear();
986 MostPopularDest = DPI->first;
987 Popularity = DPI->second;
991 // Okay, now we know the most popular destination. If there is more than
992 // destination, we need to determine one. This is arbitrary, but we need
993 // to make a deterministic decision. Pick the first one that appears in the
995 if (!SamePopularity.empty()) {
996 SamePopularity.push_back(MostPopularDest);
997 TerminatorInst *TI = BB->getTerminator();
998 for (unsigned i = 0; ; ++i) {
999 assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
1001 if (std::find(SamePopularity.begin(), SamePopularity.end(),
1002 TI->getSuccessor(i)) == SamePopularity.end())
1005 MostPopularDest = TI->getSuccessor(i);
1010 // Okay, we have finally picked the most popular destination.
1011 return MostPopularDest;
1014 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) {
1015 // If threading this would thread across a loop header, don't even try to
1017 if (LoopHeaders.count(BB))
1020 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues;
1021 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues))
1023 assert(!PredValues.empty() &&
1024 "ComputeValueKnownInPredecessors returned true with no values");
1026 DEBUG(dbgs() << "IN BB: " << *BB;
1027 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1028 dbgs() << " BB '" << BB->getName() << "': FOUND condition = ";
1029 if (PredValues[i].first)
1030 dbgs() << *PredValues[i].first;
1033 dbgs() << " for pred '" << PredValues[i].second->getName()
1037 // Decide what we want to thread through. Convert our list of known values to
1038 // a list of known destinations for each pred. This also discards duplicate
1039 // predecessors and keeps track of the undefined inputs (which are represented
1040 // as a null dest in the PredToDestList).
1041 SmallPtrSet<BasicBlock*, 16> SeenPreds;
1042 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
1044 BasicBlock *OnlyDest = 0;
1045 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1047 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1048 BasicBlock *Pred = PredValues[i].second;
1049 if (!SeenPreds.insert(Pred))
1050 continue; // Duplicate predecessor entry.
1052 // If the predecessor ends with an indirect goto, we can't change its
1054 if (isa<IndirectBrInst>(Pred->getTerminator()))
1057 ConstantInt *Val = PredValues[i].first;
1060 if (Val == 0) // Undef.
1062 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1063 DestBB = BI->getSuccessor(Val->isZero());
1065 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
1066 DestBB = SI->getSuccessor(SI->findCaseValue(Val));
1069 // If we have exactly one destination, remember it for efficiency below.
1072 else if (OnlyDest != DestBB)
1073 OnlyDest = MultipleDestSentinel;
1075 PredToDestList.push_back(std::make_pair(Pred, DestBB));
1078 // If all edges were unthreadable, we fail.
1079 if (PredToDestList.empty())
1082 // Determine which is the most common successor. If we have many inputs and
1083 // this block is a switch, we want to start by threading the batch that goes
1084 // to the most popular destination first. If we only know about one
1085 // threadable destination (the common case) we can avoid this.
1086 BasicBlock *MostPopularDest = OnlyDest;
1088 if (MostPopularDest == MultipleDestSentinel)
1089 MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1091 // Now that we know what the most popular destination is, factor all
1092 // predecessors that will jump to it into a single predecessor.
1093 SmallVector<BasicBlock*, 16> PredsToFactor;
1094 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1095 if (PredToDestList[i].second == MostPopularDest) {
1096 BasicBlock *Pred = PredToDestList[i].first;
1098 // This predecessor may be a switch or something else that has multiple
1099 // edges to the block. Factor each of these edges by listing them
1100 // according to # occurrences in PredsToFactor.
1101 TerminatorInst *PredTI = Pred->getTerminator();
1102 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
1103 if (PredTI->getSuccessor(i) == BB)
1104 PredsToFactor.push_back(Pred);
1107 // If the threadable edges are branching on an undefined value, we get to pick
1108 // the destination that these predecessors should get to.
1109 if (MostPopularDest == 0)
1110 MostPopularDest = BB->getTerminator()->
1111 getSuccessor(GetBestDestForJumpOnUndef(BB));
1113 // Ok, try to thread it!
1114 return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1117 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1118 /// a PHI node in the current block. See if there are any simplifications we
1119 /// can do based on inputs to the phi node.
1121 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
1122 BasicBlock *BB = PN->getParent();
1124 // TODO: We could make use of this to do it once for blocks with common PHI
1126 SmallVector<BasicBlock*, 1> PredBBs;
1129 // If any of the predecessor blocks end in an unconditional branch, we can
1130 // *duplicate* the conditional branch into that block in order to further
1131 // encourage jump threading and to eliminate cases where we have branch on a
1132 // phi of an icmp (branch on icmp is much better).
1133 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1134 BasicBlock *PredBB = PN->getIncomingBlock(i);
1135 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1136 if (PredBr->isUnconditional()) {
1137 PredBBs[0] = PredBB;
1138 // Try to duplicate BB into PredBB.
1139 if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
1147 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
1148 /// a xor instruction in the current block. See if there are any
1149 /// simplifications we can do based on inputs to the xor.
1151 bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
1152 BasicBlock *BB = BO->getParent();
1154 // If either the LHS or RHS of the xor is a constant, don't do this
1156 if (isa<ConstantInt>(BO->getOperand(0)) ||
1157 isa<ConstantInt>(BO->getOperand(1)))
1160 // If the first instruction in BB isn't a phi, we won't be able to infer
1161 // anything special about any particular predecessor.
1162 if (!isa<PHINode>(BB->front()))
1165 // If we have a xor as the branch input to this block, and we know that the
1166 // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1167 // the condition into the predecessor and fix that value to true, saving some
1168 // logical ops on that path and encouraging other paths to simplify.
1170 // This copies something like this:
1173 // %X = phi i1 [1], [%X']
1174 // %Y = icmp eq i32 %A, %B
1175 // %Z = xor i1 %X, %Y
1180 // %Y = icmp ne i32 %A, %B
1183 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> XorOpValues;
1185 if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) {
1186 assert(XorOpValues.empty());
1187 if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues))
1192 assert(!XorOpValues.empty() &&
1193 "ComputeValueKnownInPredecessors returned true with no values");
1195 // Scan the information to see which is most popular: true or false. The
1196 // predecessors can be of the set true, false, or undef.
1197 unsigned NumTrue = 0, NumFalse = 0;
1198 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1199 if (!XorOpValues[i].first) continue; // Ignore undefs for the count.
1200 if (XorOpValues[i].first->isZero())
1206 // Determine which value to split on, true, false, or undef if neither.
1207 ConstantInt *SplitVal = 0;
1208 if (NumTrue > NumFalse)
1209 SplitVal = ConstantInt::getTrue(BB->getContext());
1210 else if (NumTrue != 0 || NumFalse != 0)
1211 SplitVal = ConstantInt::getFalse(BB->getContext());
1213 // Collect all of the blocks that this can be folded into so that we can
1214 // factor this once and clone it once.
1215 SmallVector<BasicBlock*, 8> BlocksToFoldInto;
1216 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1217 if (XorOpValues[i].first != SplitVal && XorOpValues[i].first != 0) continue;
1219 BlocksToFoldInto.push_back(XorOpValues[i].second);
1222 // If we inferred a value for all of the predecessors, then duplication won't
1223 // help us. However, we can just replace the LHS or RHS with the constant.
1224 if (BlocksToFoldInto.size() ==
1225 cast<PHINode>(BB->front()).getNumIncomingValues()) {
1226 if (SplitVal == 0) {
1227 // If all preds provide undef, just nuke the xor, because it is undef too.
1228 BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
1229 BO->eraseFromParent();
1230 } else if (SplitVal->isZero()) {
1231 // If all preds provide 0, replace the xor with the other input.
1232 BO->replaceAllUsesWith(BO->getOperand(isLHS));
1233 BO->eraseFromParent();
1235 // If all preds provide 1, set the computed value to 1.
1236 BO->setOperand(!isLHS, SplitVal);
1242 // Try to duplicate BB into PredBB.
1243 return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
1247 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1248 /// predecessor to the PHIBB block. If it has PHI nodes, add entries for
1249 /// NewPred using the entries from OldPred (suitably mapped).
1250 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1251 BasicBlock *OldPred,
1252 BasicBlock *NewPred,
1253 DenseMap<Instruction*, Value*> &ValueMap) {
1254 for (BasicBlock::iterator PNI = PHIBB->begin();
1255 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1256 // Ok, we have a PHI node. Figure out what the incoming value was for the
1258 Value *IV = PN->getIncomingValueForBlock(OldPred);
1260 // Remap the value if necessary.
1261 if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1262 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1263 if (I != ValueMap.end())
1267 PN->addIncoming(IV, NewPred);
1271 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1272 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1273 /// across BB. Transform the IR to reflect this change.
1274 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1275 const SmallVectorImpl<BasicBlock*> &PredBBs,
1276 BasicBlock *SuccBB) {
1277 // If threading to the same block as we come from, we would infinite loop.
1279 DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
1280 << "' - would thread to self!\n");
1284 // If threading this would thread across a loop header, don't thread the edge.
1285 // See the comments above FindLoopHeaders for justifications and caveats.
1286 if (LoopHeaders.count(BB)) {
1287 DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName()
1288 << "' to dest BB '" << SuccBB->getName()
1289 << "' - it might create an irreducible loop!\n");
1293 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
1294 if (JumpThreadCost > Threshold) {
1295 DEBUG(dbgs() << " Not threading BB '" << BB->getName()
1296 << "' - Cost is too high: " << JumpThreadCost << "\n");
1300 // And finally, do it! Start by factoring the predecessors is needed.
1302 if (PredBBs.size() == 1)
1303 PredBB = PredBBs[0];
1305 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1306 << " common predecessors.\n");
1307 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1311 // And finally, do it!
1312 DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '"
1313 << SuccBB->getName() << "' with cost: " << JumpThreadCost
1314 << ", across block:\n "
1317 // We are going to have to map operands from the original BB block to the new
1318 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
1319 // account for entry from PredBB.
1320 DenseMap<Instruction*, Value*> ValueMapping;
1322 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1323 BB->getName()+".thread",
1324 BB->getParent(), BB);
1325 NewBB->moveAfter(PredBB);
1327 BasicBlock::iterator BI = BB->begin();
1328 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1329 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1331 // Clone the non-phi instructions of BB into NewBB, keeping track of the
1332 // mapping and using it to remap operands in the cloned instructions.
1333 for (; !isa<TerminatorInst>(BI); ++BI) {
1334 Instruction *New = BI->clone();
1335 New->setName(BI->getName());
1336 NewBB->getInstList().push_back(New);
1337 ValueMapping[BI] = New;
1339 // Remap operands to patch up intra-block references.
1340 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1341 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1342 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1343 if (I != ValueMapping.end())
1344 New->setOperand(i, I->second);
1348 // We didn't copy the terminator from BB over to NewBB, because there is now
1349 // an unconditional jump to SuccBB. Insert the unconditional jump.
1350 BranchInst::Create(SuccBB, NewBB);
1352 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1353 // PHI nodes for NewBB now.
1354 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1356 // If there were values defined in BB that are used outside the block, then we
1357 // now have to update all uses of the value to use either the original value,
1358 // the cloned value, or some PHI derived value. This can require arbitrary
1359 // PHI insertion, of which we are prepared to do, clean these up now.
1360 SSAUpdater SSAUpdate;
1361 SmallVector<Use*, 16> UsesToRename;
1362 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1363 // Scan all uses of this instruction to see if it is used outside of its
1364 // block, and if so, record them in UsesToRename.
1365 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1367 Instruction *User = cast<Instruction>(*UI);
1368 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1369 if (UserPN->getIncomingBlock(UI) == BB)
1371 } else if (User->getParent() == BB)
1374 UsesToRename.push_back(&UI.getUse());
1377 // If there are no uses outside the block, we're done with this instruction.
1378 if (UsesToRename.empty())
1381 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1383 // We found a use of I outside of BB. Rename all uses of I that are outside
1384 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1385 // with the two values we know.
1386 SSAUpdate.Initialize(I);
1387 SSAUpdate.AddAvailableValue(BB, I);
1388 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1390 while (!UsesToRename.empty())
1391 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1392 DEBUG(dbgs() << "\n");
1396 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to
1397 // NewBB instead of BB. This eliminates predecessors from BB, which requires
1398 // us to simplify any PHI nodes in BB.
1399 TerminatorInst *PredTerm = PredBB->getTerminator();
1400 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1401 if (PredTerm->getSuccessor(i) == BB) {
1402 RemovePredecessorAndSimplify(BB, PredBB, TD);
1403 PredTerm->setSuccessor(i, NewBB);
1406 // At this point, the IR is fully up to date and consistent. Do a quick scan
1407 // over the new instructions and zap any that are constants or dead. This
1408 // frequently happens because of phi translation.
1409 SimplifyInstructionsInBlock(NewBB, TD);
1411 // Threaded an edge!
1416 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1417 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1418 /// If we can duplicate the contents of BB up into PredBB do so now, this
1419 /// improves the odds that the branch will be on an analyzable instruction like
1421 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1422 const SmallVectorImpl<BasicBlock *> &PredBBs) {
1423 assert(!PredBBs.empty() && "Can't handle an empty set");
1425 // If BB is a loop header, then duplicating this block outside the loop would
1426 // cause us to transform this into an irreducible loop, don't do this.
1427 // See the comments above FindLoopHeaders for justifications and caveats.
1428 if (LoopHeaders.count(BB)) {
1429 DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
1430 << "' into predecessor block '" << PredBBs[0]->getName()
1431 << "' - it might create an irreducible loop!\n");
1435 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
1436 if (DuplicationCost > Threshold) {
1437 DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
1438 << "' - Cost is too high: " << DuplicationCost << "\n");
1442 // And finally, do it! Start by factoring the predecessors is needed.
1444 if (PredBBs.size() == 1)
1445 PredBB = PredBBs[0];
1447 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1448 << " common predecessors.\n");
1449 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1453 // Okay, we decided to do this! Clone all the instructions in BB onto the end
1455 DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '"
1456 << PredBB->getName() << "' to eliminate branch on phi. Cost: "
1457 << DuplicationCost << " block is:" << *BB << "\n");
1459 // Unless PredBB ends with an unconditional branch, split the edge so that we
1460 // can just clone the bits from BB into the end of the new PredBB.
1461 BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
1463 if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) {
1464 PredBB = SplitEdge(PredBB, BB, this);
1465 OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1468 // We are going to have to map operands from the original BB block into the
1469 // PredBB block. Evaluate PHI nodes in BB.
1470 DenseMap<Instruction*, Value*> ValueMapping;
1472 BasicBlock::iterator BI = BB->begin();
1473 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1474 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1476 // Clone the non-phi instructions of BB into PredBB, keeping track of the
1477 // mapping and using it to remap operands in the cloned instructions.
1478 for (; BI != BB->end(); ++BI) {
1479 Instruction *New = BI->clone();
1481 // Remap operands to patch up intra-block references.
1482 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1483 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1484 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1485 if (I != ValueMapping.end())
1486 New->setOperand(i, I->second);
1489 // If this instruction can be simplified after the operands are updated,
1490 // just use the simplified value instead. This frequently happens due to
1492 if (Value *IV = SimplifyInstruction(New, TD)) {
1494 ValueMapping[BI] = IV;
1496 // Otherwise, insert the new instruction into the block.
1497 New->setName(BI->getName());
1498 PredBB->getInstList().insert(OldPredBranch, New);
1499 ValueMapping[BI] = New;
1503 // Check to see if the targets of the branch had PHI nodes. If so, we need to
1504 // add entries to the PHI nodes for branch from PredBB now.
1505 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1506 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1508 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1511 // If there were values defined in BB that are used outside the block, then we
1512 // now have to update all uses of the value to use either the original value,
1513 // the cloned value, or some PHI derived value. This can require arbitrary
1514 // PHI insertion, of which we are prepared to do, clean these up now.
1515 SSAUpdater SSAUpdate;
1516 SmallVector<Use*, 16> UsesToRename;
1517 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1518 // Scan all uses of this instruction to see if it is used outside of its
1519 // block, and if so, record them in UsesToRename.
1520 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1522 Instruction *User = cast<Instruction>(*UI);
1523 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1524 if (UserPN->getIncomingBlock(UI) == BB)
1526 } else if (User->getParent() == BB)
1529 UsesToRename.push_back(&UI.getUse());
1532 // If there are no uses outside the block, we're done with this instruction.
1533 if (UsesToRename.empty())
1536 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1538 // We found a use of I outside of BB. Rename all uses of I that are outside
1539 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1540 // with the two values we know.
1541 SSAUpdate.Initialize(I);
1542 SSAUpdate.AddAvailableValue(BB, I);
1543 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1545 while (!UsesToRename.empty())
1546 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1547 DEBUG(dbgs() << "\n");
1550 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1552 RemovePredecessorAndSimplify(BB, PredBB, TD);
1554 // Remove the unconditional branch at the end of the PredBB block.
1555 OldPredBranch->eraseFromParent();