1 //===- JumpThreading.cpp - Thread control through conditional blocks ------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements the Jump Threading pass.
12 //===----------------------------------------------------------------------===//
14 #define DEBUG_TYPE "jump-threading"
15 #include "llvm/Transforms/Scalar.h"
16 #include "llvm/IntrinsicInst.h"
17 #include "llvm/LLVMContext.h"
18 #include "llvm/Pass.h"
19 #include "llvm/Analysis/InstructionSimplify.h"
20 #include "llvm/Analysis/LazyValueInfo.h"
21 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
22 #include "llvm/Transforms/Utils/Local.h"
23 #include "llvm/Transforms/Utils/SSAUpdater.h"
24 #include "llvm/Target/TargetData.h"
25 #include "llvm/ADT/DenseMap.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/ADT/STLExtras.h"
28 #include "llvm/ADT/SmallPtrSet.h"
29 #include "llvm/ADT/SmallSet.h"
30 #include "llvm/Support/CommandLine.h"
31 #include "llvm/Support/Debug.h"
32 #include "llvm/Support/ValueHandle.h"
33 #include "llvm/Support/raw_ostream.h"
36 STATISTIC(NumThreads, "Number of jumps threaded");
37 STATISTIC(NumFolds, "Number of terminators folded");
38 STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
40 static cl::opt<unsigned>
41 Threshold("jump-threading-threshold",
42 cl::desc("Max block size to duplicate for jump threading"),
43 cl::init(6), cl::Hidden);
45 // Turn on use of LazyValueInfo.
47 EnableLVI("enable-jump-threading-lvi", cl::ReallyHidden);
52 /// This pass performs 'jump threading', which looks at blocks that have
53 /// multiple predecessors and multiple successors. If one or more of the
54 /// predecessors of the block can be proven to always jump to one of the
55 /// successors, we forward the edge from the predecessor to the successor by
56 /// duplicating the contents of this block.
58 /// An example of when this can occur is code like this:
65 /// In this case, the unconditional branch at the end of the first if can be
66 /// revectored to the false side of the second if.
68 class JumpThreading : public FunctionPass {
72 SmallPtrSet<BasicBlock*, 16> LoopHeaders;
74 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
77 static char ID; // Pass identification
78 JumpThreading() : FunctionPass(&ID) {}
80 bool runOnFunction(Function &F);
82 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
84 AU.addRequired<LazyValueInfo>();
87 void FindLoopHeaders(Function &F);
88 bool ProcessBlock(BasicBlock *BB);
89 bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
91 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
94 typedef SmallVectorImpl<std::pair<ConstantInt*,
95 BasicBlock*> > PredValueInfo;
97 bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
98 PredValueInfo &Result);
99 bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB);
102 bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
103 bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
105 bool ProcessJumpOnPHI(PHINode *PN);
107 bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
111 char JumpThreading::ID = 0;
112 static RegisterPass<JumpThreading>
113 X("jump-threading", "Jump Threading");
115 // Public interface to the Jump Threading pass
116 FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
118 /// runOnFunction - Top level algorithm.
120 bool JumpThreading::runOnFunction(Function &F) {
121 DEBUG(errs() << "Jump threading on function '" << F.getName() << "'\n");
122 TD = getAnalysisIfAvailable<TargetData>();
123 LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0;
127 bool AnotherIteration = true, EverChanged = false;
128 while (AnotherIteration) {
129 AnotherIteration = false;
130 bool Changed = false;
131 for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
133 // Thread all of the branches we can over this block.
134 while (ProcessBlock(BB))
139 // If the block is trivially dead, zap it. This eliminates the successor
140 // edges which simplifies the CFG.
141 if (pred_begin(BB) == pred_end(BB) &&
142 BB != &BB->getParent()->getEntryBlock()) {
143 DEBUG(errs() << " JT: Deleting dead block '" << BB->getName()
144 << "' with terminator: " << *BB->getTerminator() << '\n');
145 LoopHeaders.erase(BB);
148 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
149 // Can't thread an unconditional jump, but if the block is "almost
150 // empty", we can replace uses of it with uses of the successor and make
152 if (BI->isUnconditional() &&
153 BB != &BB->getParent()->getEntryBlock()) {
154 BasicBlock::iterator BBI = BB->getFirstNonPHI();
155 // Ignore dbg intrinsics.
156 while (isa<DbgInfoIntrinsic>(BBI))
158 // If the terminator is the only non-phi instruction, try to nuke it.
159 if (BBI->isTerminator()) {
160 // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
161 // block, we have to make sure it isn't in the LoopHeaders set. We
162 // reinsert afterward if needed.
163 bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
164 BasicBlock *Succ = BI->getSuccessor(0);
166 if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
168 // If we deleted BB and BB was the header of a loop, then the
169 // successor is now the header of the loop.
173 if (ErasedFromLoopHeaders)
174 LoopHeaders.insert(BB);
179 AnotherIteration = Changed;
180 EverChanged |= Changed;
187 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
188 /// thread across it.
189 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
190 /// Ignore PHI nodes, these will be flattened when duplication happens.
191 BasicBlock::const_iterator I = BB->getFirstNonPHI();
193 // FIXME: THREADING will delete values that are just used to compute the
194 // branch, so they shouldn't count against the duplication cost.
197 // Sum up the cost of each instruction until we get to the terminator. Don't
198 // include the terminator because the copy won't include it.
200 for (; !isa<TerminatorInst>(I); ++I) {
201 // Debugger intrinsics don't incur code size.
202 if (isa<DbgInfoIntrinsic>(I)) continue;
204 // If this is a pointer->pointer bitcast, it is free.
205 if (isa<BitCastInst>(I) && isa<PointerType>(I->getType()))
208 // All other instructions count for at least one unit.
211 // Calls are more expensive. If they are non-intrinsic calls, we model them
212 // as having cost of 4. If they are a non-vector intrinsic, we model them
213 // as having cost of 2 total, and if they are a vector intrinsic, we model
214 // them as having cost 1.
215 if (const CallInst *CI = dyn_cast<CallInst>(I)) {
216 if (!isa<IntrinsicInst>(CI))
218 else if (!isa<VectorType>(CI->getType()))
223 // Threading through a switch statement is particularly profitable. If this
224 // block ends in a switch, decrease its cost to make it more likely to happen.
225 if (isa<SwitchInst>(I))
226 Size = Size > 6 ? Size-6 : 0;
231 /// FindLoopHeaders - We do not want jump threading to turn proper loop
232 /// structures into irreducible loops. Doing this breaks up the loop nesting
233 /// hierarchy and pessimizes later transformations. To prevent this from
234 /// happening, we first have to find the loop headers. Here we approximate this
235 /// by finding targets of backedges in the CFG.
237 /// Note that there definitely are cases when we want to allow threading of
238 /// edges across a loop header. For example, threading a jump from outside the
239 /// loop (the preheader) to an exit block of the loop is definitely profitable.
240 /// It is also almost always profitable to thread backedges from within the loop
241 /// to exit blocks, and is often profitable to thread backedges to other blocks
242 /// within the loop (forming a nested loop). This simple analysis is not rich
243 /// enough to track all of these properties and keep it up-to-date as the CFG
244 /// mutates, so we don't allow any of these transformations.
246 void JumpThreading::FindLoopHeaders(Function &F) {
247 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
248 FindFunctionBackedges(F, Edges);
250 for (unsigned i = 0, e = Edges.size(); i != e; ++i)
251 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
254 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
255 /// if we can infer that the value is a known ConstantInt in any of our
256 /// predecessors. If so, return the known list of value and pred BB in the
257 /// result vector. If a value is known to be undef, it is returned as null.
259 /// This returns true if there were any known values.
262 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
263 // If V is a constantint, then it is known in all predecessors.
264 if (isa<ConstantInt>(V) || isa<UndefValue>(V)) {
265 ConstantInt *CI = dyn_cast<ConstantInt>(V);
267 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
268 Result.push_back(std::make_pair(CI, *PI));
272 // If V is a non-instruction value, or an instruction in a different block,
273 // then it can't be derived from a PHI.
274 Instruction *I = dyn_cast<Instruction>(V);
275 if (I == 0 || I->getParent() != BB) {
277 // Okay, if this is a live-in value, see if it has a known value at the end
278 // of any of our predecessors.
280 // FIXME: This should be an edge property, not a block end property.
281 /// TODO: Per PR2563, we could infer value range information about a
282 /// predecessor based on its terminator.
285 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
286 // "I" is a non-local compare-with-a-constant instruction. This would be
287 // able to handle value inequalities better, for example if the compare is
288 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
289 // Perhaps getConstantOnEdge should be smart enough to do this?
291 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
292 // If the value is known by LazyValueInfo to be a constant in a
293 // predecessor, use that information to try to thread this block.
294 Constant *PredCst = LVI->getConstantOnEdge(V, *PI, BB);
296 (!isa<ConstantInt>(PredCst) && !isa<UndefValue>(PredCst)))
299 Result.push_back(std::make_pair(dyn_cast<ConstantInt>(PredCst), *PI));
302 return !Result.empty();
308 /// If I is a PHI node, then we know the incoming values for any constants.
309 if (PHINode *PN = dyn_cast<PHINode>(I)) {
310 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
311 Value *InVal = PN->getIncomingValue(i);
312 if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) {
313 ConstantInt *CI = dyn_cast<ConstantInt>(InVal);
314 Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i)));
317 return !Result.empty();
320 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals;
322 // Handle some boolean conditions.
323 if (I->getType()->getPrimitiveSizeInBits() == 1) {
325 // X & false -> false
326 if (I->getOpcode() == Instruction::Or ||
327 I->getOpcode() == Instruction::And) {
328 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals);
329 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals);
331 if (LHSVals.empty() && RHSVals.empty())
334 ConstantInt *InterestingVal;
335 if (I->getOpcode() == Instruction::Or)
336 InterestingVal = ConstantInt::getTrue(I->getContext());
338 InterestingVal = ConstantInt::getFalse(I->getContext());
340 // Scan for the sentinel.
341 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
342 if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0)
343 Result.push_back(LHSVals[i]);
344 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
345 if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0)
346 Result.push_back(RHSVals[i]);
347 return !Result.empty();
350 // Handle the NOT form of XOR.
351 if (I->getOpcode() == Instruction::Xor &&
352 isa<ConstantInt>(I->getOperand(1)) &&
353 cast<ConstantInt>(I->getOperand(1))->isOne()) {
354 ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result);
358 // Invert the known values.
359 for (unsigned i = 0, e = Result.size(); i != e; ++i)
362 cast<ConstantInt>(ConstantExpr::getNot(Result[i].first));
367 // Handle compare with phi operand, where the PHI is defined in this block.
368 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
369 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
370 if (PN && PN->getParent() == BB) {
371 // We can do this simplification if any comparisons fold to true or false.
373 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
374 BasicBlock *PredBB = PN->getIncomingBlock(i);
375 Value *LHS = PN->getIncomingValue(i);
376 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
378 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD);
380 if (!LVI || !isa<Constant>(RHS))
383 LazyValueInfo::Tristate
384 ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
385 cast<Constant>(RHS), PredBB, BB);
386 if (ResT == LazyValueInfo::Unknown)
388 Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
391 if (isa<UndefValue>(Res))
392 Result.push_back(std::make_pair((ConstantInt*)0, PredBB));
393 else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res))
394 Result.push_back(std::make_pair(CI, PredBB));
397 return !Result.empty();
401 // If comparing a live-in value against a constant, see if we know the
402 // live-in value on any predecessors.
403 if (LVI && isa<Constant>(Cmp->getOperand(1)) &&
404 Cmp->getType()->isInteger() && // Not vector compare.
405 (!isa<Instruction>(Cmp->getOperand(0)) ||
406 cast<Instruction>(Cmp->getOperand(0))->getParent() != BB)) {
407 Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
409 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
410 // If the value is known by LazyValueInfo to be a constant in a
411 // predecessor, use that information to try to thread this block.
412 LazyValueInfo::Tristate
413 Res = LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
415 if (Res == LazyValueInfo::Unknown)
418 Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
419 Result.push_back(std::make_pair(cast<ConstantInt>(ResC), *PI));
422 return !Result.empty();
430 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
431 /// in an undefined jump, decide which block is best to revector to.
433 /// Since we can pick an arbitrary destination, we pick the successor with the
434 /// fewest predecessors. This should reduce the in-degree of the others.
436 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
437 TerminatorInst *BBTerm = BB->getTerminator();
438 unsigned MinSucc = 0;
439 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
440 // Compute the successor with the minimum number of predecessors.
441 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
442 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
443 TestBB = BBTerm->getSuccessor(i);
444 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
445 if (NumPreds < MinNumPreds)
452 /// ProcessBlock - If there are any predecessors whose control can be threaded
453 /// through to a successor, transform them now.
454 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
455 // If this block has a single predecessor, and if that pred has a single
456 // successor, merge the blocks. This encourages recursive jump threading
457 // because now the condition in this block can be threaded through
458 // predecessors of our predecessor block.
459 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
460 if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
462 // If SinglePred was a loop header, BB becomes one.
463 if (LoopHeaders.erase(SinglePred))
464 LoopHeaders.insert(BB);
466 // Remember if SinglePred was the entry block of the function. If so, we
467 // will need to move BB back to the entry position.
468 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
469 MergeBasicBlockIntoOnlyPred(BB);
471 if (isEntry && BB != &BB->getParent()->getEntryBlock())
472 BB->moveBefore(&BB->getParent()->getEntryBlock());
477 // Look to see if the terminator is a branch of switch, if not we can't thread
480 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
481 // Can't thread an unconditional jump.
482 if (BI->isUnconditional()) return false;
483 Condition = BI->getCondition();
484 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
485 Condition = SI->getCondition();
487 return false; // Must be an invoke.
489 // If the terminator of this block is branching on a constant, simplify the
490 // terminator to an unconditional branch. This can occur due to threading in
492 if (isa<ConstantInt>(Condition)) {
493 DEBUG(errs() << " In block '" << BB->getName()
494 << "' folding terminator: " << *BB->getTerminator() << '\n');
496 ConstantFoldTerminator(BB);
500 // If the terminator is branching on an undef, we can pick any of the
501 // successors to branch to. Let GetBestDestForJumpOnUndef decide.
502 if (isa<UndefValue>(Condition)) {
503 unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
505 // Fold the branch/switch.
506 TerminatorInst *BBTerm = BB->getTerminator();
507 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
508 if (i == BestSucc) continue;
509 RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD);
512 DEBUG(errs() << " In block '" << BB->getName()
513 << "' folding undef terminator: " << *BBTerm << '\n');
514 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
515 BBTerm->eraseFromParent();
519 Instruction *CondInst = dyn_cast<Instruction>(Condition);
521 // If the condition is an instruction defined in another block, see if a
522 // predecessor has the same condition:
527 !Condition->hasOneUse() && // Multiple uses.
528 (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
529 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
530 if (isa<BranchInst>(BB->getTerminator())) {
531 for (; PI != E; ++PI)
532 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
533 if (PBI->isConditional() && PBI->getCondition() == Condition &&
534 ProcessBranchOnDuplicateCond(*PI, BB))
537 assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
538 for (; PI != E; ++PI)
539 if (SwitchInst *PSI = dyn_cast<SwitchInst>((*PI)->getTerminator()))
540 if (PSI->getCondition() == Condition &&
541 ProcessSwitchOnDuplicateCond(*PI, BB))
546 // All the rest of our checks depend on the condition being an instruction.
548 // FIXME: Unify this with code below.
549 if (LVI && ProcessThreadableEdges(Condition, BB))
555 // See if this is a phi node in the current block.
556 if (PHINode *PN = dyn_cast<PHINode>(CondInst))
557 if (PN->getParent() == BB)
558 return ProcessJumpOnPHI(PN);
560 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
562 (!isa<PHINode>(CondCmp->getOperand(0)) ||
563 cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB)) {
564 // If we have a comparison, loop over the predecessors to see if there is
565 // a condition with a lexically identical value.
566 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
567 for (; PI != E; ++PI)
568 if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
569 if (PBI->isConditional() && *PI != BB) {
570 if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) {
571 if (CI->getOperand(0) == CondCmp->getOperand(0) &&
572 CI->getOperand(1) == CondCmp->getOperand(1) &&
573 CI->getPredicate() == CondCmp->getPredicate()) {
574 // TODO: Could handle things like (x != 4) --> (x == 17)
575 if (ProcessBranchOnDuplicateCond(*PI, BB))
583 // Check for some cases that are worth simplifying. Right now we want to look
584 // for loads that are used by a switch or by the condition for the branch. If
585 // we see one, check to see if it's partially redundant. If so, insert a PHI
586 // which can then be used to thread the values.
588 // This is particularly important because reg2mem inserts loads and stores all
589 // over the place, and this blocks jump threading if we don't zap them.
590 Value *SimplifyValue = CondInst;
591 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
592 if (isa<Constant>(CondCmp->getOperand(1)))
593 SimplifyValue = CondCmp->getOperand(0);
595 // TODO: There are other places where load PRE would be profitable, such as
596 // more complex comparisons.
597 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
598 if (SimplifyPartiallyRedundantLoad(LI))
602 // Handle a variety of cases where we are branching on something derived from
603 // a PHI node in the current block. If we can prove that any predecessors
604 // compute a predictable value based on a PHI node, thread those predecessors.
606 if (ProcessThreadableEdges(CondInst, BB))
610 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know
611 // "(X == 4)" thread through this block.
616 /// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
617 /// block that jump on exactly the same condition. This means that we almost
618 /// always know the direction of the edge in the DESTBB:
620 /// br COND, DESTBB, BBY
622 /// br COND, BBZ, BBW
624 /// If DESTBB has multiple predecessors, we can't just constant fold the branch
625 /// in DESTBB, we have to thread over it.
626 bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
628 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
630 // If both successors of PredBB go to DESTBB, we don't know anything. We can
631 // fold the branch to an unconditional one, which allows other recursive
634 if (PredBI->getSuccessor(1) != BB)
636 else if (PredBI->getSuccessor(0) != BB)
639 DEBUG(errs() << " In block '" << PredBB->getName()
640 << "' folding terminator: " << *PredBB->getTerminator() << '\n');
642 ConstantFoldTerminator(PredBB);
646 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
648 // If the dest block has one predecessor, just fix the branch condition to a
649 // constant and fold it.
650 if (BB->getSinglePredecessor()) {
651 DEBUG(errs() << " In block '" << BB->getName()
652 << "' folding condition to '" << BranchDir << "': "
653 << *BB->getTerminator() << '\n');
655 Value *OldCond = DestBI->getCondition();
656 DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
658 ConstantFoldTerminator(BB);
659 RecursivelyDeleteTriviallyDeadInstructions(OldCond);
664 // Next, figure out which successor we are threading to.
665 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
667 SmallVector<BasicBlock*, 2> Preds;
668 Preds.push_back(PredBB);
670 // Ok, try to thread it!
671 return ThreadEdge(BB, Preds, SuccBB);
674 /// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
675 /// block that switch on exactly the same condition. This means that we almost
676 /// always know the direction of the edge in the DESTBB:
678 /// switch COND [... DESTBB, BBY ... ]
680 /// switch COND [... BBZ, BBW ]
682 /// Optimizing switches like this is very important, because simplifycfg builds
683 /// switches out of repeated 'if' conditions.
684 bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
685 BasicBlock *DestBB) {
686 // Can't thread edge to self.
687 if (PredBB == DestBB)
690 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
691 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
693 // There are a variety of optimizations that we can potentially do on these
694 // blocks: we order them from most to least preferable.
696 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB
697 // directly to their destination. This does not introduce *any* code size
698 // growth. Skip debug info first.
699 BasicBlock::iterator BBI = DestBB->begin();
700 while (isa<DbgInfoIntrinsic>(BBI))
703 // FIXME: Thread if it just contains a PHI.
704 if (isa<SwitchInst>(BBI)) {
705 bool MadeChange = false;
706 // Ignore the default edge for now.
707 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
708 ConstantInt *DestVal = DestSI->getCaseValue(i);
709 BasicBlock *DestSucc = DestSI->getSuccessor(i);
711 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if
712 // PredSI has an explicit case for it. If so, forward. If it is covered
713 // by the default case, we can't update PredSI.
714 unsigned PredCase = PredSI->findCaseValue(DestVal);
715 if (PredCase == 0) continue;
717 // If PredSI doesn't go to DestBB on this value, then it won't reach the
718 // case on this condition.
719 if (PredSI->getSuccessor(PredCase) != DestBB &&
720 DestSI->getSuccessor(i) != DestBB)
723 // Do not forward this if it already goes to this destination, this would
724 // be an infinite loop.
725 if (PredSI->getSuccessor(PredCase) == DestSucc)
728 // Otherwise, we're safe to make the change. Make sure that the edge from
729 // DestSI to DestSucc is not critical and has no PHI nodes.
730 DEBUG(errs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI);
731 DEBUG(errs() << "THROUGH: " << *DestSI);
733 // If the destination has PHI nodes, just split the edge for updating
735 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
736 SplitCriticalEdge(DestSI, i, this);
737 DestSucc = DestSI->getSuccessor(i);
739 FoldSingleEntryPHINodes(DestSucc);
740 PredSI->setSuccessor(PredCase, DestSucc);
752 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
753 /// load instruction, eliminate it by replacing it with a PHI node. This is an
754 /// important optimization that encourages jump threading, and needs to be run
755 /// interlaced with other jump threading tasks.
756 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
757 // Don't hack volatile loads.
758 if (LI->isVolatile()) return false;
760 // If the load is defined in a block with exactly one predecessor, it can't be
761 // partially redundant.
762 BasicBlock *LoadBB = LI->getParent();
763 if (LoadBB->getSinglePredecessor())
766 Value *LoadedPtr = LI->getOperand(0);
768 // If the loaded operand is defined in the LoadBB, it can't be available.
769 // TODO: Could do simple PHI translation, that would be fun :)
770 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
771 if (PtrOp->getParent() == LoadBB)
774 // Scan a few instructions up from the load, to see if it is obviously live at
775 // the entry to its block.
776 BasicBlock::iterator BBIt = LI;
778 if (Value *AvailableVal =
779 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
780 // If the value if the load is locally available within the block, just use
781 // it. This frequently occurs for reg2mem'd allocas.
782 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
784 // If the returned value is the load itself, replace with an undef. This can
785 // only happen in dead loops.
786 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
787 LI->replaceAllUsesWith(AvailableVal);
788 LI->eraseFromParent();
792 // Otherwise, if we scanned the whole block and got to the top of the block,
793 // we know the block is locally transparent to the load. If not, something
794 // might clobber its value.
795 if (BBIt != LoadBB->begin())
799 SmallPtrSet<BasicBlock*, 8> PredsScanned;
800 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
801 AvailablePredsTy AvailablePreds;
802 BasicBlock *OneUnavailablePred = 0;
804 // If we got here, the loaded value is transparent through to the start of the
805 // block. Check to see if it is available in any of the predecessor blocks.
806 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
808 BasicBlock *PredBB = *PI;
810 // If we already scanned this predecessor, skip it.
811 if (!PredsScanned.insert(PredBB))
814 // Scan the predecessor to see if the value is available in the pred.
815 BBIt = PredBB->end();
816 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6);
817 if (!PredAvailable) {
818 OneUnavailablePred = PredBB;
822 // If so, this load is partially redundant. Remember this info so that we
823 // can create a PHI node.
824 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
827 // If the loaded value isn't available in any predecessor, it isn't partially
829 if (AvailablePreds.empty()) return false;
831 // Okay, the loaded value is available in at least one (and maybe all!)
832 // predecessors. If the value is unavailable in more than one unique
833 // predecessor, we want to insert a merge block for those common predecessors.
834 // This ensures that we only have to insert one reload, thus not increasing
836 BasicBlock *UnavailablePred = 0;
838 // If there is exactly one predecessor where the value is unavailable, the
839 // already computed 'OneUnavailablePred' block is it. If it ends in an
840 // unconditional branch, we know that it isn't a critical edge.
841 if (PredsScanned.size() == AvailablePreds.size()+1 &&
842 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
843 UnavailablePred = OneUnavailablePred;
844 } else if (PredsScanned.size() != AvailablePreds.size()) {
845 // Otherwise, we had multiple unavailable predecessors or we had a critical
846 // edge from the one.
847 SmallVector<BasicBlock*, 8> PredsToSplit;
848 SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
850 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
851 AvailablePredSet.insert(AvailablePreds[i].first);
853 // Add all the unavailable predecessors to the PredsToSplit list.
854 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
856 if (!AvailablePredSet.count(*PI))
857 PredsToSplit.push_back(*PI);
859 // Split them out to their own block.
861 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
862 "thread-pre-split", this);
865 // If the value isn't available in all predecessors, then there will be
866 // exactly one where it isn't available. Insert a load on that edge and add
867 // it to the AvailablePreds list.
868 if (UnavailablePred) {
869 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
870 "Can't handle critical edge here!");
871 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
873 UnavailablePred->getTerminator());
874 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
877 // Now we know that each predecessor of this block has a value in
878 // AvailablePreds, sort them for efficient access as we're walking the preds.
879 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
881 // Create a PHI node at the start of the block for the PRE'd load value.
882 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
885 // Insert new entries into the PHI for each predecessor. A single block may
886 // have multiple entries here.
887 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
889 AvailablePredsTy::iterator I =
890 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
891 std::make_pair(*PI, (Value*)0));
893 assert(I != AvailablePreds.end() && I->first == *PI &&
894 "Didn't find entry for predecessor!");
896 PN->addIncoming(I->second, I->first);
899 //cerr << "PRE: " << *LI << *PN << "\n";
901 LI->replaceAllUsesWith(PN);
902 LI->eraseFromParent();
907 /// FindMostPopularDest - The specified list contains multiple possible
908 /// threadable destinations. Pick the one that occurs the most frequently in
911 FindMostPopularDest(BasicBlock *BB,
912 const SmallVectorImpl<std::pair<BasicBlock*,
913 BasicBlock*> > &PredToDestList) {
914 assert(!PredToDestList.empty());
916 // Determine popularity. If there are multiple possible destinations, we
917 // explicitly choose to ignore 'undef' destinations. We prefer to thread
918 // blocks with known and real destinations to threading undef. We'll handle
919 // them later if interesting.
920 DenseMap<BasicBlock*, unsigned> DestPopularity;
921 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
922 if (PredToDestList[i].second)
923 DestPopularity[PredToDestList[i].second]++;
925 // Find the most popular dest.
926 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
927 BasicBlock *MostPopularDest = DPI->first;
928 unsigned Popularity = DPI->second;
929 SmallVector<BasicBlock*, 4> SamePopularity;
931 for (++DPI; DPI != DestPopularity.end(); ++DPI) {
932 // If the popularity of this entry isn't higher than the popularity we've
933 // seen so far, ignore it.
934 if (DPI->second < Popularity)
936 else if (DPI->second == Popularity) {
937 // If it is the same as what we've seen so far, keep track of it.
938 SamePopularity.push_back(DPI->first);
940 // If it is more popular, remember it.
941 SamePopularity.clear();
942 MostPopularDest = DPI->first;
943 Popularity = DPI->second;
947 // Okay, now we know the most popular destination. If there is more than
948 // destination, we need to determine one. This is arbitrary, but we need
949 // to make a deterministic decision. Pick the first one that appears in the
951 if (!SamePopularity.empty()) {
952 SamePopularity.push_back(MostPopularDest);
953 TerminatorInst *TI = BB->getTerminator();
954 for (unsigned i = 0; ; ++i) {
955 assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
957 if (std::find(SamePopularity.begin(), SamePopularity.end(),
958 TI->getSuccessor(i)) == SamePopularity.end())
961 MostPopularDest = TI->getSuccessor(i);
966 // Okay, we have finally picked the most popular destination.
967 return MostPopularDest;
970 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) {
971 // If threading this would thread across a loop header, don't even try to
973 if (LoopHeaders.count(BB))
976 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues;
977 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues))
979 assert(!PredValues.empty() &&
980 "ComputeValueKnownInPredecessors returned true with no values");
982 DEBUG(errs() << "IN BB: " << *BB;
983 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
984 errs() << " BB '" << BB->getName() << "': FOUND condition = ";
985 if (PredValues[i].first)
986 errs() << *PredValues[i].first;
989 errs() << " for pred '" << PredValues[i].second->getName()
993 // Decide what we want to thread through. Convert our list of known values to
994 // a list of known destinations for each pred. This also discards duplicate
995 // predecessors and keeps track of the undefined inputs (which are represented
996 // as a null dest in the PredToDestList).
997 SmallPtrSet<BasicBlock*, 16> SeenPreds;
998 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
1000 BasicBlock *OnlyDest = 0;
1001 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1003 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1004 BasicBlock *Pred = PredValues[i].second;
1005 if (!SeenPreds.insert(Pred))
1006 continue; // Duplicate predecessor entry.
1008 // If the predecessor ends with an indirect goto, we can't change its
1010 if (isa<IndirectBrInst>(Pred->getTerminator()))
1013 ConstantInt *Val = PredValues[i].first;
1016 if (Val == 0) // Undef.
1018 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1019 DestBB = BI->getSuccessor(Val->isZero());
1021 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
1022 DestBB = SI->getSuccessor(SI->findCaseValue(Val));
1025 // If we have exactly one destination, remember it for efficiency below.
1028 else if (OnlyDest != DestBB)
1029 OnlyDest = MultipleDestSentinel;
1031 PredToDestList.push_back(std::make_pair(Pred, DestBB));
1034 // If all edges were unthreadable, we fail.
1035 if (PredToDestList.empty())
1038 // Determine which is the most common successor. If we have many inputs and
1039 // this block is a switch, we want to start by threading the batch that goes
1040 // to the most popular destination first. If we only know about one
1041 // threadable destination (the common case) we can avoid this.
1042 BasicBlock *MostPopularDest = OnlyDest;
1044 if (MostPopularDest == MultipleDestSentinel)
1045 MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1047 // Now that we know what the most popular destination is, factor all
1048 // predecessors that will jump to it into a single predecessor.
1049 SmallVector<BasicBlock*, 16> PredsToFactor;
1050 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1051 if (PredToDestList[i].second == MostPopularDest) {
1052 BasicBlock *Pred = PredToDestList[i].first;
1054 // This predecessor may be a switch or something else that has multiple
1055 // edges to the block. Factor each of these edges by listing them
1056 // according to # occurrences in PredsToFactor.
1057 TerminatorInst *PredTI = Pred->getTerminator();
1058 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
1059 if (PredTI->getSuccessor(i) == BB)
1060 PredsToFactor.push_back(Pred);
1063 // If the threadable edges are branching on an undefined value, we get to pick
1064 // the destination that these predecessors should get to.
1065 if (MostPopularDest == 0)
1066 MostPopularDest = BB->getTerminator()->
1067 getSuccessor(GetBestDestForJumpOnUndef(BB));
1069 // Ok, try to thread it!
1070 return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1073 /// ProcessJumpOnPHI - We have a conditional branch or switch on a PHI node in
1074 /// the current block. See if there are any simplifications we can do based on
1075 /// inputs to the phi node.
1077 bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) {
1078 BasicBlock *BB = PN->getParent();
1080 // If any of the predecessor blocks end in an unconditional branch, we can
1081 // *duplicate* the jump into that block in order to further encourage jump
1082 // threading and to eliminate cases where we have branch on a phi of an icmp
1083 // (branch on icmp is much better).
1085 // We don't want to do this tranformation for switches, because we don't
1086 // really want to duplicate a switch.
1087 if (isa<SwitchInst>(BB->getTerminator()))
1090 // Look for unconditional branch predecessors.
1091 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1092 BasicBlock *PredBB = PN->getIncomingBlock(i);
1093 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1094 if (PredBr->isUnconditional() &&
1095 // Try to duplicate BB into PredBB.
1096 DuplicateCondBranchOnPHIIntoPred(BB, PredBB))
1104 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1105 /// predecessor to the PHIBB block. If it has PHI nodes, add entries for
1106 /// NewPred using the entries from OldPred (suitably mapped).
1107 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1108 BasicBlock *OldPred,
1109 BasicBlock *NewPred,
1110 DenseMap<Instruction*, Value*> &ValueMap) {
1111 for (BasicBlock::iterator PNI = PHIBB->begin();
1112 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1113 // Ok, we have a PHI node. Figure out what the incoming value was for the
1115 Value *IV = PN->getIncomingValueForBlock(OldPred);
1117 // Remap the value if necessary.
1118 if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1119 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1120 if (I != ValueMap.end())
1124 PN->addIncoming(IV, NewPred);
1128 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1129 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1130 /// across BB. Transform the IR to reflect this change.
1131 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1132 const SmallVectorImpl<BasicBlock*> &PredBBs,
1133 BasicBlock *SuccBB) {
1134 // If threading to the same block as we come from, we would infinite loop.
1136 DEBUG(errs() << " Not threading across BB '" << BB->getName()
1137 << "' - would thread to self!\n");
1141 // If threading this would thread across a loop header, don't thread the edge.
1142 // See the comments above FindLoopHeaders for justifications and caveats.
1143 if (LoopHeaders.count(BB)) {
1144 DEBUG(errs() << " Not threading across loop header BB '" << BB->getName()
1145 << "' to dest BB '" << SuccBB->getName()
1146 << "' - it might create an irreducible loop!\n");
1150 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
1151 if (JumpThreadCost > Threshold) {
1152 DEBUG(errs() << " Not threading BB '" << BB->getName()
1153 << "' - Cost is too high: " << JumpThreadCost << "\n");
1157 // And finally, do it! Start by factoring the predecessors is needed.
1159 if (PredBBs.size() == 1)
1160 PredBB = PredBBs[0];
1162 DEBUG(errs() << " Factoring out " << PredBBs.size()
1163 << " common predecessors.\n");
1164 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1168 // And finally, do it!
1169 DEBUG(errs() << " Threading edge from '" << PredBB->getName() << "' to '"
1170 << SuccBB->getName() << "' with cost: " << JumpThreadCost
1171 << ", across block:\n "
1174 // We are going to have to map operands from the original BB block to the new
1175 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
1176 // account for entry from PredBB.
1177 DenseMap<Instruction*, Value*> ValueMapping;
1179 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1180 BB->getName()+".thread",
1181 BB->getParent(), BB);
1182 NewBB->moveAfter(PredBB);
1184 BasicBlock::iterator BI = BB->begin();
1185 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1186 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1188 // Clone the non-phi instructions of BB into NewBB, keeping track of the
1189 // mapping and using it to remap operands in the cloned instructions.
1190 for (; !isa<TerminatorInst>(BI); ++BI) {
1191 Instruction *New = BI->clone();
1192 New->setName(BI->getName());
1193 NewBB->getInstList().push_back(New);
1194 ValueMapping[BI] = New;
1196 // Remap operands to patch up intra-block references.
1197 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1198 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1199 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1200 if (I != ValueMapping.end())
1201 New->setOperand(i, I->second);
1205 // We didn't copy the terminator from BB over to NewBB, because there is now
1206 // an unconditional jump to SuccBB. Insert the unconditional jump.
1207 BranchInst::Create(SuccBB, NewBB);
1209 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1210 // PHI nodes for NewBB now.
1211 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1213 // If there were values defined in BB that are used outside the block, then we
1214 // now have to update all uses of the value to use either the original value,
1215 // the cloned value, or some PHI derived value. This can require arbitrary
1216 // PHI insertion, of which we are prepared to do, clean these up now.
1217 SSAUpdater SSAUpdate;
1218 SmallVector<Use*, 16> UsesToRename;
1219 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1220 // Scan all uses of this instruction to see if it is used outside of its
1221 // block, and if so, record them in UsesToRename.
1222 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1224 Instruction *User = cast<Instruction>(*UI);
1225 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1226 if (UserPN->getIncomingBlock(UI) == BB)
1228 } else if (User->getParent() == BB)
1231 UsesToRename.push_back(&UI.getUse());
1234 // If there are no uses outside the block, we're done with this instruction.
1235 if (UsesToRename.empty())
1238 DEBUG(errs() << "JT: Renaming non-local uses of: " << *I << "\n");
1240 // We found a use of I outside of BB. Rename all uses of I that are outside
1241 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1242 // with the two values we know.
1243 SSAUpdate.Initialize(I);
1244 SSAUpdate.AddAvailableValue(BB, I);
1245 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1247 while (!UsesToRename.empty())
1248 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1249 DEBUG(errs() << "\n");
1253 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to
1254 // NewBB instead of BB. This eliminates predecessors from BB, which requires
1255 // us to simplify any PHI nodes in BB.
1256 TerminatorInst *PredTerm = PredBB->getTerminator();
1257 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1258 if (PredTerm->getSuccessor(i) == BB) {
1259 RemovePredecessorAndSimplify(BB, PredBB, TD);
1260 PredTerm->setSuccessor(i, NewBB);
1263 // At this point, the IR is fully up to date and consistent. Do a quick scan
1264 // over the new instructions and zap any that are constants or dead. This
1265 // frequently happens because of phi translation.
1266 BI = NewBB->begin();
1267 for (BasicBlock::iterator E = NewBB->end(); BI != E; ) {
1268 Instruction *Inst = BI++;
1270 if (Value *V = SimplifyInstruction(Inst, TD)) {
1271 WeakVH BIHandle(BI);
1272 ReplaceAndSimplifyAllUses(Inst, V, TD);
1274 BI = NewBB->begin();
1278 RecursivelyDeleteTriviallyDeadInstructions(Inst);
1281 // Threaded an edge!
1286 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1287 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1288 /// If we can duplicate the contents of BB up into PredBB do so now, this
1289 /// improves the odds that the branch will be on an analyzable instruction like
1291 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1292 BasicBlock *PredBB) {
1293 // If BB is a loop header, then duplicating this block outside the loop would
1294 // cause us to transform this into an irreducible loop, don't do this.
1295 // See the comments above FindLoopHeaders for justifications and caveats.
1296 if (LoopHeaders.count(BB)) {
1297 DEBUG(errs() << " Not duplicating loop header '" << BB->getName()
1298 << "' into predecessor block '" << PredBB->getName()
1299 << "' - it might create an irreducible loop!\n");
1303 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
1304 if (DuplicationCost > Threshold) {
1305 DEBUG(errs() << " Not duplicating BB '" << BB->getName()
1306 << "' - Cost is too high: " << DuplicationCost << "\n");
1310 // Okay, we decided to do this! Clone all the instructions in BB onto the end
1312 DEBUG(errs() << " Duplicating block '" << BB->getName() << "' into end of '"
1313 << PredBB->getName() << "' to eliminate branch on phi. Cost: "
1314 << DuplicationCost << " block is:" << *BB << "\n");
1316 // We are going to have to map operands from the original BB block into the
1317 // PredBB block. Evaluate PHI nodes in BB.
1318 DenseMap<Instruction*, Value*> ValueMapping;
1320 BasicBlock::iterator BI = BB->begin();
1321 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1322 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1324 BranchInst *OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1326 // Clone the non-phi instructions of BB into PredBB, keeping track of the
1327 // mapping and using it to remap operands in the cloned instructions.
1328 for (; BI != BB->end(); ++BI) {
1329 Instruction *New = BI->clone();
1330 New->setName(BI->getName());
1331 PredBB->getInstList().insert(OldPredBranch, New);
1332 ValueMapping[BI] = New;
1334 // Remap operands to patch up intra-block references.
1335 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1336 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1337 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1338 if (I != ValueMapping.end())
1339 New->setOperand(i, I->second);
1343 // Check to see if the targets of the branch had PHI nodes. If so, we need to
1344 // add entries to the PHI nodes for branch from PredBB now.
1345 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1346 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1348 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1351 // If there were values defined in BB that are used outside the block, then we
1352 // now have to update all uses of the value to use either the original value,
1353 // the cloned value, or some PHI derived value. This can require arbitrary
1354 // PHI insertion, of which we are prepared to do, clean these up now.
1355 SSAUpdater SSAUpdate;
1356 SmallVector<Use*, 16> UsesToRename;
1357 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1358 // Scan all uses of this instruction to see if it is used outside of its
1359 // block, and if so, record them in UsesToRename.
1360 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1362 Instruction *User = cast<Instruction>(*UI);
1363 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1364 if (UserPN->getIncomingBlock(UI) == BB)
1366 } else if (User->getParent() == BB)
1369 UsesToRename.push_back(&UI.getUse());
1372 // If there are no uses outside the block, we're done with this instruction.
1373 if (UsesToRename.empty())
1376 DEBUG(errs() << "JT: Renaming non-local uses of: " << *I << "\n");
1378 // We found a use of I outside of BB. Rename all uses of I that are outside
1379 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1380 // with the two values we know.
1381 SSAUpdate.Initialize(I);
1382 SSAUpdate.AddAvailableValue(BB, I);
1383 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1385 while (!UsesToRename.empty())
1386 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1387 DEBUG(errs() << "\n");
1390 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1392 RemovePredecessorAndSimplify(BB, PredBB, TD);
1394 // Remove the unconditional branch at the end of the PredBB block.
1395 OldPredBranch->eraseFromParent();