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/ADT/DenseMap.h"
17 #include "llvm/ADT/DenseSet.h"
18 #include "llvm/ADT/STLExtras.h"
19 #include "llvm/ADT/SmallPtrSet.h"
20 #include "llvm/ADT/SmallSet.h"
21 #include "llvm/ADT/Statistic.h"
22 #include "llvm/Analysis/ConstantFolding.h"
23 #include "llvm/Analysis/InstructionSimplify.h"
24 #include "llvm/Analysis/LazyValueInfo.h"
25 #include "llvm/Analysis/Loads.h"
26 #include "llvm/DataLayout.h"
27 #include "llvm/IntrinsicInst.h"
28 #include "llvm/LLVMContext.h"
29 #include "llvm/Pass.h"
30 #include "llvm/Support/CommandLine.h"
31 #include "llvm/Support/Debug.h"
32 #include "llvm/Support/ValueHandle.h"
33 #include "llvm/Support/raw_ostream.h"
34 #include "llvm/Target/TargetLibraryInfo.h"
35 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
36 #include "llvm/Transforms/Utils/Local.h"
37 #include "llvm/Transforms/Utils/SSAUpdater.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<DataLayout>();
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. Stop scanning the block when passing the threshold.
220 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB,
221 unsigned Threshold) {
222 /// Ignore PHI nodes, these will be flattened when duplication happens.
223 BasicBlock::const_iterator I = BB->getFirstNonPHI();
225 // FIXME: THREADING will delete values that are just used to compute the
226 // 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) {
233 // Stop scanning the block if we've reached the threshold.
234 if (Size > Threshold)
237 // Debugger intrinsics don't incur code size.
238 if (isa<DbgInfoIntrinsic>(I)) continue;
240 // If this is a pointer->pointer bitcast, it is free.
241 if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
244 // All other instructions count for at least one unit.
247 // Calls are more expensive. If they are non-intrinsic calls, we model them
248 // as having cost of 4. If they are a non-vector intrinsic, we model them
249 // as having cost of 2 total, and if they are a vector intrinsic, we model
250 // them as having cost 1.
251 if (const CallInst *CI = dyn_cast<CallInst>(I)) {
252 if (!isa<IntrinsicInst>(CI))
254 else if (!CI->getType()->isVectorTy())
259 // Threading through a switch statement is particularly profitable. If this
260 // block ends in a switch, decrease its cost to make it more likely to happen.
261 if (isa<SwitchInst>(I))
262 Size = Size > 6 ? Size-6 : 0;
264 // The same holds for indirect branches, but slightly more so.
265 if (isa<IndirectBrInst>(I))
266 Size = Size > 8 ? Size-8 : 0;
271 /// FindLoopHeaders - We do not want jump threading to turn proper loop
272 /// structures into irreducible loops. Doing this breaks up the loop nesting
273 /// hierarchy and pessimizes later transformations. To prevent this from
274 /// happening, we first have to find the loop headers. Here we approximate this
275 /// by finding targets of backedges in the CFG.
277 /// Note that there definitely are cases when we want to allow threading of
278 /// edges across a loop header. For example, threading a jump from outside the
279 /// loop (the preheader) to an exit block of the loop is definitely profitable.
280 /// It is also almost always profitable to thread backedges from within the loop
281 /// to exit blocks, and is often profitable to thread backedges to other blocks
282 /// within the loop (forming a nested loop). This simple analysis is not rich
283 /// enough to track all of these properties and keep it up-to-date as the CFG
284 /// mutates, so we don't allow any of these transformations.
286 void JumpThreading::FindLoopHeaders(Function &F) {
287 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
288 FindFunctionBackedges(F, Edges);
290 for (unsigned i = 0, e = Edges.size(); i != e; ++i)
291 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
294 /// getKnownConstant - Helper method to determine if we can thread over a
295 /// terminator with the given value as its condition, and if so what value to
296 /// use for that. What kind of value this is depends on whether we want an
297 /// integer or a block address, but an undef is always accepted.
298 /// Returns null if Val is null or not an appropriate constant.
299 static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
303 // Undef is "known" enough.
304 if (UndefValue *U = dyn_cast<UndefValue>(Val))
307 if (Preference == WantBlockAddress)
308 return dyn_cast<BlockAddress>(Val->stripPointerCasts());
310 return dyn_cast<ConstantInt>(Val);
313 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
314 /// if we can infer that the value is a known ConstantInt/BlockAddress or undef
315 /// in any of our predecessors. If so, return the known list of value and pred
316 /// BB in the result vector.
318 /// This returns true if there were any known values.
321 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB, PredValueInfo &Result,
322 ConstantPreference Preference) {
323 // This method walks up use-def chains recursively. Because of this, we could
324 // get into an infinite loop going around loops in the use-def chain. To
325 // prevent this, keep track of what (value, block) pairs we've already visited
326 // and terminate the search if we loop back to them
327 if (!RecursionSet.insert(std::make_pair(V, BB)).second)
330 // An RAII help to remove this pair from the recursion set once the recursion
331 // stack pops back out again.
332 RecursionSetRemover remover(RecursionSet, std::make_pair(V, BB));
334 // If V is a constant, then it is known in all predecessors.
335 if (Constant *KC = getKnownConstant(V, Preference)) {
336 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
337 Result.push_back(std::make_pair(KC, *PI));
342 // If V is a non-instruction value, or an instruction in a different block,
343 // then it can't be derived from a PHI.
344 Instruction *I = dyn_cast<Instruction>(V);
345 if (I == 0 || I->getParent() != BB) {
347 // Okay, if this is a live-in value, see if it has a known value at the end
348 // of any of our predecessors.
350 // FIXME: This should be an edge property, not a block end property.
351 /// TODO: Per PR2563, we could infer value range information about a
352 /// predecessor based on its terminator.
354 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
355 // "I" is a non-local compare-with-a-constant instruction. This would be
356 // able to handle value inequalities better, for example if the compare is
357 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
358 // Perhaps getConstantOnEdge should be smart enough to do this?
360 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
362 // If the value is known by LazyValueInfo to be a constant in a
363 // predecessor, use that information to try to thread this block.
364 Constant *PredCst = LVI->getConstantOnEdge(V, P, BB);
365 if (Constant *KC = getKnownConstant(PredCst, Preference))
366 Result.push_back(std::make_pair(KC, P));
369 return !Result.empty();
372 /// If I is a PHI node, then we know the incoming values for any constants.
373 if (PHINode *PN = dyn_cast<PHINode>(I)) {
374 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
375 Value *InVal = PN->getIncomingValue(i);
376 if (Constant *KC = getKnownConstant(InVal, Preference)) {
377 Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
379 Constant *CI = LVI->getConstantOnEdge(InVal,
380 PN->getIncomingBlock(i), BB);
381 if (Constant *KC = getKnownConstant(CI, Preference))
382 Result.push_back(std::make_pair(KC, PN->getIncomingBlock(i)));
386 return !Result.empty();
389 PredValueInfoTy LHSVals, RHSVals;
391 // Handle some boolean conditions.
392 if (I->getType()->getPrimitiveSizeInBits() == 1) {
393 assert(Preference == WantInteger && "One-bit non-integer type?");
395 // X & false -> false
396 if (I->getOpcode() == Instruction::Or ||
397 I->getOpcode() == Instruction::And) {
398 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
400 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals,
403 if (LHSVals.empty() && RHSVals.empty())
406 ConstantInt *InterestingVal;
407 if (I->getOpcode() == Instruction::Or)
408 InterestingVal = ConstantInt::getTrue(I->getContext());
410 InterestingVal = ConstantInt::getFalse(I->getContext());
412 SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
414 // Scan for the sentinel. If we find an undef, force it to the
415 // interesting value: x|undef -> true and x&undef -> false.
416 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
417 if (LHSVals[i].first == InterestingVal ||
418 isa<UndefValue>(LHSVals[i].first)) {
419 Result.push_back(LHSVals[i]);
420 Result.back().first = InterestingVal;
421 LHSKnownBBs.insert(LHSVals[i].second);
423 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
424 if (RHSVals[i].first == InterestingVal ||
425 isa<UndefValue>(RHSVals[i].first)) {
426 // If we already inferred a value for this block on the LHS, don't
428 if (!LHSKnownBBs.count(RHSVals[i].second)) {
429 Result.push_back(RHSVals[i]);
430 Result.back().first = InterestingVal;
434 return !Result.empty();
437 // Handle the NOT form of XOR.
438 if (I->getOpcode() == Instruction::Xor &&
439 isa<ConstantInt>(I->getOperand(1)) &&
440 cast<ConstantInt>(I->getOperand(1))->isOne()) {
441 ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result,
446 // Invert the known values.
447 for (unsigned i = 0, e = Result.size(); i != e; ++i)
448 Result[i].first = ConstantExpr::getNot(Result[i].first);
453 // Try to simplify some other binary operator values.
454 } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
455 assert(Preference != WantBlockAddress
456 && "A binary operator creating a block address?");
457 if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1))) {
458 PredValueInfoTy LHSVals;
459 ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals,
462 // Try to use constant folding to simplify the binary operator.
463 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
464 Constant *V = LHSVals[i].first;
465 Constant *Folded = ConstantExpr::get(BO->getOpcode(), V, CI);
467 if (Constant *KC = getKnownConstant(Folded, WantInteger))
468 Result.push_back(std::make_pair(KC, LHSVals[i].second));
472 return !Result.empty();
475 // Handle compare with phi operand, where the PHI is defined in this block.
476 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
477 assert(Preference == WantInteger && "Compares only produce integers");
478 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
479 if (PN && PN->getParent() == BB) {
480 // We can do this simplification if any comparisons fold to true or false.
482 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
483 BasicBlock *PredBB = PN->getIncomingBlock(i);
484 Value *LHS = PN->getIncomingValue(i);
485 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
487 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD);
489 if (!isa<Constant>(RHS))
492 LazyValueInfo::Tristate
493 ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
494 cast<Constant>(RHS), PredBB, BB);
495 if (ResT == LazyValueInfo::Unknown)
497 Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
500 if (Constant *KC = getKnownConstant(Res, WantInteger))
501 Result.push_back(std::make_pair(KC, PredBB));
504 return !Result.empty();
508 // If comparing a live-in value against a constant, see if we know the
509 // live-in value on any predecessors.
510 if (isa<Constant>(Cmp->getOperand(1)) && Cmp->getType()->isIntegerTy()) {
511 if (!isa<Instruction>(Cmp->getOperand(0)) ||
512 cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) {
513 Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
515 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB);PI != E; ++PI){
517 // If the value is known by LazyValueInfo to be a constant in a
518 // predecessor, use that information to try to thread this block.
519 LazyValueInfo::Tristate Res =
520 LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
522 if (Res == LazyValueInfo::Unknown)
525 Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
526 Result.push_back(std::make_pair(ResC, P));
529 return !Result.empty();
532 // Try to find a constant value for the LHS of a comparison,
533 // and evaluate it statically if we can.
534 if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) {
535 PredValueInfoTy LHSVals;
536 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals,
539 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
540 Constant *V = LHSVals[i].first;
541 Constant *Folded = ConstantExpr::getCompare(Cmp->getPredicate(),
543 if (Constant *KC = getKnownConstant(Folded, WantInteger))
544 Result.push_back(std::make_pair(KC, LHSVals[i].second));
547 return !Result.empty();
552 if (SelectInst *SI = dyn_cast<SelectInst>(I)) {
553 // Handle select instructions where at least one operand is a known constant
554 // and we can figure out the condition value for any predecessor block.
555 Constant *TrueVal = getKnownConstant(SI->getTrueValue(), Preference);
556 Constant *FalseVal = getKnownConstant(SI->getFalseValue(), Preference);
557 PredValueInfoTy Conds;
558 if ((TrueVal || FalseVal) &&
559 ComputeValueKnownInPredecessors(SI->getCondition(), BB, Conds,
561 for (unsigned i = 0, e = Conds.size(); i != e; ++i) {
562 Constant *Cond = Conds[i].first;
564 // Figure out what value to use for the condition.
566 if (ConstantInt *CI = dyn_cast<ConstantInt>(Cond)) {
568 KnownCond = CI->isOne();
570 assert(isa<UndefValue>(Cond) && "Unexpected condition value");
571 // Either operand will do, so be sure to pick the one that's a known
573 // FIXME: Do this more cleverly if both values are known constants?
574 KnownCond = (TrueVal != 0);
577 // See if the select has a known constant value for this predecessor.
578 if (Constant *Val = KnownCond ? TrueVal : FalseVal)
579 Result.push_back(std::make_pair(Val, Conds[i].second));
582 return !Result.empty();
586 // If all else fails, see if LVI can figure out a constant value for us.
587 Constant *CI = LVI->getConstant(V, BB);
588 if (Constant *KC = getKnownConstant(CI, Preference)) {
589 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
590 Result.push_back(std::make_pair(KC, *PI));
593 return !Result.empty();
598 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
599 /// in an undefined jump, decide which block is best to revector to.
601 /// Since we can pick an arbitrary destination, we pick the successor with the
602 /// fewest predecessors. This should reduce the in-degree of the others.
604 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
605 TerminatorInst *BBTerm = BB->getTerminator();
606 unsigned MinSucc = 0;
607 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
608 // Compute the successor with the minimum number of predecessors.
609 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
610 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
611 TestBB = BBTerm->getSuccessor(i);
612 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
613 if (NumPreds < MinNumPreds) {
615 MinNumPreds = NumPreds;
622 static bool hasAddressTakenAndUsed(BasicBlock *BB) {
623 if (!BB->hasAddressTaken()) return false;
625 // If the block has its address taken, it may be a tree of dead constants
626 // hanging off of it. These shouldn't keep the block alive.
627 BlockAddress *BA = BlockAddress::get(BB);
628 BA->removeDeadConstantUsers();
629 return !BA->use_empty();
632 /// ProcessBlock - If there are any predecessors whose control can be threaded
633 /// through to a successor, transform them now.
634 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
635 // If the block is trivially dead, just return and let the caller nuke it.
636 // This simplifies other transformations.
637 if (pred_begin(BB) == pred_end(BB) &&
638 BB != &BB->getParent()->getEntryBlock())
641 // If this block has a single predecessor, and if that pred has a single
642 // successor, merge the blocks. This encourages recursive jump threading
643 // because now the condition in this block can be threaded through
644 // predecessors of our predecessor block.
645 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
646 if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
647 SinglePred != BB && !hasAddressTakenAndUsed(BB)) {
648 // If SinglePred was a loop header, BB becomes one.
649 if (LoopHeaders.erase(SinglePred))
650 LoopHeaders.insert(BB);
652 // Remember if SinglePred was the entry block of the function. If so, we
653 // will need to move BB back to the entry position.
654 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
655 LVI->eraseBlock(SinglePred);
656 MergeBasicBlockIntoOnlyPred(BB);
658 if (isEntry && BB != &BB->getParent()->getEntryBlock())
659 BB->moveBefore(&BB->getParent()->getEntryBlock());
664 // What kind of constant we're looking for.
665 ConstantPreference Preference = WantInteger;
667 // Look to see if the terminator is a conditional branch, switch or indirect
668 // branch, if not we can't thread it.
670 Instruction *Terminator = BB->getTerminator();
671 if (BranchInst *BI = dyn_cast<BranchInst>(Terminator)) {
672 // Can't thread an unconditional jump.
673 if (BI->isUnconditional()) return false;
674 Condition = BI->getCondition();
675 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Terminator)) {
676 Condition = SI->getCondition();
677 } else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Terminator)) {
678 // Can't thread indirect branch with no successors.
679 if (IB->getNumSuccessors() == 0) return false;
680 Condition = IB->getAddress()->stripPointerCasts();
681 Preference = WantBlockAddress;
683 return false; // Must be an invoke.
686 // Run constant folding to see if we can reduce the condition to a simple
688 if (Instruction *I = dyn_cast<Instruction>(Condition)) {
689 Value *SimpleVal = ConstantFoldInstruction(I, TD, TLI);
691 I->replaceAllUsesWith(SimpleVal);
692 I->eraseFromParent();
693 Condition = SimpleVal;
697 // If the terminator is branching on an undef, we can pick any of the
698 // successors to branch to. Let GetBestDestForJumpOnUndef decide.
699 if (isa<UndefValue>(Condition)) {
700 unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
702 // Fold the branch/switch.
703 TerminatorInst *BBTerm = BB->getTerminator();
704 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
705 if (i == BestSucc) continue;
706 BBTerm->getSuccessor(i)->removePredecessor(BB, true);
709 DEBUG(dbgs() << " In block '" << BB->getName()
710 << "' folding undef terminator: " << *BBTerm << '\n');
711 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
712 BBTerm->eraseFromParent();
716 // If the terminator of this block is branching on a constant, simplify the
717 // terminator to an unconditional branch. This can occur due to threading in
719 if (getKnownConstant(Condition, Preference)) {
720 DEBUG(dbgs() << " In block '" << BB->getName()
721 << "' folding terminator: " << *BB->getTerminator() << '\n');
723 ConstantFoldTerminator(BB, true);
727 Instruction *CondInst = dyn_cast<Instruction>(Condition);
729 // All the rest of our checks depend on the condition being an instruction.
731 // FIXME: Unify this with code below.
732 if (ProcessThreadableEdges(Condition, BB, Preference))
738 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
739 // For a comparison where the LHS is outside this block, it's possible
740 // that we've branched on it before. Used LVI to see if we can simplify
741 // the branch based on that.
742 BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
743 Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
744 pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
745 if (CondBr && CondConst && CondBr->isConditional() && PI != PE &&
746 (!isa<Instruction>(CondCmp->getOperand(0)) ||
747 cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) {
748 // For predecessor edge, determine if the comparison is true or false
749 // on that edge. If they're all true or all false, we can simplify the
751 // FIXME: We could handle mixed true/false by duplicating code.
752 LazyValueInfo::Tristate Baseline =
753 LVI->getPredicateOnEdge(CondCmp->getPredicate(), CondCmp->getOperand(0),
755 if (Baseline != LazyValueInfo::Unknown) {
756 // Check that all remaining incoming values match the first one.
758 LazyValueInfo::Tristate Ret =
759 LVI->getPredicateOnEdge(CondCmp->getPredicate(),
760 CondCmp->getOperand(0), CondConst, *PI, BB);
761 if (Ret != Baseline) break;
764 // If we terminated early, then one of the values didn't match.
766 unsigned ToRemove = Baseline == LazyValueInfo::True ? 1 : 0;
767 unsigned ToKeep = Baseline == LazyValueInfo::True ? 0 : 1;
768 CondBr->getSuccessor(ToRemove)->removePredecessor(BB, true);
769 BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
770 CondBr->eraseFromParent();
777 // Check for some cases that are worth simplifying. Right now we want to look
778 // for loads that are used by a switch or by the condition for the branch. If
779 // we see one, check to see if it's partially redundant. If so, insert a PHI
780 // which can then be used to thread the values.
782 Value *SimplifyValue = CondInst;
783 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
784 if (isa<Constant>(CondCmp->getOperand(1)))
785 SimplifyValue = CondCmp->getOperand(0);
787 // TODO: There are other places where load PRE would be profitable, such as
788 // more complex comparisons.
789 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
790 if (SimplifyPartiallyRedundantLoad(LI))
794 // Handle a variety of cases where we are branching on something derived from
795 // a PHI node in the current block. If we can prove that any predecessors
796 // compute a predictable value based on a PHI node, thread those predecessors.
798 if (ProcessThreadableEdges(CondInst, BB, Preference))
801 // If this is an otherwise-unfoldable branch on a phi node in the current
802 // block, see if we can simplify.
803 if (PHINode *PN = dyn_cast<PHINode>(CondInst))
804 if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
805 return ProcessBranchOnPHI(PN);
808 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
809 if (CondInst->getOpcode() == Instruction::Xor &&
810 CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
811 return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
814 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know
815 // "(X == 4)", thread through this block.
821 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
822 /// load instruction, eliminate it by replacing it with a PHI node. This is an
823 /// important optimization that encourages jump threading, and needs to be run
824 /// interlaced with other jump threading tasks.
825 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
826 // Don't hack volatile/atomic loads.
827 if (!LI->isSimple()) return false;
829 // If the load is defined in a block with exactly one predecessor, it can't be
830 // partially redundant.
831 BasicBlock *LoadBB = LI->getParent();
832 if (LoadBB->getSinglePredecessor())
835 Value *LoadedPtr = LI->getOperand(0);
837 // If the loaded operand is defined in the LoadBB, it can't be available.
838 // TODO: Could do simple PHI translation, that would be fun :)
839 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
840 if (PtrOp->getParent() == LoadBB)
843 // Scan a few instructions up from the load, to see if it is obviously live at
844 // the entry to its block.
845 BasicBlock::iterator BBIt = LI;
847 if (Value *AvailableVal =
848 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
849 // If the value if the load is locally available within the block, just use
850 // it. This frequently occurs for reg2mem'd allocas.
851 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
853 // If the returned value is the load itself, replace with an undef. This can
854 // only happen in dead loops.
855 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
856 LI->replaceAllUsesWith(AvailableVal);
857 LI->eraseFromParent();
861 // Otherwise, if we scanned the whole block and got to the top of the block,
862 // we know the block is locally transparent to the load. If not, something
863 // might clobber its value.
864 if (BBIt != LoadBB->begin())
867 // If all of the loads and stores that feed the value have the same TBAA tag,
868 // then we can propagate it onto any newly inserted loads.
869 MDNode *TBAATag = LI->getMetadata(LLVMContext::MD_tbaa);
871 SmallPtrSet<BasicBlock*, 8> PredsScanned;
872 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
873 AvailablePredsTy AvailablePreds;
874 BasicBlock *OneUnavailablePred = 0;
876 // If we got here, the loaded value is transparent through to the start of the
877 // block. Check to see if it is available in any of the predecessor blocks.
878 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
880 BasicBlock *PredBB = *PI;
882 // If we already scanned this predecessor, skip it.
883 if (!PredsScanned.insert(PredBB))
886 // Scan the predecessor to see if the value is available in the pred.
887 BBIt = PredBB->end();
888 MDNode *ThisTBAATag = 0;
889 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6,
891 if (!PredAvailable) {
892 OneUnavailablePred = PredBB;
896 // If tbaa tags disagree or are not present, forget about them.
897 if (TBAATag != ThisTBAATag) TBAATag = 0;
899 // If so, this load is partially redundant. Remember this info so that we
900 // can create a PHI node.
901 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
904 // If the loaded value isn't available in any predecessor, it isn't partially
906 if (AvailablePreds.empty()) return false;
908 // Okay, the loaded value is available in at least one (and maybe all!)
909 // predecessors. If the value is unavailable in more than one unique
910 // predecessor, we want to insert a merge block for those common predecessors.
911 // This ensures that we only have to insert one reload, thus not increasing
913 BasicBlock *UnavailablePred = 0;
915 // If there is exactly one predecessor where the value is unavailable, the
916 // already computed 'OneUnavailablePred' block is it. If it ends in an
917 // unconditional branch, we know that it isn't a critical edge.
918 if (PredsScanned.size() == AvailablePreds.size()+1 &&
919 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
920 UnavailablePred = OneUnavailablePred;
921 } else if (PredsScanned.size() != AvailablePreds.size()) {
922 // Otherwise, we had multiple unavailable predecessors or we had a critical
923 // edge from the one.
924 SmallVector<BasicBlock*, 8> PredsToSplit;
925 SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
927 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
928 AvailablePredSet.insert(AvailablePreds[i].first);
930 // Add all the unavailable predecessors to the PredsToSplit list.
931 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
934 // If the predecessor is an indirect goto, we can't split the edge.
935 if (isa<IndirectBrInst>(P->getTerminator()))
938 if (!AvailablePredSet.count(P))
939 PredsToSplit.push_back(P);
942 // Split them out to their own block.
944 SplitBlockPredecessors(LoadBB, PredsToSplit, "thread-pre-split", this);
947 // If the value isn't available in all predecessors, then there will be
948 // exactly one where it isn't available. Insert a load on that edge and add
949 // it to the AvailablePreds list.
950 if (UnavailablePred) {
951 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
952 "Can't handle critical edge here!");
953 LoadInst *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
955 UnavailablePred->getTerminator());
956 NewVal->setDebugLoc(LI->getDebugLoc());
958 NewVal->setMetadata(LLVMContext::MD_tbaa, TBAATag);
960 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
963 // Now we know that each predecessor of this block has a value in
964 // AvailablePreds, sort them for efficient access as we're walking the preds.
965 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
967 // Create a PHI node at the start of the block for the PRE'd load value.
968 pred_iterator PB = pred_begin(LoadBB), PE = pred_end(LoadBB);
969 PHINode *PN = PHINode::Create(LI->getType(), std::distance(PB, PE), "",
972 PN->setDebugLoc(LI->getDebugLoc());
974 // Insert new entries into the PHI for each predecessor. A single block may
975 // have multiple entries here.
976 for (pred_iterator PI = PB; PI != PE; ++PI) {
978 AvailablePredsTy::iterator I =
979 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
980 std::make_pair(P, (Value*)0));
982 assert(I != AvailablePreds.end() && I->first == P &&
983 "Didn't find entry for predecessor!");
985 PN->addIncoming(I->second, I->first);
988 //cerr << "PRE: " << *LI << *PN << "\n";
990 LI->replaceAllUsesWith(PN);
991 LI->eraseFromParent();
996 /// FindMostPopularDest - The specified list contains multiple possible
997 /// threadable destinations. Pick the one that occurs the most frequently in
1000 FindMostPopularDest(BasicBlock *BB,
1001 const SmallVectorImpl<std::pair<BasicBlock*,
1002 BasicBlock*> > &PredToDestList) {
1003 assert(!PredToDestList.empty());
1005 // Determine popularity. If there are multiple possible destinations, we
1006 // explicitly choose to ignore 'undef' destinations. We prefer to thread
1007 // blocks with known and real destinations to threading undef. We'll handle
1008 // them later if interesting.
1009 DenseMap<BasicBlock*, unsigned> DestPopularity;
1010 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1011 if (PredToDestList[i].second)
1012 DestPopularity[PredToDestList[i].second]++;
1014 // Find the most popular dest.
1015 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
1016 BasicBlock *MostPopularDest = DPI->first;
1017 unsigned Popularity = DPI->second;
1018 SmallVector<BasicBlock*, 4> SamePopularity;
1020 for (++DPI; DPI != DestPopularity.end(); ++DPI) {
1021 // If the popularity of this entry isn't higher than the popularity we've
1022 // seen so far, ignore it.
1023 if (DPI->second < Popularity)
1025 else if (DPI->second == Popularity) {
1026 // If it is the same as what we've seen so far, keep track of it.
1027 SamePopularity.push_back(DPI->first);
1029 // If it is more popular, remember it.
1030 SamePopularity.clear();
1031 MostPopularDest = DPI->first;
1032 Popularity = DPI->second;
1036 // Okay, now we know the most popular destination. If there is more than one
1037 // destination, we need to determine one. This is arbitrary, but we need
1038 // to make a deterministic decision. Pick the first one that appears in the
1040 if (!SamePopularity.empty()) {
1041 SamePopularity.push_back(MostPopularDest);
1042 TerminatorInst *TI = BB->getTerminator();
1043 for (unsigned i = 0; ; ++i) {
1044 assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
1046 if (std::find(SamePopularity.begin(), SamePopularity.end(),
1047 TI->getSuccessor(i)) == SamePopularity.end())
1050 MostPopularDest = TI->getSuccessor(i);
1055 // Okay, we have finally picked the most popular destination.
1056 return MostPopularDest;
1059 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB,
1060 ConstantPreference Preference) {
1061 // If threading this would thread across a loop header, don't even try to
1063 if (LoopHeaders.count(BB))
1066 PredValueInfoTy PredValues;
1067 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues, Preference))
1070 assert(!PredValues.empty() &&
1071 "ComputeValueKnownInPredecessors returned true with no values");
1073 DEBUG(dbgs() << "IN BB: " << *BB;
1074 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1075 dbgs() << " BB '" << BB->getName() << "': FOUND condition = "
1076 << *PredValues[i].first
1077 << " for pred '" << PredValues[i].second->getName() << "'.\n";
1080 // Decide what we want to thread through. Convert our list of known values to
1081 // a list of known destinations for each pred. This also discards duplicate
1082 // predecessors and keeps track of the undefined inputs (which are represented
1083 // as a null dest in the PredToDestList).
1084 SmallPtrSet<BasicBlock*, 16> SeenPreds;
1085 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
1087 BasicBlock *OnlyDest = 0;
1088 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1090 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1091 BasicBlock *Pred = PredValues[i].second;
1092 if (!SeenPreds.insert(Pred))
1093 continue; // Duplicate predecessor entry.
1095 // If the predecessor ends with an indirect goto, we can't change its
1097 if (isa<IndirectBrInst>(Pred->getTerminator()))
1100 Constant *Val = PredValues[i].first;
1103 if (isa<UndefValue>(Val))
1105 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1106 DestBB = BI->getSuccessor(cast<ConstantInt>(Val)->isZero());
1107 else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator())) {
1108 DestBB = SI->findCaseValue(cast<ConstantInt>(Val)).getCaseSuccessor();
1110 assert(isa<IndirectBrInst>(BB->getTerminator())
1111 && "Unexpected terminator");
1112 DestBB = cast<BlockAddress>(Val)->getBasicBlock();
1115 // If we have exactly one destination, remember it for efficiency below.
1116 if (PredToDestList.empty())
1118 else if (OnlyDest != DestBB)
1119 OnlyDest = MultipleDestSentinel;
1121 PredToDestList.push_back(std::make_pair(Pred, DestBB));
1124 // If all edges were unthreadable, we fail.
1125 if (PredToDestList.empty())
1128 // Determine which is the most common successor. If we have many inputs and
1129 // this block is a switch, we want to start by threading the batch that goes
1130 // to the most popular destination first. If we only know about one
1131 // threadable destination (the common case) we can avoid this.
1132 BasicBlock *MostPopularDest = OnlyDest;
1134 if (MostPopularDest == MultipleDestSentinel)
1135 MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1137 // Now that we know what the most popular destination is, factor all
1138 // predecessors that will jump to it into a single predecessor.
1139 SmallVector<BasicBlock*, 16> PredsToFactor;
1140 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1141 if (PredToDestList[i].second == MostPopularDest) {
1142 BasicBlock *Pred = PredToDestList[i].first;
1144 // This predecessor may be a switch or something else that has multiple
1145 // edges to the block. Factor each of these edges by listing them
1146 // according to # occurrences in PredsToFactor.
1147 TerminatorInst *PredTI = Pred->getTerminator();
1148 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
1149 if (PredTI->getSuccessor(i) == BB)
1150 PredsToFactor.push_back(Pred);
1153 // If the threadable edges are branching on an undefined value, we get to pick
1154 // the destination that these predecessors should get to.
1155 if (MostPopularDest == 0)
1156 MostPopularDest = BB->getTerminator()->
1157 getSuccessor(GetBestDestForJumpOnUndef(BB));
1159 // Ok, try to thread it!
1160 return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1163 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1164 /// a PHI node in the current block. See if there are any simplifications we
1165 /// can do based on inputs to the phi node.
1167 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
1168 BasicBlock *BB = PN->getParent();
1170 // TODO: We could make use of this to do it once for blocks with common PHI
1172 SmallVector<BasicBlock*, 1> PredBBs;
1175 // If any of the predecessor blocks end in an unconditional branch, we can
1176 // *duplicate* the conditional branch into that block in order to further
1177 // encourage jump threading and to eliminate cases where we have branch on a
1178 // phi of an icmp (branch on icmp is much better).
1179 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1180 BasicBlock *PredBB = PN->getIncomingBlock(i);
1181 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1182 if (PredBr->isUnconditional()) {
1183 PredBBs[0] = PredBB;
1184 // Try to duplicate BB into PredBB.
1185 if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
1193 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
1194 /// a xor instruction in the current block. See if there are any
1195 /// simplifications we can do based on inputs to the xor.
1197 bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
1198 BasicBlock *BB = BO->getParent();
1200 // If either the LHS or RHS of the xor is a constant, don't do this
1202 if (isa<ConstantInt>(BO->getOperand(0)) ||
1203 isa<ConstantInt>(BO->getOperand(1)))
1206 // If the first instruction in BB isn't a phi, we won't be able to infer
1207 // anything special about any particular predecessor.
1208 if (!isa<PHINode>(BB->front()))
1211 // If we have a xor as the branch input to this block, and we know that the
1212 // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1213 // the condition into the predecessor and fix that value to true, saving some
1214 // logical ops on that path and encouraging other paths to simplify.
1216 // This copies something like this:
1219 // %X = phi i1 [1], [%X']
1220 // %Y = icmp eq i32 %A, %B
1221 // %Z = xor i1 %X, %Y
1226 // %Y = icmp ne i32 %A, %B
1229 PredValueInfoTy XorOpValues;
1231 if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues,
1233 assert(XorOpValues.empty());
1234 if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues,
1240 assert(!XorOpValues.empty() &&
1241 "ComputeValueKnownInPredecessors returned true with no values");
1243 // Scan the information to see which is most popular: true or false. The
1244 // predecessors can be of the set true, false, or undef.
1245 unsigned NumTrue = 0, NumFalse = 0;
1246 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1247 if (isa<UndefValue>(XorOpValues[i].first))
1248 // Ignore undefs for the count.
1250 if (cast<ConstantInt>(XorOpValues[i].first)->isZero())
1256 // Determine which value to split on, true, false, or undef if neither.
1257 ConstantInt *SplitVal = 0;
1258 if (NumTrue > NumFalse)
1259 SplitVal = ConstantInt::getTrue(BB->getContext());
1260 else if (NumTrue != 0 || NumFalse != 0)
1261 SplitVal = ConstantInt::getFalse(BB->getContext());
1263 // Collect all of the blocks that this can be folded into so that we can
1264 // factor this once and clone it once.
1265 SmallVector<BasicBlock*, 8> BlocksToFoldInto;
1266 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1267 if (XorOpValues[i].first != SplitVal &&
1268 !isa<UndefValue>(XorOpValues[i].first))
1271 BlocksToFoldInto.push_back(XorOpValues[i].second);
1274 // If we inferred a value for all of the predecessors, then duplication won't
1275 // help us. However, we can just replace the LHS or RHS with the constant.
1276 if (BlocksToFoldInto.size() ==
1277 cast<PHINode>(BB->front()).getNumIncomingValues()) {
1278 if (SplitVal == 0) {
1279 // If all preds provide undef, just nuke the xor, because it is undef too.
1280 BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
1281 BO->eraseFromParent();
1282 } else if (SplitVal->isZero()) {
1283 // If all preds provide 0, replace the xor with the other input.
1284 BO->replaceAllUsesWith(BO->getOperand(isLHS));
1285 BO->eraseFromParent();
1287 // If all preds provide 1, set the computed value to 1.
1288 BO->setOperand(!isLHS, SplitVal);
1294 // Try to duplicate BB into PredBB.
1295 return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
1299 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1300 /// predecessor to the PHIBB block. If it has PHI nodes, add entries for
1301 /// NewPred using the entries from OldPred (suitably mapped).
1302 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1303 BasicBlock *OldPred,
1304 BasicBlock *NewPred,
1305 DenseMap<Instruction*, Value*> &ValueMap) {
1306 for (BasicBlock::iterator PNI = PHIBB->begin();
1307 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1308 // Ok, we have a PHI node. Figure out what the incoming value was for the
1310 Value *IV = PN->getIncomingValueForBlock(OldPred);
1312 // Remap the value if necessary.
1313 if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1314 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1315 if (I != ValueMap.end())
1319 PN->addIncoming(IV, NewPred);
1323 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1324 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1325 /// across BB. Transform the IR to reflect this change.
1326 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1327 const SmallVectorImpl<BasicBlock*> &PredBBs,
1328 BasicBlock *SuccBB) {
1329 // If threading to the same block as we come from, we would infinite loop.
1331 DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
1332 << "' - would thread to self!\n");
1336 // If threading this would thread across a loop header, don't thread the edge.
1337 // See the comments above FindLoopHeaders for justifications and caveats.
1338 if (LoopHeaders.count(BB)) {
1339 DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName()
1340 << "' to dest BB '" << SuccBB->getName()
1341 << "' - it might create an irreducible loop!\n");
1345 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB, Threshold);
1346 if (JumpThreadCost > Threshold) {
1347 DEBUG(dbgs() << " Not threading BB '" << BB->getName()
1348 << "' - Cost is too high: " << JumpThreadCost << "\n");
1352 // And finally, do it! Start by factoring the predecessors is needed.
1354 if (PredBBs.size() == 1)
1355 PredBB = PredBBs[0];
1357 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1358 << " common predecessors.\n");
1359 PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm", this);
1362 // And finally, do it!
1363 DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '"
1364 << SuccBB->getName() << "' with cost: " << JumpThreadCost
1365 << ", across block:\n "
1368 LVI->threadEdge(PredBB, BB, SuccBB);
1370 // We are going to have to map operands from the original BB block to the new
1371 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
1372 // account for entry from PredBB.
1373 DenseMap<Instruction*, Value*> ValueMapping;
1375 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1376 BB->getName()+".thread",
1377 BB->getParent(), BB);
1378 NewBB->moveAfter(PredBB);
1380 BasicBlock::iterator BI = BB->begin();
1381 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1382 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1384 // Clone the non-phi instructions of BB into NewBB, keeping track of the
1385 // mapping and using it to remap operands in the cloned instructions.
1386 for (; !isa<TerminatorInst>(BI); ++BI) {
1387 Instruction *New = BI->clone();
1388 New->setName(BI->getName());
1389 NewBB->getInstList().push_back(New);
1390 ValueMapping[BI] = New;
1392 // Remap operands to patch up intra-block references.
1393 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1394 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1395 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1396 if (I != ValueMapping.end())
1397 New->setOperand(i, I->second);
1401 // We didn't copy the terminator from BB over to NewBB, because there is now
1402 // an unconditional jump to SuccBB. Insert the unconditional jump.
1403 BranchInst *NewBI =BranchInst::Create(SuccBB, NewBB);
1404 NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
1406 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1407 // PHI nodes for NewBB now.
1408 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1410 // If there were values defined in BB that are used outside the block, then we
1411 // now have to update all uses of the value to use either the original value,
1412 // the cloned value, or some PHI derived value. This can require arbitrary
1413 // PHI insertion, of which we are prepared to do, clean these up now.
1414 SSAUpdater SSAUpdate;
1415 SmallVector<Use*, 16> UsesToRename;
1416 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1417 // Scan all uses of this instruction to see if it is used outside of its
1418 // block, and if so, record them in UsesToRename.
1419 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1421 Instruction *User = cast<Instruction>(*UI);
1422 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1423 if (UserPN->getIncomingBlock(UI) == BB)
1425 } else if (User->getParent() == BB)
1428 UsesToRename.push_back(&UI.getUse());
1431 // If there are no uses outside the block, we're done with this instruction.
1432 if (UsesToRename.empty())
1435 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1437 // We found a use of I outside of BB. Rename all uses of I that are outside
1438 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1439 // with the two values we know.
1440 SSAUpdate.Initialize(I->getType(), I->getName());
1441 SSAUpdate.AddAvailableValue(BB, I);
1442 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1444 while (!UsesToRename.empty())
1445 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1446 DEBUG(dbgs() << "\n");
1450 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to
1451 // NewBB instead of BB. This eliminates predecessors from BB, which requires
1452 // us to simplify any PHI nodes in BB.
1453 TerminatorInst *PredTerm = PredBB->getTerminator();
1454 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1455 if (PredTerm->getSuccessor(i) == BB) {
1456 BB->removePredecessor(PredBB, true);
1457 PredTerm->setSuccessor(i, NewBB);
1460 // At this point, the IR is fully up to date and consistent. Do a quick scan
1461 // over the new instructions and zap any that are constants or dead. This
1462 // frequently happens because of phi translation.
1463 SimplifyInstructionsInBlock(NewBB, TD, TLI);
1465 // Threaded an edge!
1470 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1471 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1472 /// If we can duplicate the contents of BB up into PredBB do so now, this
1473 /// improves the odds that the branch will be on an analyzable instruction like
1475 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1476 const SmallVectorImpl<BasicBlock *> &PredBBs) {
1477 assert(!PredBBs.empty() && "Can't handle an empty set");
1479 // If BB is a loop header, then duplicating this block outside the loop would
1480 // cause us to transform this into an irreducible loop, don't do this.
1481 // See the comments above FindLoopHeaders for justifications and caveats.
1482 if (LoopHeaders.count(BB)) {
1483 DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
1484 << "' into predecessor block '" << PredBBs[0]->getName()
1485 << "' - it might create an irreducible loop!\n");
1489 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB, Threshold);
1490 if (DuplicationCost > Threshold) {
1491 DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
1492 << "' - Cost is too high: " << DuplicationCost << "\n");
1496 // And finally, do it! Start by factoring the predecessors is needed.
1498 if (PredBBs.size() == 1)
1499 PredBB = PredBBs[0];
1501 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1502 << " common predecessors.\n");
1503 PredBB = SplitBlockPredecessors(BB, PredBBs, ".thr_comm", this);
1506 // Okay, we decided to do this! Clone all the instructions in BB onto the end
1508 DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '"
1509 << PredBB->getName() << "' to eliminate branch on phi. Cost: "
1510 << DuplicationCost << " block is:" << *BB << "\n");
1512 // Unless PredBB ends with an unconditional branch, split the edge so that we
1513 // can just clone the bits from BB into the end of the new PredBB.
1514 BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
1516 if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) {
1517 PredBB = SplitEdge(PredBB, BB, this);
1518 OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1521 // We are going to have to map operands from the original BB block into the
1522 // PredBB block. Evaluate PHI nodes in BB.
1523 DenseMap<Instruction*, Value*> ValueMapping;
1525 BasicBlock::iterator BI = BB->begin();
1526 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1527 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1529 // Clone the non-phi instructions of BB into PredBB, keeping track of the
1530 // mapping and using it to remap operands in the cloned instructions.
1531 for (; BI != BB->end(); ++BI) {
1532 Instruction *New = BI->clone();
1534 // Remap operands to patch up intra-block references.
1535 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1536 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1537 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1538 if (I != ValueMapping.end())
1539 New->setOperand(i, I->second);
1542 // If this instruction can be simplified after the operands are updated,
1543 // just use the simplified value instead. This frequently happens due to
1545 if (Value *IV = SimplifyInstruction(New, TD)) {
1547 ValueMapping[BI] = IV;
1549 // Otherwise, insert the new instruction into the block.
1550 New->setName(BI->getName());
1551 PredBB->getInstList().insert(OldPredBranch, New);
1552 ValueMapping[BI] = New;
1556 // Check to see if the targets of the branch had PHI nodes. If so, we need to
1557 // add entries to the PHI nodes for branch from PredBB now.
1558 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1559 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1561 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1564 // If there were values defined in BB that are used outside the block, then we
1565 // now have to update all uses of the value to use either the original value,
1566 // the cloned value, or some PHI derived value. This can require arbitrary
1567 // PHI insertion, of which we are prepared to do, clean these up now.
1568 SSAUpdater SSAUpdate;
1569 SmallVector<Use*, 16> UsesToRename;
1570 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1571 // Scan all uses of this instruction to see if it is used outside of its
1572 // block, and if so, record them in UsesToRename.
1573 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1575 Instruction *User = cast<Instruction>(*UI);
1576 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1577 if (UserPN->getIncomingBlock(UI) == BB)
1579 } else if (User->getParent() == BB)
1582 UsesToRename.push_back(&UI.getUse());
1585 // If there are no uses outside the block, we're done with this instruction.
1586 if (UsesToRename.empty())
1589 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1591 // We found a use of I outside of BB. Rename all uses of I that are outside
1592 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1593 // with the two values we know.
1594 SSAUpdate.Initialize(I->getType(), I->getName());
1595 SSAUpdate.AddAvailableValue(BB, I);
1596 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1598 while (!UsesToRename.empty())
1599 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1600 DEBUG(dbgs() << "\n");
1603 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1605 BB->removePredecessor(PredBB, true);
1607 // Remove the unconditional branch at the end of the PredBB block.
1608 OldPredBranch->eraseFromParent();