1 //===-- BasicBlockUtils.cpp - BasicBlock Utilities -------------------------==//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This family of functions perform manipulations on basic blocks, and
11 // instructions contained within basic blocks.
12 //
13 //===----------------------------------------------------------------------===//
14
15 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
16 #include "llvm/Analysis/AliasAnalysis.h"
17 #include "llvm/Analysis/CFG.h"
18 #include "llvm/Analysis/LoopInfo.h"
19 #include "llvm/Analysis/MemoryDependenceAnalysis.h"
20 #include "llvm/IR/Constant.h"
21 #include "llvm/IR/DataLayout.h"
22 #include "llvm/IR/Dominators.h"
23 #include "llvm/IR/Function.h"
24 #include "llvm/IR/Instructions.h"
25 #include "llvm/IR/IntrinsicInst.h"
26 #include "llvm/IR/Type.h"
27 #include "llvm/IR/ValueHandle.h"
28 #include "llvm/Support/ErrorHandling.h"
29 #include "llvm/Transforms/Scalar.h"
30 #include "llvm/Transforms/Utils/Local.h"
31 #include <algorithm>
32 using namespace llvm;
33
34 /// DeleteDeadBlock - Delete the specified block, which must have no
35 /// predecessors.
DeleteDeadBlock(BasicBlock * BB)36 void llvm::DeleteDeadBlock(BasicBlock *BB) {
37 assert((pred_begin(BB) == pred_end(BB) ||
38 // Can delete self loop.
39 BB->getSinglePredecessor() == BB) && "Block is not dead!");
40 TerminatorInst *BBTerm = BB->getTerminator();
41
42 // Loop through all of our successors and make sure they know that one
43 // of their predecessors is going away.
44 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i)
45 BBTerm->getSuccessor(i)->removePredecessor(BB);
46
47 // Zap all the instructions in the block.
48 while (!BB->empty()) {
49 Instruction &I = BB->back();
50 // If this instruction is used, replace uses with an arbitrary value.
51 // Because control flow can't get here, we don't care what we replace the
52 // value with. Note that since this block is unreachable, and all values
53 // contained within it must dominate their uses, that all uses will
54 // eventually be removed (they are themselves dead).
55 if (!I.use_empty())
56 I.replaceAllUsesWith(UndefValue::get(I.getType()));
57 BB->getInstList().pop_back();
58 }
59
60 // Zap the block!
61 BB->eraseFromParent();
62 }
63
64 /// FoldSingleEntryPHINodes - We know that BB has one predecessor. If there are
65 /// any single-entry PHI nodes in it, fold them away. This handles the case
66 /// when all entries to the PHI nodes in a block are guaranteed equal, such as
67 /// when the block has exactly one predecessor.
FoldSingleEntryPHINodes(BasicBlock * BB,Pass * P)68 void llvm::FoldSingleEntryPHINodes(BasicBlock *BB, Pass *P) {
69 if (!isa<PHINode>(BB->begin())) return;
70
71 AliasAnalysis *AA = nullptr;
72 MemoryDependenceAnalysis *MemDep = nullptr;
73 if (P) {
74 AA = P->getAnalysisIfAvailable<AliasAnalysis>();
75 MemDep = P->getAnalysisIfAvailable<MemoryDependenceAnalysis>();
76 }
77
78 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
79 if (PN->getIncomingValue(0) != PN)
80 PN->replaceAllUsesWith(PN->getIncomingValue(0));
81 else
82 PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
83
84 if (MemDep)
85 MemDep->removeInstruction(PN); // Memdep updates AA itself.
86 else if (AA && isa<PointerType>(PN->getType()))
87 AA->deleteValue(PN);
88
89 PN->eraseFromParent();
90 }
91 }
92
93
94 /// DeleteDeadPHIs - Examine each PHI in the given block and delete it if it
95 /// is dead. Also recursively delete any operands that become dead as
96 /// a result. This includes tracing the def-use list from the PHI to see if
97 /// it is ultimately unused or if it reaches an unused cycle.
DeleteDeadPHIs(BasicBlock * BB,const TargetLibraryInfo * TLI)98 bool llvm::DeleteDeadPHIs(BasicBlock *BB, const TargetLibraryInfo *TLI) {
99 // Recursively deleting a PHI may cause multiple PHIs to be deleted
100 // or RAUW'd undef, so use an array of WeakVH for the PHIs to delete.
101 SmallVector<WeakVH, 8> PHIs;
102 for (BasicBlock::iterator I = BB->begin();
103 PHINode *PN = dyn_cast<PHINode>(I); ++I)
104 PHIs.push_back(PN);
105
106 bool Changed = false;
107 for (unsigned i = 0, e = PHIs.size(); i != e; ++i)
108 if (PHINode *PN = dyn_cast_or_null<PHINode>(PHIs[i].operator Value*()))
109 Changed |= RecursivelyDeleteDeadPHINode(PN, TLI);
110
111 return Changed;
112 }
113
114 /// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor,
115 /// if possible. The return value indicates success or failure.
MergeBlockIntoPredecessor(BasicBlock * BB,Pass * P)116 bool llvm::MergeBlockIntoPredecessor(BasicBlock *BB, Pass *P) {
117 // Don't merge away blocks who have their address taken.
118 if (BB->hasAddressTaken()) return false;
119
120 // Can't merge if there are multiple predecessors, or no predecessors.
121 BasicBlock *PredBB = BB->getUniquePredecessor();
122 if (!PredBB) return false;
123
124 // Don't break self-loops.
125 if (PredBB == BB) return false;
126 // Don't break invokes.
127 if (isa<InvokeInst>(PredBB->getTerminator())) return false;
128
129 succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB));
130 BasicBlock *OnlySucc = BB;
131 for (; SI != SE; ++SI)
132 if (*SI != OnlySucc) {
133 OnlySucc = nullptr; // There are multiple distinct successors!
134 break;
135 }
136
137 // Can't merge if there are multiple successors.
138 if (!OnlySucc) return false;
139
140 // Can't merge if there is PHI loop.
141 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) {
142 if (PHINode *PN = dyn_cast<PHINode>(BI)) {
143 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
144 if (PN->getIncomingValue(i) == PN)
145 return false;
146 } else
147 break;
148 }
149
150 // Begin by getting rid of unneeded PHIs.
151 if (isa<PHINode>(BB->front()))
152 FoldSingleEntryPHINodes(BB, P);
153
154 // Delete the unconditional branch from the predecessor...
155 PredBB->getInstList().pop_back();
156
157 // Make all PHI nodes that referred to BB now refer to Pred as their
158 // source...
159 BB->replaceAllUsesWith(PredBB);
160
161 // Move all definitions in the successor to the predecessor...
162 PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
163
164 // Inherit predecessors name if it exists.
165 if (!PredBB->hasName())
166 PredBB->takeName(BB);
167
168 // Finally, erase the old block and update dominator info.
169 if (P) {
170 if (DominatorTreeWrapperPass *DTWP =
171 P->getAnalysisIfAvailable<DominatorTreeWrapperPass>()) {
172 DominatorTree &DT = DTWP->getDomTree();
173 if (DomTreeNode *DTN = DT.getNode(BB)) {
174 DomTreeNode *PredDTN = DT.getNode(PredBB);
175 SmallVector<DomTreeNode*, 8> Children(DTN->begin(), DTN->end());
176 for (SmallVectorImpl<DomTreeNode *>::iterator DI = Children.begin(),
177 DE = Children.end(); DI != DE; ++DI)
178 DT.changeImmediateDominator(*DI, PredDTN);
179
180 DT.eraseNode(BB);
181 }
182
183 if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>())
184 LI->removeBlock(BB);
185
186 if (MemoryDependenceAnalysis *MD =
187 P->getAnalysisIfAvailable<MemoryDependenceAnalysis>())
188 MD->invalidateCachedPredecessors();
189 }
190 }
191
192 BB->eraseFromParent();
193 return true;
194 }
195
196 /// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI)
197 /// with a value, then remove and delete the original instruction.
198 ///
ReplaceInstWithValue(BasicBlock::InstListType & BIL,BasicBlock::iterator & BI,Value * V)199 void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL,
200 BasicBlock::iterator &BI, Value *V) {
201 Instruction &I = *BI;
202 // Replaces all of the uses of the instruction with uses of the value
203 I.replaceAllUsesWith(V);
204
205 // Make sure to propagate a name if there is one already.
206 if (I.hasName() && !V->hasName())
207 V->takeName(&I);
208
209 // Delete the unnecessary instruction now...
210 BI = BIL.erase(BI);
211 }
212
213
214 /// ReplaceInstWithInst - Replace the instruction specified by BI with the
215 /// instruction specified by I. The original instruction is deleted and BI is
216 /// updated to point to the new instruction.
217 ///
ReplaceInstWithInst(BasicBlock::InstListType & BIL,BasicBlock::iterator & BI,Instruction * I)218 void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL,
219 BasicBlock::iterator &BI, Instruction *I) {
220 assert(I->getParent() == nullptr &&
221 "ReplaceInstWithInst: Instruction already inserted into basic block!");
222
223 // Insert the new instruction into the basic block...
224 BasicBlock::iterator New = BIL.insert(BI, I);
225
226 // Replace all uses of the old instruction, and delete it.
227 ReplaceInstWithValue(BIL, BI, I);
228
229 // Move BI back to point to the newly inserted instruction
230 BI = New;
231 }
232
233 /// ReplaceInstWithInst - Replace the instruction specified by From with the
234 /// instruction specified by To.
235 ///
ReplaceInstWithInst(Instruction * From,Instruction * To)236 void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) {
237 BasicBlock::iterator BI(From);
238 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To);
239 }
240
241 /// SplitEdge - Split the edge connecting specified block. Pass P must
242 /// not be NULL.
SplitEdge(BasicBlock * BB,BasicBlock * Succ,Pass * P)243 BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) {
244 unsigned SuccNum = GetSuccessorNumber(BB, Succ);
245
246 // If this is a critical edge, let SplitCriticalEdge do it.
247 TerminatorInst *LatchTerm = BB->getTerminator();
248 if (SplitCriticalEdge(LatchTerm, SuccNum, P))
249 return LatchTerm->getSuccessor(SuccNum);
250
251 // If the edge isn't critical, then BB has a single successor or Succ has a
252 // single pred. Split the block.
253 if (BasicBlock *SP = Succ->getSinglePredecessor()) {
254 // If the successor only has a single pred, split the top of the successor
255 // block.
256 assert(SP == BB && "CFG broken");
257 SP = nullptr;
258 return SplitBlock(Succ, Succ->begin(), P);
259 }
260
261 // Otherwise, if BB has a single successor, split it at the bottom of the
262 // block.
263 assert(BB->getTerminator()->getNumSuccessors() == 1 &&
264 "Should have a single succ!");
265 return SplitBlock(BB, BB->getTerminator(), P);
266 }
267
SplitAllCriticalEdges(Function & F,Pass * P)268 unsigned llvm::SplitAllCriticalEdges(Function &F, Pass *P) {
269 unsigned NumBroken = 0;
270 for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I) {
271 TerminatorInst *TI = I->getTerminator();
272 if (TI->getNumSuccessors() > 1 && !isa<IndirectBrInst>(TI))
273 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
274 if (SplitCriticalEdge(TI, i, P))
275 ++NumBroken;
276 }
277 return NumBroken;
278 }
279
280 /// SplitBlock - Split the specified block at the specified instruction - every
281 /// thing before SplitPt stays in Old and everything starting with SplitPt moves
282 /// to a new block. The two blocks are joined by an unconditional branch and
283 /// the loop info is updated.
284 ///
SplitBlock(BasicBlock * Old,Instruction * SplitPt,Pass * P)285 BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) {
286 BasicBlock::iterator SplitIt = SplitPt;
287 while (isa<PHINode>(SplitIt) || isa<LandingPadInst>(SplitIt))
288 ++SplitIt;
289 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
290
291 // The new block lives in whichever loop the old one did. This preserves
292 // LCSSA as well, because we force the split point to be after any PHI nodes.
293 if (LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>())
294 if (Loop *L = LI->getLoopFor(Old))
295 L->addBasicBlockToLoop(New, LI->getBase());
296
297 if (DominatorTreeWrapperPass *DTWP =
298 P->getAnalysisIfAvailable<DominatorTreeWrapperPass>()) {
299 DominatorTree &DT = DTWP->getDomTree();
300 // Old dominates New. New node dominates all other nodes dominated by Old.
301 if (DomTreeNode *OldNode = DT.getNode(Old)) {
302 std::vector<DomTreeNode *> Children;
303 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end();
304 I != E; ++I)
305 Children.push_back(*I);
306
307 DomTreeNode *NewNode = DT.addNewBlock(New, Old);
308 for (std::vector<DomTreeNode *>::iterator I = Children.begin(),
309 E = Children.end(); I != E; ++I)
310 DT.changeImmediateDominator(*I, NewNode);
311 }
312 }
313
314 return New;
315 }
316
317 /// UpdateAnalysisInformation - Update DominatorTree, LoopInfo, and LCCSA
318 /// analysis information.
UpdateAnalysisInformation(BasicBlock * OldBB,BasicBlock * NewBB,ArrayRef<BasicBlock * > Preds,Pass * P,bool & HasLoopExit)319 static void UpdateAnalysisInformation(BasicBlock *OldBB, BasicBlock *NewBB,
320 ArrayRef<BasicBlock *> Preds,
321 Pass *P, bool &HasLoopExit) {
322 if (!P) return;
323
324 LoopInfo *LI = P->getAnalysisIfAvailable<LoopInfo>();
325 Loop *L = LI ? LI->getLoopFor(OldBB) : nullptr;
326
327 // If we need to preserve loop analyses, collect some information about how
328 // this split will affect loops.
329 bool IsLoopEntry = !!L;
330 bool SplitMakesNewLoopHeader = false;
331 if (LI) {
332 bool PreserveLCSSA = P->mustPreserveAnalysisID(LCSSAID);
333 for (ArrayRef<BasicBlock*>::iterator
334 i = Preds.begin(), e = Preds.end(); i != e; ++i) {
335 BasicBlock *Pred = *i;
336
337 // If we need to preserve LCSSA, determine if any of the preds is a loop
338 // exit.
339 if (PreserveLCSSA)
340 if (Loop *PL = LI->getLoopFor(Pred))
341 if (!PL->contains(OldBB))
342 HasLoopExit = true;
343
344 // If we need to preserve LoopInfo, note whether any of the preds crosses
345 // an interesting loop boundary.
346 if (!L) continue;
347 if (L->contains(Pred))
348 IsLoopEntry = false;
349 else
350 SplitMakesNewLoopHeader = true;
351 }
352 }
353
354 // Update dominator tree if available.
355 if (DominatorTreeWrapperPass *DTWP =
356 P->getAnalysisIfAvailable<DominatorTreeWrapperPass>())
357 DTWP->getDomTree().splitBlock(NewBB);
358
359 if (!L) return;
360
361 if (IsLoopEntry) {
362 // Add the new block to the nearest enclosing loop (and not an adjacent
363 // loop). To find this, examine each of the predecessors and determine which
364 // loops enclose them, and select the most-nested loop which contains the
365 // loop containing the block being split.
366 Loop *InnermostPredLoop = nullptr;
367 for (ArrayRef<BasicBlock*>::iterator
368 i = Preds.begin(), e = Preds.end(); i != e; ++i) {
369 BasicBlock *Pred = *i;
370 if (Loop *PredLoop = LI->getLoopFor(Pred)) {
371 // Seek a loop which actually contains the block being split (to avoid
372 // adjacent loops).
373 while (PredLoop && !PredLoop->contains(OldBB))
374 PredLoop = PredLoop->getParentLoop();
375
376 // Select the most-nested of these loops which contains the block.
377 if (PredLoop && PredLoop->contains(OldBB) &&
378 (!InnermostPredLoop ||
379 InnermostPredLoop->getLoopDepth() < PredLoop->getLoopDepth()))
380 InnermostPredLoop = PredLoop;
381 }
382 }
383
384 if (InnermostPredLoop)
385 InnermostPredLoop->addBasicBlockToLoop(NewBB, LI->getBase());
386 } else {
387 L->addBasicBlockToLoop(NewBB, LI->getBase());
388 if (SplitMakesNewLoopHeader)
389 L->moveToHeader(NewBB);
390 }
391 }
392
393 /// UpdatePHINodes - Update the PHI nodes in OrigBB to include the values coming
394 /// from NewBB. This also updates AliasAnalysis, if available.
UpdatePHINodes(BasicBlock * OrigBB,BasicBlock * NewBB,ArrayRef<BasicBlock * > Preds,BranchInst * BI,Pass * P,bool HasLoopExit)395 static void UpdatePHINodes(BasicBlock *OrigBB, BasicBlock *NewBB,
396 ArrayRef<BasicBlock*> Preds, BranchInst *BI,
397 Pass *P, bool HasLoopExit) {
398 // Otherwise, create a new PHI node in NewBB for each PHI node in OrigBB.
399 AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : nullptr;
400 SmallPtrSet<BasicBlock *, 16> PredSet(Preds.begin(), Preds.end());
401 for (BasicBlock::iterator I = OrigBB->begin(); isa<PHINode>(I); ) {
402 PHINode *PN = cast<PHINode>(I++);
403
404 // Check to see if all of the values coming in are the same. If so, we
405 // don't need to create a new PHI node, unless it's needed for LCSSA.
406 Value *InVal = nullptr;
407 if (!HasLoopExit) {
408 InVal = PN->getIncomingValueForBlock(Preds[0]);
409 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
410 if (!PredSet.count(PN->getIncomingBlock(i)))
411 continue;
412 if (!InVal)
413 InVal = PN->getIncomingValue(i);
414 else if (InVal != PN->getIncomingValue(i)) {
415 InVal = nullptr;
416 break;
417 }
418 }
419 }
420
421 if (InVal) {
422 // If all incoming values for the new PHI would be the same, just don't
423 // make a new PHI. Instead, just remove the incoming values from the old
424 // PHI.
425
426 // NOTE! This loop walks backwards for a reason! First off, this minimizes
427 // the cost of removal if we end up removing a large number of values, and
428 // second off, this ensures that the indices for the incoming values
429 // aren't invalidated when we remove one.
430 for (int64_t i = PN->getNumIncomingValues() - 1; i >= 0; --i)
431 if (PredSet.count(PN->getIncomingBlock(i)))
432 PN->removeIncomingValue(i, false);
433
434 // Add an incoming value to the PHI node in the loop for the preheader
435 // edge.
436 PN->addIncoming(InVal, NewBB);
437 continue;
438 }
439
440 // If the values coming into the block are not the same, we need a new
441 // PHI.
442 // Create the new PHI node, insert it into NewBB at the end of the block
443 PHINode *NewPHI =
444 PHINode::Create(PN->getType(), Preds.size(), PN->getName() + ".ph", BI);
445 if (AA)
446 AA->copyValue(PN, NewPHI);
447
448 // NOTE! This loop walks backwards for a reason! First off, this minimizes
449 // the cost of removal if we end up removing a large number of values, and
450 // second off, this ensures that the indices for the incoming values aren't
451 // invalidated when we remove one.
452 for (int64_t i = PN->getNumIncomingValues() - 1; i >= 0; --i) {
453 BasicBlock *IncomingBB = PN->getIncomingBlock(i);
454 if (PredSet.count(IncomingBB)) {
455 Value *V = PN->removeIncomingValue(i, false);
456 NewPHI->addIncoming(V, IncomingBB);
457 }
458 }
459
460 PN->addIncoming(NewPHI, NewBB);
461 }
462 }
463
464 /// SplitBlockPredecessors - This method transforms BB by introducing a new
465 /// basic block into the function, and moving some of the predecessors of BB to
466 /// be predecessors of the new block. The new predecessors are indicated by the
467 /// Preds array, which has NumPreds elements in it. The new block is given a
468 /// suffix of 'Suffix'.
469 ///
470 /// This currently updates the LLVM IR, AliasAnalysis, DominatorTree,
471 /// LoopInfo, and LCCSA but no other analyses. In particular, it does not
472 /// preserve LoopSimplify (because it's complicated to handle the case where one
473 /// of the edges being split is an exit of a loop with other exits).
474 ///
SplitBlockPredecessors(BasicBlock * BB,ArrayRef<BasicBlock * > Preds,const char * Suffix,Pass * P)475 BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
476 ArrayRef<BasicBlock*> Preds,
477 const char *Suffix, Pass *P) {
478 // Create new basic block, insert right before the original block.
479 BasicBlock *NewBB = BasicBlock::Create(BB->getContext(), BB->getName()+Suffix,
480 BB->getParent(), BB);
481
482 // The new block unconditionally branches to the old block.
483 BranchInst *BI = BranchInst::Create(BB, NewBB);
484
485 // Move the edges from Preds to point to NewBB instead of BB.
486 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
487 // This is slightly more strict than necessary; the minimum requirement
488 // is that there be no more than one indirectbr branching to BB. And
489 // all BlockAddress uses would need to be updated.
490 assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) &&
491 "Cannot split an edge from an IndirectBrInst");
492 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB);
493 }
494
495 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI
496 // node becomes an incoming value for BB's phi node. However, if the Preds
497 // list is empty, we need to insert dummy entries into the PHI nodes in BB to
498 // account for the newly created predecessor.
499 if (Preds.size() == 0) {
500 // Insert dummy values as the incoming value.
501 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I)
502 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB);
503 return NewBB;
504 }
505
506 // Update DominatorTree, LoopInfo, and LCCSA analysis information.
507 bool HasLoopExit = false;
508 UpdateAnalysisInformation(BB, NewBB, Preds, P, HasLoopExit);
509
510 // Update the PHI nodes in BB with the values coming from NewBB.
511 UpdatePHINodes(BB, NewBB, Preds, BI, P, HasLoopExit);
512 return NewBB;
513 }
514
515 /// SplitLandingPadPredecessors - This method transforms the landing pad,
516 /// OrigBB, by introducing two new basic blocks into the function. One of those
517 /// new basic blocks gets the predecessors listed in Preds. The other basic
518 /// block gets the remaining predecessors of OrigBB. The landingpad instruction
519 /// OrigBB is clone into both of the new basic blocks. The new blocks are given
520 /// the suffixes 'Suffix1' and 'Suffix2', and are returned in the NewBBs vector.
521 ///
522 /// This currently updates the LLVM IR, AliasAnalysis, DominatorTree,
523 /// DominanceFrontier, LoopInfo, and LCCSA but no other analyses. In particular,
524 /// it does not preserve LoopSimplify (because it's complicated to handle the
525 /// case where one of the edges being split is an exit of a loop with other
526 /// exits).
527 ///
SplitLandingPadPredecessors(BasicBlock * OrigBB,ArrayRef<BasicBlock * > Preds,const char * Suffix1,const char * Suffix2,Pass * P,SmallVectorImpl<BasicBlock * > & NewBBs)528 void llvm::SplitLandingPadPredecessors(BasicBlock *OrigBB,
529 ArrayRef<BasicBlock*> Preds,
530 const char *Suffix1, const char *Suffix2,
531 Pass *P,
532 SmallVectorImpl<BasicBlock*> &NewBBs) {
533 assert(OrigBB->isLandingPad() && "Trying to split a non-landing pad!");
534
535 // Create a new basic block for OrigBB's predecessors listed in Preds. Insert
536 // it right before the original block.
537 BasicBlock *NewBB1 = BasicBlock::Create(OrigBB->getContext(),
538 OrigBB->getName() + Suffix1,
539 OrigBB->getParent(), OrigBB);
540 NewBBs.push_back(NewBB1);
541
542 // The new block unconditionally branches to the old block.
543 BranchInst *BI1 = BranchInst::Create(OrigBB, NewBB1);
544
545 // Move the edges from Preds to point to NewBB1 instead of OrigBB.
546 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
547 // This is slightly more strict than necessary; the minimum requirement
548 // is that there be no more than one indirectbr branching to BB. And
549 // all BlockAddress uses would need to be updated.
550 assert(!isa<IndirectBrInst>(Preds[i]->getTerminator()) &&
551 "Cannot split an edge from an IndirectBrInst");
552 Preds[i]->getTerminator()->replaceUsesOfWith(OrigBB, NewBB1);
553 }
554
555 // Update DominatorTree, LoopInfo, and LCCSA analysis information.
556 bool HasLoopExit = false;
557 UpdateAnalysisInformation(OrigBB, NewBB1, Preds, P, HasLoopExit);
558
559 // Update the PHI nodes in OrigBB with the values coming from NewBB1.
560 UpdatePHINodes(OrigBB, NewBB1, Preds, BI1, P, HasLoopExit);
561
562 // Move the remaining edges from OrigBB to point to NewBB2.
563 SmallVector<BasicBlock*, 8> NewBB2Preds;
564 for (pred_iterator i = pred_begin(OrigBB), e = pred_end(OrigBB);
565 i != e; ) {
566 BasicBlock *Pred = *i++;
567 if (Pred == NewBB1) continue;
568 assert(!isa<IndirectBrInst>(Pred->getTerminator()) &&
569 "Cannot split an edge from an IndirectBrInst");
570 NewBB2Preds.push_back(Pred);
571 e = pred_end(OrigBB);
572 }
573
574 BasicBlock *NewBB2 = nullptr;
575 if (!NewBB2Preds.empty()) {
576 // Create another basic block for the rest of OrigBB's predecessors.
577 NewBB2 = BasicBlock::Create(OrigBB->getContext(),
578 OrigBB->getName() + Suffix2,
579 OrigBB->getParent(), OrigBB);
580 NewBBs.push_back(NewBB2);
581
582 // The new block unconditionally branches to the old block.
583 BranchInst *BI2 = BranchInst::Create(OrigBB, NewBB2);
584
585 // Move the remaining edges from OrigBB to point to NewBB2.
586 for (SmallVectorImpl<BasicBlock*>::iterator
587 i = NewBB2Preds.begin(), e = NewBB2Preds.end(); i != e; ++i)
588 (*i)->getTerminator()->replaceUsesOfWith(OrigBB, NewBB2);
589
590 // Update DominatorTree, LoopInfo, and LCCSA analysis information.
591 HasLoopExit = false;
592 UpdateAnalysisInformation(OrigBB, NewBB2, NewBB2Preds, P, HasLoopExit);
593
594 // Update the PHI nodes in OrigBB with the values coming from NewBB2.
595 UpdatePHINodes(OrigBB, NewBB2, NewBB2Preds, BI2, P, HasLoopExit);
596 }
597
598 LandingPadInst *LPad = OrigBB->getLandingPadInst();
599 Instruction *Clone1 = LPad->clone();
600 Clone1->setName(Twine("lpad") + Suffix1);
601 NewBB1->getInstList().insert(NewBB1->getFirstInsertionPt(), Clone1);
602
603 if (NewBB2) {
604 Instruction *Clone2 = LPad->clone();
605 Clone2->setName(Twine("lpad") + Suffix2);
606 NewBB2->getInstList().insert(NewBB2->getFirstInsertionPt(), Clone2);
607
608 // Create a PHI node for the two cloned landingpad instructions.
609 PHINode *PN = PHINode::Create(LPad->getType(), 2, "lpad.phi", LPad);
610 PN->addIncoming(Clone1, NewBB1);
611 PN->addIncoming(Clone2, NewBB2);
612 LPad->replaceAllUsesWith(PN);
613 LPad->eraseFromParent();
614 } else {
615 // There is no second clone. Just replace the landing pad with the first
616 // clone.
617 LPad->replaceAllUsesWith(Clone1);
618 LPad->eraseFromParent();
619 }
620 }
621
622 /// FoldReturnIntoUncondBranch - This method duplicates the specified return
623 /// instruction into a predecessor which ends in an unconditional branch. If
624 /// the return instruction returns a value defined by a PHI, propagate the
625 /// right value into the return. It returns the new return instruction in the
626 /// predecessor.
FoldReturnIntoUncondBranch(ReturnInst * RI,BasicBlock * BB,BasicBlock * Pred)627 ReturnInst *llvm::FoldReturnIntoUncondBranch(ReturnInst *RI, BasicBlock *BB,
628 BasicBlock *Pred) {
629 Instruction *UncondBranch = Pred->getTerminator();
630 // Clone the return and add it to the end of the predecessor.
631 Instruction *NewRet = RI->clone();
632 Pred->getInstList().push_back(NewRet);
633
634 // If the return instruction returns a value, and if the value was a
635 // PHI node in "BB", propagate the right value into the return.
636 for (User::op_iterator i = NewRet->op_begin(), e = NewRet->op_end();
637 i != e; ++i) {
638 Value *V = *i;
639 Instruction *NewBC = nullptr;
640 if (BitCastInst *BCI = dyn_cast<BitCastInst>(V)) {
641 // Return value might be bitcasted. Clone and insert it before the
642 // return instruction.
643 V = BCI->getOperand(0);
644 NewBC = BCI->clone();
645 Pred->getInstList().insert(NewRet, NewBC);
646 *i = NewBC;
647 }
648 if (PHINode *PN = dyn_cast<PHINode>(V)) {
649 if (PN->getParent() == BB) {
650 if (NewBC)
651 NewBC->setOperand(0, PN->getIncomingValueForBlock(Pred));
652 else
653 *i = PN->getIncomingValueForBlock(Pred);
654 }
655 }
656 }
657
658 // Update any PHI nodes in the returning block to realize that we no
659 // longer branch to them.
660 BB->removePredecessor(Pred);
661 UncondBranch->eraseFromParent();
662 return cast<ReturnInst>(NewRet);
663 }
664
665 /// SplitBlockAndInsertIfThen - Split the containing block at the
666 /// specified instruction - everything before and including SplitBefore stays
667 /// in the old basic block, and everything after SplitBefore is moved to a
668 /// new block. The two blocks are connected by a conditional branch
669 /// (with value of Cmp being the condition).
670 /// Before:
671 /// Head
672 /// SplitBefore
673 /// Tail
674 /// After:
675 /// Head
676 /// if (Cond)
677 /// ThenBlock
678 /// SplitBefore
679 /// Tail
680 ///
681 /// If Unreachable is true, then ThenBlock ends with
682 /// UnreachableInst, otherwise it branches to Tail.
683 /// Returns the NewBasicBlock's terminator.
684
SplitBlockAndInsertIfThen(Value * Cond,Instruction * SplitBefore,bool Unreachable,MDNode * BranchWeights,DominatorTree * DT)685 TerminatorInst *llvm::SplitBlockAndInsertIfThen(Value *Cond,
686 Instruction *SplitBefore,
687 bool Unreachable,
688 MDNode *BranchWeights,
689 DominatorTree *DT) {
690 BasicBlock *Head = SplitBefore->getParent();
691 BasicBlock *Tail = Head->splitBasicBlock(SplitBefore);
692 TerminatorInst *HeadOldTerm = Head->getTerminator();
693 LLVMContext &C = Head->getContext();
694 BasicBlock *ThenBlock = BasicBlock::Create(C, "", Head->getParent(), Tail);
695 TerminatorInst *CheckTerm;
696 if (Unreachable)
697 CheckTerm = new UnreachableInst(C, ThenBlock);
698 else
699 CheckTerm = BranchInst::Create(Tail, ThenBlock);
700 CheckTerm->setDebugLoc(SplitBefore->getDebugLoc());
701 BranchInst *HeadNewTerm =
702 BranchInst::Create(/*ifTrue*/ThenBlock, /*ifFalse*/Tail, Cond);
703 HeadNewTerm->setDebugLoc(SplitBefore->getDebugLoc());
704 HeadNewTerm->setMetadata(LLVMContext::MD_prof, BranchWeights);
705 ReplaceInstWithInst(HeadOldTerm, HeadNewTerm);
706
707 if (DT) {
708 if (DomTreeNode *OldNode = DT->getNode(Head)) {
709 std::vector<DomTreeNode *> Children(OldNode->begin(), OldNode->end());
710
711 DomTreeNode *NewNode = DT->addNewBlock(Tail, Head);
712 for (auto Child : Children)
713 DT->changeImmediateDominator(Child, NewNode);
714
715 // Head dominates ThenBlock.
716 DT->addNewBlock(ThenBlock, Head);
717 }
718 }
719
720 return CheckTerm;
721 }
722
723 /// SplitBlockAndInsertIfThenElse is similar to SplitBlockAndInsertIfThen,
724 /// but also creates the ElseBlock.
725 /// Before:
726 /// Head
727 /// SplitBefore
728 /// Tail
729 /// After:
730 /// Head
731 /// if (Cond)
732 /// ThenBlock
733 /// else
734 /// ElseBlock
735 /// SplitBefore
736 /// Tail
SplitBlockAndInsertIfThenElse(Value * Cond,Instruction * SplitBefore,TerminatorInst ** ThenTerm,TerminatorInst ** ElseTerm,MDNode * BranchWeights)737 void llvm::SplitBlockAndInsertIfThenElse(Value *Cond, Instruction *SplitBefore,
738 TerminatorInst **ThenTerm,
739 TerminatorInst **ElseTerm,
740 MDNode *BranchWeights) {
741 BasicBlock *Head = SplitBefore->getParent();
742 BasicBlock *Tail = Head->splitBasicBlock(SplitBefore);
743 TerminatorInst *HeadOldTerm = Head->getTerminator();
744 LLVMContext &C = Head->getContext();
745 BasicBlock *ThenBlock = BasicBlock::Create(C, "", Head->getParent(), Tail);
746 BasicBlock *ElseBlock = BasicBlock::Create(C, "", Head->getParent(), Tail);
747 *ThenTerm = BranchInst::Create(Tail, ThenBlock);
748 (*ThenTerm)->setDebugLoc(SplitBefore->getDebugLoc());
749 *ElseTerm = BranchInst::Create(Tail, ElseBlock);
750 (*ElseTerm)->setDebugLoc(SplitBefore->getDebugLoc());
751 BranchInst *HeadNewTerm =
752 BranchInst::Create(/*ifTrue*/ThenBlock, /*ifFalse*/ElseBlock, Cond);
753 HeadNewTerm->setDebugLoc(SplitBefore->getDebugLoc());
754 HeadNewTerm->setMetadata(LLVMContext::MD_prof, BranchWeights);
755 ReplaceInstWithInst(HeadOldTerm, HeadNewTerm);
756 }
757
758
759 /// GetIfCondition - Given a basic block (BB) with two predecessors,
760 /// check to see if the merge at this block is due
761 /// to an "if condition". If so, return the boolean condition that determines
762 /// which entry into BB will be taken. Also, return by references the block
763 /// that will be entered from if the condition is true, and the block that will
764 /// be entered if the condition is false.
765 ///
766 /// This does no checking to see if the true/false blocks have large or unsavory
767 /// instructions in them.
GetIfCondition(BasicBlock * BB,BasicBlock * & IfTrue,BasicBlock * & IfFalse)768 Value *llvm::GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue,
769 BasicBlock *&IfFalse) {
770 PHINode *SomePHI = dyn_cast<PHINode>(BB->begin());
771 BasicBlock *Pred1 = nullptr;
772 BasicBlock *Pred2 = nullptr;
773
774 if (SomePHI) {
775 if (SomePHI->getNumIncomingValues() != 2)
776 return nullptr;
777 Pred1 = SomePHI->getIncomingBlock(0);
778 Pred2 = SomePHI->getIncomingBlock(1);
779 } else {
780 pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
781 if (PI == PE) // No predecessor
782 return nullptr;
783 Pred1 = *PI++;
784 if (PI == PE) // Only one predecessor
785 return nullptr;
786 Pred2 = *PI++;
787 if (PI != PE) // More than two predecessors
788 return nullptr;
789 }
790
791 // We can only handle branches. Other control flow will be lowered to
792 // branches if possible anyway.
793 BranchInst *Pred1Br = dyn_cast<BranchInst>(Pred1->getTerminator());
794 BranchInst *Pred2Br = dyn_cast<BranchInst>(Pred2->getTerminator());
795 if (!Pred1Br || !Pred2Br)
796 return nullptr;
797
798 // Eliminate code duplication by ensuring that Pred1Br is conditional if
799 // either are.
800 if (Pred2Br->isConditional()) {
801 // If both branches are conditional, we don't have an "if statement". In
802 // reality, we could transform this case, but since the condition will be
803 // required anyway, we stand no chance of eliminating it, so the xform is
804 // probably not profitable.
805 if (Pred1Br->isConditional())
806 return nullptr;
807
808 std::swap(Pred1, Pred2);
809 std::swap(Pred1Br, Pred2Br);
810 }
811
812 if (Pred1Br->isConditional()) {
813 // The only thing we have to watch out for here is to make sure that Pred2
814 // doesn't have incoming edges from other blocks. If it does, the condition
815 // doesn't dominate BB.
816 if (!Pred2->getSinglePredecessor())
817 return nullptr;
818
819 // If we found a conditional branch predecessor, make sure that it branches
820 // to BB and Pred2Br. If it doesn't, this isn't an "if statement".
821 if (Pred1Br->getSuccessor(0) == BB &&
822 Pred1Br->getSuccessor(1) == Pred2) {
823 IfTrue = Pred1;
824 IfFalse = Pred2;
825 } else if (Pred1Br->getSuccessor(0) == Pred2 &&
826 Pred1Br->getSuccessor(1) == BB) {
827 IfTrue = Pred2;
828 IfFalse = Pred1;
829 } else {
830 // We know that one arm of the conditional goes to BB, so the other must
831 // go somewhere unrelated, and this must not be an "if statement".
832 return nullptr;
833 }
834
835 return Pred1Br->getCondition();
836 }
837
838 // Ok, if we got here, both predecessors end with an unconditional branch to
839 // BB. Don't panic! If both blocks only have a single (identical)
840 // predecessor, and THAT is a conditional branch, then we're all ok!
841 BasicBlock *CommonPred = Pred1->getSinglePredecessor();
842 if (CommonPred == nullptr || CommonPred != Pred2->getSinglePredecessor())
843 return nullptr;
844
845 // Otherwise, if this is a conditional branch, then we can use it!
846 BranchInst *BI = dyn_cast<BranchInst>(CommonPred->getTerminator());
847 if (!BI) return nullptr;
848
849 assert(BI->isConditional() && "Two successors but not conditional?");
850 if (BI->getSuccessor(0) == Pred1) {
851 IfTrue = Pred1;
852 IfFalse = Pred2;
853 } else {
854 IfTrue = Pred2;
855 IfFalse = Pred1;
856 }
857 return BI->getCondition();
858 }
859