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/ConstantFolding.h"
20 #include "llvm/Analysis/InstructionSimplify.h"
21 #include "llvm/Analysis/LazyValueInfo.h"
22 #include "llvm/Analysis/Loads.h"
23 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
24 #include "llvm/Transforms/Utils/Local.h"
25 #include "llvm/Transforms/Utils/SSAUpdater.h"
26 #include "llvm/Target/TargetData.h"
27 #include "llvm/Target/TargetLibraryInfo.h"
28 #include "llvm/ADT/DenseMap.h"
29 #include "llvm/ADT/DenseSet.h"
30 #include "llvm/ADT/Statistic.h"
31 #include "llvm/ADT/STLExtras.h"
32 #include "llvm/ADT/SmallPtrSet.h"
33 #include "llvm/ADT/SmallSet.h"
34 #include "llvm/Support/CommandLine.h"
35 #include "llvm/Support/Debug.h"
36 #include "llvm/Support/ValueHandle.h"
37 #include "llvm/Support/raw_ostream.h"
40 STATISTIC(NumThreads, "Number of jumps threaded");
41 STATISTIC(NumFolds, "Number of terminators folded");
42 STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
44 static cl::opt<unsigned>
45 Threshold("jump-threading-threshold",
46 cl::desc("Max block size to duplicate for jump threading"),
47 cl::init(6), cl::Hidden);
50 // These are at global scope so static functions can use them too.
51 typedef SmallVectorImpl<std::pair<Constant*, BasicBlock*> > PredValueInfo;
52 typedef SmallVector<std::pair<Constant*, BasicBlock*>, 8> PredValueInfoTy;
54 // This is used to keep track of what kind of constant we're currently hoping
56 enum ConstantPreference {
61 /// This pass performs 'jump threading', which looks at blocks that have
62 /// multiple predecessors and multiple successors. If one or more of the
63 /// predecessors of the block can be proven to always jump to one of the
64 /// successors, we forward the edge from the predecessor to the successor by
65 /// duplicating the contents of this block.
67 /// An example of when this can occur is code like this:
74 /// In this case, the unconditional branch at the end of the first if can be
75 /// revectored to the false side of the second if.
77 class JumpThreading : public FunctionPass {
79 TargetLibraryInfo *TLI;
82 SmallPtrSet<BasicBlock*, 16> LoopHeaders;
84 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
86 DenseSet<std::pair<Value*, BasicBlock*> > RecursionSet;
88 // RAII helper for updating the recursion stack.
89 struct RecursionSetRemover {
90 DenseSet<std::pair<Value*, BasicBlock*> > &TheSet;
91 std::pair<Value*, BasicBlock*> ThePair;
93 RecursionSetRemover(DenseSet<std::pair<Value*, BasicBlock*> > &S,
94 std::pair<Value*, BasicBlock*> P)
95 : TheSet(S), ThePair(P) { }
97 ~RecursionSetRemover() {
98 TheSet.erase(ThePair);
102 static char ID; // Pass identification
103 JumpThreading() : FunctionPass(ID) {
104 initializeJumpThreadingPass(*PassRegistry::getPassRegistry());
107 bool runOnFunction(Function &F);
109 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
110 AU.addRequired<LazyValueInfo>();
111 AU.addPreserved<LazyValueInfo>();
112 AU.addRequired<TargetLibraryInfo>();
115 void FindLoopHeaders(Function &F);
116 bool ProcessBlock(BasicBlock *BB);
117 bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
119 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
120 const SmallVectorImpl<BasicBlock *> &PredBBs);
122 bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
123 PredValueInfo &Result,
124 ConstantPreference Preference);
125 bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
126 ConstantPreference Preference);
128 bool ProcessBranchOnPHI(PHINode *PN);
129 bool ProcessBranchOnXOR(BinaryOperator *BO);
131 bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
135 char JumpThreading::ID = 0;
136 INITIALIZE_PASS_BEGIN(JumpThreading, "jump-threading",
137 "Jump Threading", false, false)
138 INITIALIZE_PASS_DEPENDENCY(LazyValueInfo)
139 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
140 INITIALIZE_PASS_END(JumpThreading, "jump-threading",
141 "Jump Threading", false, false)
143 // Public interface to the Jump Threading pass
144 FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
146 /// runOnFunction - Top level algorithm.
148 bool JumpThreading::runOnFunction(Function &F) {
149 DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
150 TD = getAnalysisIfAvailable<TargetData>();
151 TLI = &getAnalysis<TargetLibraryInfo>();
152 LVI = &getAnalysis<LazyValueInfo>();
156 bool Changed, EverChanged = false;
159 for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
161 // Thread all of the branches we can over this block.
162 while (ProcessBlock(BB))
167 // If the block is trivially dead, zap it. This eliminates the successor
168 // edges which simplifies the CFG.
169 if (pred_begin(BB) == pred_end(BB) &&
170 BB != &BB->getParent()->getEntryBlock()) {
171 DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName()
172 << "' with terminator: " << *BB->getTerminator() << '\n');
173 LoopHeaders.erase(BB);
180 BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator());
182 // Can't thread an unconditional jump, but if the block is "almost
183 // empty", we can replace uses of it with uses of the successor and make
185 if (BI && BI->isUnconditional() &&
186 BB != &BB->getParent()->getEntryBlock() &&
187 // If the terminator is the only non-phi instruction, try to nuke it.
188 BB->getFirstNonPHIOrDbg()->isTerminator()) {
189 // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
190 // block, we have to make sure it isn't in the LoopHeaders set. We
191 // reinsert afterward if needed.
192 bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
193 BasicBlock *Succ = BI->getSuccessor(0);
195 // FIXME: It is always conservatively correct to drop the info
196 // for a block even if it doesn't get erased. This isn't totally
197 // awesome, but it allows us to use AssertingVH to prevent nasty
198 // dangling pointer issues within LazyValueInfo.
200 if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
202 // If we deleted BB and BB was the header of a loop, then the
203 // successor is now the header of the loop.
207 if (ErasedFromLoopHeaders)
208 LoopHeaders.insert(BB);
211 EverChanged |= Changed;
218 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
219 /// thread across it.
220 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
221 /// Ignore PHI nodes, these will be flattened when duplication happens.
222 BasicBlock::const_iterator I = BB->getFirstNonPHI();
224 // FIXME: THREADING will delete values that are just used to compute the
225 // branch, so they shouldn't count against the duplication cost.
228 // Sum up the cost of each instruction until we get to the terminator. Don't
229 // include the terminator because the copy won't include it.
231 for (; !isa<TerminatorInst>(I); ++I) {
232 // Debugger intrinsics don't incur code size.
233 if (isa<DbgInfoIntrinsic>(I)) continue;
235 // If this is a pointer->pointer bitcast, it is free.
236 if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
239 // All other instructions count for at least one unit.
242 // Calls are more expensive. If they are non-intrinsic calls, we model them
243 // as having cost of 4. If they are a non-vector intrinsic, we model them
244 // as having cost of 2 total, and if they are a vector intrinsic, we model
245 // them as having cost 1.
246 if (const CallInst *CI = dyn_cast<CallInst>(I)) {
247 if (!isa<IntrinsicInst>(CI))
249 else if (!CI->getType()->isVectorTy())
254 // Threading through a switch statement is particularly profitable. If this
255 // block ends in a switch, decrease its cost to make it more likely to happen.
256 if (isa<SwitchInst>(I))
257 Size = Size > 6 ? Size-6 : 0;
259 // The same holds for indirect branches, but slightly more so.
260 if (isa<IndirectBrInst>(I))
261 Size = Size > 8 ? Size-8 : 0;
266 /// FindLoopHeaders - We do not want jump threading to turn proper loop
267 /// structures into irreducible loops. Doing this breaks up the loop nesting
268 /// hierarchy and pessimizes later transformations. To prevent this from
269 /// happening, we first have to find the loop headers. Here we approximate this
270 /// by finding targets of backedges in the CFG.
272 /// Note that there definitely are cases when we want to allow threading of
273 /// edges across a loop header. For example, threading a jump from outside the
274 /// loop (the preheader) to an exit block of the loop is definitely profitable.
275 /// It is also almost always profitable to thread backedges from within the loop
276 /// to exit blocks, and is often profitable to thread backedges to other blocks
277 /// within the loop (forming a nested loop). This simple analysis is not rich
278 /// enough to track all of these properties and keep it up-to-date as the CFG
279 /// mutates, so we don't allow any of these transformations.
281 void JumpThreading::FindLoopHeaders(Function &F) {
282 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
283 FindFunctionBackedges(F, Edges);
285 for (unsigned i = 0, e = Edges.size(); i != e; ++i)
286 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
289 /// getKnownConstant - Helper method to determine if we can thread over a
290 /// terminator with the given value as its condition, and if so what value to
291 /// use for that. What kind of value this is depends on whether we want an
292 /// integer or a block address, but an undef is always accepted.
293 /// Returns null if Val is null or not an appropriate constant.
294 static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
298 // Undef is "known" enough.
299 if (UndefValue *U = dyn_cast<UndefValue>(Val))
302 if (Preference == WantBlockAddress)
303 return dyn_cast<BlockAddress>(Val->stripPointerCasts());
305 return dyn_cast<ConstantInt>(Val);
308 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
309 /// if we can infer that the value is a known ConstantInt/BlockAddress or undef
310 /// in any of our predecessors. If so, return the known list of value and pred
311 /// BB in the result vector.
313 /// This returns true if there were any known values.
316 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
317 ConstantPreference Preference) {
318 // This method walks up use-def chains recursively. Because of this, we could
319 // get into an infinite loop going around loops in the use-def chain. To
320 // prevent this, keep track of what (value, block) pairs we've already visited
321 // and terminate the search if we loop back to them
322 if (!RecursionSet.insert(std::make_pair(V, BB)).second)
325 // An RAII help to remove this pair from the recursion set once the recursion
326 // stack pops back out again.
327 RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB));
329 // If V is a constant, then it is known in all predecessors.
330 if (Constant *KC = getKnownConstant(V, Preference)) {
331 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
332 Result.push_back(std::make_pair(KC, *PI));
337 // If V is a non-instruction value, or an instruction in a different block,
338 // then it can't be derived from a PHI.
339 Instruction *I = dyn_cast<Instruction>(V);
340 if (I == 0 || I->getParent() != BB) {
342 // Okay, if this is a live-in value, see if it has a known value at the end
343 // of any of our predecessors.
345 // FIXME: This should be an edge property, not a block end property.
346 /// TODO: Per PR2563, we could infer value range information about a
347 /// predecessor based on its terminator.
349 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
350 // "I" is a non-local compare-with-a-constant instruction. This would be
351 // able to handle value inequalities better, for example if the compare is
352 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
353 // Perhaps getConstantOnEdge should be smart enough to do this?
355 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
357 // If the value is known by LazyValueInfo to be a constant in a
358 // predecessor, use that information to try to thread this block.
359 Constant *PredCst = LVI->getConstantOnEdge(V, P, BB);
360 if (Constant *KC = getKnownConstant(PredCst, Preference))
361 Result.push_back(std::make_pair(KC, P));
364 return !Result.empty();
367 /// If I is a PHI node, then we know the incoming values for any constants.
368 if (PHINode *PN = dyn_cast<PHINode>(I)) {
369 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
370 Value *InVal = PN->getIncomingValue(i);
371 if (Constant *KC = getKnownConstant(InVal, Preference)) {
372 Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
374 Constant *CI = LVI->getConstantOnEdge(InVal,
375 PN->getIncomingBlock(i), BB);
376 if (Constant *KC = getKnownConstant(CI, Preference))
377 Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
381 return !Result.empty();
384 PredValueInfoTy LHSVals, RHSVals;
386 // Handle some boolean conditions.
387 if (I->getType()->getPrimitiveSizeInBits() == 1) {
388 assert(Preference == WantInteger && "One-bit non-integer type?");
390 // X & false -> false
391 if (I->getOpcode() == Instruction::Or ||
392 I->getOpcode() == Instruction::And) {
393 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
395 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals,
398 if (LHSVals.empty() && RHSVals.empty())
401 ConstantInt *InterestingVal;
402 if (I->getOpcode() == Instruction::Or)
403 InterestingVal = ConstantInt::getTrue(I->getContext());
405 InterestingVal = ConstantInt::getFalse(I->getContext());
407 SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
409 // Scan for the sentinel. If we find an undef, force it to the
410 // interesting value: x|undef -> true and x&undef -> false.
411 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
412 if (LHSVals[i].first == InterestingVal ||
413 isa<UndefValue>(LHSVals[i].first)) {
414 Result.push_back(LHSVals[i]);
415 Result.back().first = InterestingVal;
416 LHSKnownBBs.insert(LHSVals[i].second);
418 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
419 if (RHSVals[i].first == InterestingVal ||
420 isa<UndefValue>(RHSVals[i].first)) {
421 // If we already inferred a value for this block on the LHS, don't
423 if (!LHSKnownBBs.count(RHSVals[i].second)) {
424 Result.push_back(RHSVals[i]);
425 Result.back().first = InterestingVal;
429 return !Result.empty();
432 // Handle the NOT form of XOR.
433 if (I->getOpcode() == Instruction::Xor &&
434 isa<ConstantInt>(I->getOperand(1)) &&
435 cast<ConstantInt>(I->getOperand(1))->isOne()) {
436 ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result,
441 // Invert the known values.
442 for (unsigned i = 0, e = Result.size(); i != e; ++i)
443 Result[i].first = ConstantExpr::getNot(Result[i].first);
448 // Try to simplify some other binary operator values.
449 } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
450 assert(Preference != WantBlockAddress
451 && "A binary operator creating a block address?");
452 if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
453 PredValueInfoTy LHSVals;
454 ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals,
457 // Try to use constant folding to simplify the binary operator.
458 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
459 Constant *V = LHSVals[i].first;
460 Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI);
462 if (Constant *KC = getKnownConstant(Folded, WantInteger))
463 Result.push_back(std::make_pair(KC, LHSVals[i].second));
467 return !Result.empty();
470 // Handle compare with phi operand, where the PHI is defined in this block.
471 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
472 assert(Preference == WantInteger && "Compares only produce integers");
473 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
474 if (PN && PN->getParent() == BB) {
475 // We can do this simplification if any comparisons fold to true or false.
477 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
478 BasicBlock *PredBB = PN->getIncomingBlock(i);
479 Value *LHS = PN->getIncomingValue(i);
480 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
482 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD);
484 if (!isa<Constant>(RHS))
487 LazyValueInfo::Tristate
488 ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
489 cast<Constant>(RHS), PredBB, BB);
490 if (ResT == LazyValueInfo::Unknown)
492 Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
495 if (Constant *KC = getKnownConstant(Res, WantInteger))
496 Result.push_back(std::make_pair(KC, PredBB));
499 return !Result.empty();
503 // If comparing a live-in value against a constant, see if we know the
504 // live-in value on any predecessors.
505 if (isa<Constant>(Cmp->getOperand(1)) && Cmp->getType()->isIntegerTy()) {
506 if (!isa<Instruction>(Cmp->getOperand(0)) ||
507 cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) {
508 Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
510 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB);PI != E; ++PI){
512 // If the value is known by LazyValueInfo to be a constant in a
513 // predecessor, use that information to try to thread this block.
514 LazyValueInfo::Tristate Res =
515 LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
517 if (Res == LazyValueInfo::Unknown)
520 Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
521 Result.push_back(std::make_pair(ResC, P));
524 return !Result.empty();
527 // Try to find a constant value for the LHS of a comparison,
528 // and evaluate it statically if we can.
529 if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) {
530 PredValueInfoTy LHSVals;
531 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
534 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
535 Constant *V = LHSVals[i].first;
536 Constant *Folded = ConstantExpr::getCompare(Cmp->getPredicate(),
538 if (Constant *KC = getKnownConstant(Folded, WantInteger))
539 Result.push_back(std::make_pair(KC, LHSVals[i].second));
542 return !Result.empty();
547 if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
548 // Handle select instructions where at least one operand is a known constant
549 // and we can figure out the condition value for any predecessor block.
550 Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
551 Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
552 PredValueInfoTy Conds;
553 if ((TrueVal || FalseVal) &&
554 ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
556 for (unsigned i = 0, e = Conds.size(); i != e; ++i) {
557 Constant *Cond = Conds[i].first;
559 // Figure out what value to use for the condition.
561 if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
563 KnownCond = CI->isOne();
565 assert(isa<UndefValue>(Cond) && "Unexpected condition value");
566 // Either operand will do, so be sure to pick the one that's a known
568 // FIXME: Do this more cleverly if both values are known constants?
569 KnownCond = (TrueVal != 0);
572 // See if the select has a known constant value for this predecessor.
573 if (Constant *Val = KnownCond ? TrueVal : FalseVal)
574 Result.push_back(std::make_pair(Val, Conds[i].second));
577 return !Result.empty();
581 // If all else fails, see if LVI can figure out a constant value for us.
582 Constant *CI = LVI->getConstant(V, BB);
583 if (Constant *KC = getKnownConstant(CI, Preference)) {
584 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
585 Result.push_back(std::make_pair(KC, *PI));
588 return !Result.empty();
593 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
594 /// in an undefined jump, decide which block is best to revector to.
596 /// Since we can pick an arbitrary destination, we pick the successor with the
597 /// fewest predecessors. This should reduce the in-degree of the others.
599 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
600 TerminatorInst *BBTerm = BB->getTerminator();
601 unsigned MinSucc = 0;
602 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
603 // Compute the successor with the minimum number of predecessors.
604 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
605 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
606 TestBB = BBTerm->getSuccessor(i);
607 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
608 if (NumPreds < MinNumPreds) {
610 MinNumPreds = NumPreds;
617 static bool hasAddressTakenAndUsed(BasicBlock *BB) {
618 if (!BB->hasAddressTaken()) return false;
620 // If the block has its address taken, it may be a tree of dead constants
621 // hanging off of it. These shouldn't keep the block alive.
622 BlockAddress *BA = BlockAddress::get(BB);
623 BA->removeDeadConstantUsers();
624 return !BA->use_empty();
627 /// ProcessBlock - If there are any predecessors whose control can be threaded
628 /// through to a successor, transform them now.
629 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
630 // If the block is trivially dead, just return and let the caller nuke it.
631 // This simplifies other transformations.
632 if (pred_begin(BB) == pred_end(BB) &&
633 BB != &BB->getParent()->getEntryBlock())
636 // If this block has a single predecessor, and if that pred has a single
637 // successor, merge the blocks. This encourages recursive jump threading
638 // because now the condition in this block can be threaded through
639 // predecessors of our predecessor block.
640 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
641 if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
642 SinglePred != BB && !hasAddressTakenAndUsed(BB)) {
643 // If SinglePred was a loop header, BB becomes one.
644 if (LoopHeaders.erase(SinglePred))
645 LoopHeaders.insert(BB);
647 // Remember if SinglePred was the entry block of the function. If so, we
648 // will need to move BB back to the entry position.
649 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
650 LVI->eraseBlock(SinglePred);
651 MergeBasicBlockIntoOnlyPred(BB);
653 if (isEntry && BB != &BB->getParent()->getEntryBlock())
654 BB->moveBefore(&BB->getParent()->getEntryBlock());
659 // What kind of constant we're looking for.
660 ConstantPreference Preference = WantInteger;
662 // Look to see if the terminator is a conditional branch, switch or indirect
663 // branch, if not we can't thread it.
665 Instruction *Terminator = BB->getTerminator();
666 if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) {
667 // Can't thread an unconditional jump.
668 if (BI->isUnconditional()) return false;
669 Condition = BI->getCondition();
670 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) {
671 Condition = SI->getCondition();
672 } else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) {
673 Condition = IB->getAddress()->stripPointerCasts();
674 Preference = WantBlockAddress;
676 return false; // Must be an invoke.
679 // Run constant folding to see if we can reduce the condition to a simple
681 if (Instruction *I = dyn_cast<Instruction>(Condition)) {
682 Value *SimpleVal = ConstantFoldInstruction(I, TD, TLI);
684 I->replaceAllUsesWith(SimpleVal);
685 I->eraseFromParent();
686 Condition = SimpleVal;
690 // If the terminator is branching on an undef, we can pick any of the
691 // successors to branch to. Let GetBestDestForJumpOnUndef decide.
692 if (isa<UndefValue>(Condition)) {
693 unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
695 // Fold the branch/switch.
696 TerminatorInst *BBTerm = BB->getTerminator();
697 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
698 if (i == BestSucc) continue;
699 BBTerm->getSuccessor(i)->removePredecessor(BB, true);
702 DEBUG(dbgs() << " In block '" << BB->getName()
703 << "' folding undef terminator: " << *BBTerm << '\n');
704 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
705 BBTerm->eraseFromParent();
709 // If the terminator of this block is branching on a constant, simplify the
710 // terminator to an unconditional branch. This can occur due to threading in
712 if (getKnownConstant(Condition, Preference)) {
713 DEBUG(dbgs() << " In block '" << BB->getName()
714 << "' folding terminator: " << *BB->getTerminator() << '\n');
716 ConstantFoldTerminator(BB, true);
720 Instruction *CondInst = dyn_cast<Instruction>(Condition);
722 // All the rest of our checks depend on the condition being an instruction.
724 // FIXME: Unify this with code below.
725 if (ProcessThreadableEdges(Condition, BB, Preference))
731 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
732 // For a comparison where the LHS is outside this block, it's possible
733 // that we've branched on it before. Used LVI to see if we can simplify
734 // the branch based on that.
735 BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
736 Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
737 pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
738 if (CondBr && CondConst && CondBr->isConditional() && PI != PE &&
739 (!isa<Instruction>(CondCmp->getOperand(0)) ||
740 cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) {
741 // For predecessor edge, determine if the comparison is true or false
742 // on that edge. If they're all true or all false, we can simplify the
744 // FIXME: We could handle mixed true/false by duplicating code.
745 LazyValueInfo::Tristate Baseline =
746 LVI->getPredicateOnEdge(CondCmp->getPredicate(), CondCmp->getOperand(0),
748 if (Baseline != LazyValueInfo::Unknown) {
749 // Check that all remaining incoming values match the first one.
751 LazyValueInfo::Tristate Ret =
752 LVI->getPredicateOnEdge(CondCmp->getPredicate(),
753 CondCmp->getOperand(0), CondConst, *PI, BB);
754 if (Ret != Baseline) break;
757 // If we terminated early, then one of the values didn't match.
759 unsigned ToRemove = Baseline == LazyValueInfo::True ? 1 : 0;
760 unsigned ToKeep = Baseline == LazyValueInfo::True ? 0 : 1;
761 CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
762 BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
763 CondBr->eraseFromParent();
770 // Check for some cases that are worth simplifying. Right now we want to look
771 // for loads that are used by a switch or by the condition for the branch. If
772 // we see one, check to see if it's partially redundant. If so, insert a PHI
773 // which can then be used to thread the values.
775 Value *SimplifyValue = CondInst;
776 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
777 if (isa<Constant>(CondCmp->getOperand(1)))
778 SimplifyValue = CondCmp->getOperand(0);
780 // TODO: There are other places where load PRE would be profitable, such as
781 // more complex comparisons.
782 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
783 if (SimplifyPartiallyRedundantLoad(LI))
787 // Handle a variety of cases where we are branching on something derived from
788 // a PHI node in the current block. If we can prove that any predecessors
789 // compute a predictable value based on a PHI node, thread those predecessors.
791 if (ProcessThreadableEdges(CondInst, BB, Preference))
794 // If this is an otherwise-unfoldable branch on a phi node in the current
795 // block, see if we can simplify.
796 if (PHINode *PN = dyn_cast<PHINode>(CondInst))
797 if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
798 return ProcessBranchOnPHI(PN);
801 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
802 if (CondInst->getOpcode() == Instruction::Xor &&
803 CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
804 return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
807 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know
808 // "(X == 4)", thread through this block.
814 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
815 /// load instruction, eliminate it by replacing it with a PHI node. This is an
816 /// important optimization that encourages jump threading, and needs to be run
817 /// interlaced with other jump threading tasks.
818 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
819 // Don't hack volatile/atomic loads.
820 if (!LI->isSimple()) return false;
822 // If the load is defined in a block with exactly one predecessor, it can't be
823 // partially redundant.
824 BasicBlock *LoadBB = LI->getParent();
825 if (LoadBB->getSinglePredecessor())
828 Value *LoadedPtr = LI->getOperand(0);
830 // If the loaded operand is defined in the LoadBB, it can't be available.
831 // TODO: Could do simple PHI translation, that would be fun :)
832 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
833 if (PtrOp->getParent() == LoadBB)
836 // Scan a few instructions up from the load, to see if it is obviously live at
837 // the entry to its block.
838 BasicBlock::iterator BBIt = LI;
840 if (Value *AvailableVal =
841 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
842 // If the value if the load is locally available within the block, just use
843 // it. This frequently occurs for reg2mem'd allocas.
844 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
846 // If the returned value is the load itself, replace with an undef. This can
847 // only happen in dead loops.
848 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
849 LI->replaceAllUsesWith(AvailableVal);
850 LI->eraseFromParent();
854 // Otherwise, if we scanned the whole block and got to the top of the block,
855 // we know the block is locally transparent to the load. If not, something
856 // might clobber its value.
857 if (BBIt != LoadBB->begin())
860 // If all of the loads and stores that feed the value have the same TBAA tag,
861 // then we can propagate it onto any newly inserted loads.
862 MDNode *TBAATag = LI->getMetadata(LLVMContext::MD_tbaa);
864 SmallPtrSet<BasicBlock*, 8> PredsScanned;
865 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
866 AvailablePredsTy AvailablePreds;
867 BasicBlock *OneUnavailablePred = 0;
869 // If we got here, the loaded value is transparent through to the start of the
870 // block. Check to see if it is available in any of the predecessor blocks.
871 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
873 BasicBlock *PredBB = *PI;
875 // If we already scanned this predecessor, skip it.
876 if (!PredsScanned.insert(PredBB))
879 // Scan the predecessor to see if the value is available in the pred.
880 BBIt = PredBB->end();
881 MDNode *ThisTBAATag = 0;
882 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6,
884 if (!PredAvailable) {
885 OneUnavailablePred = PredBB;
889 // If tbaa tags disagree or are not present, forget about them.
890 if (TBAATag != ThisTBAATag) TBAATag = 0;
892 // If so, this load is partially redundant. Remember this info so that we
893 // can create a PHI node.
894 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
897 // If the loaded value isn't available in any predecessor, it isn't partially
899 if (AvailablePreds.empty()) return false;
901 // Okay, the loaded value is available in at least one (and maybe all!)
902 // predecessors. If the value is unavailable in more than one unique
903 // predecessor, we want to insert a merge block for those common predecessors.
904 // This ensures that we only have to insert one reload, thus not increasing
906 BasicBlock *UnavailablePred = 0;
908 // If there is exactly one predecessor where the value is unavailable, the
909 // already computed 'OneUnavailablePred' block is it. If it ends in an
910 // unconditional branch, we know that it isn't a critical edge.
911 if (PredsScanned.size() == AvailablePreds.size()+1 &&
912 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
913 UnavailablePred = OneUnavailablePred;
914 } else if (PredsScanned.size() != AvailablePreds.size()) {
915 // Otherwise, we had multiple unavailable predecessors or we had a critical
916 // edge from the one.
917 SmallVector<BasicBlock*, 8> PredsToSplit;
918 SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
920 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
921 AvailablePredSet.insert(AvailablePreds[i].first);
923 // Add all the unavailable predecessors to the PredsToSplit list.
924 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
927 // If the predecessor is an indirect goto, we can't split the edge.
928 if (isa<IndirectBrInst>(P->getTerminator()))
931 if (!AvailablePredSet.count(P))
932 PredsToSplit.push_back(P);
935 // Split them out to their own block.
937 SplitBlockPredecessors(LoadBB, PredsToSplit, "thread-pre-split", this);
940 // If the value isn't available in all predecessors, then there will be
941 // exactly one where it isn't available. Insert a load on that edge and add
942 // it to the AvailablePreds list.
943 if (UnavailablePred) {
944 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
945 "Can't handle critical edge here!");
946 LoadInst *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
948 UnavailablePred->getTerminator());
949 NewVal->setDebugLoc(LI->getDebugLoc());
951 NewVal->setMetadata(LLVMContext::MD_tbaa, TBAATag);
953 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
956 // Now we know that each predecessor of this block has a value in
957 // AvailablePreds, sort them for efficient access as we're walking the preds.
958 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
960 // Create a PHI node at the start of the block for the PRE'd load value.
961 pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB);
962 PHINode *PN = PHINode::Create(LI->getType(), std::distance(PB, PE), "",
965 PN->setDebugLoc(LI->getDebugLoc());
967 // Insert new entries into the PHI for each predecessor. A single block may
968 // have multiple entries here.
969 for (pred_iterator PI = PB; PI != PE; ++PI) {
971 AvailablePredsTy::iterator I =
972 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
973 std::make_pair(P, (Value*)0));
975 assert(I != AvailablePreds.end() && I->first == P &&
976 "Didn't find entry for predecessor!");
978 PN->addIncoming(I->second, I->first);
981 //cerr << "PRE: " << *LI << *PN << "\n";
983 LI->replaceAllUsesWith(PN);
984 LI->eraseFromParent();
989 /// FindMostPopularDest - The specified list contains multiple possible
990 /// threadable destinations. Pick the one that occurs the most frequently in
993 FindMostPopularDest(BasicBlock *BB,
994 const SmallVectorImpl<std::pair<BasicBlock*,
995 BasicBlock*> > &PredToDestList) {
996 assert(!PredToDestList.empty());
998 // Determine popularity. If there are multiple possible destinations, we
999 // explicitly choose to ignore 'undef' destinations. We prefer to thread
1000 // blocks with known and real destinations to threading undef. We'll handle
1001 // them later if interesting.
1002 DenseMap<BasicBlock*, unsigned> DestPopularity;
1003 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1004 if (PredToDestList[i].second)
1005 DestPopularity[PredToDestList[i].second]++;
1007 // Find the most popular dest.
1008 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
1009 BasicBlock *MostPopularDest = DPI->first;
1010 unsigned Popularity = DPI->second;
1011 SmallVector<BasicBlock*, 4> SamePopularity;
1013 for (++DPI; DPI != DestPopularity.end(); ++DPI) {
1014 // If the popularity of this entry isn't higher than the popularity we've
1015 // seen so far, ignore it.
1016 if (DPI->second < Popularity)
1018 else if (DPI->second == Popularity) {
1019 // If it is the same as what we've seen so far, keep track of it.
1020 SamePopularity.push_back(DPI->first);
1022 // If it is more popular, remember it.
1023 SamePopularity.clear();
1024 MostPopularDest = DPI->first;
1025 Popularity = DPI->second;
1029 // Okay, now we know the most popular destination. If there is more than one
1030 // destination, we need to determine one. This is arbitrary, but we need
1031 // to make a deterministic decision. Pick the first one that appears in the
1033 if (!SamePopularity.empty()) {
1034 SamePopularity.push_back(MostPopularDest);
1035 TerminatorInst *TI = BB->getTerminator();
1036 for (unsigned i = 0; ; ++i) {
1037 assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
1039 if (std::find(SamePopularity.begin(), SamePopularity.end(),
1040 TI->getSuccessor(i)) == SamePopularity.end())
1043 MostPopularDest = TI->getSuccessor(i);
1048 // Okay, we have finally picked the most popular destination.
1049 return MostPopularDest;
1052 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
1053 ConstantPreference Preference) {
1054 // If threading this would thread across a loop header, don't even try to
1056 if (LoopHeaders.count(BB))
1059 PredValueInfoTy PredValues;
1060 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference))
1063 assert(!PredValues.empty() &&
1064 "ComputeValueKnownInPredecessors returned true with no values");
1066 DEBUG(dbgs() << "IN BB: " << *BB;
1067 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1068 dbgs() << " BB '" << BB->getName() << "': FOUND condition = "
1069 << *PredValues[i].first
1070 << " for pred '" << PredValues[i].second->getName() << "'.\n";
1073 // Decide what we want to thread through. Convert our list of known values to
1074 // a list of known destinations for each pred. This also discards duplicate
1075 // predecessors and keeps track of the undefined inputs (which are represented
1076 // as a null dest in the PredToDestList).
1077 SmallPtrSet<BasicBlock*, 16> SeenPreds;
1078 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
1080 BasicBlock *OnlyDest = 0;
1081 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1083 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1084 BasicBlock *Pred = PredValues[i].second;
1085 if (!SeenPreds.insert(Pred))
1086 continue; // Duplicate predecessor entry.
1088 // If the predecessor ends with an indirect goto, we can't change its
1090 if (isa<IndirectBrInst>(Pred->getTerminator()))
1093 Constant *Val = PredValues[i].first;
1096 if (isa<UndefValue>(Val))
1098 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1099 DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero());
1100 else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
1101 DestBB = SI->findCaseValue(cast<ConstantInt>(Val)).getCaseSuccessor();
1103 assert(isa<IndirectBrInst>(BB->getTerminator())
1104 && "Unexpected terminator");
1105 DestBB = cast<BlockAddress>(Val)->getBasicBlock();
1108 // If we have exactly one destination, remember it for efficiency below.
1109 if (PredToDestList.empty())
1111 else if (OnlyDest != DestBB)
1112 OnlyDest = MultipleDestSentinel;
1114 PredToDestList.push_back(std::make_pair(Pred, DestBB));
1117 // If all edges were unthreadable, we fail.
1118 if (PredToDestList.empty())
1121 // Determine which is the most common successor. If we have many inputs and
1122 // this block is a switch, we want to start by threading the batch that goes
1123 // to the most popular destination first. If we only know about one
1124 // threadable destination (the common case) we can avoid this.
1125 BasicBlock *MostPopularDest = OnlyDest;
1127 if (MostPopularDest == MultipleDestSentinel)
1128 MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1130 // Now that we know what the most popular destination is, factor all
1131 // predecessors that will jump to it into a single predecessor.
1132 SmallVector<BasicBlock*, 16> PredsToFactor;
1133 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1134 if (PredToDestList[i].second == MostPopularDest) {
1135 BasicBlock *Pred = PredToDestList[i].first;
1137 // This predecessor may be a switch or something else that has multiple
1138 // edges to the block. Factor each of these edges by listing them
1139 // according to # occurrences in PredsToFactor.
1140 TerminatorInst *PredTI = Pred->getTerminator();
1141 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
1142 if (PredTI->getSuccessor(i) == BB)
1143 PredsToFactor.push_back(Pred);
1146 // If the threadable edges are branching on an undefined value, we get to pick
1147 // the destination that these predecessors should get to.
1148 if (MostPopularDest == 0)
1149 MostPopularDest = BB->getTerminator()->
1150 getSuccessor(GetBestDestForJumpOnUndef(BB));
1152 // Ok, try to thread it!
1153 return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1156 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1157 /// a PHI node in the current block. See if there are any simplifications we
1158 /// can do based on inputs to the phi node.
1160 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
1161 BasicBlock *BB = PN->getParent();
1163 // TODO: We could make use of this to do it once for blocks with common PHI
1165 SmallVector<BasicBlock*, 1> PredBBs;
1168 // If any of the predecessor blocks end in an unconditional branch, we can
1169 // *duplicate* the conditional branch into that block in order to further
1170 // encourage jump threading and to eliminate cases where we have branch on a
1171 // phi of an icmp (branch on icmp is much better).
1172 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1173 BasicBlock *PredBB = PN->getIncomingBlock(i);
1174 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1175 if (PredBr->isUnconditional()) {
1176 PredBBs[0] = PredBB;
1177 // Try to duplicate BB into PredBB.
1178 if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
1186 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
1187 /// a xor instruction in the current block. See if there are any
1188 /// simplifications we can do based on inputs to the xor.
1190 bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
1191 BasicBlock *BB = BO->getParent();
1193 // If either the LHS or RHS of the xor is a constant, don't do this
1195 if (isa<ConstantInt>(BO->getOperand(0)) ||
1196 isa<ConstantInt>(BO->getOperand(1)))
1199 // If the first instruction in BB isn't a phi, we won't be able to infer
1200 // anything special about any particular predecessor.
1201 if (!isa<PHINode>(BB->front()))
1204 // If we have a xor as the branch input to this block, and we know that the
1205 // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1206 // the condition into the predecessor and fix that value to true, saving some
1207 // logical ops on that path and encouraging other paths to simplify.
1209 // This copies something like this:
1212 // %X = phi i1 [1], [%X']
1213 // %Y = icmp eq i32 %A, %B
1214 // %Z = xor i1 %X, %Y
1219 // %Y = icmp ne i32 %A, %B
1222 PredValueInfoTy XorOpValues;
1224 if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
1226 assert(XorOpValues.empty());
1227 if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
1233 assert(!XorOpValues.empty() &&
1234 "ComputeValueKnownInPredecessors returned true with no values");
1236 // Scan the information to see which is most popular: true or false. The
1237 // predecessors can be of the set true, false, or undef.
1238 unsigned NumTrue = 0, NumFalse = 0;
1239 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1240 if (isa<UndefValue>(XorOpValues[i].first))
1241 // Ignore undefs for the count.
1243 if (cast<ConstantInt>(XorOpValues[i].first)->isZero())
1249 // Determine which value to split on, true, false, or undef if neither.
1250 ConstantInt *SplitVal = 0;
1251 if (NumTrue > NumFalse)
1252 SplitVal = ConstantInt::getTrue(BB->getContext());
1253 else if (NumTrue != 0 || NumFalse != 0)
1254 SplitVal = ConstantInt::getFalse(BB->getContext());
1256 // Collect all of the blocks that this can be folded into so that we can
1257 // factor this once and clone it once.
1258 SmallVector<BasicBlock*, 8> BlocksToFoldInto;
1259 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1260 if (XorOpValues[i].first != SplitVal &&
1261 !isa<UndefValue>(XorOpValues[i].first))
1264 BlocksToFoldInto.push_back(XorOpValues[i].second);
1267 // If we inferred a value for all of the predecessors, then duplication won't
1268 // help us. However, we can just replace the LHS or RHS with the constant.
1269 if (BlocksToFoldInto.size() ==
1270 cast<PHINode>(BB->front()).getNumIncomingValues()) {
1271 if (SplitVal == 0) {
1272 // If all preds provide undef, just nuke the xor, because it is undef too.
1273 BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
1274 BO->eraseFromParent();
1275 } else if (SplitVal->isZero()) {
1276 // If all preds provide 0, replace the xor with the other input.
1277 BO->replaceAllUsesWith(BO->getOperand(isLHS));
1278 BO->eraseFromParent();
1280 // If all preds provide 1, set the computed value to 1.
1281 BO->setOperand(!isLHS, SplitVal);
1287 // Try to duplicate BB into PredBB.
1288 return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
1292 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1293 /// predecessor to the PHIBB block. If it has PHI nodes, add entries for
1294 /// NewPred using the entries from OldPred (suitably mapped).
1295 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1296 BasicBlock *OldPred,
1297 BasicBlock *NewPred,
1298 DenseMap<Instruction*, Value*> &ValueMap) {
1299 for (BasicBlock::iterator PNI = PHIBB->begin();
1300 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1301 // Ok, we have a PHI node. Figure out what the incoming value was for the
1303 Value *IV = PN->getIncomingValueForBlock(OldPred);
1305 // Remap the value if necessary.
1306 if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1307 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1308 if (I != ValueMap.end())
1312 PN->addIncoming(IV, NewPred);
1316 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1317 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1318 /// across BB. Transform the IR to reflect this change.
1319 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1320 const SmallVectorImpl<BasicBlock*> &PredBBs,
1321 BasicBlock *SuccBB) {
1322 // If threading to the same block as we come from, we would infinite loop.
1324 DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
1325 << "' - would thread to self!\n");
1329 // If threading this would thread across a loop header, don't thread the edge.
1330 // See the comments above FindLoopHeaders for justifications and caveats.
1331 if (LoopHeaders.count(BB)) {
1332 DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName()
1333 << "' to dest BB '" << SuccBB->getName()
1334 << "' - it might create an irreducible loop!\n");
1338 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
1339 if (JumpThreadCost > Threshold) {
1340 DEBUG(dbgs() << " Not threading BB '" << BB->getName()
1341 << "' - Cost is too high: " << JumpThreadCost << "\n");
1345 // And finally, do it! Start by factoring the predecessors is needed.
1347 if (PredBBs.size() == 1)
1348 PredBB = PredBBs[0];
1350 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1351 << " common predecessors.\n");
1352 PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm", this);
1355 // And finally, do it!
1356 DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '"
1357 << SuccBB->getName() << "' with cost: " << JumpThreadCost
1358 << ", across block:\n "
1361 LVI->threadEdge(PredBB, BB, SuccBB);
1363 // We are going to have to map operands from the original BB block to the new
1364 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
1365 // account for entry from PredBB.
1366 DenseMap<Instruction*, Value*> ValueMapping;
1368 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1369 BB->getName()+".thread",
1370 BB->getParent(), BB);
1371 NewBB->moveAfter(PredBB);
1373 BasicBlock::iterator BI = BB->begin();
1374 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1375 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1377 // Clone the non-phi instructions of BB into NewBB, keeping track of the
1378 // mapping and using it to remap operands in the cloned instructions.
1379 for (; !isa<TerminatorInst>(BI); ++BI) {
1380 Instruction *New = BI->clone();
1381 New->setName(BI->getName());
1382 NewBB->getInstList().push_back(New);
1383 ValueMapping[BI] = New;
1385 // Remap operands to patch up intra-block references.
1386 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1387 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1388 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1389 if (I != ValueMapping.end())
1390 New->setOperand(i, I->second);
1394 // We didn't copy the terminator from BB over to NewBB, because there is now
1395 // an unconditional jump to SuccBB. Insert the unconditional jump.
1396 BranchInst *NewBI =BranchInst::Create(SuccBB, NewBB);
1397 NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
1399 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1400 // PHI nodes for NewBB now.
1401 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1403 // If there were values defined in BB that are used outside the block, then we
1404 // now have to update all uses of the value to use either the original value,
1405 // the cloned value, or some PHI derived value. This can require arbitrary
1406 // PHI insertion, of which we are prepared to do, clean these up now.
1407 SSAUpdater SSAUpdate;
1408 SmallVector<Use*, 16> UsesToRename;
1409 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1410 // Scan all uses of this instruction to see if it is used outside of its
1411 // block, and if so, record them in UsesToRename.
1412 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1414 Instruction *User = cast<Instruction>(*UI);
1415 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1416 if (UserPN->getIncomingBlock(UI) == BB)
1418 } else if (User->getParent() == BB)
1421 UsesToRename.push_back(&UI.getUse());
1424 // If there are no uses outside the block, we're done with this instruction.
1425 if (UsesToRename.empty())
1428 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1430 // We found a use of I outside of BB. Rename all uses of I that are outside
1431 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1432 // with the two values we know.
1433 SSAUpdate.Initialize(I->getType(), I->getName());
1434 SSAUpdate.AddAvailableValue(BB, I);
1435 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1437 while (!UsesToRename.empty())
1438 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1439 DEBUG(dbgs() << "\n");
1443 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to
1444 // NewBB instead of BB. This eliminates predecessors from BB, which requires
1445 // us to simplify any PHI nodes in BB.
1446 TerminatorInst *PredTerm = PredBB->getTerminator();
1447 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1448 if (PredTerm->getSuccessor(i) == BB) {
1449 BB->removePredecessor(PredBB, true);
1450 PredTerm->setSuccessor(i, NewBB);
1453 // At this point, the IR is fully up to date and consistent. Do a quick scan
1454 // over the new instructions and zap any that are constants or dead. This
1455 // frequently happens because of phi translation.
1456 SimplifyInstructionsInBlock(NewBB, TD);
1458 // Threaded an edge!
1463 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1464 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1465 /// If we can duplicate the contents of BB up into PredBB do so now, this
1466 /// improves the odds that the branch will be on an analyzable instruction like
1468 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1469 const SmallVectorImpl<BasicBlock *> &PredBBs) {
1470 assert(!PredBBs.empty() && "Can't handle an empty set");
1472 // If BB is a loop header, then duplicating this block outside the loop would
1473 // cause us to transform this into an irreducible loop, don't do this.
1474 // See the comments above FindLoopHeaders for justifications and caveats.
1475 if (LoopHeaders.count(BB)) {
1476 DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
1477 << "' into predecessor block '" << PredBBs[0]->getName()
1478 << "' - it might create an irreducible loop!\n");
1482 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
1483 if (DuplicationCost > Threshold) {
1484 DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
1485 << "' - Cost is too high: " << DuplicationCost << "\n");
1489 // And finally, do it! Start by factoring the predecessors is needed.
1491 if (PredBBs.size() == 1)
1492 PredBB = PredBBs[0];
1494 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1495 << " common predecessors.\n");
1496 PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm", this);
1499 // Okay, we decided to do this! Clone all the instructions in BB onto the end
1501 DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '"
1502 << PredBB->getName() << "' to eliminate branch on phi. Cost: "
1503 << DuplicationCost << " block is:" << *BB << "\n");
1505 // Unless PredBB ends with an unconditional branch, split the edge so that we
1506 // can just clone the bits from BB into the end of the new PredBB.
1507 BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
1509 if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) {
1510 PredBB = SplitEdge(PredBB, BB, this);
1511 OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1514 // We are going to have to map operands from the original BB block into the
1515 // PredBB block. Evaluate PHI nodes in BB.
1516 DenseMap<Instruction*, Value*> ValueMapping;
1518 BasicBlock::iterator BI = BB->begin();
1519 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1520 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1522 // Clone the non-phi instructions of BB into PredBB, keeping track of the
1523 // mapping and using it to remap operands in the cloned instructions.
1524 for (; BI != BB->end(); ++BI) {
1525 Instruction *New = BI->clone();
1527 // Remap operands to patch up intra-block references.
1528 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1529 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1530 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1531 if (I != ValueMapping.end())
1532 New->setOperand(i, I->second);
1535 // If this instruction can be simplified after the operands are updated,
1536 // just use the simplified value instead. This frequently happens due to
1538 if (Value *IV = SimplifyInstruction(New, TD)) {
1540 ValueMapping[BI] = IV;
1542 // Otherwise, insert the new instruction into the block.
1543 New->setName(BI->getName());
1544 PredBB->getInstList().insert(OldPredBranch, New);
1545 ValueMapping[BI] = New;
1549 // Check to see if the targets of the branch had PHI nodes. If so, we need to
1550 // add entries to the PHI nodes for branch from PredBB now.
1551 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1552 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1554 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1557 // If there were values defined in BB that are used outside the block, then we
1558 // now have to update all uses of the value to use either the original value,
1559 // the cloned value, or some PHI derived value. This can require arbitrary
1560 // PHI insertion, of which we are prepared to do, clean these up now.
1561 SSAUpdater SSAUpdate;
1562 SmallVector<Use*, 16> UsesToRename;
1563 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1564 // Scan all uses of this instruction to see if it is used outside of its
1565 // block, and if so, record them in UsesToRename.
1566 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1568 Instruction *User = cast<Instruction>(*UI);
1569 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1570 if (UserPN->getIncomingBlock(UI) == BB)
1572 } else if (User->getParent() == BB)
1575 UsesToRename.push_back(&UI.getUse());
1578 // If there are no uses outside the block, we're done with this instruction.
1579 if (UsesToRename.empty())
1582 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1584 // We found a use of I outside of BB. Rename all uses of I that are outside
1585 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1586 // with the two values we know.
1587 SSAUpdate.Initialize(I->getType(), I->getName());
1588 SSAUpdate.AddAvailableValue(BB, I);
1589 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1591 while (!UsesToRename.empty())
1592 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1593 DEBUG(dbgs() << "\n");
1596 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1598 BB->removePredecessor(PredBB, true);
1600 // Remove the unconditional branch at the end of the PredBB block.
1601 OldPredBranch->eraseFromParent();