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"
26 /// MergeBlockIntoPredecessor - Attempts to merge a block into its predecessor,
27 /// if possible. The return value indicates success or failure.
28 bool llvm::MergeBlockIntoPredecessor(BasicBlock* BB, Pass* P) {
29 pred_iterator PI(pred_begin(BB)), PE(pred_end(BB));
30 // Can't merge the entry block.
31 if (pred_begin(BB) == pred_end(BB)) return false;
33 BasicBlock *PredBB = *PI++;
34 for (; PI != PE; ++PI) // Search all predecessors, see if they are all same
36 PredBB = 0; // There are multiple different predecessors...
40 // Can't merge if there are multiple predecessors.
41 if (!PredBB) return false;
42 // Don't break self-loops.
43 if (PredBB == BB) return false;
44 // Don't break invokes.
45 if (isa<InvokeInst>(PredBB->getTerminator())) return false;
47 succ_iterator SI(succ_begin(PredBB)), SE(succ_end(PredBB));
48 BasicBlock* OnlySucc = BB;
49 for (; SI != SE; ++SI)
50 if (*SI != OnlySucc) {
51 OnlySucc = 0; // There are multiple distinct successors!
55 // Can't merge if there are multiple successors.
56 if (!OnlySucc) return false;
58 // Can't merge if there is PHI loop.
59 for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE; ++BI) {
60 if (PHINode *PN = dyn_cast<PHINode>(BI)) {
61 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
62 if (PN->getIncomingValue(i) == PN)
68 // Begin by getting rid of unneeded PHIs.
69 while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
70 PN->replaceAllUsesWith(PN->getIncomingValue(0));
71 BB->getInstList().pop_front(); // Delete the phi node...
74 // Delete the unconditional branch from the predecessor...
75 PredBB->getInstList().pop_back();
77 // Move all definitions in the successor to the predecessor...
78 PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
80 // Make all PHI nodes that referred to BB now refer to Pred as their
82 BB->replaceAllUsesWith(PredBB);
84 // Inherit predecessors name if it exists.
85 if (!PredBB->hasName())
88 // Finally, erase the old block and update dominator info.
90 if (DominatorTree* DT = P->getAnalysisToUpdate<DominatorTree>()) {
91 DomTreeNode* DTN = DT->getNode(BB);
92 DomTreeNode* PredDTN = DT->getNode(PredBB);
95 SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end());
96 for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(),
97 DE = Children.end(); DI != DE; ++DI)
98 DT->changeImmediateDominator(*DI, PredDTN);
105 BB->eraseFromParent();
111 /// ReplaceInstWithValue - Replace all uses of an instruction (specified by BI)
112 /// with a value, then remove and delete the original instruction.
114 void llvm::ReplaceInstWithValue(BasicBlock::InstListType &BIL,
115 BasicBlock::iterator &BI, Value *V) {
116 Instruction &I = *BI;
117 // Replaces all of the uses of the instruction with uses of the value
118 I.replaceAllUsesWith(V);
120 // Make sure to propagate a name if there is one already.
121 if (I.hasName() && !V->hasName())
124 // Delete the unnecessary instruction now...
129 /// ReplaceInstWithInst - Replace the instruction specified by BI with the
130 /// instruction specified by I. The original instruction is deleted and BI is
131 /// updated to point to the new instruction.
133 void llvm::ReplaceInstWithInst(BasicBlock::InstListType &BIL,
134 BasicBlock::iterator &BI, Instruction *I) {
135 assert(I->getParent() == 0 &&
136 "ReplaceInstWithInst: Instruction already inserted into basic block!");
138 // Insert the new instruction into the basic block...
139 BasicBlock::iterator New = BIL.insert(BI, I);
141 // Replace all uses of the old instruction, and delete it.
142 ReplaceInstWithValue(BIL, BI, I);
144 // Move BI back to point to the newly inserted instruction
148 /// ReplaceInstWithInst - Replace the instruction specified by From with the
149 /// instruction specified by To.
151 void llvm::ReplaceInstWithInst(Instruction *From, Instruction *To) {
152 BasicBlock::iterator BI(From);
153 ReplaceInstWithInst(From->getParent()->getInstList(), BI, To);
156 /// RemoveSuccessor - Change the specified terminator instruction such that its
157 /// successor SuccNum no longer exists. Because this reduces the outgoing
158 /// degree of the current basic block, the actual terminator instruction itself
159 /// may have to be changed. In the case where the last successor of the block
160 /// is deleted, a return instruction is inserted in its place which can cause a
161 /// surprising change in program behavior if it is not expected.
163 void llvm::RemoveSuccessor(TerminatorInst *TI, unsigned SuccNum) {
164 assert(SuccNum < TI->getNumSuccessors() &&
165 "Trying to remove a nonexistant successor!");
167 // If our old successor block contains any PHI nodes, remove the entry in the
168 // PHI nodes that comes from this branch...
170 BasicBlock *BB = TI->getParent();
171 TI->getSuccessor(SuccNum)->removePredecessor(BB);
173 TerminatorInst *NewTI = 0;
174 switch (TI->getOpcode()) {
175 case Instruction::Br:
176 // If this is a conditional branch... convert to unconditional branch.
177 if (TI->getNumSuccessors() == 2) {
178 cast<BranchInst>(TI)->setUnconditionalDest(TI->getSuccessor(1-SuccNum));
179 } else { // Otherwise convert to a return instruction...
182 // Create a value to return... if the function doesn't return null...
183 if (BB->getParent()->getReturnType() != Type::VoidTy)
184 RetVal = Constant::getNullValue(BB->getParent()->getReturnType());
186 // Create the return...
187 NewTI = ReturnInst::Create(RetVal);
191 case Instruction::Invoke: // Should convert to call
192 case Instruction::Switch: // Should remove entry
194 case Instruction::Ret: // Cannot happen, has no successors!
195 assert(0 && "Unhandled terminator instruction type in RemoveSuccessor!");
199 if (NewTI) // If it's a different instruction, replace.
200 ReplaceInstWithInst(TI, NewTI);
203 /// SplitEdge - Split the edge connecting specified block. Pass P must
205 BasicBlock *llvm::SplitEdge(BasicBlock *BB, BasicBlock *Succ, Pass *P) {
206 TerminatorInst *LatchTerm = BB->getTerminator();
207 unsigned SuccNum = 0;
209 unsigned e = LatchTerm->getNumSuccessors();
211 for (unsigned i = 0; ; ++i) {
212 assert(i != e && "Didn't find edge?");
213 if (LatchTerm->getSuccessor(i) == Succ) {
219 // If this is a critical edge, let SplitCriticalEdge do it.
220 if (SplitCriticalEdge(BB->getTerminator(), SuccNum, P))
221 return LatchTerm->getSuccessor(SuccNum);
223 // If the edge isn't critical, then BB has a single successor or Succ has a
224 // single pred. Split the block.
225 BasicBlock::iterator SplitPoint;
226 if (BasicBlock *SP = Succ->getSinglePredecessor()) {
227 // If the successor only has a single pred, split the top of the successor
229 assert(SP == BB && "CFG broken");
231 return SplitBlock(Succ, Succ->begin(), P);
233 // Otherwise, if BB has a single successor, split it at the bottom of the
235 assert(BB->getTerminator()->getNumSuccessors() == 1 &&
236 "Should have a single succ!");
237 return SplitBlock(BB, BB->getTerminator(), P);
241 /// SplitBlock - Split the specified block at the specified instruction - every
242 /// thing before SplitPt stays in Old and everything starting with SplitPt moves
243 /// to a new block. The two blocks are joined by an unconditional branch and
244 /// the loop info is updated.
246 BasicBlock *llvm::SplitBlock(BasicBlock *Old, Instruction *SplitPt, Pass *P) {
247 BasicBlock::iterator SplitIt = SplitPt;
248 while (isa<PHINode>(SplitIt))
250 BasicBlock *New = Old->splitBasicBlock(SplitIt, Old->getName()+".split");
252 // The new block lives in whichever loop the old one did.
253 if (LoopInfo* LI = P->getAnalysisToUpdate<LoopInfo>())
254 if (Loop *L = LI->getLoopFor(Old))
255 L->addBasicBlockToLoop(New, LI->getBase());
257 if (DominatorTree *DT = P->getAnalysisToUpdate<DominatorTree>())
259 // Old dominates New. New node domiantes all other nodes dominated by Old.
260 DomTreeNode *OldNode = DT->getNode(Old);
261 std::vector<DomTreeNode *> Children;
262 for (DomTreeNode::iterator I = OldNode->begin(), E = OldNode->end();
264 Children.push_back(*I);
266 DomTreeNode *NewNode = DT->addNewBlock(New,Old);
268 for (std::vector<DomTreeNode *>::iterator I = Children.begin(),
269 E = Children.end(); I != E; ++I)
270 DT->changeImmediateDominator(*I, NewNode);
273 if (DominanceFrontier *DF = P->getAnalysisToUpdate<DominanceFrontier>())
280 /// SplitBlockPredecessors - This method transforms BB by introducing a new
281 /// basic block into the function, and moving some of the predecessors of BB to
282 /// be predecessors of the new block. The new predecessors are indicated by the
283 /// Preds array, which has NumPreds elements in it. The new block is given a
284 /// suffix of 'Suffix'.
286 /// This currently updates the LLVM IR, AliasAnalysis, DominatorTree and
287 /// DominanceFrontier, but no other analyses.
288 BasicBlock *llvm::SplitBlockPredecessors(BasicBlock *BB,
289 BasicBlock *const *Preds,
290 unsigned NumPreds, const char *Suffix,
292 // Create new basic block, insert right before the original block.
294 BasicBlock::Create(BB->getName()+Suffix, BB->getParent(), BB);
296 // The new block unconditionally branches to the old block.
297 BranchInst *BI = BranchInst::Create(BB, NewBB);
299 // Move the edges from Preds to point to NewBB instead of BB.
300 for (unsigned i = 0; i != NumPreds; ++i)
301 Preds[i]->getTerminator()->replaceUsesOfWith(BB, NewBB);
303 // Update dominator tree and dominator frontier if available.
304 DominatorTree *DT = P ? P->getAnalysisToUpdate<DominatorTree>() : 0;
306 DT->splitBlock(NewBB);
307 if (DominanceFrontier *DF = P ? P->getAnalysisToUpdate<DominanceFrontier>():0)
308 DF->splitBlock(NewBB);
309 AliasAnalysis *AA = P ? P->getAnalysisToUpdate<AliasAnalysis>() : 0;
312 // Insert a new PHI node into NewBB for every PHI node in BB and that new PHI
313 // node becomes an incoming value for BB's phi node. However, if the Preds
314 // list is empty, we need to insert dummy entries into the PHI nodes in BB to
315 // account for the newly created predecessor.
317 // Insert dummy values as the incoming value.
318 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I)
319 cast<PHINode>(I)->addIncoming(UndefValue::get(I->getType()), NewBB);
323 // Otherwise, create a new PHI node in NewBB for each PHI node in BB.
324 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) {
325 PHINode *PN = cast<PHINode>(I++);
327 // Check to see if all of the values coming in are the same. If so, we
328 // don't need to create a new PHI node.
329 Value *InVal = PN->getIncomingValueForBlock(Preds[0]);
330 for (unsigned i = 1; i != NumPreds; ++i)
331 if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
337 // If all incoming values for the new PHI would be the same, just don't
338 // make a new PHI. Instead, just remove the incoming values from the old
340 for (unsigned i = 0; i != NumPreds; ++i)
341 PN->removeIncomingValue(Preds[i], false);
343 // If the values coming into the block are not the same, we need a PHI.
344 // Create the new PHI node, insert it into NewBB at the end of the block
346 PHINode::Create(PN->getType(), PN->getName()+".ph", BI);
347 if (AA) AA->copyValue(PN, NewPHI);
349 // Move all of the PHI values for 'Preds' to the new PHI.
350 for (unsigned i = 0; i != NumPreds; ++i) {
351 Value *V = PN->removeIncomingValue(Preds[i], false);
352 NewPHI->addIncoming(V, Preds[i]);
357 // Add an incoming value to the PHI node in the loop for the preheader
359 PN->addIncoming(InVal, NewBB);
361 // Check to see if we can eliminate this phi node.
362 if (Value *V = PN->hasConstantValue(DT != 0)) {
363 Instruction *I = dyn_cast<Instruction>(V);
364 if (!I || DT == 0 || DT->dominates(I, PN)) {
365 PN->replaceAllUsesWith(V);
366 if (AA) AA->deleteValue(PN);
367 PN->eraseFromParent();
375 /// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at the
376 /// instruction before ScanFrom) checking to see if we have the value at the
377 /// memory address *Ptr locally available within a small number of instructions.
378 /// If the value is available, return it.
380 /// If not, return the iterator for the last validated instruction that the
381 /// value would be live through. If we scanned the entire block and didn't find
382 /// something that invalidates *Ptr or provides it, ScanFrom would be left at
383 /// begin() and this returns null. ScanFrom could also be left
385 /// MaxInstsToScan specifies the maximum instructions to scan in the block. If
386 /// it is set to 0, it will scan the whole block. You can also optionally
387 /// specify an alias analysis implementation, which makes this more precise.
388 Value *llvm::FindAvailableLoadedValue(Value *Ptr, BasicBlock *ScanBB,
389 BasicBlock::iterator &ScanFrom,
390 unsigned MaxInstsToScan,
392 if (MaxInstsToScan == 0) MaxInstsToScan = ~0U;
394 while (ScanFrom != ScanBB->begin()) {
395 // Don't scan huge blocks.
396 if (MaxInstsToScan-- == 0) return 0;
398 Instruction *Inst = --ScanFrom;
400 // If this is a load of Ptr, the loaded value is available.
401 if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
402 if (LI->getOperand(0) == Ptr)
405 if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
406 // If this is a store through Ptr, the value is available!
407 if (SI->getOperand(1) == Ptr)
408 return SI->getOperand(0);
410 // If Ptr is an alloca and this is a store to a different alloca, ignore
411 // the store. This is a trivial form of alias analysis that is important
412 // for reg2mem'd code.
413 if ((isa<AllocaInst>(Ptr) || isa<GlobalVariable>(Ptr)) &&
414 (isa<AllocaInst>(SI->getOperand(1)) ||
415 isa<GlobalVariable>(SI->getOperand(1))))
418 // Otherwise the store that may or may not alias the pointer, bail out.
424 // If this is some other instruction that may clobber Ptr, bail out.
425 if (Inst->mayWriteToMemory()) {
426 // May modify the pointer, bail out.
432 // Got to the start of the block, we didn't find it, but are done for this