1 //===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
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 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 // Indirectbr instructions introduce several complications. If the loop
27 // contains or is entered by an indirectbr instruction, it may not be possible
28 // to transform the loop and make these guarantees. Client code should check
29 // that these conditions are true before relying on them.
31 // Note that the simplifycfg pass will clean up blocks which are split out but
32 // end up being unnecessary, so usage of this pass should not pessimize
35 // This pass obviously modifies the CFG, but updates loop information and
36 // dominator information.
38 //===----------------------------------------------------------------------===//
40 #define DEBUG_TYPE "loop-simplify"
41 #include "llvm/Transforms/Scalar.h"
42 #include "llvm/ADT/DepthFirstIterator.h"
43 #include "llvm/ADT/SetOperations.h"
44 #include "llvm/ADT/SetVector.h"
45 #include "llvm/ADT/SmallVector.h"
46 #include "llvm/ADT/Statistic.h"
47 #include "llvm/Analysis/AliasAnalysis.h"
48 #include "llvm/Analysis/DependenceAnalysis.h"
49 #include "llvm/Analysis/InstructionSimplify.h"
50 #include "llvm/Analysis/LoopInfo.h"
51 #include "llvm/Analysis/ScalarEvolution.h"
52 #include "llvm/IR/CFG.h"
53 #include "llvm/IR/Constants.h"
54 #include "llvm/IR/Dominators.h"
55 #include "llvm/IR/Function.h"
56 #include "llvm/IR/Instructions.h"
57 #include "llvm/IR/IntrinsicInst.h"
58 #include "llvm/IR/LLVMContext.h"
59 #include "llvm/IR/Type.h"
60 #include "llvm/Support/Debug.h"
61 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
62 #include "llvm/Transforms/Utils/Local.h"
63 #include "llvm/Transforms/Utils/LoopUtils.h"
66 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
67 STATISTIC(NumNested , "Number of nested loops split out");
69 // If the block isn't already, move the new block to right after some 'outside
70 // block' block. This prevents the preheader from being placed inside the loop
71 // body, e.g. when the loop hasn't been rotated.
72 static void placeSplitBlockCarefully(BasicBlock *NewBB,
73 SmallVectorImpl<BasicBlock *> &SplitPreds,
75 // Check to see if NewBB is already well placed.
76 Function::iterator BBI = NewBB; --BBI;
77 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
78 if (&*BBI == SplitPreds[i])
82 // If it isn't already after an outside block, move it after one. This is
83 // always good as it makes the uncond branch from the outside block into a
86 // Figure out *which* outside block to put this after. Prefer an outside
87 // block that neighbors a BB actually in the loop.
88 BasicBlock *FoundBB = 0;
89 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
90 Function::iterator BBI = SplitPreds[i];
91 if (++BBI != NewBB->getParent()->end() &&
93 FoundBB = SplitPreds[i];
98 // If our heuristic for a *good* bb to place this after doesn't find
99 // anything, just pick something. It's likely better than leaving it within
102 FoundBB = SplitPreds[0];
103 NewBB->moveAfter(FoundBB);
106 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
107 /// preheader, this method is called to insert one. This method has two phases:
108 /// preheader insertion and analysis updating.
110 BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, Pass *PP) {
111 BasicBlock *Header = L->getHeader();
113 // Compute the set of predecessors of the loop that are not in the loop.
114 SmallVector<BasicBlock*, 8> OutsideBlocks;
115 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
118 if (!L->contains(P)) { // Coming in from outside the loop?
119 // If the loop is branched to from an indirect branch, we won't
120 // be able to fully transform the loop, because it prohibits
122 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
125 OutsideBlocks.push_back(P);
129 // Split out the loop pre-header.
130 BasicBlock *PreheaderBB;
131 if (!Header->isLandingPad()) {
132 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader",
135 SmallVector<BasicBlock*, 2> NewBBs;
136 SplitLandingPadPredecessors(Header, OutsideBlocks, ".preheader",
137 ".split-lp", PP, NewBBs);
138 PreheaderBB = NewBBs[0];
141 PreheaderBB->getTerminator()->setDebugLoc(
142 Header->getFirstNonPHI()->getDebugLoc());
143 DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
144 << PreheaderBB->getName() << "\n");
146 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
147 // code layout too horribly.
148 placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
153 /// \brief Ensure that the loop preheader dominates all exit blocks.
155 /// This method is used to split exit blocks that have predecessors outside of
157 static BasicBlock *rewriteLoopExitBlock(Loop *L, BasicBlock *Exit, Pass *PP) {
158 SmallVector<BasicBlock*, 8> LoopBlocks;
159 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
161 if (L->contains(P)) {
162 // Don't do this if the loop is exited via an indirect branch.
163 if (isa<IndirectBrInst>(P->getTerminator())) return 0;
165 LoopBlocks.push_back(P);
169 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
170 BasicBlock *NewExitBB = 0;
172 if (Exit->isLandingPad()) {
173 SmallVector<BasicBlock*, 2> NewBBs;
174 SplitLandingPadPredecessors(Exit, ArrayRef<BasicBlock*>(&LoopBlocks[0],
176 ".loopexit", ".nonloopexit",
178 NewExitBB = NewBBs[0];
180 NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", PP);
183 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
184 << NewExitBB->getName() << "\n");
188 /// Add the specified block, and all of its predecessors, to the specified set,
189 /// if it's not already in there. Stop predecessor traversal when we reach
191 static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
192 std::set<BasicBlock*> &Blocks) {
193 SmallVector<BasicBlock *, 8> Worklist;
194 Worklist.push_back(InputBB);
196 BasicBlock *BB = Worklist.pop_back_val();
197 if (Blocks.insert(BB).second && BB != StopBlock)
198 // If BB is not already processed and it is not a stop block then
199 // insert its predecessor in the work list
200 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
201 BasicBlock *WBB = *I;
202 Worklist.push_back(WBB);
204 } while (!Worklist.empty());
207 /// \brief The first part of loop-nestification is to find a PHI node that tells
208 /// us how to partition the loops.
209 static PHINode *findPHIToPartitionLoops(Loop *L, AliasAnalysis *AA,
211 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
212 PHINode *PN = cast<PHINode>(I);
214 if (Value *V = SimplifyInstruction(PN, 0, 0, DT)) {
215 // This is a degenerate PHI already, don't modify it!
216 PN->replaceAllUsesWith(V);
217 if (AA) AA->deleteValue(PN);
218 PN->eraseFromParent();
222 // Scan this PHI node looking for a use of the PHI node by itself.
223 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
224 if (PN->getIncomingValue(i) == PN &&
225 L->contains(PN->getIncomingBlock(i)))
226 // We found something tasty to remove.
232 /// \brief If this loop has multiple backedges, try to pull one of them out into
235 /// This is important for code that looks like
240 /// br cond, Loop, Next
242 /// br cond2, Loop, Out
244 /// To identify this common case, we look at the PHI nodes in the header of the
245 /// loop. PHI nodes with unchanging values on one backedge correspond to values
246 /// that change in the "outer" loop, but not in the "inner" loop.
248 /// If we are able to separate out a loop, return the new outer loop that was
251 static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
252 AliasAnalysis *AA, DominatorTree *DT,
253 LoopInfo *LI, ScalarEvolution *SE, Pass *PP) {
254 // Don't try to separate loops without a preheader.
258 // The header is not a landing pad; preheader insertion should ensure this.
259 assert(!L->getHeader()->isLandingPad() &&
260 "Can't insert backedge to landing pad");
262 PHINode *PN = findPHIToPartitionLoops(L, AA, DT);
263 if (PN == 0) return 0; // No known way to partition.
265 // Pull out all predecessors that have varying values in the loop. This
266 // handles the case when a PHI node has multiple instances of itself as
268 SmallVector<BasicBlock*, 8> OuterLoopPreds;
269 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
270 if (PN->getIncomingValue(i) != PN ||
271 !L->contains(PN->getIncomingBlock(i))) {
272 // We can't split indirectbr edges.
273 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
275 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
278 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
280 // If ScalarEvolution is around and knows anything about values in
281 // this loop, tell it to forget them, because we're about to
282 // substantially change it.
286 BasicBlock *Header = L->getHeader();
288 SplitBlockPredecessors(Header, OuterLoopPreds, ".outer", PP);
290 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
291 // code layout too horribly.
292 placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
294 // Create the new outer loop.
295 Loop *NewOuter = new Loop();
297 // Change the parent loop to use the outer loop as its child now.
298 if (Loop *Parent = L->getParentLoop())
299 Parent->replaceChildLoopWith(L, NewOuter);
301 LI->changeTopLevelLoop(L, NewOuter);
303 // L is now a subloop of our outer loop.
304 NewOuter->addChildLoop(L);
306 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
308 NewOuter->addBlockEntry(*I);
310 // Now reset the header in L, which had been moved by
311 // SplitBlockPredecessors for the outer loop.
312 L->moveToHeader(Header);
314 // Determine which blocks should stay in L and which should be moved out to
315 // the Outer loop now.
316 std::set<BasicBlock*> BlocksInL;
317 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
319 if (DT->dominates(Header, P))
320 addBlockAndPredsToSet(P, Header, BlocksInL);
323 // Scan all of the loop children of L, moving them to OuterLoop if they are
324 // not part of the inner loop.
325 const std::vector<Loop*> &SubLoops = L->getSubLoops();
326 for (size_t I = 0; I != SubLoops.size(); )
327 if (BlocksInL.count(SubLoops[I]->getHeader()))
328 ++I; // Loop remains in L
330 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
332 // Now that we know which blocks are in L and which need to be moved to
333 // OuterLoop, move any blocks that need it.
334 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
335 BasicBlock *BB = L->getBlocks()[i];
336 if (!BlocksInL.count(BB)) {
337 // Move this block to the parent, updating the exit blocks sets
338 L->removeBlockFromLoop(BB);
340 LI->changeLoopFor(BB, NewOuter);
348 /// \brief This method is called when the specified loop has more than one
351 /// If this occurs, revector all of these backedges to target a new basic block
352 /// and have that block branch to the loop header. This ensures that loops
353 /// have exactly one backedge.
354 static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
356 DominatorTree *DT, LoopInfo *LI) {
357 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
359 // Get information about the loop
360 BasicBlock *Header = L->getHeader();
361 Function *F = Header->getParent();
363 // Unique backedge insertion currently depends on having a preheader.
367 // The header is not a landing pad; preheader insertion should ensure this.
368 assert(!Header->isLandingPad() && "Can't insert backedge to landing pad");
370 // Figure out which basic blocks contain back-edges to the loop header.
371 std::vector<BasicBlock*> BackedgeBlocks;
372 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
375 // Indirectbr edges cannot be split, so we must fail if we find one.
376 if (isa<IndirectBrInst>(P->getTerminator()))
379 if (P != Preheader) BackedgeBlocks.push_back(P);
382 // Create and insert the new backedge block...
383 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
384 Header->getName()+".backedge", F);
385 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
387 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
388 << BEBlock->getName() << "\n");
390 // Move the new backedge block to right after the last backedge block.
391 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
392 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
394 // Now that the block has been inserted into the function, create PHI nodes in
395 // the backedge block which correspond to any PHI nodes in the header block.
396 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
397 PHINode *PN = cast<PHINode>(I);
398 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
399 PN->getName()+".be", BETerminator);
400 if (AA) AA->copyValue(PN, NewPN);
402 // Loop over the PHI node, moving all entries except the one for the
403 // preheader over to the new PHI node.
404 unsigned PreheaderIdx = ~0U;
405 bool HasUniqueIncomingValue = true;
406 Value *UniqueValue = 0;
407 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
408 BasicBlock *IBB = PN->getIncomingBlock(i);
409 Value *IV = PN->getIncomingValue(i);
410 if (IBB == Preheader) {
413 NewPN->addIncoming(IV, IBB);
414 if (HasUniqueIncomingValue) {
415 if (UniqueValue == 0)
417 else if (UniqueValue != IV)
418 HasUniqueIncomingValue = false;
423 // Delete all of the incoming values from the old PN except the preheader's
424 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
425 if (PreheaderIdx != 0) {
426 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
427 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
429 // Nuke all entries except the zero'th.
430 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
431 PN->removeIncomingValue(e-i, false);
433 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
434 PN->addIncoming(NewPN, BEBlock);
436 // As an optimization, if all incoming values in the new PhiNode (which is a
437 // subset of the incoming values of the old PHI node) have the same value,
438 // eliminate the PHI Node.
439 if (HasUniqueIncomingValue) {
440 NewPN->replaceAllUsesWith(UniqueValue);
441 if (AA) AA->deleteValue(NewPN);
442 BEBlock->getInstList().erase(NewPN);
446 // Now that all of the PHI nodes have been inserted and adjusted, modify the
447 // backedge blocks to just to the BEBlock instead of the header.
448 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
449 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
450 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
451 if (TI->getSuccessor(Op) == Header)
452 TI->setSuccessor(Op, BEBlock);
455 //===--- Update all analyses which we must preserve now -----------------===//
457 // Update Loop Information - we know that this block is now in the current
458 // loop and all parent loops.
459 L->addBasicBlockToLoop(BEBlock, LI->getBase());
461 // Update dominator information
462 DT->splitBlock(BEBlock);
467 /// \brief Simplify one loop and queue further loops for simplification.
469 /// FIXME: Currently this accepts both lots of analyses that it uses and a raw
470 /// Pass pointer. The Pass pointer is used by numerous utilities to update
471 /// specific analyses. Rather than a pass it would be much cleaner and more
472 /// explicit if they accepted the analysis directly and then updated it.
473 static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
474 AliasAnalysis *AA, DominatorTree *DT, LoopInfo *LI,
475 ScalarEvolution *SE, Pass *PP) {
476 bool Changed = false;
479 // Check to see that no blocks (other than the header) in this loop have
480 // predecessors that are not in the loop. This is not valid for natural
481 // loops, but can occur if the blocks are unreachable. Since they are
482 // unreachable we can just shamelessly delete those CFG edges!
483 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
485 if (*BB == L->getHeader()) continue;
487 SmallPtrSet<BasicBlock*, 4> BadPreds;
488 for (pred_iterator PI = pred_begin(*BB),
489 PE = pred_end(*BB); PI != PE; ++PI) {
495 // Delete each unique out-of-loop (and thus dead) predecessor.
496 for (SmallPtrSet<BasicBlock*, 4>::iterator I = BadPreds.begin(),
497 E = BadPreds.end(); I != E; ++I) {
499 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
500 << (*I)->getName() << "\n");
502 // Inform each successor of each dead pred.
503 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
504 (*SI)->removePredecessor(*I);
505 // Zap the dead pred's terminator and replace it with unreachable.
506 TerminatorInst *TI = (*I)->getTerminator();
507 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
508 (*I)->getTerminator()->eraseFromParent();
509 new UnreachableInst((*I)->getContext(), *I);
514 // If there are exiting blocks with branches on undef, resolve the undef in
515 // the direction which will exit the loop. This will help simplify loop
516 // trip count computations.
517 SmallVector<BasicBlock*, 8> ExitingBlocks;
518 L->getExitingBlocks(ExitingBlocks);
519 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
520 E = ExitingBlocks.end(); I != E; ++I)
521 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
522 if (BI->isConditional()) {
523 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
525 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
526 << (*I)->getName() << "\n");
528 BI->setCondition(ConstantInt::get(Cond->getType(),
529 !L->contains(BI->getSuccessor(0))));
531 // This may make the loop analyzable, force SCEV recomputation.
539 // Does the loop already have a preheader? If so, don't insert one.
540 BasicBlock *Preheader = L->getLoopPreheader();
542 Preheader = InsertPreheaderForLoop(L, PP);
549 // Next, check to make sure that all exit nodes of the loop only have
550 // predecessors that are inside of the loop. This check guarantees that the
551 // loop preheader/header will dominate the exit blocks. If the exit block has
552 // predecessors from outside of the loop, split the edge now.
553 SmallVector<BasicBlock*, 8> ExitBlocks;
554 L->getExitBlocks(ExitBlocks);
556 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
558 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
559 E = ExitBlockSet.end(); I != E; ++I) {
560 BasicBlock *ExitBlock = *I;
561 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
563 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
565 if (!L->contains(*PI)) {
566 if (rewriteLoopExitBlock(L, ExitBlock, PP)) {
574 // If the header has more than two predecessors at this point (from the
575 // preheader and from multiple backedges), we must adjust the loop.
576 BasicBlock *LoopLatch = L->getLoopLatch();
578 // If this is really a nested loop, rip it out into a child loop. Don't do
579 // this for loops with a giant number of backedges, just factor them into a
580 // common backedge instead.
581 if (L->getNumBackEdges() < 8) {
582 if (Loop *OuterL = separateNestedLoop(L, Preheader, AA, DT, LI, SE, PP)) {
584 // Enqueue the outer loop as it should be processed next in our
585 // depth-first nest walk.
586 Worklist.push_back(OuterL);
588 // This is a big restructuring change, reprocess the whole loop.
590 // GCC doesn't tail recursion eliminate this.
591 // FIXME: It isn't clear we can't rely on LLVM to TRE this.
596 // If we either couldn't, or didn't want to, identify nesting of the loops,
597 // insert a new block that all backedges target, then make it jump to the
599 LoopLatch = insertUniqueBackedgeBlock(L, Preheader, AA, DT, LI);
606 // Scan over the PHI nodes in the loop header. Since they now have only two
607 // incoming values (the loop is canonicalized), we may have simplified the PHI
608 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
610 for (BasicBlock::iterator I = L->getHeader()->begin();
611 (PN = dyn_cast<PHINode>(I++)); )
612 if (Value *V = SimplifyInstruction(PN, 0, 0, DT)) {
613 if (AA) AA->deleteValue(PN);
614 if (SE) SE->forgetValue(PN);
615 PN->replaceAllUsesWith(V);
616 PN->eraseFromParent();
619 // If this loop has multiple exits and the exits all go to the same
620 // block, attempt to merge the exits. This helps several passes, such
621 // as LoopRotation, which do not support loops with multiple exits.
622 // SimplifyCFG also does this (and this code uses the same utility
623 // function), however this code is loop-aware, where SimplifyCFG is
624 // not. That gives it the advantage of being able to hoist
625 // loop-invariant instructions out of the way to open up more
626 // opportunities, and the disadvantage of having the responsibility
627 // to preserve dominator information.
628 bool UniqueExit = true;
629 if (!ExitBlocks.empty())
630 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
631 if (ExitBlocks[i] != ExitBlocks[0]) {
636 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
637 BasicBlock *ExitingBlock = ExitingBlocks[i];
638 if (!ExitingBlock->getSinglePredecessor()) continue;
639 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
640 if (!BI || !BI->isConditional()) continue;
641 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
642 if (!CI || CI->getParent() != ExitingBlock) continue;
644 // Attempt to hoist out all instructions except for the
645 // comparison and the branch.
646 bool AllInvariant = true;
647 bool AnyInvariant = false;
648 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
649 Instruction *Inst = I++;
650 // Skip debug info intrinsics.
651 if (isa<DbgInfoIntrinsic>(Inst))
655 if (!L->makeLoopInvariant(Inst, AnyInvariant,
656 Preheader ? Preheader->getTerminator() : 0)) {
657 AllInvariant = false;
663 // The loop disposition of all SCEV expressions that depend on any
664 // hoisted values have also changed.
666 SE->forgetLoopDispositions(L);
668 if (!AllInvariant) continue;
670 // The block has now been cleared of all instructions except for
671 // a comparison and a conditional branch. SimplifyCFG may be able
673 if (!FoldBranchToCommonDest(BI)) continue;
675 // Success. The block is now dead, so remove it from the loop,
676 // update the dominator tree and delete it.
677 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
678 << ExitingBlock->getName() << "\n");
680 // Notify ScalarEvolution before deleting this block. Currently assume the
681 // parent loop doesn't change (spliting edges doesn't count). If blocks,
682 // CFG edges, or other values in the parent loop change, then we need call
683 // to forgetLoop() for the parent instead.
687 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
689 LI->removeBlock(ExitingBlock);
691 DomTreeNode *Node = DT->getNode(ExitingBlock);
692 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
694 while (!Children.empty()) {
695 DomTreeNode *Child = Children.front();
696 DT->changeImmediateDominator(Child, Node->getIDom());
698 DT->eraseNode(ExitingBlock);
700 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
701 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
702 ExitingBlock->eraseFromParent();
709 bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI, Pass *PP,
710 AliasAnalysis *AA, ScalarEvolution *SE) {
711 bool Changed = false;
713 // Worklist maintains our depth-first queue of loops in this nest to process.
714 SmallVector<Loop *, 4> Worklist;
715 Worklist.push_back(L);
717 // Walk the worklist from front to back, pushing newly found sub loops onto
718 // the back. This will let us process loops from back to front in depth-first
719 // order. We can use this simple process because loops form a tree.
720 for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
721 Loop *L2 = Worklist[Idx];
722 for (Loop::iterator I = L2->begin(), E = L2->end(); I != E; ++I)
723 Worklist.push_back(*I);
726 while (!Worklist.empty())
727 Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, AA, DT, LI, SE, PP);
733 struct LoopSimplify : public FunctionPass {
734 static char ID; // Pass identification, replacement for typeid
735 LoopSimplify() : FunctionPass(ID) {
736 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
739 // AA - If we have an alias analysis object to update, this is it, otherwise
746 bool runOnFunction(Function &F) override;
748 void getAnalysisUsage(AnalysisUsage &AU) const override {
749 // We need loop information to identify the loops...
750 AU.addRequired<DominatorTreeWrapperPass>();
751 AU.addPreserved<DominatorTreeWrapperPass>();
753 AU.addRequired<LoopInfo>();
754 AU.addPreserved<LoopInfo>();
756 AU.addPreserved<AliasAnalysis>();
757 AU.addPreserved<ScalarEvolution>();
758 AU.addPreserved<DependenceAnalysis>();
759 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
762 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
763 void verifyAnalysis() const override;
767 char LoopSimplify::ID = 0;
768 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
769 "Canonicalize natural loops", true, false)
770 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
771 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
772 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
773 "Canonicalize natural loops", true, false)
775 // Publicly exposed interface to pass...
776 char &llvm::LoopSimplifyID = LoopSimplify::ID;
777 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
779 /// runOnLoop - Run down all loops in the CFG (recursively, but we could do
780 /// it in any convenient order) inserting preheaders...
782 bool LoopSimplify::runOnFunction(Function &F) {
783 bool Changed = false;
784 AA = getAnalysisIfAvailable<AliasAnalysis>();
785 LI = &getAnalysis<LoopInfo>();
786 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
787 SE = getAnalysisIfAvailable<ScalarEvolution>();
789 // Simplify each loop nest in the function.
790 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
791 Changed |= simplifyLoop(*I, DT, LI, this, AA, SE);
796 // FIXME: Restore this code when we re-enable verification in verifyAnalysis
799 static void verifyLoop(Loop *L) {
801 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
804 // It used to be possible to just assert L->isLoopSimplifyForm(), however
805 // with the introduction of indirectbr, there are now cases where it's
806 // not possible to transform a loop as necessary. We can at least check
807 // that there is an indirectbr near any time there's trouble.
809 // Indirectbr can interfere with preheader and unique backedge insertion.
810 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
811 bool HasIndBrPred = false;
812 for (pred_iterator PI = pred_begin(L->getHeader()),
813 PE = pred_end(L->getHeader()); PI != PE; ++PI)
814 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
818 assert(HasIndBrPred &&
819 "LoopSimplify has no excuse for missing loop header info!");
823 // Indirectbr can interfere with exit block canonicalization.
824 if (!L->hasDedicatedExits()) {
825 bool HasIndBrExiting = false;
826 SmallVector<BasicBlock*, 8> ExitingBlocks;
827 L->getExitingBlocks(ExitingBlocks);
828 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
829 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
830 HasIndBrExiting = true;
835 assert(HasIndBrExiting &&
836 "LoopSimplify has no excuse for missing exit block info!");
837 (void)HasIndBrExiting;
842 void LoopSimplify::verifyAnalysis() const {
843 // FIXME: This routine is being called mid-way through the loop pass manager
844 // as loop passes destroy this analysis. That's actually fine, but we have no
845 // way of expressing that here. Once all of the passes that destroy this are
846 // hoisted out of the loop pass manager we can add back verification here.
848 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)