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/Constant.h"
19 #include "llvm/Type.h"
20 #include "llvm/Analysis/AliasAnalysis.h"
21 #include "llvm/Analysis/LoopInfo.h"
22 #include "llvm/Analysis/Dominators.h"
23 #include "llvm/Target/TargetData.h"
27 /// DeleteDeadBlock - Delete the specified block, which must have no
29 void llvm::DeleteDeadBlock(BasicBlock *BB) {
30 assert((pred_begin(BB) == pred_end(BB) ||
31 // Can delete self loop.
32 BB->getSinglePredecessor() == BB) && "Block is not dead!");
33 TerminatorInst *BBTerm = BB->getTerminator();
35 // Loop through all of our successors and make sure they know that one
36 // of their predecessors is going away.
37 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i)
38 BBTerm->getSuccessor(i)->removePredecessor(BB);
40 // Zap all the instructions in the block.
41 while (!BB->empty()) {
42 Instruction &I = BB->back();
43 // If this instruction is used, replace uses with an arbitrary value.
44 // Because control flow can't get here, we don't care what we replace the
45 // value with. Note that since this block is unreachable, and all values
46 // contained within it must dominate their uses, that all uses will
47 // eventually be removed (they are themselves dead).
49 I.replaceAllUsesWith(UndefValue::get(I.getType()));
50 BB->getInstList().pop_back();
54 BB->eraseFromParent();
57 /// FoldSingleEntryPHINodes - We know that BB has one predecessor. If there are
58 /// any single-entry PHI nodes in it, fold them away. This handles the case
59 /// when all entries to the PHI nodes in a block are guaranteed equal, such as
60 /// when the block has exactly one predecessor.
61 void llvm::FoldSingleEntryPHINodes(BasicBlock *BB) {
62 if (!isa<PHINode>(BB->begin()))
65 while (PHINode *PN = dyn_cast<PHINode>(BB->begin())) {
66 if (PN->getIncomingValue(0) != PN)
67 PN->replaceAllUsesWith(PN->getIncomingValue(0));
69 PN->replaceAllUsesWith(UndefValue::get(PN->getType()));
70 PN->eraseFromParent();
75 /// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor,
76 /// if possible. The return value indicates success or failure.
77 bool llvm::MergeBlockIntoPredecessor(BasicBlock* BB, Pass* P) {
78 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
79 // Can't merge the entry block.
80 if (pred_begin(BB) == pred_end(BB)) return false;
82 BasicBlock *PredBB = *PI++;
83 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
85 PredBB = 0; // There are multiple different predecessors...
89 // Can't merge if there are multiple predecessors.
90 if (!PredBB) return false;
91 // Don't break self-loops.
92 if (PredBB == BB) return false;
93 // Don't break invokes.
94 if (isa<InvokeInst>(PredBB->getTerminator())) return false;
96 succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB));
97 BasicBlock* OnlySucc = BB;
98 for (; SI != SE; ++SI)
99 if (*SI != OnlySucc) {
100 OnlySucc = 0; // There are multiple distinct successors!
104 // Can't merge if there are multiple successors.
105 if (!OnlySucc) return false;
107 // Can't merge if there is PHI loop.
108 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) {
109 if (PHINode *PN = dyn_cast<PHINode>(BI)) {
110 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
111 if (PN->getIncomingValue(i) == PN)
117 // Begin by getting rid of unneeded PHIs.
118 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
119 PN->replaceAllUsesWith(PN->getIncomingValue(0));
120 BB->getInstList().pop_front(); // Delete the phi node...
123 // Delete the unconditional branch from the predecessor...
124 PredBB->getInstList().pop_back();
126 // Move all definitions in the successor to the predecessor...
127 PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
129 // Make all PHI nodes that referred to BB now refer to Pred as their
131 BB->replaceAllUsesWith(PredBB);
133 // Inherit predecessors name if it exists.
134 if (!PredBB->hasName())
135 PredBB->takeName(BB);
137 // Finally, erase the old block and update dominator info.
139 if (DominatorTree* DT = P->getAnalysisToUpdate<DominatorTree>()) {
140 DomTreeNode* DTN = DT->getNode(BB);
141 DomTreeNode* PredDTN = DT->getNode(PredBB);
144 SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end());
145 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(),
146 DE = Children.end(); DI != DE; ++DI)
147 DT->changeImmediateDominator(*DI, PredDTN);
154 BB->eraseFromParent();
160 /// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI)
161 /// with a value, then remove and delete the original instruction.
163 void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL,
164 BasicBlock::iterator &BI, Value *V) {
165 Instruction &I = *BI;
166 // Replaces all of the uses of the instruction with uses of the value
167 I.replaceAllUsesWith(V);
169 // Make sure to propagate a name if there is one already.
170 if (I.hasName() && !V->hasName())
173 // Delete the unnecessary instruction now...
178 /// ReplaceInstWithInst - Replace the instruction specified by BI with the
179 /// instruction specified by I. The original instruction is deleted and BI is
180 /// updated to point to the new instruction.
182 void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL,
183 BasicBlock::iterator &BI, Instruction *I) {
184 assert(I->getParent() == 0 &&
185 "ReplaceInstWithInst: Instruction already inserted into basic block!");
187 // Insert the new instruction into the basic block...
188 BasicBlock::iterator New = BIL.insert(BI, I);
190 // Replace all uses of the old instruction, and delete it.
191 ReplaceInstWithValue(BIL, BI, I);
193 // Move BI back to point to the newly inserted instruction
197 /// ReplaceInstWithInst - Replace the instruction specified by From with the
198 /// instruction specified by To.
200 void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) {
201 BasicBlock::iterator BI(From);
202 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To);
205 /// RemoveSuccessor - Change the specified terminator instruction such that its
206 /// successor SuccNum no longer exists. Because this reduces the outgoing
207 /// degree of the current basic block, the actual terminator instruction itself
208 /// may have to be changed. In the case where the last successor of the block
209 /// is deleted, a return instruction is inserted in its place which can cause a
210 /// surprising change in program behavior if it is not expected.
212 void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) {
213 assert(SuccNum < TI->getNumSuccessors() &&
214 "Trying to remove a nonexistant successor!");
216 // If our old successor block contains any PHI nodes, remove the entry in the
217 // PHI nodes that comes from this branch...
219 BasicBlock *BB = TI->getParent();
220 TI->getSuccessor(SuccNum)->removePredecessor(BB);
222 TerminatorInst *NewTI = 0;
223 switch (TI->getOpcode()) {
224 case Instruction::Br:
225 // If this is a conditional branch... convert to unconditional branch.
226 if (TI->getNumSuccessors() == 2) {
227 cast<BranchInst>(TI)->setUnconditionalDest(TI->getSuccessor(1-SuccNum));
228 } else { // Otherwise convert to a return instruction...
231 // Create a value to return... if the function doesn't return null...
232 if (BB->getParent()->getReturnType() != Type::VoidTy)
233 RetVal = Constant::getNullValue(BB->getParent()->getReturnType());
235 // Create the return...
236 NewTI = ReturnInst::Create(RetVal);
240 case Instruction::Invoke: // Should convert to call
241 case Instruction::Switch: // Should remove entry
243 case Instruction::Ret: // Cannot happen, has no successors!
244 assert(0 && "Unhandled terminator instruction type in RemoveSuccessor!");
248 if (NewTI) // If it's a different instruction, replace.
249 ReplaceInstWithInst(TI, NewTI);
252 /// SplitEdge - Split the edge connecting specified block. Pass P must
254 BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) {
255 TerminatorInst *LatchTerm = BB->getTerminator();
256 unsigned SuccNum = 0;
258 unsigned e = LatchTerm->getNumSuccessors();
260 for (unsigned i = 0; ; ++i) {
261 assert(i != e && "Didn't find edge?");
262 if (LatchTerm->getSuccessor(i) == Succ) {
268 // If this is a critical edge, let SplitCriticalEdge do it.
269 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, P))
270 return LatchTerm->getSuccessor(SuccNum);
272 // If the edge isn't critical, then BB has a single successor or Succ has a
273 // single pred. Split the block.
274 BasicBlock::iterator SplitPoint;
275 if (BasicBlock *SP = Succ->getSinglePredecessor()) {
276 // If the successor only has a single pred, split the top of the successor
278 assert(SP == BB && "CFG broken");
280 return SplitBlock(Succ, Succ->begin(), P);
282 // Otherwise, if BB has a single successor, split it at the bottom of the
284 assert(BB->getTerminator()->getNumSuccessors() == 1 &&
285 "Should have a single succ!");
286 return SplitBlock(BB, BB->getTerminator(), P);
290 /// SplitBlock - Split the specified block at the specified instruction - every
291 /// thing before SplitPt stays in Old and everything starting with SplitPt moves
292 /// to a new block. The two blocks are joined by an unconditional branch and
293 /// the loop info is updated.
295 BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) {
296 BasicBlock::iterator SplitIt = SplitPt;
297 while (isa<PHINode>(SplitIt))
299 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
301 // The new block lives in whichever loop the old one did.
302 if (LoopInfo* LI = P->getAnalysisToUpdate<LoopInfo>())
303 if (Loop *L = LI->getLoopFor(Old))
304 L->addBasicBlockToLoop(New, LI->getBase());
306 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>())
308 // Old dominates New. New node domiantes all other nodes dominated by Old.
309 DomTreeNode *OldNode = DT->getNode(Old);
310 std::vector<DomTreeNode *> Children;
311 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end();
313 Children.push_back(*I);
315 DomTreeNode *NewNode = DT->addNewBlock(New,Old);
317 for (std::vector<DomTreeNode *>::iterator I = Children.begin(),
318 E = Children.end(); I != E; ++I)
319 DT->changeImmediateDominator(*I, NewNode);
322 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>())
329 /// SplitBlockPredecessors - This method transforms BB by introducing a new
330 /// basic block into the function, and moving some of the predecessors of BB to
331 /// be predecessors of the new block. The new predecessors are indicated by the
332 /// Preds array, which has NumPreds elements in it. The new block is given a
333 /// suffix of 'Suffix'.
335 /// This currently updates the LLVM IR, AliasAnalysis, DominatorTree and
336 /// DominanceFrontier, but no other analyses.
337 BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
338 BasicBlock *const *Preds,
339 unsigned NumPreds, const char *Suffix,
341 // Create new basic block, insert right before the original block.
343 BasicBlock::Create(BB->getName()+Suffix, BB->getParent(), BB);
345 // The new block unconditionally branches to the old block.
346 BranchInst *BI = BranchInst::Create(BB, NewBB);
348 // Move the edges from Preds to point to NewBB instead of BB.
349 for (unsigned i = 0; i != NumPreds; ++i)
350 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB);
352 // Update dominator tree and dominator frontier if available.
353 DominatorTree *DT = P ? P->getAnalysisToUpdate<DominatorTree>() : 0;
355 DT->splitBlock(NewBB);
356 if (DominanceFrontier *DF = P ? P->getAnalysisToUpdate<DominanceFrontier>():0)
357 DF->splitBlock(NewBB);
358 AliasAnalysis *AA = P ? P->getAnalysisToUpdate<AliasAnalysis>() : 0;
361 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI
362 // node becomes an incoming value for BB's phi node. However, if the Preds
363 // list is empty, we need to insert dummy entries into the PHI nodes in BB to
364 // account for the newly created predecessor.
366 // Insert dummy values as the incoming value.
367 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I)
368 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB);
372 // Otherwise, create a new PHI node in NewBB for each PHI node in BB.
373 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) {
374 PHINode *PN = cast<PHINode>(I++);
376 // Check to see if all of the values coming in are the same. If so, we
377 // don't need to create a new PHI node.
378 Value *InVal = PN->getIncomingValueForBlock(Preds[0]);
379 for (unsigned i = 1; i != NumPreds; ++i)
380 if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
386 // If all incoming values for the new PHI would be the same, just don't
387 // make a new PHI. Instead, just remove the incoming values from the old
389 for (unsigned i = 0; i != NumPreds; ++i)
390 PN->removeIncomingValue(Preds[i], false);
392 // If the values coming into the block are not the same, we need a PHI.
393 // Create the new PHI node, insert it into NewBB at the end of the block
395 PHINode::Create(PN->getType(), PN->getName()+".ph", BI);
396 if (AA) AA->copyValue(PN, NewPHI);
398 // Move all of the PHI values for 'Preds' to the new PHI.
399 for (unsigned i = 0; i != NumPreds; ++i) {
400 Value *V = PN->removeIncomingValue(Preds[i], false);
401 NewPHI->addIncoming(V, Preds[i]);
406 // Add an incoming value to the PHI node in the loop for the preheader
408 PN->addIncoming(InVal, NewBB);
410 // Check to see if we can eliminate this phi node.
411 if (Value *V = PN->hasConstantValue(DT != 0)) {
412 Instruction *I = dyn_cast<Instruction>(V);
413 if (!I || DT == 0 || DT->dominates(I, PN)) {
414 PN->replaceAllUsesWith(V);
415 if (AA) AA->deleteValue(PN);
416 PN->eraseFromParent();
424 /// AreEquivalentAddressValues - Test if A and B will obviously have the same
425 /// value. This includes recognizing that %t0 and %t1 will have the same
426 /// value in code like this:
427 /// %t0 = getelementptr @a, 0, 3
428 /// store i32 0, i32* %t0
429 /// %t1 = getelementptr @a, 0, 3
430 /// %t2 = load i32* %t1
432 static bool AreEquivalentAddressValues(const Value *A, const Value *B) {
433 // Test if the values are trivially equivalent.
434 if (A == B) return true;
436 // Test if the values come form identical arithmetic instructions.
437 if (isa<BinaryOperator>(A) || isa<CastInst>(A) ||
438 isa<PHINode>(A) || isa<GetElementPtrInst>(A))
439 if (const Instruction *BI = dyn_cast<Instruction>(B))
440 if (cast<Instruction>(A)->isIdenticalTo(BI))
443 // Otherwise they may not be equivalent.
447 /// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the
448 /// instruction before ScanFrom) checking to see if we have the value at the
449 /// memory address *Ptr locally available within a small number of instructions.
450 /// If the value is available, return it.
452 /// If not, return the iterator for the last validated instruction that the
453 /// value would be live through. If we scanned the entire block and didn't find
454 /// something that invalidates *Ptr or provides it, ScanFrom would be left at
455 /// begin() and this returns null. ScanFrom could also be left
457 /// MaxInstsToScan specifies the maximum instructions to scan in the block. If
458 /// it is set to 0, it will scan the whole block. You can also optionally
459 /// specify an alias analysis implementation, which makes this more precise.
460 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
461 BasicBlock::iterator &ScanFrom,
462 unsigned MaxInstsToScan,
464 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U;
466 // If we're using alias analysis to disambiguate get the size of *Ptr.
467 unsigned AccessSize = 0;
469 const Type *AccessTy = cast<PointerType>(Ptr->getType())->getElementType();
470 AccessSize = AA->getTargetData().getTypeStoreSizeInBits(AccessTy);
473 while (ScanFrom != ScanBB->begin()) {
474 // Don't scan huge blocks.
475 if (MaxInstsToScan-- == 0) return 0;
477 Instruction *Inst = --ScanFrom;
479 // If this is a load of Ptr, the loaded value is available.
480 if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
481 if (AreEquivalentAddressValues(LI->getOperand(0), Ptr))
484 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
485 // If this is a store through Ptr, the value is available!
486 if (AreEquivalentAddressValues(SI->getOperand(1), Ptr))
487 return SI->getOperand(0);
489 // If Ptr is an alloca and this is a store to a different alloca, ignore
490 // the store. This is a trivial form of alias analysis that is important
491 // for reg2mem'd code.
492 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
493 (isa<AllocaInst>(SI->getOperand(1)) ||
494 isa<GlobalVariable>(SI->getOperand(1))))
497 // If we have alias analysis and it says the store won't modify the loaded
498 // value, ignore the store.
500 (AA->getModRefInfo(SI, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
503 // Otherwise the store that may or may not alias the pointer, bail out.
508 // If this is some other instruction that may clobber Ptr, bail out.
509 if (Inst->mayWriteToMemory()) {
510 // If alias analysis claims that it really won't modify the load,
513 (AA->getModRefInfo(Inst, Ptr, AccessSize) & AliasAnalysis::Mod) == 0)
516 // May modify the pointer, bail out.
522 // Got to the start of the block, we didn't find it, but are done for this