1 //===- JumpThreading.cpp - Thread control through conditional blocks ------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements the Jump Threading pass.
12 //===----------------------------------------------------------------------===//
14 #define DEBUG_TYPE "jump-threading"
15 #include "llvm/Transforms/Scalar.h"
16 #include "llvm/IntrinsicInst.h"
17 #include "llvm/LLVMContext.h"
18 #include "llvm/Pass.h"
19 #include "llvm/Analysis/InstructionSimplify.h"
20 #include "llvm/Analysis/LazyValueInfo.h"
21 #include "llvm/Analysis/Loads.h"
22 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
23 #include "llvm/Transforms/Utils/Local.h"
24 #include "llvm/Transforms/Utils/SSAUpdater.h"
25 #include "llvm/Target/TargetData.h"
26 #include "llvm/ADT/DenseMap.h"
27 #include "llvm/ADT/DenseSet.h"
28 #include "llvm/ADT/Statistic.h"
29 #include "llvm/ADT/STLExtras.h"
30 #include "llvm/ADT/SmallPtrSet.h"
31 #include "llvm/ADT/SmallSet.h"
32 #include "llvm/Support/CommandLine.h"
33 #include "llvm/Support/Debug.h"
34 #include "llvm/Support/ValueHandle.h"
35 #include "llvm/Support/raw_ostream.h"
38 STATISTIC(NumThreads, "Number of jumps threaded");
39 STATISTIC(NumFolds, "Number of terminators folded");
40 STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
42 static cl::opt<unsigned>
43 Threshold("jump-threading-threshold",
44 cl::desc("Max block size to duplicate for jump threading"),
45 cl::init(6), cl::Hidden);
47 // Turn on use of LazyValueInfo.
49 EnableLVI("enable-jump-threading-lvi",
50 cl::desc("Use LVI for jump threading"),
57 /// This pass performs 'jump threading', which looks at blocks that have
58 /// multiple predecessors and multiple successors. If one or more of the
59 /// predecessors of the block can be proven to always jump to one of the
60 /// successors, we forward the edge from the predecessor to the successor by
61 /// duplicating the contents of this block.
63 /// An example of when this can occur is code like this:
70 /// In this case, the unconditional branch at the end of the first if can be
71 /// revectored to the false side of the second if.
73 class JumpThreading : public FunctionPass {
77 SmallPtrSet<BasicBlock*, 16> LoopHeaders;
79 SmallSet<AssertingVH<BasicBlock>, 16> LoopHeaders;
81 DenseSet<std::pair<Value*, BasicBlock*> > RecursionSet;
83 static char ID; // Pass identification
84 JumpThreading() : FunctionPass(ID) {}
86 bool runOnFunction(Function &F);
88 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
90 AU.addRequired<LazyValueInfo>();
93 void FindLoopHeaders(Function &F);
94 bool ProcessBlock(BasicBlock *BB);
95 bool ThreadEdge(BasicBlock *BB, const SmallVectorImpl<BasicBlock*> &PredBBs,
97 bool DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
98 const SmallVectorImpl<BasicBlock *> &PredBBs);
100 typedef SmallVectorImpl<std::pair<ConstantInt*,
101 BasicBlock*> > PredValueInfo;
103 bool ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,
104 PredValueInfo &Result);
105 bool ProcessThreadableEdges(Value *Cond, BasicBlock *BB);
108 bool ProcessBranchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
109 bool ProcessSwitchOnDuplicateCond(BasicBlock *PredBB, BasicBlock *DestBB);
111 bool ProcessBranchOnPHI(PHINode *PN);
112 bool ProcessBranchOnXOR(BinaryOperator *BO);
114 bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
118 char JumpThreading::ID = 0;
119 INITIALIZE_PASS(JumpThreading, "jump-threading",
120 "Jump Threading", false, false);
122 // Public interface to the Jump Threading pass
123 FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
125 /// runOnFunction - Top level algorithm.
127 bool JumpThreading::runOnFunction(Function &F) {
128 DEBUG(dbgs() << "Jump threading on function '" << F.getName() << "'\n");
129 TD = getAnalysisIfAvailable<TargetData>();
130 LVI = EnableLVI ? &getAnalysis<LazyValueInfo>() : 0;
134 bool Changed, EverChanged = false;
137 for (Function::iterator I = F.begin(), E = F.end(); I != E;) {
139 // Thread all of the branches we can over this block.
140 while (ProcessBlock(BB))
145 // If the block is trivially dead, zap it. This eliminates the successor
146 // edges which simplifies the CFG.
147 if (pred_begin(BB) == pred_end(BB) &&
148 BB != &BB->getParent()->getEntryBlock()) {
149 DEBUG(dbgs() << " JT: Deleting dead block '" << BB->getName()
150 << "' with terminator: " << *BB->getTerminator() << '\n');
151 LoopHeaders.erase(BB);
152 if (LVI) LVI->eraseBlock(BB);
155 } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
156 // Can't thread an unconditional jump, but if the block is "almost
157 // empty", we can replace uses of it with uses of the successor and make
159 if (BI->isUnconditional() &&
160 BB != &BB->getParent()->getEntryBlock()) {
161 BasicBlock::iterator BBI = BB->getFirstNonPHI();
162 // Ignore dbg intrinsics.
163 while (isa<DbgInfoIntrinsic>(BBI))
165 // If the terminator is the only non-phi instruction, try to nuke it.
166 if (BBI->isTerminator()) {
167 // Since TryToSimplifyUncondBranchFromEmptyBlock may delete the
168 // block, we have to make sure it isn't in the LoopHeaders set. We
169 // reinsert afterward if needed.
170 bool ErasedFromLoopHeaders = LoopHeaders.erase(BB);
171 BasicBlock *Succ = BI->getSuccessor(0);
173 // FIXME: It is always conservatively correct to drop the info
174 // for a block even if it doesn't get erased. This isn't totally
175 // awesome, but it allows us to use AssertingVH to prevent nasty
176 // dangling pointer issues within LazyValueInfo.
177 if (LVI) LVI->eraseBlock(BB);
178 if (TryToSimplifyUncondBranchFromEmptyBlock(BB)) {
180 // If we deleted BB and BB was the header of a loop, then the
181 // successor is now the header of the loop.
185 if (ErasedFromLoopHeaders)
186 LoopHeaders.insert(BB);
191 EverChanged |= Changed;
198 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
199 /// thread across it.
200 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
201 /// Ignore PHI nodes, these will be flattened when duplication happens.
202 BasicBlock::const_iterator I = BB->getFirstNonPHI();
204 // FIXME: THREADING will delete values that are just used to compute the
205 // branch, so they shouldn't count against the duplication cost.
208 // Sum up the cost of each instruction until we get to the terminator. Don't
209 // include the terminator because the copy won't include it.
211 for (; !isa<TerminatorInst>(I); ++I) {
212 // Debugger intrinsics don't incur code size.
213 if (isa<DbgInfoIntrinsic>(I)) continue;
215 // If this is a pointer->pointer bitcast, it is free.
216 if (isa<BitCastInst>(I) && I->getType()->isPointerTy())
219 // All other instructions count for at least one unit.
222 // Calls are more expensive. If they are non-intrinsic calls, we model them
223 // as having cost of 4. If they are a non-vector intrinsic, we model them
224 // as having cost of 2 total, and if they are a vector intrinsic, we model
225 // them as having cost 1.
226 if (const CallInst *CI = dyn_cast<CallInst>(I)) {
227 if (!isa<IntrinsicInst>(CI))
229 else if (!CI->getType()->isVectorTy())
234 // Threading through a switch statement is particularly profitable. If this
235 // block ends in a switch, decrease its cost to make it more likely to happen.
236 if (isa<SwitchInst>(I))
237 Size = Size > 6 ? Size-6 : 0;
242 /// FindLoopHeaders - We do not want jump threading to turn proper loop
243 /// structures into irreducible loops. Doing this breaks up the loop nesting
244 /// hierarchy and pessimizes later transformations. To prevent this from
245 /// happening, we first have to find the loop headers. Here we approximate this
246 /// by finding targets of backedges in the CFG.
248 /// Note that there definitely are cases when we want to allow threading of
249 /// edges across a loop header. For example, threading a jump from outside the
250 /// loop (the preheader) to an exit block of the loop is definitely profitable.
251 /// It is also almost always profitable to thread backedges from within the loop
252 /// to exit blocks, and is often profitable to thread backedges to other blocks
253 /// within the loop (forming a nested loop). This simple analysis is not rich
254 /// enough to track all of these properties and keep it up-to-date as the CFG
255 /// mutates, so we don't allow any of these transformations.
257 void JumpThreading::FindLoopHeaders(Function &F) {
258 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
259 FindFunctionBackedges(F, Edges);
261 for (unsigned i = 0, e = Edges.size(); i != e; ++i)
262 LoopHeaders.insert(const_cast<BasicBlock*>(Edges[i].second));
265 /// ComputeValueKnownInPredecessors - Given a basic block BB and a value V, see
266 /// if we can infer that the value is a known ConstantInt in any of our
267 /// predecessors. If so, return the known list of value and pred BB in the
268 /// result vector. If a value is known to be undef, it is returned as null.
270 /// This returns true if there were any known values.
273 ComputeValueKnownInPredecessors(Value *V, BasicBlock *BB,PredValueInfo &Result){
274 if (!RecursionSet.insert(std::make_pair(V, BB)).second)
277 // If V is a constantint, then it is known in all predecessors.
278 if (isa<ConstantInt>(V) || isa<UndefValue>(V)) {
279 ConstantInt *CI = dyn_cast<ConstantInt>(V);
281 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
282 Result.push_back(std::make_pair(CI, *PI));
284 RecursionSet.erase(std::make_pair(V, BB));
288 // If V is a non-instruction value, or an instruction in a different block,
289 // then it can't be derived from a PHI.
290 Instruction *I = dyn_cast<Instruction>(V);
291 if (I == 0 || I->getParent() != BB) {
293 // Okay, if this is a live-in value, see if it has a known value at the end
294 // of any of our predecessors.
296 // FIXME: This should be an edge property, not a block end property.
297 /// TODO: Per PR2563, we could infer value range information about a
298 /// predecessor based on its terminator.
301 // FIXME: change this to use the more-rich 'getPredicateOnEdge' method if
302 // "I" is a non-local compare-with-a-constant instruction. This would be
303 // able to handle value inequalities better, for example if the compare is
304 // "X < 4" and "X < 3" is known true but "X < 4" itself is not available.
305 // Perhaps getConstantOnEdge should be smart enough to do this?
307 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
309 // If the value is known by LazyValueInfo to be a constant in a
310 // predecessor, use that information to try to thread this block.
311 Constant *PredCst = LVI->getConstantOnEdge(V, P, BB);
313 (!isa<ConstantInt>(PredCst) && !isa<UndefValue>(PredCst)))
316 Result.push_back(std::make_pair(dyn_cast<ConstantInt>(PredCst), P));
319 RecursionSet.erase(std::make_pair(V, BB));
320 return !Result.empty();
323 RecursionSet.erase(std::make_pair(V, BB));
327 /// If I is a PHI node, then we know the incoming values for any constants.
328 if (PHINode *PN = dyn_cast<PHINode>(I)) {
329 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
330 Value *InVal = PN->getIncomingValue(i);
331 if (isa<ConstantInt>(InVal) || isa<UndefValue>(InVal)) {
332 ConstantInt *CI = dyn_cast<ConstantInt>(InVal);
333 Result.push_back(std::make_pair(CI, PN->getIncomingBlock(i)));
335 Constant *CI = LVI->getConstantOnEdge(InVal,
336 PN->getIncomingBlock(i), BB);
337 // LVI returns null is no value could be determined.
339 if (ConstantInt *CInt = dyn_cast<ConstantInt>(CI))
340 Result.push_back(std::make_pair(CInt, PN->getIncomingBlock(i)));
341 else if (isa<UndefValue>(CI))
342 Result.push_back(std::make_pair((ConstantInt*)0,
343 PN->getIncomingBlock(i)));
347 RecursionSet.erase(std::make_pair(V, BB));
348 return !Result.empty();
351 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals, RHSVals;
353 // Handle some boolean conditions.
354 if (I->getType()->getPrimitiveSizeInBits() == 1) {
356 // X & false -> false
357 if (I->getOpcode() == Instruction::Or ||
358 I->getOpcode() == Instruction::And) {
359 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals);
360 ComputeValueKnownInPredecessors(I->getOperand(1), BB, RHSVals);
362 if (LHSVals.empty() && RHSVals.empty()) {
363 RecursionSet.erase(std::make_pair(V, BB));
367 ConstantInt *InterestingVal;
368 if (I->getOpcode() == Instruction::Or)
369 InterestingVal = ConstantInt::getTrue(I->getContext());
371 InterestingVal = ConstantInt::getFalse(I->getContext());
373 SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
375 // Scan for the sentinel. If we find an undef, force it to the
376 // interesting value: x|undef -> true and x&undef -> false.
377 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i)
378 if (LHSVals[i].first == InterestingVal || LHSVals[i].first == 0) {
379 Result.push_back(LHSVals[i]);
380 Result.back().first = InterestingVal;
381 LHSKnownBBs.insert(LHSVals[i].second);
383 for (unsigned i = 0, e = RHSVals.size(); i != e; ++i)
384 if (RHSVals[i].first == InterestingVal || RHSVals[i].first == 0) {
385 // If we already inferred a value for this block on the LHS, don't
387 if (!LHSKnownBBs.count(RHSVals[i].second)) {
388 Result.push_back(RHSVals[i]);
389 Result.back().first = InterestingVal;
393 RecursionSet.erase(std::make_pair(V, BB));
394 return !Result.empty();
397 // Handle the NOT form of XOR.
398 if (I->getOpcode() == Instruction::Xor &&
399 isa<ConstantInt>(I->getOperand(1)) &&
400 cast<ConstantInt>(I->getOperand(1))->isOne()) {
401 ComputeValueKnownInPredecessors(I->getOperand(0), BB, Result);
402 if (Result.empty()) {
403 RecursionSet.erase(std::make_pair(V, BB));
407 // Invert the known values.
408 for (unsigned i = 0, e = Result.size(); i != e; ++i)
411 cast<ConstantInt>(ConstantExpr::getNot(Result[i].first));
413 RecursionSet.erase(std::make_pair(V, BB));
417 // Try to simplify some other binary operator values.
418 } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(I)) {
419 ConstantInt *CI = dyn_cast<ConstantInt>(BO->getOperand(1));
421 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals;
422 ComputeValueKnownInPredecessors(BO->getOperand(0), BB, LHSVals);
424 // Try to use constant folding to simplify the binary operator.
425 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
426 Constant *Folded = 0;
427 if (LHSVals[i].first == 0) {
428 Folded = ConstantExpr::get(BO->getOpcode(),
429 UndefValue::get(BO->getType()),
432 Folded = ConstantExpr::get(BO->getOpcode(), LHSVals[i].first, CI);
435 if (ConstantInt *FoldedCInt = dyn_cast<ConstantInt>(Folded))
436 Result.push_back(std::make_pair(FoldedCInt, LHSVals[i].second));
437 else if (isa<UndefValue>(Folded))
438 Result.push_back(std::make_pair((ConstantInt*)0, LHSVals[i].second));
442 RecursionSet.erase(std::make_pair(V, BB));
443 return !Result.empty();
446 // Handle compare with phi operand, where the PHI is defined in this block.
447 if (CmpInst *Cmp = dyn_cast<CmpInst>(I)) {
448 PHINode *PN = dyn_cast<PHINode>(Cmp->getOperand(0));
449 if (PN && PN->getParent() == BB) {
450 // We can do this simplification if any comparisons fold to true or false.
452 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
453 BasicBlock *PredBB = PN->getIncomingBlock(i);
454 Value *LHS = PN->getIncomingValue(i);
455 Value *RHS = Cmp->getOperand(1)->DoPHITranslation(BB, PredBB);
457 Value *Res = SimplifyCmpInst(Cmp->getPredicate(), LHS, RHS, TD);
459 if (!LVI || !isa<Constant>(RHS))
462 LazyValueInfo::Tristate
463 ResT = LVI->getPredicateOnEdge(Cmp->getPredicate(), LHS,
464 cast<Constant>(RHS), PredBB, BB);
465 if (ResT == LazyValueInfo::Unknown)
467 Res = ConstantInt::get(Type::getInt1Ty(LHS->getContext()), ResT);
470 if (isa<UndefValue>(Res))
471 Result.push_back(std::make_pair((ConstantInt*)0, PredBB));
472 else if (ConstantInt *CI = dyn_cast<ConstantInt>(Res))
473 Result.push_back(std::make_pair(CI, PredBB));
476 RecursionSet.erase(std::make_pair(V, BB));
477 return !Result.empty();
481 // If comparing a live-in value against a constant, see if we know the
482 // live-in value on any predecessors.
483 if (LVI && isa<Constant>(Cmp->getOperand(1)) &&
484 Cmp->getType()->isIntegerTy()) {
485 if (!isa<Instruction>(Cmp->getOperand(0)) ||
486 cast<Instruction>(Cmp->getOperand(0))->getParent() != BB) {
487 Constant *RHSCst = cast<Constant>(Cmp->getOperand(1));
489 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB);PI != E; ++PI){
491 // If the value is known by LazyValueInfo to be a constant in a
492 // predecessor, use that information to try to thread this block.
493 LazyValueInfo::Tristate Res =
494 LVI->getPredicateOnEdge(Cmp->getPredicate(), Cmp->getOperand(0),
496 if (Res == LazyValueInfo::Unknown)
499 Constant *ResC = ConstantInt::get(Cmp->getType(), Res);
500 Result.push_back(std::make_pair(cast<ConstantInt>(ResC), P));
503 RecursionSet.erase(std::make_pair(V, BB));
504 return !Result.empty();
507 // Try to find a constant value for the LHS of a comparison,
508 // and evaluate it statically if we can.
509 if (Constant *CmpConst = dyn_cast<Constant>(Cmp->getOperand(1))) {
510 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> LHSVals;
511 ComputeValueKnownInPredecessors(I->getOperand(0), BB, LHSVals);
513 for (unsigned i = 0, e = LHSVals.size(); i != e; ++i) {
514 Constant * Folded = 0;
515 if (LHSVals[i].first == 0)
516 Folded = ConstantExpr::getCompare(Cmp->getPredicate(),
517 UndefValue::get(CmpConst->getType()), CmpConst);
519 Folded = ConstantExpr::getCompare(Cmp->getPredicate(),
520 LHSVals[i].first, CmpConst);
522 if (ConstantInt *FoldedCInt = dyn_cast<ConstantInt>(Folded))
523 Result.push_back(std::make_pair(FoldedCInt, LHSVals[i].second));
524 else if (isa<UndefValue>(Folded))
525 Result.push_back(std::make_pair((ConstantInt*)0,LHSVals[i].second));
528 RecursionSet.erase(std::make_pair(V, BB));
529 return !Result.empty();
535 // If all else fails, see if LVI can figure out a constant value for us.
536 Constant *CI = LVI->getConstant(V, BB);
537 ConstantInt *CInt = dyn_cast_or_null<ConstantInt>(CI);
539 for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
540 Result.push_back(std::make_pair(CInt, *PI));
543 RecursionSet.erase(std::make_pair(V, BB));
544 return !Result.empty();
547 RecursionSet.erase(std::make_pair(V, BB));
553 /// GetBestDestForBranchOnUndef - If we determine that the specified block ends
554 /// in an undefined jump, decide which block is best to revector to.
556 /// Since we can pick an arbitrary destination, we pick the successor with the
557 /// fewest predecessors. This should reduce the in-degree of the others.
559 static unsigned GetBestDestForJumpOnUndef(BasicBlock *BB) {
560 TerminatorInst *BBTerm = BB->getTerminator();
561 unsigned MinSucc = 0;
562 BasicBlock *TestBB = BBTerm->getSuccessor(MinSucc);
563 // Compute the successor with the minimum number of predecessors.
564 unsigned MinNumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
565 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
566 TestBB = BBTerm->getSuccessor(i);
567 unsigned NumPreds = std::distance(pred_begin(TestBB), pred_end(TestBB));
568 if (NumPreds < MinNumPreds)
575 /// ProcessBlock - If there are any predecessors whose control can be threaded
576 /// through to a successor, transform them now.
577 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
578 // If the block is trivially dead, just return and let the caller nuke it.
579 // This simplifies other transformations.
580 if (pred_begin(BB) == pred_end(BB) &&
581 BB != &BB->getParent()->getEntryBlock())
584 // If this block has a single predecessor, and if that pred has a single
585 // successor, merge the blocks. This encourages recursive jump threading
586 // because now the condition in this block can be threaded through
587 // predecessors of our predecessor block.
588 if (BasicBlock *SinglePred = BB->getSinglePredecessor()) {
589 if (SinglePred->getTerminator()->getNumSuccessors() == 1 &&
591 // If SinglePred was a loop header, BB becomes one.
592 if (LoopHeaders.erase(SinglePred))
593 LoopHeaders.insert(BB);
595 // Remember if SinglePred was the entry block of the function. If so, we
596 // will need to move BB back to the entry position.
597 bool isEntry = SinglePred == &SinglePred->getParent()->getEntryBlock();
598 if (LVI) LVI->eraseBlock(SinglePred);
599 MergeBasicBlockIntoOnlyPred(BB);
601 if (isEntry && BB != &BB->getParent()->getEntryBlock())
602 BB->moveBefore(&BB->getParent()->getEntryBlock());
607 // Look to see if the terminator is a branch of switch, if not we can't thread
610 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
611 // Can't thread an unconditional jump.
612 if (BI->isUnconditional()) return false;
613 Condition = BI->getCondition();
614 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
615 Condition = SI->getCondition();
617 return false; // Must be an invoke.
619 // If the terminator of this block is branching on a constant, simplify the
620 // terminator to an unconditional branch. This can occur due to threading in
622 if (isa<ConstantInt>(Condition)) {
623 DEBUG(dbgs() << " In block '" << BB->getName()
624 << "' folding terminator: " << *BB->getTerminator() << '\n');
626 ConstantFoldTerminator(BB);
630 // If the terminator is branching on an undef, we can pick any of the
631 // successors to branch to. Let GetBestDestForJumpOnUndef decide.
632 if (isa<UndefValue>(Condition)) {
633 unsigned BestSucc = GetBestDestForJumpOnUndef(BB);
635 // Fold the branch/switch.
636 TerminatorInst *BBTerm = BB->getTerminator();
637 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
638 if (i == BestSucc) continue;
639 RemovePredecessorAndSimplify(BBTerm->getSuccessor(i), BB, TD);
642 DEBUG(dbgs() << " In block '" << BB->getName()
643 << "' folding undef terminator: " << *BBTerm << '\n');
644 BranchInst::Create(BBTerm->getSuccessor(BestSucc), BBTerm);
645 BBTerm->eraseFromParent();
649 Instruction *CondInst = dyn_cast<Instruction>(Condition);
651 // If the condition is an instruction defined in another block, see if a
652 // predecessor has the same condition:
657 !Condition->hasOneUse() && // Multiple uses.
658 (CondInst == 0 || CondInst->getParent() != BB)) { // Non-local definition.
659 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
660 if (isa<BranchInst>(BB->getTerminator())) {
661 for (; PI != E; ++PI) {
663 if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator()))
664 if (PBI->isConditional() && PBI->getCondition() == Condition &&
665 ProcessBranchOnDuplicateCond(P, BB))
669 assert(isa<SwitchInst>(BB->getTerminator()) && "Unknown jump terminator");
670 for (; PI != E; ++PI) {
672 if (SwitchInst *PSI = dyn_cast<SwitchInst>(P->getTerminator()))
673 if (PSI->getCondition() == Condition &&
674 ProcessSwitchOnDuplicateCond(P, BB))
680 // All the rest of our checks depend on the condition being an instruction.
682 // FIXME: Unify this with code below.
683 if (LVI && ProcessThreadableEdges(Condition, BB))
689 if (CmpInst *CondCmp = dyn_cast<CmpInst>(CondInst)) {
691 (!isa<PHINode>(CondCmp->getOperand(0)) ||
692 cast<PHINode>(CondCmp->getOperand(0))->getParent() != BB)) {
693 // If we have a comparison, loop over the predecessors to see if there is
694 // a condition with a lexically identical value.
695 pred_iterator PI = pred_begin(BB), E = pred_end(BB);
696 for (; PI != E; ++PI) {
698 if (BranchInst *PBI = dyn_cast<BranchInst>(P->getTerminator()))
699 if (PBI->isConditional() && P != BB) {
700 if (CmpInst *CI = dyn_cast<CmpInst>(PBI->getCondition())) {
701 if (CI->getOperand(0) == CondCmp->getOperand(0) &&
702 CI->getOperand(1) == CondCmp->getOperand(1) &&
703 CI->getPredicate() == CondCmp->getPredicate()) {
704 // TODO: Could handle things like (x != 4) --> (x == 17)
705 if (ProcessBranchOnDuplicateCond(P, BB))
713 // For a comparison where the LHS is outside this block, it's possible
714 // that we've branched on it before. Used LVI to see if we can simplify
715 // the branch based on that.
716 BranchInst *CondBr = dyn_cast<BranchInst>(BB->getTerminator());
717 Constant *CondConst = dyn_cast<Constant>(CondCmp->getOperand(1));
718 pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
719 if (LVI && CondBr && CondConst && CondBr->isConditional() && PI != PE &&
720 (!isa<Instruction>(CondCmp->getOperand(0)) ||
721 cast<Instruction>(CondCmp->getOperand(0))->getParent() != BB)) {
722 // For predecessor edge, determine if the comparison is true or false
723 // on that edge. If they're all true or all false, we can simplify the
725 // FIXME: We could handle mixed true/false by duplicating code.
726 LazyValueInfo::Tristate Baseline =
727 LVI->getPredicateOnEdge(CondCmp->getPredicate(), CondCmp->getOperand(0),
729 if (Baseline != LazyValueInfo::Unknown) {
730 // Check that all remaining incoming values match the first one.
732 LazyValueInfo::Tristate Ret = LVI->getPredicateOnEdge(
733 CondCmp->getPredicate(),
734 CondCmp->getOperand(0),
736 if (Ret != Baseline) break;
739 // If we terminated early, then one of the values didn't match.
741 unsigned ToRemove = Baseline == LazyValueInfo::True ? 1 : 0;
742 unsigned ToKeep = Baseline == LazyValueInfo::True ? 0 : 1;
743 RemovePredecessorAndSimplify(CondBr->getSuccessor(ToRemove), BB, TD);
744 BranchInst::Create(CondBr->getSuccessor(ToKeep), CondBr);
745 CondBr->eraseFromParent();
752 // Check for some cases that are worth simplifying. Right now we want to look
753 // for loads that are used by a switch or by the condition for the branch. If
754 // we see one, check to see if it's partially redundant. If so, insert a PHI
755 // which can then be used to thread the values.
757 Value *SimplifyValue = CondInst;
758 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
759 if (isa<Constant>(CondCmp->getOperand(1)))
760 SimplifyValue = CondCmp->getOperand(0);
762 // TODO: There are other places where load PRE would be profitable, such as
763 // more complex comparisons.
764 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
765 if (SimplifyPartiallyRedundantLoad(LI))
769 // Handle a variety of cases where we are branching on something derived from
770 // a PHI node in the current block. If we can prove that any predecessors
771 // compute a predictable value based on a PHI node, thread those predecessors.
773 if (ProcessThreadableEdges(CondInst, BB))
776 // If this is an otherwise-unfoldable branch on a phi node in the current
777 // block, see if we can simplify.
778 if (PHINode *PN = dyn_cast<PHINode>(CondInst))
779 if (PN->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
780 return ProcessBranchOnPHI(PN);
783 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
784 if (CondInst->getOpcode() == Instruction::Xor &&
785 CondInst->getParent() == BB && isa<BranchInst>(BB->getTerminator()))
786 return ProcessBranchOnXOR(cast<BinaryOperator>(CondInst));
789 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know
790 // "(X == 4)", thread through this block.
795 /// ProcessBranchOnDuplicateCond - We found a block and a predecessor of that
796 /// block that jump on exactly the same condition. This means that we almost
797 /// always know the direction of the edge in the DESTBB:
799 /// br COND, DESTBB, BBY
801 /// br COND, BBZ, BBW
803 /// If DESTBB has multiple predecessors, we can't just constant fold the branch
804 /// in DESTBB, we have to thread over it.
805 bool JumpThreading::ProcessBranchOnDuplicateCond(BasicBlock *PredBB,
807 BranchInst *PredBI = cast<BranchInst>(PredBB->getTerminator());
809 // If both successors of PredBB go to DESTBB, we don't know anything. We can
810 // fold the branch to an unconditional one, which allows other recursive
813 if (PredBI->getSuccessor(1) != BB)
815 else if (PredBI->getSuccessor(0) != BB)
818 DEBUG(dbgs() << " In block '" << PredBB->getName()
819 << "' folding terminator: " << *PredBB->getTerminator() << '\n');
821 ConstantFoldTerminator(PredBB);
825 BranchInst *DestBI = cast<BranchInst>(BB->getTerminator());
827 // If the dest block has one predecessor, just fix the branch condition to a
828 // constant and fold it.
829 if (BB->getSinglePredecessor()) {
830 DEBUG(dbgs() << " In block '" << BB->getName()
831 << "' folding condition to '" << BranchDir << "': "
832 << *BB->getTerminator() << '\n');
834 Value *OldCond = DestBI->getCondition();
835 DestBI->setCondition(ConstantInt::get(Type::getInt1Ty(BB->getContext()),
837 // Delete dead instructions before we fold the branch. Folding the branch
838 // can eliminate edges from the CFG which can end up deleting OldCond.
839 RecursivelyDeleteTriviallyDeadInstructions(OldCond);
840 ConstantFoldTerminator(BB);
845 // Next, figure out which successor we are threading to.
846 BasicBlock *SuccBB = DestBI->getSuccessor(!BranchDir);
848 SmallVector<BasicBlock*, 2> Preds;
849 Preds.push_back(PredBB);
851 // Ok, try to thread it!
852 return ThreadEdge(BB, Preds, SuccBB);
855 /// ProcessSwitchOnDuplicateCond - We found a block and a predecessor of that
856 /// block that switch on exactly the same condition. This means that we almost
857 /// always know the direction of the edge in the DESTBB:
859 /// switch COND [... DESTBB, BBY ... ]
861 /// switch COND [... BBZ, BBW ]
863 /// Optimizing switches like this is very important, because simplifycfg builds
864 /// switches out of repeated 'if' conditions.
865 bool JumpThreading::ProcessSwitchOnDuplicateCond(BasicBlock *PredBB,
866 BasicBlock *DestBB) {
867 // Can't thread edge to self.
868 if (PredBB == DestBB)
871 SwitchInst *PredSI = cast<SwitchInst>(PredBB->getTerminator());
872 SwitchInst *DestSI = cast<SwitchInst>(DestBB->getTerminator());
874 // There are a variety of optimizations that we can potentially do on these
875 // blocks: we order them from most to least preferable.
877 // If DESTBB *just* contains the switch, then we can forward edges from PREDBB
878 // directly to their destination. This does not introduce *any* code size
879 // growth. Skip debug info first.
880 BasicBlock::iterator BBI = DestBB->begin();
881 while (isa<DbgInfoIntrinsic>(BBI))
884 // FIXME: Thread if it just contains a PHI.
885 if (isa<SwitchInst>(BBI)) {
886 bool MadeChange = false;
887 // Ignore the default edge for now.
888 for (unsigned i = 1, e = DestSI->getNumSuccessors(); i != e; ++i) {
889 ConstantInt *DestVal = DestSI->getCaseValue(i);
890 BasicBlock *DestSucc = DestSI->getSuccessor(i);
892 // Okay, DestSI has a case for 'DestVal' that goes to 'DestSucc'. See if
893 // PredSI has an explicit case for it. If so, forward. If it is covered
894 // by the default case, we can't update PredSI.
895 unsigned PredCase = PredSI->findCaseValue(DestVal);
896 if (PredCase == 0) continue;
898 // If PredSI doesn't go to DestBB on this value, then it won't reach the
899 // case on this condition.
900 if (PredSI->getSuccessor(PredCase) != DestBB &&
901 DestSI->getSuccessor(i) != DestBB)
904 // Do not forward this if it already goes to this destination, this would
905 // be an infinite loop.
906 if (PredSI->getSuccessor(PredCase) == DestSucc)
909 // Otherwise, we're safe to make the change. Make sure that the edge from
910 // DestSI to DestSucc is not critical and has no PHI nodes.
911 DEBUG(dbgs() << "FORWARDING EDGE " << *DestVal << " FROM: " << *PredSI);
912 DEBUG(dbgs() << "THROUGH: " << *DestSI);
914 // If the destination has PHI nodes, just split the edge for updating
916 if (isa<PHINode>(DestSucc->begin()) && !DestSucc->getSinglePredecessor()){
917 SplitCriticalEdge(DestSI, i, this);
918 DestSucc = DestSI->getSuccessor(i);
920 FoldSingleEntryPHINodes(DestSucc);
921 PredSI->setSuccessor(PredCase, DestSucc);
933 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
934 /// load instruction, eliminate it by replacing it with a PHI node. This is an
935 /// important optimization that encourages jump threading, and needs to be run
936 /// interlaced with other jump threading tasks.
937 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
938 // Don't hack volatile loads.
939 if (LI->isVolatile()) return false;
941 // If the load is defined in a block with exactly one predecessor, it can't be
942 // partially redundant.
943 BasicBlock *LoadBB = LI->getParent();
944 if (LoadBB->getSinglePredecessor())
947 Value *LoadedPtr = LI->getOperand(0);
949 // If the loaded operand is defined in the LoadBB, it can't be available.
950 // TODO: Could do simple PHI translation, that would be fun :)
951 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
952 if (PtrOp->getParent() == LoadBB)
955 // Scan a few instructions up from the load, to see if it is obviously live at
956 // the entry to its block.
957 BasicBlock::iterator BBIt = LI;
959 if (Value *AvailableVal =
960 FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt, 6)) {
961 // If the value if the load is locally available within the block, just use
962 // it. This frequently occurs for reg2mem'd allocas.
963 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
965 // If the returned value is the load itself, replace with an undef. This can
966 // only happen in dead loops.
967 if (AvailableVal == LI) AvailableVal = UndefValue::get(LI->getType());
968 LI->replaceAllUsesWith(AvailableVal);
969 LI->eraseFromParent();
973 // Otherwise, if we scanned the whole block and got to the top of the block,
974 // we know the block is locally transparent to the load. If not, something
975 // might clobber its value.
976 if (BBIt != LoadBB->begin())
980 SmallPtrSet<BasicBlock*, 8> PredsScanned;
981 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
982 AvailablePredsTy AvailablePreds;
983 BasicBlock *OneUnavailablePred = 0;
985 // If we got here, the loaded value is transparent through to the start of the
986 // block. Check to see if it is available in any of the predecessor blocks.
987 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
989 BasicBlock *PredBB = *PI;
991 // If we already scanned this predecessor, skip it.
992 if (!PredsScanned.insert(PredBB))
995 // Scan the predecessor to see if the value is available in the pred.
996 BBIt = PredBB->end();
997 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt, 6);
998 if (!PredAvailable) {
999 OneUnavailablePred = PredBB;
1003 // If so, this load is partially redundant. Remember this info so that we
1004 // can create a PHI node.
1005 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
1008 // If the loaded value isn't available in any predecessor, it isn't partially
1010 if (AvailablePreds.empty()) return false;
1012 // Okay, the loaded value is available in at least one (and maybe all!)
1013 // predecessors. If the value is unavailable in more than one unique
1014 // predecessor, we want to insert a merge block for those common predecessors.
1015 // This ensures that we only have to insert one reload, thus not increasing
1017 BasicBlock *UnavailablePred = 0;
1019 // If there is exactly one predecessor where the value is unavailable, the
1020 // already computed 'OneUnavailablePred' block is it. If it ends in an
1021 // unconditional branch, we know that it isn't a critical edge.
1022 if (PredsScanned.size() == AvailablePreds.size()+1 &&
1023 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
1024 UnavailablePred = OneUnavailablePred;
1025 } else if (PredsScanned.size() != AvailablePreds.size()) {
1026 // Otherwise, we had multiple unavailable predecessors or we had a critical
1027 // edge from the one.
1028 SmallVector<BasicBlock*, 8> PredsToSplit;
1029 SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
1031 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
1032 AvailablePredSet.insert(AvailablePreds[i].first);
1034 // Add all the unavailable predecessors to the PredsToSplit list.
1035 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
1037 BasicBlock *P = *PI;
1038 // If the predecessor is an indirect goto, we can't split the edge.
1039 if (isa<IndirectBrInst>(P->getTerminator()))
1042 if (!AvailablePredSet.count(P))
1043 PredsToSplit.push_back(P);
1046 // Split them out to their own block.
1048 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
1049 "thread-pre-split", this);
1052 // If the value isn't available in all predecessors, then there will be
1053 // exactly one where it isn't available. Insert a load on that edge and add
1054 // it to the AvailablePreds list.
1055 if (UnavailablePred) {
1056 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
1057 "Can't handle critical edge here!");
1058 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr", false,
1060 UnavailablePred->getTerminator());
1061 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
1064 // Now we know that each predecessor of this block has a value in
1065 // AvailablePreds, sort them for efficient access as we're walking the preds.
1066 array_pod_sort(AvailablePreds.begin(), AvailablePreds.end());
1068 // Create a PHI node at the start of the block for the PRE'd load value.
1069 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
1072 // Insert new entries into the PHI for each predecessor. A single block may
1073 // have multiple entries here.
1074 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
1076 BasicBlock *P = *PI;
1077 AvailablePredsTy::iterator I =
1078 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
1079 std::make_pair(P, (Value*)0));
1081 assert(I != AvailablePreds.end() && I->first == P &&
1082 "Didn't find entry for predecessor!");
1084 PN->addIncoming(I->second, I->first);
1087 //cerr << "PRE: " << *LI << *PN << "\n";
1089 LI->replaceAllUsesWith(PN);
1090 LI->eraseFromParent();
1095 /// FindMostPopularDest - The specified list contains multiple possible
1096 /// threadable destinations. Pick the one that occurs the most frequently in
1099 FindMostPopularDest(BasicBlock *BB,
1100 const SmallVectorImpl<std::pair<BasicBlock*,
1101 BasicBlock*> > &PredToDestList) {
1102 assert(!PredToDestList.empty());
1104 // Determine popularity. If there are multiple possible destinations, we
1105 // explicitly choose to ignore 'undef' destinations. We prefer to thread
1106 // blocks with known and real destinations to threading undef. We'll handle
1107 // them later if interesting.
1108 DenseMap<BasicBlock*, unsigned> DestPopularity;
1109 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1110 if (PredToDestList[i].second)
1111 DestPopularity[PredToDestList[i].second]++;
1113 // Find the most popular dest.
1114 DenseMap<BasicBlock*, unsigned>::iterator DPI = DestPopularity.begin();
1115 BasicBlock *MostPopularDest = DPI->first;
1116 unsigned Popularity = DPI->second;
1117 SmallVector<BasicBlock*, 4> SamePopularity;
1119 for (++DPI; DPI != DestPopularity.end(); ++DPI) {
1120 // If the popularity of this entry isn't higher than the popularity we've
1121 // seen so far, ignore it.
1122 if (DPI->second < Popularity)
1124 else if (DPI->second == Popularity) {
1125 // If it is the same as what we've seen so far, keep track of it.
1126 SamePopularity.push_back(DPI->first);
1128 // If it is more popular, remember it.
1129 SamePopularity.clear();
1130 MostPopularDest = DPI->first;
1131 Popularity = DPI->second;
1135 // Okay, now we know the most popular destination. If there is more than
1136 // destination, we need to determine one. This is arbitrary, but we need
1137 // to make a deterministic decision. Pick the first one that appears in the
1139 if (!SamePopularity.empty()) {
1140 SamePopularity.push_back(MostPopularDest);
1141 TerminatorInst *TI = BB->getTerminator();
1142 for (unsigned i = 0; ; ++i) {
1143 assert(i != TI->getNumSuccessors() && "Didn't find any successor!");
1145 if (std::find(SamePopularity.begin(), SamePopularity.end(),
1146 TI->getSuccessor(i)) == SamePopularity.end())
1149 MostPopularDest = TI->getSuccessor(i);
1154 // Okay, we have finally picked the most popular destination.
1155 return MostPopularDest;
1158 bool JumpThreading::ProcessThreadableEdges(Value *Cond, BasicBlock *BB) {
1159 // If threading this would thread across a loop header, don't even try to
1161 if (LoopHeaders.count(BB))
1164 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> PredValues;
1165 if (!ComputeValueKnownInPredecessors(Cond, BB, PredValues)) {
1168 assert(!PredValues.empty() &&
1169 "ComputeValueKnownInPredecessors returned true with no values");
1171 DEBUG(dbgs() << "IN BB: " << *BB;
1172 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1173 dbgs() << " BB '" << BB->getName() << "': FOUND condition = ";
1174 if (PredValues[i].first)
1175 dbgs() << *PredValues[i].first;
1178 dbgs() << " for pred '" << PredValues[i].second->getName()
1182 // Decide what we want to thread through. Convert our list of known values to
1183 // a list of known destinations for each pred. This also discards duplicate
1184 // predecessors and keeps track of the undefined inputs (which are represented
1185 // as a null dest in the PredToDestList).
1186 SmallPtrSet<BasicBlock*, 16> SeenPreds;
1187 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
1189 BasicBlock *OnlyDest = 0;
1190 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1192 for (unsigned i = 0, e = PredValues.size(); i != e; ++i) {
1193 BasicBlock *Pred = PredValues[i].second;
1194 if (!SeenPreds.insert(Pred))
1195 continue; // Duplicate predecessor entry.
1197 // If the predecessor ends with an indirect goto, we can't change its
1199 if (isa<IndirectBrInst>(Pred->getTerminator()))
1202 ConstantInt *Val = PredValues[i].first;
1205 if (Val == 0) // Undef.
1207 else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
1208 DestBB = BI->getSuccessor(Val->isZero());
1210 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
1211 DestBB = SI->getSuccessor(SI->findCaseValue(Val));
1214 // If we have exactly one destination, remember it for efficiency below.
1217 else if (OnlyDest != DestBB)
1218 OnlyDest = MultipleDestSentinel;
1220 PredToDestList.push_back(std::make_pair(Pred, DestBB));
1223 // If all edges were unthreadable, we fail.
1224 if (PredToDestList.empty())
1227 // Determine which is the most common successor. If we have many inputs and
1228 // this block is a switch, we want to start by threading the batch that goes
1229 // to the most popular destination first. If we only know about one
1230 // threadable destination (the common case) we can avoid this.
1231 BasicBlock *MostPopularDest = OnlyDest;
1233 if (MostPopularDest == MultipleDestSentinel)
1234 MostPopularDest = FindMostPopularDest(BB, PredToDestList);
1236 // Now that we know what the most popular destination is, factor all
1237 // predecessors that will jump to it into a single predecessor.
1238 SmallVector<BasicBlock*, 16> PredsToFactor;
1239 for (unsigned i = 0, e = PredToDestList.size(); i != e; ++i)
1240 if (PredToDestList[i].second == MostPopularDest) {
1241 BasicBlock *Pred = PredToDestList[i].first;
1243 // This predecessor may be a switch or something else that has multiple
1244 // edges to the block. Factor each of these edges by listing them
1245 // according to # occurrences in PredsToFactor.
1246 TerminatorInst *PredTI = Pred->getTerminator();
1247 for (unsigned i = 0, e = PredTI->getNumSuccessors(); i != e; ++i)
1248 if (PredTI->getSuccessor(i) == BB)
1249 PredsToFactor.push_back(Pred);
1252 // If the threadable edges are branching on an undefined value, we get to pick
1253 // the destination that these predecessors should get to.
1254 if (MostPopularDest == 0)
1255 MostPopularDest = BB->getTerminator()->
1256 getSuccessor(GetBestDestForJumpOnUndef(BB));
1258 // Ok, try to thread it!
1259 return ThreadEdge(BB, PredsToFactor, MostPopularDest);
1262 /// ProcessBranchOnPHI - We have an otherwise unthreadable conditional branch on
1263 /// a PHI node in the current block. See if there are any simplifications we
1264 /// can do based on inputs to the phi node.
1266 bool JumpThreading::ProcessBranchOnPHI(PHINode *PN) {
1267 BasicBlock *BB = PN->getParent();
1269 // TODO: We could make use of this to do it once for blocks with common PHI
1271 SmallVector<BasicBlock*, 1> PredBBs;
1274 // If any of the predecessor blocks end in an unconditional branch, we can
1275 // *duplicate* the conditional branch into that block in order to further
1276 // encourage jump threading and to eliminate cases where we have branch on a
1277 // phi of an icmp (branch on icmp is much better).
1278 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1279 BasicBlock *PredBB = PN->getIncomingBlock(i);
1280 if (BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator()))
1281 if (PredBr->isUnconditional()) {
1282 PredBBs[0] = PredBB;
1283 // Try to duplicate BB into PredBB.
1284 if (DuplicateCondBranchOnPHIIntoPred(BB, PredBBs))
1292 /// ProcessBranchOnXOR - We have an otherwise unthreadable conditional branch on
1293 /// a xor instruction in the current block. See if there are any
1294 /// simplifications we can do based on inputs to the xor.
1296 bool JumpThreading::ProcessBranchOnXOR(BinaryOperator *BO) {
1297 BasicBlock *BB = BO->getParent();
1299 // If either the LHS or RHS of the xor is a constant, don't do this
1301 if (isa<ConstantInt>(BO->getOperand(0)) ||
1302 isa<ConstantInt>(BO->getOperand(1)))
1305 // If the first instruction in BB isn't a phi, we won't be able to infer
1306 // anything special about any particular predecessor.
1307 if (!isa<PHINode>(BB->front()))
1310 // If we have a xor as the branch input to this block, and we know that the
1311 // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1312 // the condition into the predecessor and fix that value to true, saving some
1313 // logical ops on that path and encouraging other paths to simplify.
1315 // This copies something like this:
1318 // %X = phi i1 [1], [%X']
1319 // %Y = icmp eq i32 %A, %B
1320 // %Z = xor i1 %X, %Y
1325 // %Y = icmp ne i32 %A, %B
1328 SmallVector<std::pair<ConstantInt*, BasicBlock*>, 8> XorOpValues;
1330 if (!ComputeValueKnownInPredecessors(BO->getOperand(0), BB, XorOpValues)) {
1331 assert(XorOpValues.empty());
1332 if (!ComputeValueKnownInPredecessors(BO->getOperand(1), BB, XorOpValues))
1337 assert(!XorOpValues.empty() &&
1338 "ComputeValueKnownInPredecessors returned true with no values");
1340 // Scan the information to see which is most popular: true or false. The
1341 // predecessors can be of the set true, false, or undef.
1342 unsigned NumTrue = 0, NumFalse = 0;
1343 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1344 if (!XorOpValues[i].first) continue; // Ignore undefs for the count.
1345 if (XorOpValues[i].first->isZero())
1351 // Determine which value to split on, true, false, or undef if neither.
1352 ConstantInt *SplitVal = 0;
1353 if (NumTrue > NumFalse)
1354 SplitVal = ConstantInt::getTrue(BB->getContext());
1355 else if (NumTrue != 0 || NumFalse != 0)
1356 SplitVal = ConstantInt::getFalse(BB->getContext());
1358 // Collect all of the blocks that this can be folded into so that we can
1359 // factor this once and clone it once.
1360 SmallVector<BasicBlock*, 8> BlocksToFoldInto;
1361 for (unsigned i = 0, e = XorOpValues.size(); i != e; ++i) {
1362 if (XorOpValues[i].first != SplitVal && XorOpValues[i].first != 0) continue;
1364 BlocksToFoldInto.push_back(XorOpValues[i].second);
1367 // If we inferred a value for all of the predecessors, then duplication won't
1368 // help us. However, we can just replace the LHS or RHS with the constant.
1369 if (BlocksToFoldInto.size() ==
1370 cast<PHINode>(BB->front()).getNumIncomingValues()) {
1371 if (SplitVal == 0) {
1372 // If all preds provide undef, just nuke the xor, because it is undef too.
1373 BO->replaceAllUsesWith(UndefValue::get(BO->getType()));
1374 BO->eraseFromParent();
1375 } else if (SplitVal->isZero()) {
1376 // If all preds provide 0, replace the xor with the other input.
1377 BO->replaceAllUsesWith(BO->getOperand(isLHS));
1378 BO->eraseFromParent();
1380 // If all preds provide 1, set the computed value to 1.
1381 BO->setOperand(!isLHS, SplitVal);
1387 // Try to duplicate BB into PredBB.
1388 return DuplicateCondBranchOnPHIIntoPred(BB, BlocksToFoldInto);
1392 /// AddPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1393 /// predecessor to the PHIBB block. If it has PHI nodes, add entries for
1394 /// NewPred using the entries from OldPred (suitably mapped).
1395 static void AddPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1396 BasicBlock *OldPred,
1397 BasicBlock *NewPred,
1398 DenseMap<Instruction*, Value*> &ValueMap) {
1399 for (BasicBlock::iterator PNI = PHIBB->begin();
1400 PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
1401 // Ok, we have a PHI node. Figure out what the incoming value was for the
1403 Value *IV = PN->getIncomingValueForBlock(OldPred);
1405 // Remap the value if necessary.
1406 if (Instruction *Inst = dyn_cast<Instruction>(IV)) {
1407 DenseMap<Instruction*, Value*>::iterator I = ValueMap.find(Inst);
1408 if (I != ValueMap.end())
1412 PN->addIncoming(IV, NewPred);
1416 /// ThreadEdge - We have decided that it is safe and profitable to factor the
1417 /// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
1418 /// across BB. Transform the IR to reflect this change.
1419 bool JumpThreading::ThreadEdge(BasicBlock *BB,
1420 const SmallVectorImpl<BasicBlock*> &PredBBs,
1421 BasicBlock *SuccBB) {
1422 // If threading to the same block as we come from, we would infinite loop.
1424 DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
1425 << "' - would thread to self!\n");
1429 // If threading this would thread across a loop header, don't thread the edge.
1430 // See the comments above FindLoopHeaders for justifications and caveats.
1431 if (LoopHeaders.count(BB)) {
1432 DEBUG(dbgs() << " Not threading across loop header BB '" << BB->getName()
1433 << "' to dest BB '" << SuccBB->getName()
1434 << "' - it might create an irreducible loop!\n");
1438 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
1439 if (JumpThreadCost > Threshold) {
1440 DEBUG(dbgs() << " Not threading BB '" << BB->getName()
1441 << "' - Cost is too high: " << JumpThreadCost << "\n");
1445 // And finally, do it! Start by factoring the predecessors is needed.
1447 if (PredBBs.size() == 1)
1448 PredBB = PredBBs[0];
1450 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1451 << " common predecessors.\n");
1452 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1456 // And finally, do it!
1457 DEBUG(dbgs() << " Threading edge from '" << PredBB->getName() << "' to '"
1458 << SuccBB->getName() << "' with cost: " << JumpThreadCost
1459 << ", across block:\n "
1463 LVI->threadEdge(PredBB, BB, SuccBB);
1465 // We are going to have to map operands from the original BB block to the new
1466 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
1467 // account for entry from PredBB.
1468 DenseMap<Instruction*, Value*> ValueMapping;
1470 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(),
1471 BB->getName()+".thread",
1472 BB->getParent(), BB);
1473 NewBB->moveAfter(PredBB);
1475 BasicBlock::iterator BI = BB->begin();
1476 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1477 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1479 // Clone the non-phi instructions of BB into NewBB, keeping track of the
1480 // mapping and using it to remap operands in the cloned instructions.
1481 for (; !isa<TerminatorInst>(BI); ++BI) {
1482 Instruction *New = BI->clone();
1483 New->setName(BI->getName());
1484 NewBB->getInstList().push_back(New);
1485 ValueMapping[BI] = New;
1487 // Remap operands to patch up intra-block references.
1488 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1489 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1490 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1491 if (I != ValueMapping.end())
1492 New->setOperand(i, I->second);
1496 // We didn't copy the terminator from BB over to NewBB, because there is now
1497 // an unconditional jump to SuccBB. Insert the unconditional jump.
1498 BranchInst::Create(SuccBB, NewBB);
1500 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
1501 // PHI nodes for NewBB now.
1502 AddPHINodeEntriesForMappedBlock(SuccBB, BB, NewBB, ValueMapping);
1504 // If there were values defined in BB that are used outside the block, then we
1505 // now have to update all uses of the value to use either the original value,
1506 // the cloned value, or some PHI derived value. This can require arbitrary
1507 // PHI insertion, of which we are prepared to do, clean these up now.
1508 SSAUpdater SSAUpdate;
1509 SmallVector<Use*, 16> UsesToRename;
1510 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1511 // Scan all uses of this instruction to see if it is used outside of its
1512 // block, and if so, record them in UsesToRename.
1513 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1515 Instruction *User = cast<Instruction>(*UI);
1516 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1517 if (UserPN->getIncomingBlock(UI) == BB)
1519 } else if (User->getParent() == BB)
1522 UsesToRename.push_back(&UI.getUse());
1525 // If there are no uses outside the block, we're done with this instruction.
1526 if (UsesToRename.empty())
1529 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1531 // We found a use of I outside of BB. Rename all uses of I that are outside
1532 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1533 // with the two values we know.
1534 SSAUpdate.Initialize(I);
1535 SSAUpdate.AddAvailableValue(BB, I);
1536 SSAUpdate.AddAvailableValue(NewBB, ValueMapping[I]);
1538 while (!UsesToRename.empty())
1539 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1540 DEBUG(dbgs() << "\n");
1544 // Ok, NewBB is good to go. Update the terminator of PredBB to jump to
1545 // NewBB instead of BB. This eliminates predecessors from BB, which requires
1546 // us to simplify any PHI nodes in BB.
1547 TerminatorInst *PredTerm = PredBB->getTerminator();
1548 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
1549 if (PredTerm->getSuccessor(i) == BB) {
1550 RemovePredecessorAndSimplify(BB, PredBB, TD);
1551 PredTerm->setSuccessor(i, NewBB);
1554 // At this point, the IR is fully up to date and consistent. Do a quick scan
1555 // over the new instructions and zap any that are constants or dead. This
1556 // frequently happens because of phi translation.
1557 SimplifyInstructionsInBlock(NewBB, TD);
1559 // Threaded an edge!
1564 /// DuplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
1565 /// to BB which contains an i1 PHI node and a conditional branch on that PHI.
1566 /// If we can duplicate the contents of BB up into PredBB do so now, this
1567 /// improves the odds that the branch will be on an analyzable instruction like
1569 bool JumpThreading::DuplicateCondBranchOnPHIIntoPred(BasicBlock *BB,
1570 const SmallVectorImpl<BasicBlock *> &PredBBs) {
1571 assert(!PredBBs.empty() && "Can't handle an empty set");
1573 // If BB is a loop header, then duplicating this block outside the loop would
1574 // cause us to transform this into an irreducible loop, don't do this.
1575 // See the comments above FindLoopHeaders for justifications and caveats.
1576 if (LoopHeaders.count(BB)) {
1577 DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
1578 << "' into predecessor block '" << PredBBs[0]->getName()
1579 << "' - it might create an irreducible loop!\n");
1583 unsigned DuplicationCost = getJumpThreadDuplicationCost(BB);
1584 if (DuplicationCost > Threshold) {
1585 DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
1586 << "' - Cost is too high: " << DuplicationCost << "\n");
1590 // And finally, do it! Start by factoring the predecessors is needed.
1592 if (PredBBs.size() == 1)
1593 PredBB = PredBBs[0];
1595 DEBUG(dbgs() << " Factoring out " << PredBBs.size()
1596 << " common predecessors.\n");
1597 PredBB = SplitBlockPredecessors(BB, &PredBBs[0], PredBBs.size(),
1601 // Okay, we decided to do this! Clone all the instructions in BB onto the end
1603 DEBUG(dbgs() << " Duplicating block '" << BB->getName() << "' into end of '"
1604 << PredBB->getName() << "' to eliminate branch on phi. Cost: "
1605 << DuplicationCost << " block is:" << *BB << "\n");
1607 // Unless PredBB ends with an unconditional branch, split the edge so that we
1608 // can just clone the bits from BB into the end of the new PredBB.
1609 BranchInst *OldPredBranch = dyn_cast<BranchInst>(PredBB->getTerminator());
1611 if (OldPredBranch == 0 || !OldPredBranch->isUnconditional()) {
1612 PredBB = SplitEdge(PredBB, BB, this);
1613 OldPredBranch = cast<BranchInst>(PredBB->getTerminator());
1616 // We are going to have to map operands from the original BB block into the
1617 // PredBB block. Evaluate PHI nodes in BB.
1618 DenseMap<Instruction*, Value*> ValueMapping;
1620 BasicBlock::iterator BI = BB->begin();
1621 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
1622 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
1624 // Clone the non-phi instructions of BB into PredBB, keeping track of the
1625 // mapping and using it to remap operands in the cloned instructions.
1626 for (; BI != BB->end(); ++BI) {
1627 Instruction *New = BI->clone();
1629 // Remap operands to patch up intra-block references.
1630 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
1631 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i))) {
1632 DenseMap<Instruction*, Value*>::iterator I = ValueMapping.find(Inst);
1633 if (I != ValueMapping.end())
1634 New->setOperand(i, I->second);
1637 // If this instruction can be simplified after the operands are updated,
1638 // just use the simplified value instead. This frequently happens due to
1640 if (Value *IV = SimplifyInstruction(New, TD)) {
1642 ValueMapping[BI] = IV;
1644 // Otherwise, insert the new instruction into the block.
1645 New->setName(BI->getName());
1646 PredBB->getInstList().insert(OldPredBranch, New);
1647 ValueMapping[BI] = New;
1651 // Check to see if the targets of the branch had PHI nodes. If so, we need to
1652 // add entries to the PHI nodes for branch from PredBB now.
1653 BranchInst *BBBranch = cast<BranchInst>(BB->getTerminator());
1654 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(0), BB, PredBB,
1656 AddPHINodeEntriesForMappedBlock(BBBranch->getSuccessor(1), BB, PredBB,
1659 // If there were values defined in BB that are used outside the block, then we
1660 // now have to update all uses of the value to use either the original value,
1661 // the cloned value, or some PHI derived value. This can require arbitrary
1662 // PHI insertion, of which we are prepared to do, clean these up now.
1663 SSAUpdater SSAUpdate;
1664 SmallVector<Use*, 16> UsesToRename;
1665 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
1666 // Scan all uses of this instruction to see if it is used outside of its
1667 // block, and if so, record them in UsesToRename.
1668 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
1670 Instruction *User = cast<Instruction>(*UI);
1671 if (PHINode *UserPN = dyn_cast<PHINode>(User)) {
1672 if (UserPN->getIncomingBlock(UI) == BB)
1674 } else if (User->getParent() == BB)
1677 UsesToRename.push_back(&UI.getUse());
1680 // If there are no uses outside the block, we're done with this instruction.
1681 if (UsesToRename.empty())
1684 DEBUG(dbgs() << "JT: Renaming non-local uses of: " << *I << "\n");
1686 // We found a use of I outside of BB. Rename all uses of I that are outside
1687 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
1688 // with the two values we know.
1689 SSAUpdate.Initialize(I);
1690 SSAUpdate.AddAvailableValue(BB, I);
1691 SSAUpdate.AddAvailableValue(PredBB, ValueMapping[I]);
1693 while (!UsesToRename.empty())
1694 SSAUpdate.RewriteUse(*UsesToRename.pop_back_val());
1695 DEBUG(dbgs() << "\n");
1698 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
1700 RemovePredecessorAndSimplify(BB, PredBB, TD);
1702 // Remove the unconditional branch at the end of the PredBB block.
1703 OldPredBranch->eraseFromParent();