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/Pass.h"
18 #include "llvm/ADT/DenseMap.h"
19 #include "llvm/ADT/Statistic.h"
20 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
21 #include "llvm/Transforms/Utils/Local.h"
22 #include "llvm/Support/CommandLine.h"
23 #include "llvm/Support/Compiler.h"
24 #include "llvm/Support/Debug.h"
25 #include "llvm/ADT/SmallPtrSet.h"
28 STATISTIC(NumThreads, "Number of jumps threaded");
29 STATISTIC(NumFolds, "Number of terminators folded");
31 static cl::opt<unsigned>
32 Threshold("jump-threading-threshold",
33 cl::desc("Max block size to duplicate for jump threading"),
34 cl::init(6), cl::Hidden);
37 /// This pass performs 'jump threading', which looks at blocks that have
38 /// multiple predecessors and multiple successors. If one or more of the
39 /// predecessors of the block can be proven to always jump to one of the
40 /// successors, we forward the edge from the predecessor to the successor by
41 /// duplicating the contents of this block.
43 /// An example of when this can occur is code like this:
50 /// In this case, the unconditional branch at the end of the first if can be
51 /// revectored to the false side of the second if.
53 class VISIBILITY_HIDDEN JumpThreading : public FunctionPass {
55 static char ID; // Pass identification
56 JumpThreading() : FunctionPass(&ID) {}
58 bool runOnFunction(Function &F);
59 bool ProcessBlock(BasicBlock *BB);
60 void ThreadEdge(BasicBlock *BB, BasicBlock *PredBB, BasicBlock *SuccBB);
61 BasicBlock *FactorCommonPHIPreds(PHINode *PN, Constant *CstVal);
63 bool ProcessJumpOnPHI(PHINode *PN);
64 bool ProcessBranchOnLogical(Value *V, BasicBlock *BB, bool isAnd);
65 bool ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB);
67 bool SimplifyPartiallyRedundantLoad(LoadInst *LI);
71 char JumpThreading::ID = 0;
72 static RegisterPass<JumpThreading>
73 X("jump-threading", "Jump Threading");
75 // Public interface to the Jump Threading pass
76 FunctionPass *llvm::createJumpThreadingPass() { return new JumpThreading(); }
78 /// runOnFunction - Top level algorithm.
80 bool JumpThreading::runOnFunction(Function &F) {
81 DOUT << "Jump threading on function '" << F.getNameStart() << "'\n";
83 bool AnotherIteration = true, EverChanged = false;
84 while (AnotherIteration) {
85 AnotherIteration = false;
87 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
88 while (ProcessBlock(I))
90 AnotherIteration = Changed;
91 EverChanged |= Changed;
96 /// FactorCommonPHIPreds - If there are multiple preds with the same incoming
97 /// value for the PHI, factor them together so we get one block to thread for
99 /// This is important for things like "phi i1 [true, true, false, true, x]"
100 /// where we only need to clone the block for the true blocks once.
102 BasicBlock *JumpThreading::FactorCommonPHIPreds(PHINode *PN, Constant *CstVal) {
103 SmallVector<BasicBlock*, 16> CommonPreds;
104 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
105 if (PN->getIncomingValue(i) == CstVal)
106 CommonPreds.push_back(PN->getIncomingBlock(i));
108 if (CommonPreds.size() == 1)
109 return CommonPreds[0];
111 DOUT << " Factoring out " << CommonPreds.size()
112 << " common predecessors.\n";
113 return SplitBlockPredecessors(PN->getParent(),
114 &CommonPreds[0], CommonPreds.size(),
119 /// getJumpThreadDuplicationCost - Return the cost of duplicating this block to
120 /// thread across it.
121 static unsigned getJumpThreadDuplicationCost(const BasicBlock *BB) {
122 /// Ignore PHI nodes, these will be flattened when duplication happens.
123 BasicBlock::const_iterator I = BB->getFirstNonPHI();
125 // Sum up the cost of each instruction until we get to the terminator. Don't
126 // include the terminator because the copy won't include it.
128 for (; !isa<TerminatorInst>(I); ++I) {
129 // Debugger intrinsics don't incur code size.
130 if (isa<DbgInfoIntrinsic>(I)) continue;
132 // If this is a pointer->pointer bitcast, it is free.
133 if (isa<BitCastInst>(I) && isa<PointerType>(I->getType()))
136 // All other instructions count for at least one unit.
139 // Calls are more expensive. If they are non-intrinsic calls, we model them
140 // as having cost of 4. If they are a non-vector intrinsic, we model them
141 // as having cost of 2 total, and if they are a vector intrinsic, we model
142 // them as having cost 1.
143 if (const CallInst *CI = dyn_cast<CallInst>(I)) {
144 if (!isa<IntrinsicInst>(CI))
146 else if (isa<VectorType>(CI->getType()))
151 // Threading through a switch statement is particularly profitable. If this
152 // block ends in a switch, decrease its cost to make it more likely to happen.
153 if (isa<SwitchInst>(I))
154 Size = Size > 6 ? Size-6 : 0;
159 /// MergeBasicBlockIntoOnlyPred - DestBB is a block with one predecessor and its
160 /// predecessor is known to have one successor (DestBB!). Eliminate the edge
161 /// between them, moving the instructions in the predecessor into DestBB and
162 /// deleting the predecessor block.
164 /// FIXME: Move to TransformUtils to share with simplifycfg and codegenprepare.
165 static void MergeBasicBlockIntoOnlyPred(BasicBlock *DestBB) {
166 // If BB has single-entry PHI nodes, fold them.
167 while (PHINode *PN = dyn_cast<PHINode>(DestBB->begin())) {
168 Value *NewVal = PN->getIncomingValue(0);
169 // Replace self referencing PHI with undef, it must be dead.
170 if (NewVal == PN) NewVal = UndefValue::get(PN->getType());
171 PN->replaceAllUsesWith(NewVal);
172 PN->eraseFromParent();
175 BasicBlock *PredBB = DestBB->getSinglePredecessor();
176 assert(PredBB && "Block doesn't have a single predecessor!");
178 // Splice all the instructions from PredBB to DestBB.
179 PredBB->getTerminator()->eraseFromParent();
180 DestBB->getInstList().splice(DestBB->begin(), PredBB->getInstList());
182 // Anything that branched to PredBB now branches to DestBB.
183 PredBB->replaceAllUsesWith(DestBB);
186 PredBB->eraseFromParent();
190 /// ProcessBlock - If there are any predecessors whose control can be threaded
191 /// through to a successor, transform them now.
192 bool JumpThreading::ProcessBlock(BasicBlock *BB) {
193 // If this block has a single predecessor, and if that pred has a single
194 // successor, merge the blocks. This encourages recursive jump threading
195 // because now the condition in this block can be threaded through
196 // predecessors of our predecessor block.
197 if (BasicBlock *SinglePred = BB->getSinglePredecessor())
198 if (SinglePred->getTerminator()->getNumSuccessors() == 1) {
199 MergeBasicBlockIntoOnlyPred(BB);
203 // See if this block ends with a branch or switch. If so, see if the
204 // condition is a phi node. If so, and if an entry of the phi node is a
205 // constant, we can thread the block.
207 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
208 // Can't thread an unconditional jump.
209 if (BI->isUnconditional()) return false;
210 Condition = BI->getCondition();
211 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(BB->getTerminator()))
212 Condition = SI->getCondition();
214 return false; // Must be an invoke.
216 // If the terminator of this block is branching on a constant, simplify the
217 // terminator to an unconditional branch. This can occur due to threading in
219 if (isa<ConstantInt>(Condition)) {
220 DOUT << " In block '" << BB->getNameStart()
221 << "' folding terminator: " << *BB->getTerminator();
223 ConstantFoldTerminator(BB);
227 // If there is only a single predecessor of this block, nothing to fold.
228 if (BB->getSinglePredecessor())
231 // See if this is a phi node in the current block.
232 PHINode *PN = dyn_cast<PHINode>(Condition);
233 if (PN && PN->getParent() == BB)
234 return ProcessJumpOnPHI(PN);
236 // If this is a conditional branch whose condition is and/or of a phi, try to
238 if (BinaryOperator *CondI = dyn_cast<BinaryOperator>(Condition)) {
239 if ((CondI->getOpcode() == Instruction::And ||
240 CondI->getOpcode() == Instruction::Or) &&
241 isa<BranchInst>(BB->getTerminator()) &&
242 ProcessBranchOnLogical(CondI, BB,
243 CondI->getOpcode() == Instruction::And))
247 // If we have "br (phi != 42)" and the phi node has any constant values as
248 // operands, we can thread through this block.
249 if (CmpInst *CondCmp = dyn_cast<CmpInst>(Condition))
250 if (isa<PHINode>(CondCmp->getOperand(0)) &&
251 isa<Constant>(CondCmp->getOperand(1)) &&
252 ProcessBranchOnCompare(CondCmp, BB))
255 // Check for some cases that are worth simplifying. Right now we want to look
256 // for loads that are used by a switch or by the condition for the branch. If
257 // we see one, check to see if it's partially redundant. If so, insert a PHI
258 // which can then be used to thread the values.
260 // This is particularly important because reg2mem inserts loads and stores all
261 // over the place, and this blocks jump threading if we don't zap them.
262 Value *SimplifyValue = Condition;
263 if (CmpInst *CondCmp = dyn_cast<CmpInst>(SimplifyValue))
264 if (isa<Constant>(CondCmp->getOperand(1)))
265 SimplifyValue = CondCmp->getOperand(0);
267 if (LoadInst *LI = dyn_cast<LoadInst>(SimplifyValue))
268 if (SimplifyPartiallyRedundantLoad(LI))
271 // TODO: If we have: "br (X > 0)" and we have a predecessor where we know
272 // "(X == 4)" thread through this block.
278 /// FindAvailableLoadedValue - Scan backwards from ScanFrom checking to see if
279 /// we have the value at the memory address *Ptr locally available within a
280 /// small number of instructions. If the value is available, return it.
282 /// If not, return the iterator for the last validated instruction that the
283 /// value would be live through. If we scanned the entire block, ScanFrom would
284 /// be left at begin().
286 /// FIXME: Move this to transform utils and use from
287 /// InstCombiner::visitLoadInst. It would also be nice to optionally take AA so
288 /// that GVN could do this.
289 static Value *FindAvailableLoadedValue(Value *Ptr,
291 BasicBlock::iterator &ScanFrom) {
293 unsigned NumToScan = 6;
294 while (ScanFrom != ScanBB->begin()) {
295 // Don't scan huge blocks.
296 if (--NumToScan == 0) return 0;
298 Instruction *Inst = --ScanFrom;
300 // If this is a load of Ptr, the loaded value is available.
301 if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
302 if (LI->getOperand(0) == Ptr)
305 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
306 // If this is a store through Ptr, the value is available!
307 if (SI->getOperand(1) == Ptr)
308 return SI->getOperand(0);
310 // If Ptr is an alloca and this is a store to a different alloca, ignore
311 // the store. This is a trivial form of alias analysis that is important
312 // for reg2mem'd code.
313 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
314 (isa<AllocaInst>(SI->getOperand(1)) ||
315 isa<GlobalVariable>(SI->getOperand(1))))
318 // Otherwise the store that may or may not alias the pointer, bail out.
324 // If this is some other instruction that may clobber Ptr, bail out.
325 if (Inst->mayWriteToMemory()) {
326 // May modify the pointer, bail out.
332 // Got to the start of the block, we didn't find it, but are done for this
338 /// SimplifyPartiallyRedundantLoad - If LI is an obviously partially redundant
339 /// load instruction, eliminate it by replacing it with a PHI node. This is an
340 /// important optimization that encourages jump threading, and needs to be run
341 /// interlaced with other jump threading tasks.
342 bool JumpThreading::SimplifyPartiallyRedundantLoad(LoadInst *LI) {
343 // Don't hack volatile loads.
344 if (LI->isVolatile()) return false;
346 // If the load is defined in a block with exactly one predecessor, it can't be
347 // partially redundant.
348 BasicBlock *LoadBB = LI->getParent();
349 if (LoadBB->getSinglePredecessor())
352 Value *LoadedPtr = LI->getOperand(0);
354 // If the loaded operand is defined in the LoadBB, it can't be available.
355 // FIXME: Could do PHI translation, that would be fun :)
356 if (Instruction *PtrOp = dyn_cast<Instruction>(LoadedPtr))
357 if (PtrOp->getParent() == LoadBB)
360 // Scan a few instructions up from the load, to see if it is obviously live at
361 // the entry to its block.
362 BasicBlock::iterator BBIt = LI;
364 if (Value *AvailableVal = FindAvailableLoadedValue(LoadedPtr, LoadBB, BBIt)) {
365 // If the value if the load is locally available within the block, just use
366 // it. This frequently occurs for reg2mem'd allocas.
367 //cerr << "LOAD ELIMINATED:\n" << *BBIt << *LI << "\n";
368 LI->replaceAllUsesWith(AvailableVal);
369 LI->eraseFromParent();
373 // Otherwise, if we scanned the whole block and got to the top of the block,
374 // we know the block is locally transparent to the load. If not, something
375 // might clobber its value.
376 if (BBIt != LoadBB->begin())
380 SmallPtrSet<BasicBlock*, 8> PredsScanned;
381 typedef SmallVector<std::pair<BasicBlock*, Value*>, 8> AvailablePredsTy;
382 AvailablePredsTy AvailablePreds;
383 BasicBlock *OneUnavailablePred = 0;
385 // If we got here, the loaded value is transparent through to the start of the
386 // block. Check to see if it is available in any of the predecessor blocks.
387 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
389 BasicBlock *PredBB = *PI;
391 // If we already scanned this predecessor, skip it.
392 if (!PredsScanned.insert(PredBB))
395 // Scan the predecessor to see if the value is available in the pred.
396 BBIt = PredBB->end();
397 Value *PredAvailable = FindAvailableLoadedValue(LoadedPtr, PredBB, BBIt);
398 if (!PredAvailable) {
399 OneUnavailablePred = PredBB;
403 // If so, this load is partially redundant. Remember this info so that we
404 // can create a PHI node.
405 AvailablePreds.push_back(std::make_pair(PredBB, PredAvailable));
408 // If the loaded value isn't available in any predecessor, it isn't partially
410 if (AvailablePreds.empty()) return false;
412 // Okay, the loaded value is available in at least one (and maybe all!)
413 // predecessors. If the value is unavailable in more than one unique
414 // predecessor, we want to insert a merge block for those common predecessors.
415 // This ensures that we only have to insert one reload, thus not increasing
417 BasicBlock *UnavailablePred = 0;
419 // If there is exactly one predecessor where the value is unavailable, the
420 // already computed 'OneUnavailablePred' block is it. If it ends in an
421 // unconditional branch, we know that it isn't a critical edge.
422 if (PredsScanned.size() == AvailablePreds.size()+1 &&
423 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
424 UnavailablePred = OneUnavailablePred;
425 } else if (PredsScanned.size() != AvailablePreds.size()) {
426 // Otherwise, we had multiple unavailable predecessors or we had a critical
427 // edge from the one.
428 SmallVector<BasicBlock*, 8> PredsToSplit;
429 SmallPtrSet<BasicBlock*, 8> AvailablePredSet;
431 for (unsigned i = 0, e = AvailablePreds.size(); i != e; ++i)
432 AvailablePredSet.insert(AvailablePreds[i].first);
434 // Add all the unavailable predecessors to the PredsToSplit list.
435 for (pred_iterator PI = pred_begin(LoadBB), PE = pred_end(LoadBB);
437 if (!AvailablePredSet.count(*PI))
438 PredsToSplit.push_back(*PI);
440 // Split them out to their own block.
442 SplitBlockPredecessors(LoadBB, &PredsToSplit[0], PredsToSplit.size(),
443 "thread-split", this);
446 // If the value isn't available in all predecessors, then there will be
447 // exactly one where it isn't available. Insert a load on that edge and add
448 // it to the AvailablePreds list.
449 if (UnavailablePred) {
450 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
451 "Can't handle critical edge here!");
452 Value *NewVal = new LoadInst(LoadedPtr, LI->getName()+".pr",
453 UnavailablePred->getTerminator());
454 AvailablePreds.push_back(std::make_pair(UnavailablePred, NewVal));
457 // Now we know that each predecessor of this block has a value in
458 // AvailablePreds, sort them for efficient access as we're walking the preds.
459 std::sort(AvailablePreds.begin(), AvailablePreds.end());
461 // Create a PHI node at the start of the block for the PRE'd load value.
462 PHINode *PN = PHINode::Create(LI->getType(), "", LoadBB->begin());
465 // Insert new entries into the PHI for each predecessor. A single block may
466 // have multiple entries here.
467 for (pred_iterator PI = pred_begin(LoadBB), E = pred_end(LoadBB); PI != E;
469 AvailablePredsTy::iterator I =
470 std::lower_bound(AvailablePreds.begin(), AvailablePreds.end(),
471 std::make_pair(*PI, (Value*)0));
473 assert(I != AvailablePreds.end() && I->first == *PI &&
474 "Didn't find entry for predecessor!");
476 PN->addIncoming(I->second, I->first);
479 //cerr << "PRE: " << *LI << *PN << "\n";
481 LI->replaceAllUsesWith(PN);
482 LI->eraseFromParent();
488 /// ProcessJumpOnPHI - We have a conditional branch of switch on a PHI node in
489 /// the current block. See if there are any simplifications we can do based on
490 /// inputs to the phi node.
492 bool JumpThreading::ProcessJumpOnPHI(PHINode *PN) {
493 // See if the phi node has any constant values. If so, we can determine where
494 // the corresponding predecessor will branch.
495 ConstantInt *PredCst = 0;
496 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
497 if ((PredCst = dyn_cast<ConstantInt>(PN->getIncomingValue(i))))
500 // If no incoming value has a constant, we don't know the destination of any
505 // See if the cost of duplicating this block is low enough.
506 BasicBlock *BB = PN->getParent();
507 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
508 if (JumpThreadCost > Threshold) {
509 DOUT << " Not threading BB '" << BB->getNameStart()
510 << "' - Cost is too high: " << JumpThreadCost << "\n";
514 // If so, we can actually do this threading. Merge any common predecessors
515 // that will act the same.
516 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
518 // Next, figure out which successor we are threading to.
520 if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator()))
521 SuccBB = BI->getSuccessor(PredCst == ConstantInt::getFalse());
523 SwitchInst *SI = cast<SwitchInst>(BB->getTerminator());
524 SuccBB = SI->getSuccessor(SI->findCaseValue(PredCst));
527 // If threading to the same block as we come from, we would infinite loop.
529 DOUT << " Not threading BB '" << BB->getNameStart()
530 << "' - would thread to self!\n";
534 // And finally, do it!
535 DOUT << " Threading edge from '" << PredBB->getNameStart() << "' to '"
536 << SuccBB->getNameStart() << "' with cost: " << JumpThreadCost
537 << ", across block:\n "
540 ThreadEdge(BB, PredBB, SuccBB);
545 /// ProcessJumpOnLogicalPHI - PN's basic block contains a conditional branch
546 /// whose condition is an AND/OR where one side is PN. If PN has constant
547 /// operands that permit us to evaluate the condition for some operand, thread
548 /// through the block. For example with:
549 /// br (and X, phi(Y, Z, false))
550 /// the predecessor corresponding to the 'false' will always jump to the false
551 /// destination of the branch.
553 bool JumpThreading::ProcessBranchOnLogical(Value *V, BasicBlock *BB,
555 // If this is a binary operator tree of the same AND/OR opcode, check the
557 if (BinaryOperator *BO = dyn_cast<BinaryOperator>(V))
558 if ((isAnd && BO->getOpcode() == Instruction::And) ||
559 (!isAnd && BO->getOpcode() == Instruction::Or)) {
560 if (ProcessBranchOnLogical(BO->getOperand(0), BB, isAnd))
562 if (ProcessBranchOnLogical(BO->getOperand(1), BB, isAnd))
566 // If this isn't a PHI node, we can't handle it.
567 PHINode *PN = dyn_cast<PHINode>(V);
568 if (!PN || PN->getParent() != BB) return false;
570 // We can only do the simplification for phi nodes of 'false' with AND or
571 // 'true' with OR. See if we have any entries in the phi for this.
572 unsigned PredNo = ~0U;
573 ConstantInt *PredCst = ConstantInt::get(Type::Int1Ty, !isAnd);
574 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
575 if (PN->getIncomingValue(i) == PredCst) {
581 // If no match, bail out.
585 // See if the cost of duplicating this block is low enough.
586 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
587 if (JumpThreadCost > Threshold) {
588 DOUT << " Not threading BB '" << BB->getNameStart()
589 << "' - Cost is too high: " << JumpThreadCost << "\n";
593 // If so, we can actually do this threading. Merge any common predecessors
594 // that will act the same.
595 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
597 // Next, figure out which successor we are threading to. If this was an AND,
598 // the constant must be FALSE, and we must be targeting the 'false' block.
599 // If this is an OR, the constant must be TRUE, and we must be targeting the
601 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(isAnd);
603 // If threading to the same block as we come from, we would infinite loop.
605 DOUT << " Not threading BB '" << BB->getNameStart()
606 << "' - would thread to self!\n";
610 // And finally, do it!
611 DOUT << " Threading edge through bool from '" << PredBB->getNameStart()
612 << "' to '" << SuccBB->getNameStart() << "' with cost: "
613 << JumpThreadCost << ", across block:\n "
616 ThreadEdge(BB, PredBB, SuccBB);
621 /// ProcessBranchOnCompare - We found a branch on a comparison between a phi
622 /// node and a constant. If the PHI node contains any constants as inputs, we
623 /// can fold the compare for that edge and thread through it.
624 bool JumpThreading::ProcessBranchOnCompare(CmpInst *Cmp, BasicBlock *BB) {
625 PHINode *PN = cast<PHINode>(Cmp->getOperand(0));
626 Constant *RHS = cast<Constant>(Cmp->getOperand(1));
628 // If the phi isn't in the current block, an incoming edge to this block
629 // doesn't control the destination.
630 if (PN->getParent() != BB)
633 // We can do this simplification if any comparisons fold to true or false.
635 Constant *PredCst = 0;
636 bool TrueDirection = false;
637 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
638 PredCst = dyn_cast<Constant>(PN->getIncomingValue(i));
639 if (PredCst == 0) continue;
642 if (ICmpInst *ICI = dyn_cast<ICmpInst>(Cmp))
643 Res = ConstantExpr::getICmp(ICI->getPredicate(), PredCst, RHS);
645 Res = ConstantExpr::getFCmp(cast<FCmpInst>(Cmp)->getPredicate(),
647 // If this folded to a constant expr, we can't do anything.
648 if (ConstantInt *ResC = dyn_cast<ConstantInt>(Res)) {
649 TrueDirection = ResC->getZExtValue();
652 // If this folded to undef, just go the false way.
653 if (isa<UndefValue>(Res)) {
654 TrueDirection = false;
658 // Otherwise, we can't fold this input.
662 // If no match, bail out.
666 // See if the cost of duplicating this block is low enough.
667 unsigned JumpThreadCost = getJumpThreadDuplicationCost(BB);
668 if (JumpThreadCost > Threshold) {
669 DOUT << " Not threading BB '" << BB->getNameStart()
670 << "' - Cost is too high: " << JumpThreadCost << "\n";
674 // If so, we can actually do this threading. Merge any common predecessors
675 // that will act the same.
676 BasicBlock *PredBB = FactorCommonPHIPreds(PN, PredCst);
678 // Next, get our successor.
679 BasicBlock *SuccBB = BB->getTerminator()->getSuccessor(!TrueDirection);
681 // If threading to the same block as we come from, we would infinite loop.
683 DOUT << " Not threading BB '" << BB->getNameStart()
684 << "' - would thread to self!\n";
689 // And finally, do it!
690 DOUT << " Threading edge through bool from '" << PredBB->getNameStart()
691 << "' to '" << SuccBB->getNameStart() << "' with cost: "
692 << JumpThreadCost << ", across block:\n "
695 ThreadEdge(BB, PredBB, SuccBB);
701 /// ThreadEdge - We have decided that it is safe and profitable to thread an
702 /// edge from PredBB to SuccBB across BB. Transform the IR to reflect this
704 void JumpThreading::ThreadEdge(BasicBlock *BB, BasicBlock *PredBB,
705 BasicBlock *SuccBB) {
707 // Jump Threading can not update SSA properties correctly if the values
708 // defined in the duplicated block are used outside of the block itself. For
709 // this reason, we spill all values that are used outside of BB to the stack.
710 for (BasicBlock::iterator I = BB->begin(); I != BB->end(); ++I) {
711 if (!I->isUsedOutsideOfBlock(BB))
714 // We found a use of I outside of BB. Create a new stack slot to
715 // break this inter-block usage pattern.
716 DemoteRegToStack(*I);
719 // We are going to have to map operands from the original BB block to the new
720 // copy of the block 'NewBB'. If there are PHI nodes in BB, evaluate them to
721 // account for entry from PredBB.
722 DenseMap<Instruction*, Value*> ValueMapping;
725 BasicBlock::Create(BB->getName()+".thread", BB->getParent(), BB);
726 NewBB->moveAfter(PredBB);
728 BasicBlock::iterator BI = BB->begin();
729 for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
730 ValueMapping[PN] = PN->getIncomingValueForBlock(PredBB);
732 // Clone the non-phi instructions of BB into NewBB, keeping track of the
733 // mapping and using it to remap operands in the cloned instructions.
734 for (; !isa<TerminatorInst>(BI); ++BI) {
735 Instruction *New = BI->clone();
736 New->setName(BI->getNameStart());
737 NewBB->getInstList().push_back(New);
738 ValueMapping[BI] = New;
740 // Remap operands to patch up intra-block references.
741 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
742 if (Instruction *Inst = dyn_cast<Instruction>(New->getOperand(i)))
743 if (Value *Remapped = ValueMapping[Inst])
744 New->setOperand(i, Remapped);
747 // We didn't copy the terminator from BB over to NewBB, because there is now
748 // an unconditional jump to SuccBB. Insert the unconditional jump.
749 BranchInst::Create(SuccBB, NewBB);
751 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
752 // PHI nodes for NewBB now.
753 for (BasicBlock::iterator PNI = SuccBB->begin(); isa<PHINode>(PNI); ++PNI) {
754 PHINode *PN = cast<PHINode>(PNI);
755 // Ok, we have a PHI node. Figure out what the incoming value was for the
757 Value *IV = PN->getIncomingValueForBlock(BB);
759 // Remap the value if necessary.
760 if (Instruction *Inst = dyn_cast<Instruction>(IV))
761 if (Value *MappedIV = ValueMapping[Inst])
763 PN->addIncoming(IV, NewBB);
766 // Finally, NewBB is good to go. Update the terminator of PredBB to jump to
767 // NewBB instead of BB. This eliminates predecessors from BB, which requires
768 // us to simplify any PHI nodes in BB.
769 TerminatorInst *PredTerm = PredBB->getTerminator();
770 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
771 if (PredTerm->getSuccessor(i) == BB) {
772 BB->removePredecessor(PredBB);
773 PredTerm->setSuccessor(i, NewBB);