1 //===-- BasicBlockUtils.cpp - BasicBlock Utilities -------------------------==//
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 family of functions perform manipulations on basic blocks, and
11 // instructions contained within basic blocks.
13 //===----------------------------------------------------------------------===//
15 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
16 #include "llvm/Function.h"
17 #include "llvm/Instructions.h"
18 #include "llvm/IntrinsicInst.h"
19 #include "llvm/LLVMContext.h"
20 #include "llvm/Constant.h"
21 #include "llvm/Type.h"
22 #include "llvm/Analysis/AliasAnalysis.h"
23 #include "llvm/Analysis/LoopInfo.h"
24 #include "llvm/Analysis/Dominators.h"
25 #include "llvm/Target/TargetData.h"
26 #include "llvm/Transforms/Utils/Local.h"
27 #include "llvm/Support/ErrorHandling.h"
28 #include "llvm/Support/ValueHandle.h"
32 /// DeleteDeadBlock - Delete the specified block, which must have no
34 void llvm::DeleteDeadBlock(BasicBlock *BB) {
35 assert((pred_begin(BB) == pred_end(BB) ||
36 // Can delete self loop.
37 BB->getSinglePredecessor() == BB) && "Block is not dead!");
38 TerminatorInst *BBTerm = BB->getTerminator();
40 // Loop through all of our successors and make sure they know that one
41 // of their predecessors is going away.
42 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i)
43 BBTerm->getSuccessor(i)->removePredecessor(BB);
45 // Zap all the instructions in the block.
46 while (!BB->empty()) {
47 Instruction &I = BB->back();
48 // If this instruction is used, replace uses with an arbitrary value.
49 // Because control flow can't get here, we don't care what we replace the
50 // value with. Note that since this block is unreachable, and all values
51 // contained within it must dominate their uses, that all uses will
52 // eventually be removed (they are themselves dead).
54 I.replaceAllUsesWith(UndefValue::get(I.getType()));
55 BB->getInstList().pop_back();
59 BB->eraseFromParent();
62 /// FoldSingleEntryPHINodes - We know that BB has one predecessor. If there are
63 /// any single-entry PHI nodes in it, fold them away. This handles the case
64 /// when all entries to the PHI nodes in a block are guaranteed equal, such as
65 /// when the block has exactly one predecessor.
66 void llvm::FoldSingleEntryPHINodes(BasicBlock *BB) {
67 if (!isa<PHINode>(BB->begin()))
70 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
71 if (PN->getIncomingValue(0) != PN)
72 PN->replaceAllUsesWith(PN->getIncomingValue(0));
74 PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
75 PN->eraseFromParent();
80 /// DeleteDeadPHIs - Examine each PHI in the given block and delete it if it
81 /// is dead. Also recursively delete any operands that become dead as
82 /// a result. This includes tracing the def-use list from the PHI to see if
83 /// it is ultimately unused or if it reaches an unused cycle.
84 void llvm::DeleteDeadPHIs(BasicBlock *BB) {
85 // Recursively deleting a PHI may cause multiple PHIs to be deleted
86 // or RAUW'd undef, so use an array of WeakVH for the PHIs to delete.
87 SmallVector<WeakVH, 8> PHIs;
88 for (BasicBlock::iterator I = BB->begin();
89 PHINode *PN = dyn_cast<PHINode>(I); ++I)
92 for (unsigned i = 0, e = PHIs.size(); i != e; ++i)
93 if (PHINode *PN = dyn_cast_or_null<PHINode>(PHIs[i].operator Value*()))
94 RecursivelyDeleteDeadPHINode(PN);
97 /// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor,
98 /// if possible. The return value indicates success or failure.
99 bool llvm::MergeBlockIntoPredecessor(BasicBlock* BB, Pass* P) {
100 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
101 // Can't merge the entry block.
102 if (pred_begin(BB) == pred_end(BB)) return false;
104 BasicBlock *PredBB = *PI++;
105 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
107 PredBB = 0; // There are multiple different predecessors...
111 // Can't merge if there are multiple predecessors.
112 if (!PredBB) return false;
113 // Don't break self-loops.
114 if (PredBB == BB) return false;
115 // Don't break invokes.
116 if (isa<InvokeInst>(PredBB->getTerminator())) return false;
118 succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB));
119 BasicBlock* OnlySucc = BB;
120 for (; SI != SE; ++SI)
121 if (*SI != OnlySucc) {
122 OnlySucc = 0; // There are multiple distinct successors!
126 // Can't merge if there are multiple successors.
127 if (!OnlySucc) return false;
129 // Can't merge if there is PHI loop.
130 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) {
131 if (PHINode *PN = dyn_cast<PHINode>(BI)) {
132 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
133 if (PN->getIncomingValue(i) == PN)
139 // Begin by getting rid of unneeded PHIs.
140 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
141 PN->replaceAllUsesWith(PN->getIncomingValue(0));
142 BB->getInstList().pop_front(); // Delete the phi node...
145 // Delete the unconditional branch from the predecessor...
146 PredBB->getInstList().pop_back();
148 // Move all definitions in the successor to the predecessor...
149 PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
151 // Make all PHI nodes that referred to BB now refer to Pred as their
153 BB->replaceAllUsesWith(PredBB);
155 // Inherit predecessors name if it exists.
156 if (!PredBB->hasName())
157 PredBB->takeName(BB);
159 // Finally, erase the old block and update dominator info.
161 if (DominatorTree* DT = P->getAnalysisIfAvailable<DominatorTree>()) {
162 DomTreeNode* DTN = DT->getNode(BB);
163 DomTreeNode* PredDTN = DT->getNode(PredBB);
166 SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end());
167 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(),
168 DE = Children.end(); DI != DE; ++DI)
169 DT->changeImmediateDominator(*DI, PredDTN);
176 BB->eraseFromParent();
182 /// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI)
183 /// with a value, then remove and delete the original instruction.
185 void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL,
186 BasicBlock::iterator &BI, Value *V) {
187 Instruction &I = *BI;
188 // Replaces all of the uses of the instruction with uses of the value
189 I.replaceAllUsesWith(V);
191 // Make sure to propagate a name if there is one already.
192 if (I.hasName() && !V->hasName())
195 // Delete the unnecessary instruction now...
200 /// ReplaceInstWithInst - Replace the instruction specified by BI with the
201 /// instruction specified by I. The original instruction is deleted and BI is
202 /// updated to point to the new instruction.
204 void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL,
205 BasicBlock::iterator &BI, Instruction *I) {
206 assert(I->getParent() == 0 &&
207 "ReplaceInstWithInst: Instruction already inserted into basic block!");
209 // Insert the new instruction into the basic block...
210 BasicBlock::iterator New = BIL.insert(BI, I);
212 // Replace all uses of the old instruction, and delete it.
213 ReplaceInstWithValue(BIL, BI, I);
215 // Move BI back to point to the newly inserted instruction
219 /// ReplaceInstWithInst - Replace the instruction specified by From with the
220 /// instruction specified by To.
222 void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) {
223 BasicBlock::iterator BI(From);
224 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To);
227 /// RemoveSuccessor - Change the specified terminator instruction such that its
228 /// successor SuccNum no longer exists. Because this reduces the outgoing
229 /// degree of the current basic block, the actual terminator instruction itself
230 /// may have to be changed. In the case where the last successor of the block
231 /// is deleted, a return instruction is inserted in its place which can cause a
232 /// surprising change in program behavior if it is not expected.
234 void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) {
235 assert(SuccNum < TI->getNumSuccessors() &&
236 "Trying to remove a nonexistant successor!");
238 // If our old successor block contains any PHI nodes, remove the entry in the
239 // PHI nodes that comes from this branch...
241 BasicBlock *BB = TI->getParent();
242 TI->getSuccessor(SuccNum)->removePredecessor(BB);
244 TerminatorInst *NewTI = 0;
245 switch (TI->getOpcode()) {
246 case Instruction::Br:
247 // If this is a conditional branch... convert to unconditional branch.
248 if (TI->getNumSuccessors() == 2) {
249 cast<BranchInst>(TI)->setUnconditionalDest(TI->getSuccessor(1-SuccNum));
250 } else { // Otherwise convert to a return instruction...
253 // Create a value to return... if the function doesn't return null...
254 if (BB->getParent()->getReturnType() != Type::VoidTy)
255 RetVal = TI->getContext().getNullValue(
256 BB->getParent()->getReturnType());
258 // Create the return...
259 NewTI = ReturnInst::Create(RetVal);
263 case Instruction::Invoke: // Should convert to call
264 case Instruction::Switch: // Should remove entry
266 case Instruction::Ret: // Cannot happen, has no successors!
267 llvm_unreachable("Unhandled terminator instruction type in RemoveSuccessor!");
270 if (NewTI) // If it's a different instruction, replace.
271 ReplaceInstWithInst(TI, NewTI);
274 /// SplitEdge - Split the edge connecting specified block. Pass P must
276 BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) {
277 TerminatorInst *LatchTerm = BB->getTerminator();
278 unsigned SuccNum = 0;
280 unsigned e = LatchTerm->getNumSuccessors();
282 for (unsigned i = 0; ; ++i) {
283 assert(i != e && "Didn't find edge?");
284 if (LatchTerm->getSuccessor(i) == Succ) {
290 // If this is a critical edge, let SplitCriticalEdge do it.
291 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, P))
292 return LatchTerm->getSuccessor(SuccNum);
294 // If the edge isn't critical, then BB has a single successor or Succ has a
295 // single pred. Split the block.
296 BasicBlock::iterator SplitPoint;
297 if (BasicBlock *SP = Succ->getSinglePredecessor()) {
298 // If the successor only has a single pred, split the top of the successor
300 assert(SP == BB && "CFG broken");
302 return SplitBlock(Succ, Succ->begin(), P);
304 // Otherwise, if BB has a single successor, split it at the bottom of the
306 assert(BB->getTerminator()->getNumSuccessors() == 1 &&
307 "Should have a single succ!");
308 return SplitBlock(BB, BB->getTerminator(), P);
312 /// SplitBlock - Split the specified block at the specified instruction - every
313 /// thing before SplitPt stays in Old and everything starting with SplitPt moves
314 /// to a new block. The two blocks are joined by an unconditional branch and
315 /// the loop info is updated.
317 BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) {
318 BasicBlock::iterator SplitIt = SplitPt;
319 while (isa<PHINode>(SplitIt))
321 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
323 // The new block lives in whichever loop the old one did.
324 if (LoopInfo* LI = P->getAnalysisIfAvailable<LoopInfo>())
325 if (Loop *L = LI->getLoopFor(Old))
326 L->addBasicBlockToLoop(New, LI->getBase());
328 if (DominatorTree *DT = P->getAnalysisIfAvailable<DominatorTree>())
330 // Old dominates New. New node domiantes all other nodes dominated by Old.
331 DomTreeNode *OldNode = DT->getNode(Old);
332 std::vector<DomTreeNode *> Children;
333 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end();
335 Children.push_back(*I);
337 DomTreeNode *NewNode = DT->addNewBlock(New,Old);
339 for (std::vector<DomTreeNode *>::iterator I = Children.begin(),
340 E = Children.end(); I != E; ++I)
341 DT->changeImmediateDominator(*I, NewNode);
344 if (DominanceFrontier *DF = P->getAnalysisIfAvailable<DominanceFrontier>())
351 /// SplitBlockPredecessors - This method transforms BB by introducing a new
352 /// basic block into the function, and moving some of the predecessors of BB to
353 /// be predecessors of the new block. The new predecessors are indicated by the
354 /// Preds array, which has NumPreds elements in it. The new block is given a
355 /// suffix of 'Suffix'.
357 /// This currently updates the LLVM IR, AliasAnalysis, DominatorTree and
358 /// DominanceFrontier, but no other analyses.
359 BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
360 BasicBlock *const *Preds,
361 unsigned NumPreds, const char *Suffix,
363 // Create new basic block, insert right before the original block.
365 BasicBlock::Create(BB->getName()+Suffix, BB->getParent(), BB);
367 // The new block unconditionally branches to the old block.
368 BranchInst *BI = BranchInst::Create(BB, NewBB);
370 // Move the edges from Preds to point to NewBB instead of BB.
371 for (unsigned i = 0; i != NumPreds; ++i)
372 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB);
374 // Update dominator tree and dominator frontier if available.
375 DominatorTree *DT = P ? P->getAnalysisIfAvailable<DominatorTree>() : 0;
377 DT->splitBlock(NewBB);
378 if (DominanceFrontier *DF = P ? P->getAnalysisIfAvailable<DominanceFrontier>():0)
379 DF->splitBlock(NewBB);
380 AliasAnalysis *AA = P ? P->getAnalysisIfAvailable<AliasAnalysis>() : 0;
383 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI
384 // node becomes an incoming value for BB's phi node. However, if the Preds
385 // list is empty, we need to insert dummy entries into the PHI nodes in BB to
386 // account for the newly created predecessor.
388 // Insert dummy values as the incoming value.
389 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I)
390 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB);
394 // Otherwise, create a new PHI node in NewBB for each PHI node in BB.
395 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) {
396 PHINode *PN = cast<PHINode>(I++);
398 // Check to see if all of the values coming in are the same. If so, we
399 // don't need to create a new PHI node.
400 Value *InVal = PN->getIncomingValueForBlock(Preds[0]);
401 for (unsigned i = 1; i != NumPreds; ++i)
402 if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
408 // If all incoming values for the new PHI would be the same, just don't
409 // make a new PHI. Instead, just remove the incoming values from the old
411 for (unsigned i = 0; i != NumPreds; ++i)
412 PN->removeIncomingValue(Preds[i], false);
414 // If the values coming into the block are not the same, we need a PHI.
415 // Create the new PHI node, insert it into NewBB at the end of the block
417 PHINode::Create(PN->getType(), PN->getName()+".ph", BI);
418 if (AA) AA->copyValue(PN, NewPHI);
420 // Move all of the PHI values for 'Preds' to the new PHI.
421 for (unsigned i = 0; i != NumPreds; ++i) {
422 Value *V = PN->removeIncomingValue(Preds[i], false);
423 NewPHI->addIncoming(V, Preds[i]);
428 // Add an incoming value to the PHI node in the loop for the preheader
430 PN->addIncoming(InVal, NewBB);
432 // Check to see if we can eliminate this phi node.
433 if (Value *V = PN->hasConstantValue(DT != 0)) {
434 Instruction *I = dyn_cast<Instruction>(V);
435 if (!I || DT == 0 || DT->dominates(I, PN)) {
436 PN->replaceAllUsesWith(V);
437 if (AA) AA->deleteValue(PN);
438 PN->eraseFromParent();
446 /// FindFunctionBackedges - Analyze the specified function to find all of the
447 /// loop backedges in the function and return them. This is a relatively cheap
448 /// (compared to computing dominators and loop info) analysis.
450 /// The output is added to Result, as pairs of <from,to> edge info.
451 void llvm::FindFunctionBackedges(const Function &F,
452 SmallVectorImpl<std::pair<const BasicBlock*,const BasicBlock*> > &Result) {
453 const BasicBlock *BB = &F.getEntryBlock();
454 if (succ_begin(BB) == succ_end(BB))
457 SmallPtrSet<const BasicBlock*, 8> Visited;
458 SmallVector<std::pair<const BasicBlock*, succ_const_iterator>, 8> VisitStack;
459 SmallPtrSet<const BasicBlock*, 8> InStack;
462 VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
465 std::pair<const BasicBlock*, succ_const_iterator> &Top = VisitStack.back();
466 const BasicBlock *ParentBB = Top.first;
467 succ_const_iterator &I = Top.second;
469 bool FoundNew = false;
470 while (I != succ_end(ParentBB)) {
472 if (Visited.insert(BB)) {
476 // Successor is in VisitStack, it's a back edge.
477 if (InStack.count(BB))
478 Result.push_back(std::make_pair(ParentBB, BB));
482 // Go down one level if there is a unvisited successor.
484 VisitStack.push_back(std::make_pair(BB, succ_begin(BB)));
487 InStack.erase(VisitStack.pop_back_val().first);
489 } while (!VisitStack.empty());
496 /// AreEquivalentAddressValues - Test if A and B will obviously have the same
497 /// value. This includes recognizing that %t0 and %t1 will have the same
498 /// value in code like this:
499 /// %t0 = getelementptr \@a, 0, 3
500 /// store i32 0, i32* %t0
501 /// %t1 = getelementptr \@a, 0, 3
502 /// %t2 = load i32* %t1
504 static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
505 // Test if the values are trivially equivalent.
506 if (A == B) return true;
508 // Test if the values come form identical arithmetic instructions.
509 if (isa<BinaryOperator>(A) || isa<CastInst>(A) ||
510 isa<PHINode>(A) || isa<GetElementPtrInst>(A))
511 if (const Instruction *BI = dyn_cast<Instruction>(B))
512 if (cast<Instruction>(A)->isIdenticalTo(BI))
515 // Otherwise they may not be equivalent.
519 /// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the
520 /// instruction before ScanFrom) checking to see if we have the value at the
521 /// memory address *Ptr locally available within a small number of instructions.
522 /// If the value is available, return it.
524 /// If not, return the iterator for the last validated instruction that the
525 /// value would be live through. If we scanned the entire block and didn't find
526 /// something that invalidates *Ptr or provides it, ScanFrom would be left at
527 /// begin() and this returns null. ScanFrom could also be left
529 /// MaxInstsToScan specifies the maximum instructions to scan in the block. If
530 /// it is set to 0, it will scan the whole block. You can also optionally
531 /// specify an alias analysis implementation, which makes this more precise.
532 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
533 BasicBlock::iterator &ScanFrom,
534 unsigned MaxInstsToScan,
536 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U;
538 // If we're using alias analysis to disambiguate get the size of *Ptr.
539 unsigned AccessSize = 0;
541 const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
542 AccessSize = AA->getTypeStoreSize(AccessTy);
545 while (ScanFrom != ScanBB->begin()) {
546 // We must ignore debug info directives when counting (otherwise they
547 // would affect codegen).
548 Instruction *Inst = --ScanFrom;
549 if (isa<DbgInfoIntrinsic>(Inst))
551 // We skip pointer-to-pointer bitcasts, which are NOPs.
552 // It is necessary for correctness to skip those that feed into a
553 // llvm.dbg.declare, as these are not present when debugging is off.
554 if (isa<BitCastInst>(Inst) && isa<PointerType>(Inst->getType()))
557 // Restore ScanFrom to expected value in case next test succeeds
560 // Don't scan huge blocks.
561 if (MaxInstsToScan-- == 0) return 0;
564 // If this is a load of Ptr, the loaded value is available.
565 if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
566 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr))
569 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
570 // If this is a store through Ptr, the value is available!
571 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr))
572 return SI->getOperand(0);
574 // If Ptr is an alloca and this is a store to a different alloca, ignore
575 // the store. This is a trivial form of alias analysis that is important
576 // for reg2mem'd code.
577 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
578 (isa<AllocaInst>(SI->getOperand(1)) ||
579 isa<GlobalVariable>(SI->getOperand(1))))
582 // If we have alias analysis and it says the store won't modify the loaded
583 // value, ignore the store.
585 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
588 // Otherwise the store that may or may not alias the pointer, bail out.
593 // If this is some other instruction that may clobber Ptr, bail out.
594 if (Inst->mayWriteToMemory()) {
595 // If alias analysis claims that it really won't modify the load,
598 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
601 // May modify the pointer, bail out.
607 // Got to the start of the block, we didn't find it, but are done for this
612 /// CopyPrecedingStopPoint - If I is immediately preceded by a StopPoint,
613 /// make a copy of the stoppoint before InsertPos (presumably before copying
615 void llvm::CopyPrecedingStopPoint(Instruction *I,
616 BasicBlock::iterator InsertPos) {
617 if (I != I->getParent()->begin()) {
618 BasicBlock::iterator BBI = I; --BBI;
619 if (DbgStopPointInst *DSPI = dyn_cast<DbgStopPointInst>(BBI)) {
620 CallInst *newDSPI = DSPI->clone(I->getContext());
621 newDSPI->insertBefore(InsertPos);