1 //===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass performs several transformations to transform natural loops into a
11 // simpler form, which makes subsequent analyses and transformations simpler and
14 // Loop pre-header insertion guarantees that there is a single, non-critical
15 // entry edge from outside of the loop to the loop header. This simplifies a
16 // number of analyses and transformations, such as LICM.
18 // Loop exit-block insertion guarantees that all exit blocks from the loop
19 // (blocks which are outside of the loop that have predecessors inside of the
20 // loop) only have predecessors from inside of the loop (and are thus dominated
21 // by the loop header). This simplifies transformations such as store-sinking
22 // that are built into LICM.
24 // This pass also guarantees that loops will have exactly one backedge.
26 // Note that the simplifycfg pass will clean up blocks which are split out but
27 // end up being unnecessary, so usage of this pass should not pessimize
30 // This pass obviously modifies the CFG, but updates loop information and
31 // dominator information.
33 //===----------------------------------------------------------------------===//
35 #define DEBUG_TYPE "loopsimplify"
36 #include "llvm/Transforms/Scalar.h"
37 #include "llvm/Constant.h"
38 #include "llvm/Instructions.h"
39 #include "llvm/Function.h"
40 #include "llvm/Type.h"
41 #include "llvm/Analysis/AliasAnalysis.h"
42 #include "llvm/Analysis/Dominators.h"
43 #include "llvm/Analysis/LoopInfo.h"
44 #include "llvm/Support/CFG.h"
45 #include "llvm/Support/Compiler.h"
46 #include "llvm/ADT/SetOperations.h"
47 #include "llvm/ADT/SetVector.h"
48 #include "llvm/ADT/Statistic.h"
49 #include "llvm/ADT/DepthFirstIterator.h"
52 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
53 STATISTIC(NumNested , "Number of nested loops split out");
56 struct VISIBILITY_HIDDEN LoopSimplify : public FunctionPass {
57 static char ID; // Pass identification, replacement for typeid
58 LoopSimplify() : FunctionPass((intptr_t)&ID) {}
60 // AA - If we have an alias analysis object to update, this is it, otherwise
65 virtual bool runOnFunction(Function &F);
67 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
68 // We need loop information to identify the loops...
69 AU.addRequired<LoopInfo>();
70 AU.addRequired<DominatorTree>();
72 AU.addPreserved<LoopInfo>();
73 AU.addPreserved<DominatorTree>();
74 AU.addPreserved<DominanceFrontier>();
75 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
78 bool ProcessLoop(Loop *L);
79 BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix,
80 const std::vector<BasicBlock*> &Preds);
81 BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
82 void InsertPreheaderForLoop(Loop *L);
83 Loop *SeparateNestedLoop(Loop *L);
84 void InsertUniqueBackedgeBlock(Loop *L);
85 void PlaceSplitBlockCarefully(BasicBlock *NewBB,
86 std::vector<BasicBlock*> &SplitPreds,
89 void UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB,
90 std::vector<BasicBlock*> &PredBlocks);
93 char LoopSimplify::ID = 0;
94 RegisterPass<LoopSimplify>
95 X("loopsimplify", "Canonicalize natural loops", true);
98 // Publically exposed interface to pass...
99 const PassInfo *llvm::LoopSimplifyID = X.getPassInfo();
100 FunctionPass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
102 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
103 /// it in any convenient order) inserting preheaders...
105 bool LoopSimplify::runOnFunction(Function &F) {
106 bool Changed = false;
107 LI = &getAnalysis<LoopInfo>();
108 AA = getAnalysisToUpdate<AliasAnalysis>();
110 // Check to see that no blocks (other than the header) in loops have
111 // predecessors that are not in loops. This is not valid for natural loops,
112 // but can occur if the blocks are unreachable. Since they are unreachable we
113 // can just shamelessly destroy their terminators to make them not branch into
115 for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
116 // This case can only occur for unreachable blocks. Blocks that are
117 // unreachable can't be in loops, so filter those blocks out.
118 if (LI->getLoopFor(BB)) continue;
120 bool BlockUnreachable = false;
121 TerminatorInst *TI = BB->getTerminator();
123 // Check to see if any successors of this block are non-loop-header loops
124 // that are not the header.
125 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
126 // If this successor is not in a loop, BB is clearly ok.
127 Loop *L = LI->getLoopFor(TI->getSuccessor(i));
130 // If the succ is the loop header, and if L is a top-level loop, then this
131 // is an entrance into a loop through the header, which is also ok.
132 if (L->getHeader() == TI->getSuccessor(i) && L->getParentLoop() == 0)
135 // Otherwise, this is an entrance into a loop from some place invalid.
136 // Either the loop structure is invalid and this is not a natural loop (in
137 // which case the compiler is buggy somewhere else) or BB is unreachable.
138 BlockUnreachable = true;
142 // If this block is ok, check the next one.
143 if (!BlockUnreachable) continue;
145 // Otherwise, this block is dead. To clean up the CFG and to allow later
146 // loop transformations to ignore this case, we delete the edges into the
147 // loop by replacing the terminator.
149 // Remove PHI entries from the successors.
150 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
151 TI->getSuccessor(i)->removePredecessor(BB);
153 // Add a new unreachable instruction.
154 new UnreachableInst(TI);
156 // Delete the dead terminator.
157 if (AA) AA->deleteValue(&BB->back());
158 BB->getInstList().pop_back();
162 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
163 Changed |= ProcessLoop(*I);
168 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
169 /// all loops have preheaders.
171 bool LoopSimplify::ProcessLoop(Loop *L) {
172 bool Changed = false;
175 // Canonicalize inner loops before outer loops. Inner loop canonicalization
176 // can provide work for the outer loop to canonicalize.
177 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
178 Changed |= ProcessLoop(*I);
180 assert(L->getBlocks()[0] == L->getHeader() &&
181 "Header isn't first block in loop?");
183 // Does the loop already have a preheader? If so, don't insert one.
184 if (L->getLoopPreheader() == 0) {
185 InsertPreheaderForLoop(L);
190 // Next, check to make sure that all exit nodes of the loop only have
191 // predecessors that are inside of the loop. This check guarantees that the
192 // loop preheader/header will dominate the exit blocks. If the exit block has
193 // predecessors from outside of the loop, split the edge now.
194 std::vector<BasicBlock*> ExitBlocks;
195 L->getExitBlocks(ExitBlocks);
197 SetVector<BasicBlock*> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end());
198 for (SetVector<BasicBlock*>::iterator I = ExitBlockSet.begin(),
199 E = ExitBlockSet.end(); I != E; ++I) {
200 BasicBlock *ExitBlock = *I;
201 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
203 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
205 if (!L->contains(*PI)) {
206 RewriteLoopExitBlock(L, ExitBlock);
213 // If the header has more than two predecessors at this point (from the
214 // preheader and from multiple backedges), we must adjust the loop.
215 unsigned NumBackedges = L->getNumBackEdges();
216 if (NumBackedges != 1) {
217 // If this is really a nested loop, rip it out into a child loop. Don't do
218 // this for loops with a giant number of backedges, just factor them into a
219 // common backedge instead.
220 if (NumBackedges < 8) {
221 if (Loop *NL = SeparateNestedLoop(L)) {
223 // This is a big restructuring change, reprocess the whole loop.
226 // GCC doesn't tail recursion eliminate this.
231 // If we either couldn't, or didn't want to, identify nesting of the loops,
232 // insert a new block that all backedges target, then make it jump to the
234 InsertUniqueBackedgeBlock(L);
239 // Scan over the PHI nodes in the loop header. Since they now have only two
240 // incoming values (the loop is canonicalized), we may have simplified the PHI
241 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
243 for (BasicBlock::iterator I = L->getHeader()->begin();
244 (PN = dyn_cast<PHINode>(I++)); )
245 if (Value *V = PN->hasConstantValue()) {
246 PN->replaceAllUsesWith(V);
247 PN->eraseFromParent();
253 /// SplitBlockPredecessors - Split the specified block into two blocks. We want
254 /// to move the predecessors specified in the Preds list to point to the new
255 /// block, leaving the remaining predecessors pointing to BB. This method
256 /// updates the SSA PHINode's, but no other analyses.
258 BasicBlock *LoopSimplify::SplitBlockPredecessors(BasicBlock *BB,
260 const std::vector<BasicBlock*> &Preds) {
262 // Create new basic block, insert right before the original block...
263 BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB->getParent(), BB);
265 // The preheader first gets an unconditional branch to the loop header...
266 BranchInst *BI = new BranchInst(BB, NewBB);
268 // For every PHI node in the block, insert a PHI node into NewBB where the
269 // incoming values from the out of loop edges are moved to NewBB. We have two
270 // possible cases here. If the loop is dead, we just insert dummy entries
271 // into the PHI nodes for the new edge. If the loop is not dead, we move the
272 // incoming edges in BB into new PHI nodes in NewBB.
274 if (!Preds.empty()) { // Is the loop not obviously dead?
275 // Check to see if the values being merged into the new block need PHI
276 // nodes. If so, insert them.
277 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ) {
278 PHINode *PN = cast<PHINode>(I);
281 // Check to see if all of the values coming in are the same. If so, we
282 // don't need to create a new PHI node.
283 Value *InVal = PN->getIncomingValueForBlock(Preds[0]);
284 for (unsigned i = 1, e = Preds.size(); i != e; ++i)
285 if (InVal != PN->getIncomingValueForBlock(Preds[i])) {
290 // If the values coming into the block are not the same, we need a PHI.
292 // Create the new PHI node, insert it into NewBB at the end of the block
293 PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
294 if (AA) AA->copyValue(PN, NewPHI);
296 // Move all of the edges from blocks outside the loop to the new PHI
297 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
298 Value *V = PN->removeIncomingValue(Preds[i], false);
299 NewPHI->addIncoming(V, Preds[i]);
303 // Remove all of the edges coming into the PHI nodes from outside of the
305 for (unsigned i = 0, e = Preds.size(); i != e; ++i)
306 PN->removeIncomingValue(Preds[i], false);
309 // Add an incoming value to the PHI node in the loop for the preheader
311 PN->addIncoming(InVal, NewBB);
313 // Can we eliminate this phi node now?
314 if (Value *V = PN->hasConstantValue(true)) {
315 Instruction *I = dyn_cast<Instruction>(V);
316 // If I is in NewBB, the Dominator call will fail, because NewBB isn't
317 // registered in DominatorTree yet. Handle this case explicitly.
318 if (!I || (I->getParent() != NewBB &&
319 getAnalysis<DominatorTree>().dominates(I, PN))) {
320 PN->replaceAllUsesWith(V);
321 if (AA) AA->deleteValue(PN);
322 BB->getInstList().erase(PN);
327 // Now that the PHI nodes are updated, actually move the edges from
328 // Preds to point to NewBB instead of BB.
330 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
331 TerminatorInst *TI = Preds[i]->getTerminator();
332 for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
333 if (TI->getSuccessor(s) == BB)
334 TI->setSuccessor(s, NewBB);
337 } else { // Otherwise the loop is dead...
338 for (BasicBlock::iterator I = BB->begin(); isa<PHINode>(I); ++I) {
339 PHINode *PN = cast<PHINode>(I);
340 // Insert dummy values as the incoming value...
341 PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
347 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
348 /// preheader, this method is called to insert one. This method has two phases:
349 /// preheader insertion and analysis updating.
351 void LoopSimplify::InsertPreheaderForLoop(Loop *L) {
352 BasicBlock *Header = L->getHeader();
354 // Compute the set of predecessors of the loop that are not in the loop.
355 std::vector<BasicBlock*> OutsideBlocks;
356 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
358 if (!L->contains(*PI)) // Coming in from outside the loop?
359 OutsideBlocks.push_back(*PI); // Keep track of it...
361 // Split out the loop pre-header.
363 SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);
366 //===--------------------------------------------------------------------===//
367 // Update analysis results now that we have performed the transformation
370 // We know that we have loop information to update... update it now.
371 if (Loop *Parent = L->getParentLoop())
372 Parent->addBasicBlockToLoop(NewBB, *LI);
374 UpdateDomInfoForRevectoredPreds(NewBB, OutsideBlocks);
376 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
377 // code layout too horribly.
378 PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
381 /// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
382 /// blocks. This method is used to split exit blocks that have predecessors
383 /// outside of the loop.
384 BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
385 std::vector<BasicBlock*> LoopBlocks;
386 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
388 LoopBlocks.push_back(*I);
390 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
391 BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks);
393 // Update Loop Information - we know that the new block will be in whichever
394 // loop the Exit block is in. Note that it may not be in that immediate loop,
395 // if the successor is some other loop header. In that case, we continue
396 // walking up the loop tree to find a loop that contains both the successor
397 // block and the predecessor block.
398 Loop *SuccLoop = LI->getLoopFor(Exit);
399 while (SuccLoop && !SuccLoop->contains(L->getHeader()))
400 SuccLoop = SuccLoop->getParentLoop();
402 SuccLoop->addBasicBlockToLoop(NewBB, *LI);
404 // Update dominator information (set, immdom, domtree, and domfrontier)
405 UpdateDomInfoForRevectoredPreds(NewBB, LoopBlocks);
409 /// AddBlockAndPredsToSet - Add the specified block, and all of its
410 /// predecessors, to the specified set, if it's not already in there. Stop
411 /// predecessor traversal when we reach StopBlock.
412 static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
413 std::set<BasicBlock*> &Blocks) {
414 std::vector<BasicBlock *> WorkList;
415 WorkList.push_back(InputBB);
417 BasicBlock *BB = WorkList.back(); WorkList.pop_back();
418 if (Blocks.insert(BB).second && BB != StopBlock)
419 // If BB is not already processed and it is not a stop block then
420 // insert its predecessor in the work list
421 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
422 BasicBlock *WBB = *I;
423 WorkList.push_back(WBB);
425 } while(!WorkList.empty());
428 /// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
429 /// PHI node that tells us how to partition the loops.
430 static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
432 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
433 PHINode *PN = cast<PHINode>(I);
435 if (Value *V = PN->hasConstantValue())
436 if (!isa<Instruction>(V) || DT->dominates(cast<Instruction>(V), PN)) {
437 // This is a degenerate PHI already, don't modify it!
438 PN->replaceAllUsesWith(V);
439 if (AA) AA->deleteValue(PN);
440 PN->eraseFromParent();
444 // Scan this PHI node looking for a use of the PHI node by itself.
445 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
446 if (PN->getIncomingValue(i) == PN &&
447 L->contains(PN->getIncomingBlock(i)))
448 // We found something tasty to remove.
454 // PlaceSplitBlockCarefully - If the block isn't already, move the new block to
455 // right after some 'outside block' block. This prevents the preheader from
456 // being placed inside the loop body, e.g. when the loop hasn't been rotated.
457 void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
458 std::vector<BasicBlock*>&SplitPreds,
460 // Check to see if NewBB is already well placed.
461 Function::iterator BBI = NewBB; --BBI;
462 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
463 if (&*BBI == SplitPreds[i])
467 // If it isn't already after an outside block, move it after one. This is
468 // always good as it makes the uncond branch from the outside block into a
471 // Figure out *which* outside block to put this after. Prefer an outside
472 // block that neighbors a BB actually in the loop.
473 BasicBlock *FoundBB = 0;
474 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
475 Function::iterator BBI = SplitPreds[i];
476 if (++BBI != NewBB->getParent()->end() &&
478 FoundBB = SplitPreds[i];
483 // If our heuristic for a *good* bb to place this after doesn't find
484 // anything, just pick something. It's likely better than leaving it within
487 FoundBB = SplitPreds[0];
488 NewBB->moveAfter(FoundBB);
492 /// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
493 /// them out into a nested loop. This is important for code that looks like
498 /// br cond, Loop, Next
500 /// br cond2, Loop, Out
502 /// To identify this common case, we look at the PHI nodes in the header of the
503 /// loop. PHI nodes with unchanging values on one backedge correspond to values
504 /// that change in the "outer" loop, but not in the "inner" loop.
506 /// If we are able to separate out a loop, return the new outer loop that was
509 Loop *LoopSimplify::SeparateNestedLoop(Loop *L) {
510 DominatorTree *DT = getAnalysisToUpdate<DominatorTree>();
511 PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
512 if (PN == 0) return 0; // No known way to partition.
514 // Pull out all predecessors that have varying values in the loop. This
515 // handles the case when a PHI node has multiple instances of itself as
517 std::vector<BasicBlock*> OuterLoopPreds;
518 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
519 if (PN->getIncomingValue(i) != PN ||
520 !L->contains(PN->getIncomingBlock(i)))
521 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
523 BasicBlock *Header = L->getHeader();
524 BasicBlock *NewBB = SplitBlockPredecessors(Header, ".outer", OuterLoopPreds);
526 // Update dominator information (set, immdom, domtree, and domfrontier)
527 UpdateDomInfoForRevectoredPreds(NewBB, OuterLoopPreds);
529 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
530 // code layout too horribly.
531 PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
533 // Create the new outer loop.
534 Loop *NewOuter = new Loop();
536 // Change the parent loop to use the outer loop as its child now.
537 if (Loop *Parent = L->getParentLoop())
538 Parent->replaceChildLoopWith(L, NewOuter);
540 LI->changeTopLevelLoop(L, NewOuter);
542 // This block is going to be our new header block: add it to this loop and all
544 NewOuter->addBasicBlockToLoop(NewBB, *LI);
546 // L is now a subloop of our outer loop.
547 NewOuter->addChildLoop(L);
549 for (unsigned i = 0, e = L->getBlocks().size(); i != e; ++i)
550 NewOuter->addBlockEntry(L->getBlocks()[i]);
552 // Determine which blocks should stay in L and which should be moved out to
553 // the Outer loop now.
554 std::set<BasicBlock*> BlocksInL;
555 for (pred_iterator PI = pred_begin(Header), E = pred_end(Header); PI!=E; ++PI)
556 if (DT->dominates(Header, *PI))
557 AddBlockAndPredsToSet(*PI, Header, BlocksInL);
560 // Scan all of the loop children of L, moving them to OuterLoop if they are
561 // not part of the inner loop.
562 for (Loop::iterator I = L->begin(); I != L->end(); )
563 if (BlocksInL.count((*I)->getHeader()))
564 ++I; // Loop remains in L
566 NewOuter->addChildLoop(L->removeChildLoop(I));
568 // Now that we know which blocks are in L and which need to be moved to
569 // OuterLoop, move any blocks that need it.
570 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
571 BasicBlock *BB = L->getBlocks()[i];
572 if (!BlocksInL.count(BB)) {
573 // Move this block to the parent, updating the exit blocks sets
574 L->removeBlockFromLoop(BB);
576 LI->changeLoopFor(BB, NewOuter);
586 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
587 /// has more than one backedge in it. If this occurs, revector all of these
588 /// backedges to target a new basic block and have that block branch to the loop
589 /// header. This ensures that loops have exactly one backedge.
591 void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) {
592 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
594 // Get information about the loop
595 BasicBlock *Preheader = L->getLoopPreheader();
596 BasicBlock *Header = L->getHeader();
597 Function *F = Header->getParent();
599 // Figure out which basic blocks contain back-edges to the loop header.
600 std::vector<BasicBlock*> BackedgeBlocks;
601 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I)
602 if (*I != Preheader) BackedgeBlocks.push_back(*I);
604 // Create and insert the new backedge block...
605 BasicBlock *BEBlock = new BasicBlock(Header->getName()+".backedge", F);
606 BranchInst *BETerminator = new BranchInst(Header, BEBlock);
608 // Move the new backedge block to right after the last backedge block.
609 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
610 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
612 // Now that the block has been inserted into the function, create PHI nodes in
613 // the backedge block which correspond to any PHI nodes in the header block.
614 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
615 PHINode *PN = cast<PHINode>(I);
616 PHINode *NewPN = new PHINode(PN->getType(), PN->getName()+".be",
618 NewPN->reserveOperandSpace(BackedgeBlocks.size());
619 if (AA) AA->copyValue(PN, NewPN);
621 // Loop over the PHI node, moving all entries except the one for the
622 // preheader over to the new PHI node.
623 unsigned PreheaderIdx = ~0U;
624 bool HasUniqueIncomingValue = true;
625 Value *UniqueValue = 0;
626 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
627 BasicBlock *IBB = PN->getIncomingBlock(i);
628 Value *IV = PN->getIncomingValue(i);
629 if (IBB == Preheader) {
632 NewPN->addIncoming(IV, IBB);
633 if (HasUniqueIncomingValue) {
634 if (UniqueValue == 0)
636 else if (UniqueValue != IV)
637 HasUniqueIncomingValue = false;
642 // Delete all of the incoming values from the old PN except the preheader's
643 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
644 if (PreheaderIdx != 0) {
645 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
646 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
648 // Nuke all entries except the zero'th.
649 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
650 PN->removeIncomingValue(e-i, false);
652 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
653 PN->addIncoming(NewPN, BEBlock);
655 // As an optimization, if all incoming values in the new PhiNode (which is a
656 // subset of the incoming values of the old PHI node) have the same value,
657 // eliminate the PHI Node.
658 if (HasUniqueIncomingValue) {
659 NewPN->replaceAllUsesWith(UniqueValue);
660 if (AA) AA->deleteValue(NewPN);
661 BEBlock->getInstList().erase(NewPN);
665 // Now that all of the PHI nodes have been inserted and adjusted, modify the
666 // backedge blocks to just to the BEBlock instead of the header.
667 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
668 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
669 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
670 if (TI->getSuccessor(Op) == Header)
671 TI->setSuccessor(Op, BEBlock);
674 //===--- Update all analyses which we must preserve now -----------------===//
676 // Update Loop Information - we know that this block is now in the current
677 // loop and all parent loops.
678 L->addBasicBlockToLoop(BEBlock, *LI);
680 // Update dominator information (set, immdom, domtree, and domfrontier)
681 UpdateDomInfoForRevectoredPreds(BEBlock, BackedgeBlocks);
684 // Returns true if BasicBlock A dominates at least one block in vector B
685 // Helper function for UpdateDomInfoForRevectoredPreds
686 static bool BlockDominatesAny(BasicBlock* A, const std::vector<BasicBlock*>& B,
688 for (std::vector<BasicBlock*>::const_iterator BI = B.begin(), BE = B.end();
690 if (DT.dominates(A, *BI))
696 /// UpdateDomInfoForRevectoredPreds - This method is used to update
697 /// dominator trees and dominance frontiers after a new block has
698 /// been added to the CFG.
700 /// This only supports the case when an existing block (known as "NewBBSucc"),
701 /// had some of its predecessors factored into a new basic block. This
702 /// transformation inserts a new basic block ("NewBB"), with a single
703 /// unconditional branch to NewBBSucc, and moves some predecessors of
704 /// "NewBBSucc" to now branch to NewBB. These predecessors are listed in
705 /// PredBlocks, even though they are the same as
706 /// pred_begin(NewBB)/pred_end(NewBB).
708 void LoopSimplify::UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB,
709 std::vector<BasicBlock*> &PredBlocks) {
710 assert(!PredBlocks.empty() && "No predblocks??");
711 assert(NewBB->getTerminator()->getNumSuccessors() == 1
712 && "NewBB should have a single successor!");
713 BasicBlock *NewBBSucc = NewBB->getTerminator()->getSuccessor(0);
714 DominatorTree &DT = getAnalysis<DominatorTree>();
716 // The newly inserted basic block will dominate existing basic blocks iff the
717 // PredBlocks dominate all of the non-pred blocks. If all predblocks dominate
718 // the non-pred blocks, then they all must be the same block!
720 bool NewBBDominatesNewBBSucc = true;
722 BasicBlock *OnePred = PredBlocks[0];
723 unsigned i = 1, e = PredBlocks.size();
724 for (i = 1; !DT.isReachableFromEntry(OnePred); ++i) {
725 assert(i != e && "Didn't find reachable pred?");
726 OnePred = PredBlocks[i];
730 if (PredBlocks[i] != OnePred && DT.isReachableFromEntry(OnePred)){
731 NewBBDominatesNewBBSucc = false;
735 if (NewBBDominatesNewBBSucc)
736 for (pred_iterator PI = pred_begin(NewBBSucc), E = pred_end(NewBBSucc);
738 if (*PI != NewBB && !DT.dominates(NewBBSucc, *PI)) {
739 NewBBDominatesNewBBSucc = false;
744 // The other scenario where the new block can dominate its successors are when
745 // all predecessors of NewBBSucc that are not NewBB are dominated by NewBBSucc
747 if (!NewBBDominatesNewBBSucc) {
748 NewBBDominatesNewBBSucc = true;
749 for (pred_iterator PI = pred_begin(NewBBSucc), E = pred_end(NewBBSucc);
751 if (*PI != NewBB && !DT.dominates(NewBBSucc, *PI)) {
752 NewBBDominatesNewBBSucc = false;
758 // Update DominatorTree information if it is active.
760 // Find NewBB's immediate dominator and create new dominator tree node for NewBB.
761 BasicBlock *NewBBIDom = 0;
763 for (i = 0; i < PredBlocks.size(); ++i)
764 if (DT.isReachableFromEntry(PredBlocks[i])) {
765 NewBBIDom = PredBlocks[i];
768 assert(i != PredBlocks.size() && "No reachable preds?");
769 for (i = i + 1; i < PredBlocks.size(); ++i) {
770 if (DT.isReachableFromEntry(PredBlocks[i]))
771 NewBBIDom = DT.findNearestCommonDominator(NewBBIDom, PredBlocks[i]);
773 assert(NewBBIDom && "No immediate dominator found??");
775 // Create the new dominator tree node... and set the idom of NewBB.
776 DomTreeNode *NewBBNode = DT.addNewBlock(NewBB, NewBBIDom);
778 // If NewBB strictly dominates other blocks, then it is now the immediate
779 // dominator of NewBBSucc. Update the dominator tree as appropriate.
780 if (NewBBDominatesNewBBSucc) {
781 DomTreeNode *NewBBSuccNode = DT.getNode(NewBBSucc);
782 DT.changeImmediateDominator(NewBBSuccNode, NewBBNode);
785 // Update dominance frontier information...
786 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
787 // If NewBB dominates NewBBSucc, then DF(NewBB) is now going to be the
788 // DF(PredBlocks[0]) without the stuff that the new block does not dominate
790 if (NewBBDominatesNewBBSucc) {
791 DominanceFrontier::iterator DFI = DF->find(PredBlocks[0]);
792 if (DFI != DF->end()) {
793 DominanceFrontier::DomSetType Set = DFI->second;
794 // Filter out stuff in Set that we do not dominate a predecessor of.
795 for (DominanceFrontier::DomSetType::iterator SetI = Set.begin(),
796 E = Set.end(); SetI != E;) {
797 bool DominatesPred = false;
798 for (pred_iterator PI = pred_begin(*SetI), E = pred_end(*SetI);
800 if (DT.dominates(NewBB, *PI))
801 DominatesPred = true;
808 DF->addBasicBlock(NewBB, Set);
812 // DF(NewBB) is {NewBBSucc} because NewBB does not strictly dominate
813 // NewBBSucc, but it does dominate itself (and there is an edge (NewBB ->
814 // NewBBSucc)). NewBBSucc is the single successor of NewBB.
815 DominanceFrontier::DomSetType NewDFSet;
816 NewDFSet.insert(NewBBSucc);
817 DF->addBasicBlock(NewBB, NewDFSet);
820 // Now we must loop over all of the dominance frontiers in the function,
821 // replacing occurrences of NewBBSucc with NewBB in some cases. All
822 // blocks that dominate a block in PredBlocks and contained NewBBSucc in
823 // their dominance frontier must be updated to contain NewBB instead.
825 for (Function::iterator FI = NewBB->getParent()->begin(),
826 FE = NewBB->getParent()->end(); FI != FE; ++FI) {
827 DominanceFrontier::iterator DFI = DF->find(FI);
828 if (DFI == DF->end()) continue; // unreachable block.
830 // Only consider dominators of NewBBSucc
831 if (!DFI->second.count(NewBBSucc)) continue;
833 if (BlockDominatesAny(FI, PredBlocks, DT)) {
834 // If NewBBSucc should not stay in our dominator frontier, remove it.
835 // We remove it unless there is a predecessor of NewBBSucc that we
836 // dominate, but we don't strictly dominate NewBBSucc.
837 bool ShouldRemove = true;
838 if ((BasicBlock*)FI == NewBBSucc
839 || !DT.dominates(FI, NewBBSucc)) {
840 // Okay, we know that PredDom does not strictly dominate NewBBSucc.
841 // Check to see if it dominates any predecessors of NewBBSucc.
842 for (pred_iterator PI = pred_begin(NewBBSucc),
843 E = pred_end(NewBBSucc); PI != E; ++PI)
844 if (DT.dominates(FI, *PI)) {
845 ShouldRemove = false;
850 DF->removeFromFrontier(DFI, NewBBSucc);
851 DF->addToFrontier(DFI, NewBB);