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 #include "llvm/Transforms/Scalar.h"
41 #include "llvm/ADT/DepthFirstIterator.h"
42 #include "llvm/ADT/SetOperations.h"
43 #include "llvm/ADT/SetVector.h"
44 #include "llvm/ADT/SmallVector.h"
45 #include "llvm/ADT/Statistic.h"
46 #include "llvm/Analysis/AliasAnalysis.h"
47 #include "llvm/Analysis/AssumptionTracker.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/DataLayout.h"
55 #include "llvm/IR/Dominators.h"
56 #include "llvm/IR/Function.h"
57 #include "llvm/IR/Instructions.h"
58 #include "llvm/IR/IntrinsicInst.h"
59 #include "llvm/IR/LLVMContext.h"
60 #include "llvm/IR/Type.h"
61 #include "llvm/Support/Debug.h"
62 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
63 #include "llvm/Transforms/Utils/Local.h"
64 #include "llvm/Transforms/Utils/LoopUtils.h"
67 #define DEBUG_TYPE "loop-simplify"
69 STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
70 STATISTIC(NumNested , "Number of nested loops split out");
72 // If the block isn't already, move the new block to right after some 'outside
73 // block' block. This prevents the preheader from being placed inside the loop
74 // body, e.g. when the loop hasn't been rotated.
75 static void placeSplitBlockCarefully(BasicBlock *NewBB,
76 SmallVectorImpl<BasicBlock *> &SplitPreds,
78 // Check to see if NewBB is already well placed.
79 Function::iterator BBI = NewBB; --BBI;
80 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
81 if (&*BBI == SplitPreds[i])
85 // If it isn't already after an outside block, move it after one. This is
86 // always good as it makes the uncond branch from the outside block into a
89 // Figure out *which* outside block to put this after. Prefer an outside
90 // block that neighbors a BB actually in the loop.
91 BasicBlock *FoundBB = nullptr;
92 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
93 Function::iterator BBI = SplitPreds[i];
94 if (++BBI != NewBB->getParent()->end() &&
96 FoundBB = SplitPreds[i];
101 // If our heuristic for a *good* bb to place this after doesn't find
102 // anything, just pick something. It's likely better than leaving it within
105 FoundBB = SplitPreds[0];
106 NewBB->moveAfter(FoundBB);
109 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
110 /// preheader, this method is called to insert one. This method has two phases:
111 /// preheader insertion and analysis updating.
113 BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, Pass *PP) {
114 BasicBlock *Header = L->getHeader();
116 // Compute the set of predecessors of the loop that are not in the loop.
117 SmallVector<BasicBlock*, 8> OutsideBlocks;
118 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
121 if (!L->contains(P)) { // Coming in from outside the loop?
122 // If the loop is branched to from an indirect branch, we won't
123 // be able to fully transform the loop, because it prohibits
125 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
128 OutsideBlocks.push_back(P);
132 // Split out the loop pre-header.
133 BasicBlock *PreheaderBB;
134 if (!Header->isLandingPad()) {
135 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader",
138 SmallVector<BasicBlock*, 2> NewBBs;
139 SplitLandingPadPredecessors(Header, OutsideBlocks, ".preheader",
140 ".split-lp", PP, NewBBs);
141 PreheaderBB = NewBBs[0];
144 PreheaderBB->getTerminator()->setDebugLoc(
145 Header->getFirstNonPHI()->getDebugLoc());
146 DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
147 << PreheaderBB->getName() << "\n");
149 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
150 // code layout too horribly.
151 placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
156 /// \brief Ensure that the loop preheader dominates all exit blocks.
158 /// This method is used to split exit blocks that have predecessors outside of
160 static BasicBlock *rewriteLoopExitBlock(Loop *L, BasicBlock *Exit, Pass *PP) {
161 SmallVector<BasicBlock*, 8> LoopBlocks;
162 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I) {
164 if (L->contains(P)) {
165 // Don't do this if the loop is exited via an indirect branch.
166 if (isa<IndirectBrInst>(P->getTerminator())) return nullptr;
168 LoopBlocks.push_back(P);
172 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
173 BasicBlock *NewExitBB = nullptr;
175 if (Exit->isLandingPad()) {
176 SmallVector<BasicBlock*, 2> NewBBs;
177 SplitLandingPadPredecessors(Exit, LoopBlocks,
178 ".loopexit", ".nonloopexit",
180 NewExitBB = NewBBs[0];
182 NewExitBB = SplitBlockPredecessors(Exit, LoopBlocks, ".loopexit", PP);
185 DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
186 << NewExitBB->getName() << "\n");
190 /// Add the specified block, and all of its predecessors, to the specified set,
191 /// if it's not already in there. Stop predecessor traversal when we reach
193 static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
194 std::set<BasicBlock*> &Blocks) {
195 SmallVector<BasicBlock *, 8> Worklist;
196 Worklist.push_back(InputBB);
198 BasicBlock *BB = Worklist.pop_back_val();
199 if (Blocks.insert(BB).second && BB != StopBlock)
200 // If BB is not already processed and it is not a stop block then
201 // insert its predecessor in the work list
202 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
203 BasicBlock *WBB = *I;
204 Worklist.push_back(WBB);
206 } while (!Worklist.empty());
209 /// \brief The first part of loop-nestification is to find a PHI node that tells
210 /// us how to partition the loops.
211 static PHINode *findPHIToPartitionLoops(Loop *L, AliasAnalysis *AA,
213 AssumptionTracker *AT) {
214 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
215 PHINode *PN = cast<PHINode>(I);
217 if (Value *V = SimplifyInstruction(PN, nullptr, nullptr, DT, AT)) {
218 // This is a degenerate PHI already, don't modify it!
219 PN->replaceAllUsesWith(V);
220 if (AA) AA->deleteValue(PN);
221 PN->eraseFromParent();
225 // Scan this PHI node looking for a use of the PHI node by itself.
226 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
227 if (PN->getIncomingValue(i) == PN &&
228 L->contains(PN->getIncomingBlock(i)))
229 // We found something tasty to remove.
235 /// \brief If this loop has multiple backedges, try to pull one of them out into
238 /// This is important for code that looks like
243 /// br cond, Loop, Next
245 /// br cond2, Loop, Out
247 /// To identify this common case, we look at the PHI nodes in the header of the
248 /// loop. PHI nodes with unchanging values on one backedge correspond to values
249 /// that change in the "outer" loop, but not in the "inner" loop.
251 /// If we are able to separate out a loop, return the new outer loop that was
254 static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
255 AliasAnalysis *AA, DominatorTree *DT,
256 LoopInfo *LI, ScalarEvolution *SE, Pass *PP,
257 AssumptionTracker *AT) {
258 // Don't try to separate loops without a preheader.
262 // The header is not a landing pad; preheader insertion should ensure this.
263 assert(!L->getHeader()->isLandingPad() &&
264 "Can't insert backedge to landing pad");
266 PHINode *PN = findPHIToPartitionLoops(L, AA, DT, AT);
267 if (!PN) return nullptr; // No known way to partition.
269 // Pull out all predecessors that have varying values in the loop. This
270 // handles the case when a PHI node has multiple instances of itself as
272 SmallVector<BasicBlock*, 8> OuterLoopPreds;
273 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
274 if (PN->getIncomingValue(i) != PN ||
275 !L->contains(PN->getIncomingBlock(i))) {
276 // We can't split indirectbr edges.
277 if (isa<IndirectBrInst>(PN->getIncomingBlock(i)->getTerminator()))
279 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
282 DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
284 // If ScalarEvolution is around and knows anything about values in
285 // this loop, tell it to forget them, because we're about to
286 // substantially change it.
290 BasicBlock *Header = L->getHeader();
292 SplitBlockPredecessors(Header, OuterLoopPreds, ".outer", PP);
294 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
295 // code layout too horribly.
296 placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
298 // Create the new outer loop.
299 Loop *NewOuter = new Loop();
301 // Change the parent loop to use the outer loop as its child now.
302 if (Loop *Parent = L->getParentLoop())
303 Parent->replaceChildLoopWith(L, NewOuter);
305 LI->changeTopLevelLoop(L, NewOuter);
307 // L is now a subloop of our outer loop.
308 NewOuter->addChildLoop(L);
310 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
312 NewOuter->addBlockEntry(*I);
314 // Now reset the header in L, which had been moved by
315 // SplitBlockPredecessors for the outer loop.
316 L->moveToHeader(Header);
318 // Determine which blocks should stay in L and which should be moved out to
319 // the Outer loop now.
320 std::set<BasicBlock*> BlocksInL;
321 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
323 if (DT->dominates(Header, P))
324 addBlockAndPredsToSet(P, Header, BlocksInL);
327 // Scan all of the loop children of L, moving them to OuterLoop if they are
328 // not part of the inner loop.
329 const std::vector<Loop*> &SubLoops = L->getSubLoops();
330 for (size_t I = 0; I != SubLoops.size(); )
331 if (BlocksInL.count(SubLoops[I]->getHeader()))
332 ++I; // Loop remains in L
334 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
336 // Now that we know which blocks are in L and which need to be moved to
337 // OuterLoop, move any blocks that need it.
338 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
339 BasicBlock *BB = L->getBlocks()[i];
340 if (!BlocksInL.count(BB)) {
341 // Move this block to the parent, updating the exit blocks sets
342 L->removeBlockFromLoop(BB);
344 LI->changeLoopFor(BB, NewOuter);
352 /// \brief This method is called when the specified loop has more than one
355 /// If this occurs, revector all of these backedges to target a new basic block
356 /// and have that block branch to the loop header. This ensures that loops
357 /// have exactly one backedge.
358 static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
360 DominatorTree *DT, LoopInfo *LI) {
361 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
363 // Get information about the loop
364 BasicBlock *Header = L->getHeader();
365 Function *F = Header->getParent();
367 // Unique backedge insertion currently depends on having a preheader.
371 // The header is not a landing pad; preheader insertion should ensure this.
372 assert(!Header->isLandingPad() && "Can't insert backedge to landing pad");
374 // Figure out which basic blocks contain back-edges to the loop header.
375 std::vector<BasicBlock*> BackedgeBlocks;
376 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
379 // Indirectbr edges cannot be split, so we must fail if we find one.
380 if (isa<IndirectBrInst>(P->getTerminator()))
383 if (P != Preheader) BackedgeBlocks.push_back(P);
386 // Create and insert the new backedge block...
387 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
388 Header->getName()+".backedge", F);
389 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
391 DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
392 << BEBlock->getName() << "\n");
394 // Move the new backedge block to right after the last backedge block.
395 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
396 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
398 // Now that the block has been inserted into the function, create PHI nodes in
399 // the backedge block which correspond to any PHI nodes in the header block.
400 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
401 PHINode *PN = cast<PHINode>(I);
402 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
403 PN->getName()+".be", BETerminator);
404 if (AA) AA->copyValue(PN, NewPN);
406 // Loop over the PHI node, moving all entries except the one for the
407 // preheader over to the new PHI node.
408 unsigned PreheaderIdx = ~0U;
409 bool HasUniqueIncomingValue = true;
410 Value *UniqueValue = nullptr;
411 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
412 BasicBlock *IBB = PN->getIncomingBlock(i);
413 Value *IV = PN->getIncomingValue(i);
414 if (IBB == Preheader) {
417 NewPN->addIncoming(IV, IBB);
418 if (HasUniqueIncomingValue) {
421 else if (UniqueValue != IV)
422 HasUniqueIncomingValue = false;
427 // Delete all of the incoming values from the old PN except the preheader's
428 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
429 if (PreheaderIdx != 0) {
430 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
431 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
433 // Nuke all entries except the zero'th.
434 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
435 PN->removeIncomingValue(e-i, false);
437 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
438 PN->addIncoming(NewPN, BEBlock);
440 // As an optimization, if all incoming values in the new PhiNode (which is a
441 // subset of the incoming values of the old PHI node) have the same value,
442 // eliminate the PHI Node.
443 if (HasUniqueIncomingValue) {
444 NewPN->replaceAllUsesWith(UniqueValue);
445 if (AA) AA->deleteValue(NewPN);
446 BEBlock->getInstList().erase(NewPN);
450 // Now that all of the PHI nodes have been inserted and adjusted, modify the
451 // backedge blocks to just to the BEBlock instead of the header.
452 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
453 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
454 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
455 if (TI->getSuccessor(Op) == Header)
456 TI->setSuccessor(Op, BEBlock);
459 //===--- Update all analyses which we must preserve now -----------------===//
461 // Update Loop Information - we know that this block is now in the current
462 // loop and all parent loops.
463 L->addBasicBlockToLoop(BEBlock, LI->getBase());
465 // Update dominator information
466 DT->splitBlock(BEBlock);
471 /// \brief Simplify one loop and queue further loops for simplification.
473 /// FIXME: Currently this accepts both lots of analyses that it uses and a raw
474 /// Pass pointer. The Pass pointer is used by numerous utilities to update
475 /// specific analyses. Rather than a pass it would be much cleaner and more
476 /// explicit if they accepted the analysis directly and then updated it.
477 static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
478 AliasAnalysis *AA, DominatorTree *DT, LoopInfo *LI,
479 ScalarEvolution *SE, Pass *PP,
480 const DataLayout *DL, AssumptionTracker *AT) {
481 bool Changed = false;
484 // Check to see that no blocks (other than the header) in this loop have
485 // predecessors that are not in the loop. This is not valid for natural
486 // loops, but can occur if the blocks are unreachable. Since they are
487 // unreachable we can just shamelessly delete those CFG edges!
488 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
490 if (*BB == L->getHeader()) continue;
492 SmallPtrSet<BasicBlock*, 4> BadPreds;
493 for (pred_iterator PI = pred_begin(*BB),
494 PE = pred_end(*BB); PI != PE; ++PI) {
500 // Delete each unique out-of-loop (and thus dead) predecessor.
501 for (BasicBlock *P : BadPreds) {
503 DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
504 << P->getName() << "\n");
506 // Inform each successor of each dead pred.
507 for (succ_iterator SI = succ_begin(P), SE = succ_end(P); SI != SE; ++SI)
508 (*SI)->removePredecessor(P);
509 // Zap the dead pred's terminator and replace it with unreachable.
510 TerminatorInst *TI = P->getTerminator();
511 TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
512 P->getTerminator()->eraseFromParent();
513 new UnreachableInst(P->getContext(), P);
518 // If there are exiting blocks with branches on undef, resolve the undef in
519 // the direction which will exit the loop. This will help simplify loop
520 // trip count computations.
521 SmallVector<BasicBlock*, 8> ExitingBlocks;
522 L->getExitingBlocks(ExitingBlocks);
523 for (SmallVectorImpl<BasicBlock *>::iterator I = ExitingBlocks.begin(),
524 E = ExitingBlocks.end(); I != E; ++I)
525 if (BranchInst *BI = dyn_cast<BranchInst>((*I)->getTerminator()))
526 if (BI->isConditional()) {
527 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
529 DEBUG(dbgs() << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
530 << (*I)->getName() << "\n");
532 BI->setCondition(ConstantInt::get(Cond->getType(),
533 !L->contains(BI->getSuccessor(0))));
535 // This may make the loop analyzable, force SCEV recomputation.
543 // Does the loop already have a preheader? If so, don't insert one.
544 BasicBlock *Preheader = L->getLoopPreheader();
546 Preheader = InsertPreheaderForLoop(L, PP);
553 // Next, check to make sure that all exit nodes of the loop only have
554 // predecessors that are inside of the loop. This check guarantees that the
555 // loop preheader/header will dominate the exit blocks. If the exit block has
556 // predecessors from outside of the loop, split the edge now.
557 SmallVector<BasicBlock*, 8> ExitBlocks;
558 L->getExitBlocks(ExitBlocks);
560 SmallSetVector<BasicBlock *, 8> ExitBlockSet(ExitBlocks.begin(),
562 for (SmallSetVector<BasicBlock *, 8>::iterator I = ExitBlockSet.begin(),
563 E = ExitBlockSet.end(); I != E; ++I) {
564 BasicBlock *ExitBlock = *I;
565 for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
567 // Must be exactly this loop: no subloops, parent loops, or non-loop preds
569 if (!L->contains(*PI)) {
570 if (rewriteLoopExitBlock(L, ExitBlock, PP)) {
578 // If the header has more than two predecessors at this point (from the
579 // preheader and from multiple backedges), we must adjust the loop.
580 BasicBlock *LoopLatch = L->getLoopLatch();
582 // If this is really a nested loop, rip it out into a child loop. Don't do
583 // this for loops with a giant number of backedges, just factor them into a
584 // common backedge instead.
585 if (L->getNumBackEdges() < 8) {
586 if (Loop *OuterL = separateNestedLoop(L, Preheader, AA, DT, LI, SE,
589 // Enqueue the outer loop as it should be processed next in our
590 // depth-first nest walk.
591 Worklist.push_back(OuterL);
593 // This is a big restructuring change, reprocess the whole loop.
595 // GCC doesn't tail recursion eliminate this.
596 // FIXME: It isn't clear we can't rely on LLVM to TRE this.
601 // If we either couldn't, or didn't want to, identify nesting of the loops,
602 // insert a new block that all backedges target, then make it jump to the
604 LoopLatch = insertUniqueBackedgeBlock(L, Preheader, AA, DT, LI);
611 // Scan over the PHI nodes in the loop header. Since they now have only two
612 // incoming values (the loop is canonicalized), we may have simplified the PHI
613 // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
615 for (BasicBlock::iterator I = L->getHeader()->begin();
616 (PN = dyn_cast<PHINode>(I++)); )
617 if (Value *V = SimplifyInstruction(PN, nullptr, nullptr, DT, AT)) {
618 if (AA) AA->deleteValue(PN);
619 if (SE) SE->forgetValue(PN);
620 PN->replaceAllUsesWith(V);
621 PN->eraseFromParent();
624 // If this loop has multiple exits and the exits all go to the same
625 // block, attempt to merge the exits. This helps several passes, such
626 // as LoopRotation, which do not support loops with multiple exits.
627 // SimplifyCFG also does this (and this code uses the same utility
628 // function), however this code is loop-aware, where SimplifyCFG is
629 // not. That gives it the advantage of being able to hoist
630 // loop-invariant instructions out of the way to open up more
631 // opportunities, and the disadvantage of having the responsibility
632 // to preserve dominator information.
633 bool UniqueExit = true;
634 if (!ExitBlocks.empty())
635 for (unsigned i = 1, e = ExitBlocks.size(); i != e; ++i)
636 if (ExitBlocks[i] != ExitBlocks[0]) {
641 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
642 BasicBlock *ExitingBlock = ExitingBlocks[i];
643 if (!ExitingBlock->getSinglePredecessor()) continue;
644 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
645 if (!BI || !BI->isConditional()) continue;
646 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
647 if (!CI || CI->getParent() != ExitingBlock) continue;
649 // Attempt to hoist out all instructions except for the
650 // comparison and the branch.
651 bool AllInvariant = true;
652 bool AnyInvariant = false;
653 for (BasicBlock::iterator I = ExitingBlock->begin(); &*I != BI; ) {
654 Instruction *Inst = I++;
655 // Skip debug info intrinsics.
656 if (isa<DbgInfoIntrinsic>(Inst))
660 if (!L->makeLoopInvariant(Inst, AnyInvariant,
661 Preheader ? Preheader->getTerminator()
663 AllInvariant = false;
669 // The loop disposition of all SCEV expressions that depend on any
670 // hoisted values have also changed.
672 SE->forgetLoopDispositions(L);
674 if (!AllInvariant) continue;
676 // The block has now been cleared of all instructions except for
677 // a comparison and a conditional branch. SimplifyCFG may be able
679 if (!FoldBranchToCommonDest(BI, DL)) continue;
681 // Success. The block is now dead, so remove it from the loop,
682 // update the dominator tree and delete it.
683 DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
684 << ExitingBlock->getName() << "\n");
686 // Notify ScalarEvolution before deleting this block. Currently assume the
687 // parent loop doesn't change (spliting edges doesn't count). If blocks,
688 // CFG edges, or other values in the parent loop change, then we need call
689 // to forgetLoop() for the parent instead.
693 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
695 LI->removeBlock(ExitingBlock);
697 DomTreeNode *Node = DT->getNode(ExitingBlock);
698 const std::vector<DomTreeNodeBase<BasicBlock> *> &Children =
700 while (!Children.empty()) {
701 DomTreeNode *Child = Children.front();
702 DT->changeImmediateDominator(Child, Node->getIDom());
704 DT->eraseNode(ExitingBlock);
706 BI->getSuccessor(0)->removePredecessor(ExitingBlock);
707 BI->getSuccessor(1)->removePredecessor(ExitingBlock);
708 ExitingBlock->eraseFromParent();
715 bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI, Pass *PP,
716 AliasAnalysis *AA, ScalarEvolution *SE,
717 const DataLayout *DL, AssumptionTracker *AT) {
718 bool Changed = false;
720 // Worklist maintains our depth-first queue of loops in this nest to process.
721 SmallVector<Loop *, 4> Worklist;
722 Worklist.push_back(L);
724 // Walk the worklist from front to back, pushing newly found sub loops onto
725 // the back. This will let us process loops from back to front in depth-first
726 // order. We can use this simple process because loops form a tree.
727 for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
728 Loop *L2 = Worklist[Idx];
729 for (Loop::iterator I = L2->begin(), E = L2->end(); I != E; ++I)
730 Worklist.push_back(*I);
733 while (!Worklist.empty())
734 Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, AA, DT, LI,
741 struct LoopSimplify : public FunctionPass {
742 static char ID; // Pass identification, replacement for typeid
743 LoopSimplify() : FunctionPass(ID) {
744 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
747 // AA - If we have an alias analysis object to update, this is it, otherwise
753 const DataLayout *DL;
754 AssumptionTracker *AT;
756 bool runOnFunction(Function &F) override;
758 void getAnalysisUsage(AnalysisUsage &AU) const override {
759 AU.addRequired<AssumptionTracker>();
761 // We need loop information to identify the loops...
762 AU.addRequired<DominatorTreeWrapperPass>();
763 AU.addPreserved<DominatorTreeWrapperPass>();
765 AU.addRequired<LoopInfo>();
766 AU.addPreserved<LoopInfo>();
768 AU.addPreserved<AliasAnalysis>();
769 AU.addPreserved<ScalarEvolution>();
770 AU.addPreserved<DependenceAnalysis>();
771 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
774 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
775 void verifyAnalysis() const override;
779 char LoopSimplify::ID = 0;
780 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
781 "Canonicalize natural loops", true, false)
782 INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
783 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
784 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
785 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
786 "Canonicalize natural loops", true, false)
788 // Publicly exposed interface to pass...
789 char &llvm::LoopSimplifyID = LoopSimplify::ID;
790 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
792 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
793 /// it in any convenient order) inserting preheaders...
795 bool LoopSimplify::runOnFunction(Function &F) {
796 bool Changed = false;
797 AA = getAnalysisIfAvailable<AliasAnalysis>();
798 LI = &getAnalysis<LoopInfo>();
799 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
800 SE = getAnalysisIfAvailable<ScalarEvolution>();
801 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
802 DL = DLP ? &DLP->getDataLayout() : nullptr;
803 AT = &getAnalysis<AssumptionTracker>();
805 // Simplify each loop nest in the function.
806 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
807 Changed |= simplifyLoop(*I, DT, LI, this, AA, SE, DL, AT);
812 // FIXME: Restore this code when we re-enable verification in verifyAnalysis
815 static void verifyLoop(Loop *L) {
817 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
820 // It used to be possible to just assert L->isLoopSimplifyForm(), however
821 // with the introduction of indirectbr, there are now cases where it's
822 // not possible to transform a loop as necessary. We can at least check
823 // that there is an indirectbr near any time there's trouble.
825 // Indirectbr can interfere with preheader and unique backedge insertion.
826 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
827 bool HasIndBrPred = false;
828 for (pred_iterator PI = pred_begin(L->getHeader()),
829 PE = pred_end(L->getHeader()); PI != PE; ++PI)
830 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
834 assert(HasIndBrPred &&
835 "LoopSimplify has no excuse for missing loop header info!");
839 // Indirectbr can interfere with exit block canonicalization.
840 if (!L->hasDedicatedExits()) {
841 bool HasIndBrExiting = false;
842 SmallVector<BasicBlock*, 8> ExitingBlocks;
843 L->getExitingBlocks(ExitingBlocks);
844 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
845 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
846 HasIndBrExiting = true;
851 assert(HasIndBrExiting &&
852 "LoopSimplify has no excuse for missing exit block info!");
853 (void)HasIndBrExiting;
858 void LoopSimplify::verifyAnalysis() const {
859 // FIXME: This routine is being called mid-way through the loop pass manager
860 // as loop passes destroy this analysis. That's actually fine, but we have no
861 // way of expressing that here. Once all of the passes that destroy this are
862 // hoisted out of the loop pass manager we can add back verification here.
864 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)