1 //===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
7 //===----------------------------------------------------------------------===//
9 // This pass performs several transformations to transform natural loops into a
10 // simpler form, which makes subsequent analyses and transformations simpler and
13 // Loop pre-header insertion guarantees that there is a single, non-critical
14 // entry edge from outside of the loop to the loop header. This simplifies a
15 // number of analyses and transformations, such as LICM.
17 // Loop exit-block insertion guarantees that all exit blocks from the loop
18 // (blocks which are outside of the loop that have predecessors inside of the
19 // loop) only have predecessors from inside of the loop (and are thus dominated
20 // by the loop header). This simplifies transformations such as store-sinking
21 // that are built into LICM.
23 // This pass also guarantees that loops will have exactly one backedge.
25 // Indirectbr instructions introduce several complications. If the loop
26 // contains or is entered by an indirectbr instruction, it may not be possible
27 // to transform the loop and make these guarantees. Client code should check
28 // that these conditions are true before relying on them.
30 // Similar complications arise from callbr instructions, particularly in
31 // asm-goto where blockaddress expressions are used.
33 // Note that the simplifycfg pass will clean up blocks which are split out but
34 // end up being unnecessary, so usage of this pass should not pessimize
37 // This pass obviously modifies the CFG, but updates loop information and
38 // dominator information.
40 //===----------------------------------------------------------------------===//
42 #include "llvm/Transforms/Utils/LoopSimplify.h"
43 #include "llvm/ADT/DepthFirstIterator.h"
44 #include "llvm/ADT/SetOperations.h"
45 #include "llvm/ADT/SetVector.h"
46 #include "llvm/ADT/SmallVector.h"
47 #include "llvm/ADT/Statistic.h"
48 #include "llvm/Analysis/AliasAnalysis.h"
49 #include "llvm/Analysis/AssumptionCache.h"
50 #include "llvm/Analysis/BasicAliasAnalysis.h"
51 #include "llvm/Analysis/BranchProbabilityInfo.h"
52 #include "llvm/Analysis/DependenceAnalysis.h"
53 #include "llvm/Analysis/GlobalsModRef.h"
54 #include "llvm/Analysis/InstructionSimplify.h"
55 #include "llvm/Analysis/LoopInfo.h"
56 #include "llvm/Analysis/MemorySSA.h"
57 #include "llvm/Analysis/MemorySSAUpdater.h"
58 #include "llvm/Analysis/ScalarEvolution.h"
59 #include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
60 #include "llvm/IR/CFG.h"
61 #include "llvm/IR/Constants.h"
62 #include "llvm/IR/DataLayout.h"
63 #include "llvm/IR/Dominators.h"
64 #include "llvm/IR/Function.h"
65 #include "llvm/IR/Instructions.h"
66 #include "llvm/IR/IntrinsicInst.h"
67 #include "llvm/IR/LLVMContext.h"
68 #include "llvm/IR/Module.h"
69 #include "llvm/IR/Type.h"
70 #include "llvm/InitializePasses.h"
71 #include "llvm/Support/Debug.h"
72 #include "llvm/Support/raw_ostream.h"
73 #include "llvm/Transforms/Utils.h"
74 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
75 #include "llvm/Transforms/Utils/Local.h"
76 #include "llvm/Transforms/Utils/LoopUtils.h"
79 #define DEBUG_TYPE "loop-simplify"
81 STATISTIC(NumNested , "Number of nested loops split out");
83 // If the block isn't already, move the new block to right after some 'outside
84 // block' block. This prevents the preheader from being placed inside the loop
85 // body, e.g. when the loop hasn't been rotated.
86 static void placeSplitBlockCarefully(BasicBlock *NewBB,
87 SmallVectorImpl<BasicBlock *> &SplitPreds,
89 // Check to see if NewBB is already well placed.
90 Function::iterator BBI = --NewBB->getIterator();
91 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
92 if (&*BBI == SplitPreds[i])
96 // If it isn't already after an outside block, move it after one. This is
97 // always good as it makes the uncond branch from the outside block into a
100 // Figure out *which* outside block to put this after. Prefer an outside
101 // block that neighbors a BB actually in the loop.
102 BasicBlock *FoundBB = nullptr;
103 for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
104 Function::iterator BBI = SplitPreds[i]->getIterator();
105 if (++BBI != NewBB->getParent()->end() && L->contains(&*BBI)) {
106 FoundBB = SplitPreds[i];
111 // If our heuristic for a *good* bb to place this after doesn't find
112 // anything, just pick something. It's likely better than leaving it within
115 FoundBB = SplitPreds[0];
116 NewBB->moveAfter(FoundBB);
119 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
120 /// preheader, this method is called to insert one. This method has two phases:
121 /// preheader insertion and analysis updating.
123 BasicBlock *llvm::InsertPreheaderForLoop(Loop *L, DominatorTree *DT,
124 LoopInfo *LI, MemorySSAUpdater *MSSAU,
125 bool PreserveLCSSA) {
126 BasicBlock *Header = L->getHeader();
128 // Compute the set of predecessors of the loop that are not in the loop.
129 SmallVector<BasicBlock*, 8> OutsideBlocks;
130 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
133 if (!L->contains(P)) { // Coming in from outside the loop?
134 // If the loop is branched to from an indirect terminator, we won't
135 // be able to fully transform the loop, because it prohibits
137 if (P->getTerminator()->isIndirectTerminator())
141 OutsideBlocks.push_back(P);
145 // Split out the loop pre-header.
146 BasicBlock *PreheaderBB;
147 PreheaderBB = SplitBlockPredecessors(Header, OutsideBlocks, ".preheader", DT,
148 LI, MSSAU, PreserveLCSSA);
152 LLVM_DEBUG(dbgs() << "LoopSimplify: Creating pre-header "
153 << PreheaderBB->getName() << "\n");
155 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
156 // code layout too horribly.
157 placeSplitBlockCarefully(PreheaderBB, OutsideBlocks, L);
162 /// Add the specified block, and all of its predecessors, to the specified set,
163 /// if it's not already in there. Stop predecessor traversal when we reach
165 static void addBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
166 std::set<BasicBlock*> &Blocks) {
167 SmallVector<BasicBlock *, 8> Worklist;
168 Worklist.push_back(InputBB);
170 BasicBlock *BB = Worklist.pop_back_val();
171 if (Blocks.insert(BB).second && BB != StopBlock)
172 // If BB is not already processed and it is not a stop block then
173 // insert its predecessor in the work list
174 for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
175 BasicBlock *WBB = *I;
176 Worklist.push_back(WBB);
178 } while (!Worklist.empty());
181 /// The first part of loop-nestification is to find a PHI node that tells
182 /// us how to partition the loops.
183 static PHINode *findPHIToPartitionLoops(Loop *L, DominatorTree *DT,
184 AssumptionCache *AC) {
185 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
186 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
187 PHINode *PN = cast<PHINode>(I);
189 if (Value *V = SimplifyInstruction(PN, {DL, nullptr, DT, AC})) {
190 // This is a degenerate PHI already, don't modify it!
191 PN->replaceAllUsesWith(V);
192 PN->eraseFromParent();
196 // Scan this PHI node looking for a use of the PHI node by itself.
197 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
198 if (PN->getIncomingValue(i) == PN &&
199 L->contains(PN->getIncomingBlock(i)))
200 // We found something tasty to remove.
206 /// If this loop has multiple backedges, try to pull one of them out into
209 /// This is important for code that looks like
214 /// br cond, Loop, Next
216 /// br cond2, Loop, Out
218 /// To identify this common case, we look at the PHI nodes in the header of the
219 /// loop. PHI nodes with unchanging values on one backedge correspond to values
220 /// that change in the "outer" loop, but not in the "inner" loop.
222 /// If we are able to separate out a loop, return the new outer loop that was
225 static Loop *separateNestedLoop(Loop *L, BasicBlock *Preheader,
226 DominatorTree *DT, LoopInfo *LI,
227 ScalarEvolution *SE, bool PreserveLCSSA,
228 AssumptionCache *AC, MemorySSAUpdater *MSSAU) {
229 // Don't try to separate loops without a preheader.
233 // Treat the presence of convergent functions conservatively. The
234 // transformation is invalid if calls to certain convergent
235 // functions (like an AMDGPU barrier) get included in the resulting
236 // inner loop. But blocks meant for the inner loop will be
237 // identified later at a point where it's too late to abort the
238 // transformation. Also, the convergent attribute is not really
239 // sufficient to express the semantics of functions that are
240 // affected by this transformation. So we choose to back off if such
241 // a function call is present until a better alternative becomes
242 // available. This is similar to the conservative treatment of
243 // convergent function calls in GVNHoist and JumpThreading.
244 for (auto BB : L->blocks()) {
245 for (auto &II : *BB) {
246 if (auto CI = dyn_cast<CallBase>(&II)) {
247 if (CI->isConvergent()) {
254 // The header is not a landing pad; preheader insertion should ensure this.
255 BasicBlock *Header = L->getHeader();
256 assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
258 PHINode *PN = findPHIToPartitionLoops(L, DT, AC);
259 if (!PN) return nullptr; // No known way to partition.
261 // Pull out all predecessors that have varying values in the loop. This
262 // handles the case when a PHI node has multiple instances of itself as
264 SmallVector<BasicBlock*, 8> OuterLoopPreds;
265 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
266 if (PN->getIncomingValue(i) != PN ||
267 !L->contains(PN->getIncomingBlock(i))) {
268 // We can't split indirect control flow edges.
269 if (PN->getIncomingBlock(i)->getTerminator()->isIndirectTerminator())
271 OuterLoopPreds.push_back(PN->getIncomingBlock(i));
274 LLVM_DEBUG(dbgs() << "LoopSimplify: Splitting out a new outer loop\n");
276 // If ScalarEvolution is around and knows anything about values in
277 // this loop, tell it to forget them, because we're about to
278 // substantially change it.
282 BasicBlock *NewBB = SplitBlockPredecessors(Header, OuterLoopPreds, ".outer",
283 DT, LI, MSSAU, PreserveLCSSA);
285 // Make sure that NewBB is put someplace intelligent, which doesn't mess up
286 // code layout too horribly.
287 placeSplitBlockCarefully(NewBB, OuterLoopPreds, L);
289 // Create the new outer loop.
290 Loop *NewOuter = LI->AllocateLoop();
292 // Change the parent loop to use the outer loop as its child now.
293 if (Loop *Parent = L->getParentLoop())
294 Parent->replaceChildLoopWith(L, NewOuter);
296 LI->changeTopLevelLoop(L, NewOuter);
298 // L is now a subloop of our outer loop.
299 NewOuter->addChildLoop(L);
301 for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
303 NewOuter->addBlockEntry(*I);
305 // Now reset the header in L, which had been moved by
306 // SplitBlockPredecessors for the outer loop.
307 L->moveToHeader(Header);
309 // Determine which blocks should stay in L and which should be moved out to
310 // the Outer loop now.
311 std::set<BasicBlock*> BlocksInL;
312 for (pred_iterator PI=pred_begin(Header), E = pred_end(Header); PI!=E; ++PI) {
314 if (DT->dominates(Header, P))
315 addBlockAndPredsToSet(P, Header, BlocksInL);
318 // Scan all of the loop children of L, moving them to OuterLoop if they are
319 // not part of the inner loop.
320 const std::vector<Loop*> &SubLoops = L->getSubLoops();
321 for (size_t I = 0; I != SubLoops.size(); )
322 if (BlocksInL.count(SubLoops[I]->getHeader()))
323 ++I; // Loop remains in L
325 NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
327 SmallVector<BasicBlock *, 8> OuterLoopBlocks;
328 OuterLoopBlocks.push_back(NewBB);
329 // Now that we know which blocks are in L and which need to be moved to
330 // OuterLoop, move any blocks that need it.
331 for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
332 BasicBlock *BB = L->getBlocks()[i];
333 if (!BlocksInL.count(BB)) {
334 // Move this block to the parent, updating the exit blocks sets
335 L->removeBlockFromLoop(BB);
336 if ((*LI)[BB] == L) {
337 LI->changeLoopFor(BB, NewOuter);
338 OuterLoopBlocks.push_back(BB);
344 // Split edges to exit blocks from the inner loop, if they emerged in the
345 // process of separating the outer one.
346 formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA);
349 // Fix LCSSA form for L. Some values, which previously were only used inside
350 // L, can now be used in NewOuter loop. We need to insert phi-nodes for them
351 // in corresponding exit blocks.
352 // We don't need to form LCSSA recursively, because there cannot be uses
353 // inside a newly created loop of defs from inner loops as those would
354 // already be a use of an LCSSA phi node.
355 formLCSSA(*L, *DT, LI, SE);
357 assert(NewOuter->isRecursivelyLCSSAForm(*DT, *LI) &&
358 "LCSSA is broken after separating nested loops!");
364 /// This method is called when the specified loop has more than one
367 /// If this occurs, revector all of these backedges to target a new basic block
368 /// and have that block branch to the loop header. This ensures that loops
369 /// have exactly one backedge.
370 static BasicBlock *insertUniqueBackedgeBlock(Loop *L, BasicBlock *Preheader,
371 DominatorTree *DT, LoopInfo *LI,
372 MemorySSAUpdater *MSSAU) {
373 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
375 // Get information about the loop
376 BasicBlock *Header = L->getHeader();
377 Function *F = Header->getParent();
379 // Unique backedge insertion currently depends on having a preheader.
383 // The header is not an EH pad; preheader insertion should ensure this.
384 assert(!Header->isEHPad() && "Can't insert backedge to EH pad");
386 // Figure out which basic blocks contain back-edges to the loop header.
387 std::vector<BasicBlock*> BackedgeBlocks;
388 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I){
391 // Indirect edges cannot be split, so we must fail if we find one.
392 if (P->getTerminator()->isIndirectTerminator())
395 if (P != Preheader) BackedgeBlocks.push_back(P);
398 // Create and insert the new backedge block...
399 BasicBlock *BEBlock = BasicBlock::Create(Header->getContext(),
400 Header->getName() + ".backedge", F);
401 BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
402 BETerminator->setDebugLoc(Header->getFirstNonPHI()->getDebugLoc());
404 LLVM_DEBUG(dbgs() << "LoopSimplify: Inserting unique backedge block "
405 << BEBlock->getName() << "\n");
407 // Move the new backedge block to right after the last backedge block.
408 Function::iterator InsertPos = ++BackedgeBlocks.back()->getIterator();
409 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
411 // Now that the block has been inserted into the function, create PHI nodes in
412 // the backedge block which correspond to any PHI nodes in the header block.
413 for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
414 PHINode *PN = cast<PHINode>(I);
415 PHINode *NewPN = PHINode::Create(PN->getType(), BackedgeBlocks.size(),
416 PN->getName()+".be", BETerminator);
418 // Loop over the PHI node, moving all entries except the one for the
419 // preheader over to the new PHI node.
420 unsigned PreheaderIdx = ~0U;
421 bool HasUniqueIncomingValue = true;
422 Value *UniqueValue = nullptr;
423 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
424 BasicBlock *IBB = PN->getIncomingBlock(i);
425 Value *IV = PN->getIncomingValue(i);
426 if (IBB == Preheader) {
429 NewPN->addIncoming(IV, IBB);
430 if (HasUniqueIncomingValue) {
433 else if (UniqueValue != IV)
434 HasUniqueIncomingValue = false;
439 // Delete all of the incoming values from the old PN except the preheader's
440 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
441 if (PreheaderIdx != 0) {
442 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
443 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
445 // Nuke all entries except the zero'th.
446 for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
447 PN->removeIncomingValue(e-i, false);
449 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
450 PN->addIncoming(NewPN, BEBlock);
452 // As an optimization, if all incoming values in the new PhiNode (which is a
453 // subset of the incoming values of the old PHI node) have the same value,
454 // eliminate the PHI Node.
455 if (HasUniqueIncomingValue) {
456 NewPN->replaceAllUsesWith(UniqueValue);
457 BEBlock->getInstList().erase(NewPN);
461 // Now that all of the PHI nodes have been inserted and adjusted, modify the
462 // backedge blocks to jump to the BEBlock instead of the header.
463 // If one of the backedges has llvm.loop metadata attached, we remove
464 // it from the backedge and add it to BEBlock.
465 unsigned LoopMDKind = BEBlock->getContext().getMDKindID("llvm.loop");
466 MDNode *LoopMD = nullptr;
467 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
468 Instruction *TI = BackedgeBlocks[i]->getTerminator();
470 LoopMD = TI->getMetadata(LoopMDKind);
471 TI->setMetadata(LoopMDKind, nullptr);
472 TI->replaceSuccessorWith(Header, BEBlock);
474 BEBlock->getTerminator()->setMetadata(LoopMDKind, LoopMD);
476 //===--- Update all analyses which we must preserve now -----------------===//
478 // Update Loop Information - we know that this block is now in the current
479 // loop and all parent loops.
480 L->addBasicBlockToLoop(BEBlock, *LI);
482 // Update dominator information
483 DT->splitBlock(BEBlock);
486 MSSAU->updatePhisWhenInsertingUniqueBackedgeBlock(Header, Preheader,
492 /// Simplify one loop and queue further loops for simplification.
493 static bool simplifyOneLoop(Loop *L, SmallVectorImpl<Loop *> &Worklist,
494 DominatorTree *DT, LoopInfo *LI,
495 ScalarEvolution *SE, AssumptionCache *AC,
496 MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {
497 bool Changed = false;
498 if (MSSAU && VerifyMemorySSA)
499 MSSAU->getMemorySSA()->verifyMemorySSA();
503 // Check to see that no blocks (other than the header) in this loop have
504 // predecessors that are not in the loop. This is not valid for natural
505 // loops, but can occur if the blocks are unreachable. Since they are
506 // unreachable we can just shamelessly delete those CFG edges!
507 for (Loop::block_iterator BB = L->block_begin(), E = L->block_end();
509 if (*BB == L->getHeader()) continue;
511 SmallPtrSet<BasicBlock*, 4> BadPreds;
512 for (pred_iterator PI = pred_begin(*BB),
513 PE = pred_end(*BB); PI != PE; ++PI) {
519 // Delete each unique out-of-loop (and thus dead) predecessor.
520 for (BasicBlock *P : BadPreds) {
522 LLVM_DEBUG(dbgs() << "LoopSimplify: Deleting edge from dead predecessor "
523 << P->getName() << "\n");
525 // Zap the dead pred's terminator and replace it with unreachable.
526 Instruction *TI = P->getTerminator();
527 changeToUnreachable(TI, /*UseLLVMTrap=*/false, PreserveLCSSA,
528 /*DTU=*/nullptr, MSSAU);
533 if (MSSAU && VerifyMemorySSA)
534 MSSAU->getMemorySSA()->verifyMemorySSA();
536 // If there are exiting blocks with branches on undef, resolve the undef in
537 // the direction which will exit the loop. This will help simplify loop
538 // trip count computations.
539 SmallVector<BasicBlock*, 8> ExitingBlocks;
540 L->getExitingBlocks(ExitingBlocks);
541 for (BasicBlock *ExitingBlock : ExitingBlocks)
542 if (BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator()))
543 if (BI->isConditional()) {
544 if (UndefValue *Cond = dyn_cast<UndefValue>(BI->getCondition())) {
547 << "LoopSimplify: Resolving \"br i1 undef\" to exit in "
548 << ExitingBlock->getName() << "\n");
550 BI->setCondition(ConstantInt::get(Cond->getType(),
551 !L->contains(BI->getSuccessor(0))));
557 // Does the loop already have a preheader? If so, don't insert one.
558 BasicBlock *Preheader = L->getLoopPreheader();
560 Preheader = InsertPreheaderForLoop(L, DT, LI, MSSAU, PreserveLCSSA);
565 // Next, check to make sure that all exit nodes of the loop only have
566 // predecessors that are inside of the loop. This check guarantees that the
567 // loop preheader/header will dominate the exit blocks. If the exit block has
568 // predecessors from outside of the loop, split the edge now.
569 if (formDedicatedExitBlocks(L, DT, LI, MSSAU, PreserveLCSSA))
572 if (MSSAU && VerifyMemorySSA)
573 MSSAU->getMemorySSA()->verifyMemorySSA();
575 // If the header has more than two predecessors at this point (from the
576 // preheader and from multiple backedges), we must adjust the loop.
577 BasicBlock *LoopLatch = L->getLoopLatch();
579 // If this is really a nested loop, rip it out into a child loop. Don't do
580 // this for loops with a giant number of backedges, just factor them into a
581 // common backedge instead.
582 if (L->getNumBackEdges() < 8) {
583 if (Loop *OuterL = separateNestedLoop(L, Preheader, DT, LI, SE,
584 PreserveLCSSA, AC, MSSAU)) {
586 // Enqueue the outer loop as it should be processed next in our
587 // depth-first nest walk.
588 Worklist.push_back(OuterL);
590 // This is a big restructuring change, reprocess the whole loop.
592 // GCC doesn't tail recursion eliminate this.
593 // FIXME: It isn't clear we can't rely on LLVM to TRE this.
598 // If we either couldn't, or didn't want to, identify nesting of the loops,
599 // insert a new block that all backedges target, then make it jump to the
601 LoopLatch = insertUniqueBackedgeBlock(L, Preheader, DT, LI, MSSAU);
606 if (MSSAU && VerifyMemorySSA)
607 MSSAU->getMemorySSA()->verifyMemorySSA();
609 const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
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, {DL, nullptr, DT, AC})) {
618 if (SE) SE->forgetValue(PN);
619 if (!PreserveLCSSA || LI->replacementPreservesLCSSAForm(PN, V)) {
620 PN->replaceAllUsesWith(V);
621 PN->eraseFromParent();
626 // If this loop has multiple exits and the exits all go to the same
627 // block, attempt to merge the exits. This helps several passes, such
628 // as LoopRotation, which do not support loops with multiple exits.
629 // SimplifyCFG also does this (and this code uses the same utility
630 // function), however this code is loop-aware, where SimplifyCFG is
631 // not. That gives it the advantage of being able to hoist
632 // loop-invariant instructions out of the way to open up more
633 // opportunities, and the disadvantage of having the responsibility
634 // to preserve dominator information.
635 auto HasUniqueExitBlock = [&]() {
636 BasicBlock *UniqueExit = nullptr;
637 for (auto *ExitingBB : ExitingBlocks)
638 for (auto *SuccBB : successors(ExitingBB)) {
639 if (L->contains(SuccBB))
644 else if (UniqueExit != SuccBB)
650 if (HasUniqueExitBlock()) {
651 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
652 BasicBlock *ExitingBlock = ExitingBlocks[i];
653 if (!ExitingBlock->getSinglePredecessor()) continue;
654 BranchInst *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
655 if (!BI || !BI->isConditional()) continue;
656 CmpInst *CI = dyn_cast<CmpInst>(BI->getCondition());
657 if (!CI || CI->getParent() != ExitingBlock) continue;
659 // Attempt to hoist out all instructions except for the
660 // comparison and the branch.
661 bool AllInvariant = true;
662 bool AnyInvariant = false;
663 for (auto I = ExitingBlock->instructionsWithoutDebug().begin(); &*I != BI; ) {
664 Instruction *Inst = &*I++;
667 if (!L->makeLoopInvariant(
669 Preheader ? Preheader->getTerminator() : nullptr, MSSAU)) {
670 AllInvariant = false;
676 // The loop disposition of all SCEV expressions that depend on any
677 // hoisted values have also changed.
679 SE->forgetLoopDispositions(L);
681 if (!AllInvariant) continue;
683 // The block has now been cleared of all instructions except for
684 // a comparison and a conditional branch. SimplifyCFG may be able
686 if (!FoldBranchToCommonDest(BI, MSSAU))
689 // Success. The block is now dead, so remove it from the loop,
690 // update the dominator tree and delete it.
691 LLVM_DEBUG(dbgs() << "LoopSimplify: Eliminating exiting block "
692 << ExitingBlock->getName() << "\n");
694 assert(pred_begin(ExitingBlock) == pred_end(ExitingBlock));
696 LI->removeBlock(ExitingBlock);
698 DomTreeNode *Node = DT->getNode(ExitingBlock);
699 while (!Node->isLeaf()) {
700 DomTreeNode *Child = Node->back();
701 DT->changeImmediateDominator(Child, Node->getIDom());
703 DT->eraseNode(ExitingBlock);
705 SmallSetVector<BasicBlock *, 8> ExitBlockSet;
706 ExitBlockSet.insert(ExitingBlock);
707 MSSAU->removeBlocks(ExitBlockSet);
710 BI->getSuccessor(0)->removePredecessor(
711 ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);
712 BI->getSuccessor(1)->removePredecessor(
713 ExitingBlock, /* KeepOneInputPHIs */ PreserveLCSSA);
714 ExitingBlock->eraseFromParent();
718 // Changing exit conditions for blocks may affect exit counts of this loop and
719 // any of its paretns, so we must invalidate the entire subtree if we've made
722 SE->forgetTopmostLoop(L);
724 if (MSSAU && VerifyMemorySSA)
725 MSSAU->getMemorySSA()->verifyMemorySSA();
730 bool llvm::simplifyLoop(Loop *L, DominatorTree *DT, LoopInfo *LI,
731 ScalarEvolution *SE, AssumptionCache *AC,
732 MemorySSAUpdater *MSSAU, bool PreserveLCSSA) {
733 bool Changed = false;
736 // If we're asked to preserve LCSSA, the loop nest needs to start in LCSSA
739 assert(DT && "DT not available.");
740 assert(LI && "LI not available.");
741 assert(L->isRecursivelyLCSSAForm(*DT, *LI) &&
742 "Requested to preserve LCSSA, but it's already broken.");
746 // Worklist maintains our depth-first queue of loops in this nest to process.
747 SmallVector<Loop *, 4> Worklist;
748 Worklist.push_back(L);
750 // Walk the worklist from front to back, pushing newly found sub loops onto
751 // the back. This will let us process loops from back to front in depth-first
752 // order. We can use this simple process because loops form a tree.
753 for (unsigned Idx = 0; Idx != Worklist.size(); ++Idx) {
754 Loop *L2 = Worklist[Idx];
755 Worklist.append(L2->begin(), L2->end());
758 while (!Worklist.empty())
759 Changed |= simplifyOneLoop(Worklist.pop_back_val(), Worklist, DT, LI, SE,
760 AC, MSSAU, PreserveLCSSA);
766 struct LoopSimplify : public FunctionPass {
767 static char ID; // Pass identification, replacement for typeid
768 LoopSimplify() : FunctionPass(ID) {
769 initializeLoopSimplifyPass(*PassRegistry::getPassRegistry());
772 bool runOnFunction(Function &F) override;
774 void getAnalysisUsage(AnalysisUsage &AU) const override {
775 AU.addRequired<AssumptionCacheTracker>();
777 // We need loop information to identify the loops...
778 AU.addRequired<DominatorTreeWrapperPass>();
779 AU.addPreserved<DominatorTreeWrapperPass>();
781 AU.addRequired<LoopInfoWrapperPass>();
782 AU.addPreserved<LoopInfoWrapperPass>();
784 AU.addPreserved<BasicAAWrapperPass>();
785 AU.addPreserved<AAResultsWrapperPass>();
786 AU.addPreserved<GlobalsAAWrapperPass>();
787 AU.addPreserved<ScalarEvolutionWrapperPass>();
788 AU.addPreserved<SCEVAAWrapperPass>();
789 AU.addPreservedID(LCSSAID);
790 AU.addPreserved<DependenceAnalysisWrapperPass>();
791 AU.addPreservedID(BreakCriticalEdgesID); // No critical edges added.
792 AU.addPreserved<BranchProbabilityInfoWrapperPass>();
793 if (EnableMSSALoopDependency)
794 AU.addPreserved<MemorySSAWrapperPass>();
797 /// verifyAnalysis() - Verify LoopSimplifyForm's guarantees.
798 void verifyAnalysis() const override;
802 char LoopSimplify::ID = 0;
803 INITIALIZE_PASS_BEGIN(LoopSimplify, "loop-simplify",
804 "Canonicalize natural loops", false, false)
805 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
806 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
807 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
808 INITIALIZE_PASS_END(LoopSimplify, "loop-simplify",
809 "Canonicalize natural loops", false, false)
811 // Publicly exposed interface to pass...
812 char &llvm::LoopSimplifyID = LoopSimplify::ID;
813 Pass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
815 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
816 /// it in any convenient order) inserting preheaders...
818 bool LoopSimplify::runOnFunction(Function &F) {
819 bool Changed = false;
820 LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
821 DominatorTree *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
822 auto *SEWP = getAnalysisIfAvailable<ScalarEvolutionWrapperPass>();
823 ScalarEvolution *SE = SEWP ? &SEWP->getSE() : nullptr;
824 AssumptionCache *AC =
825 &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
826 MemorySSA *MSSA = nullptr;
827 std::unique_ptr<MemorySSAUpdater> MSSAU;
828 if (EnableMSSALoopDependency) {
829 auto *MSSAAnalysis = getAnalysisIfAvailable<MemorySSAWrapperPass>();
831 MSSA = &MSSAAnalysis->getMSSA();
832 MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
836 bool PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
838 // Simplify each loop nest in the function.
839 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
840 Changed |= simplifyLoop(*I, DT, LI, SE, AC, MSSAU.get(), PreserveLCSSA);
844 bool InLCSSA = all_of(
845 *LI, [&](Loop *L) { return L->isRecursivelyLCSSAForm(*DT, *LI); });
846 assert(InLCSSA && "LCSSA is broken after loop-simplify.");
852 PreservedAnalyses LoopSimplifyPass::run(Function &F,
853 FunctionAnalysisManager &AM) {
854 bool Changed = false;
855 LoopInfo *LI = &AM.getResult<LoopAnalysis>(F);
856 DominatorTree *DT = &AM.getResult<DominatorTreeAnalysis>(F);
857 ScalarEvolution *SE = AM.getCachedResult<ScalarEvolutionAnalysis>(F);
858 AssumptionCache *AC = &AM.getResult<AssumptionAnalysis>(F);
859 auto *MSSAAnalysis = AM.getCachedResult<MemorySSAAnalysis>(F);
860 std::unique_ptr<MemorySSAUpdater> MSSAU;
862 auto *MSSA = &MSSAAnalysis->getMSSA();
863 MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
867 // Note that we don't preserve LCSSA in the new PM, if you need it run LCSSA
868 // after simplifying the loops. MemorySSA is preserved if it exists.
869 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
871 simplifyLoop(*I, DT, LI, SE, AC, MSSAU.get(), /*PreserveLCSSA*/ false);
874 return PreservedAnalyses::all();
876 PreservedAnalyses PA;
877 PA.preserve<DominatorTreeAnalysis>();
878 PA.preserve<LoopAnalysis>();
879 PA.preserve<BasicAA>();
880 PA.preserve<GlobalsAA>();
881 PA.preserve<SCEVAA>();
882 PA.preserve<ScalarEvolutionAnalysis>();
883 PA.preserve<DependenceAnalysis>();
885 PA.preserve<MemorySSAAnalysis>();
886 // BPI maps conditional terminators to probabilities, LoopSimplify can insert
887 // blocks, but it does so only by splitting existing blocks and edges. This
888 // results in the interesting property that all new terminators inserted are
889 // unconditional branches which do not appear in BPI. All deletions are
890 // handled via ValueHandle callbacks w/in BPI.
891 PA.preserve<BranchProbabilityAnalysis>();
895 // FIXME: Restore this code when we re-enable verification in verifyAnalysis
898 static void verifyLoop(Loop *L) {
900 for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
903 // It used to be possible to just assert L->isLoopSimplifyForm(), however
904 // with the introduction of indirectbr, there are now cases where it's
905 // not possible to transform a loop as necessary. We can at least check
906 // that there is an indirectbr near any time there's trouble.
908 // Indirectbr can interfere with preheader and unique backedge insertion.
909 if (!L->getLoopPreheader() || !L->getLoopLatch()) {
910 bool HasIndBrPred = false;
911 for (pred_iterator PI = pred_begin(L->getHeader()),
912 PE = pred_end(L->getHeader()); PI != PE; ++PI)
913 if (isa<IndirectBrInst>((*PI)->getTerminator())) {
917 assert(HasIndBrPred &&
918 "LoopSimplify has no excuse for missing loop header info!");
922 // Indirectbr can interfere with exit block canonicalization.
923 if (!L->hasDedicatedExits()) {
924 bool HasIndBrExiting = false;
925 SmallVector<BasicBlock*, 8> ExitingBlocks;
926 L->getExitingBlocks(ExitingBlocks);
927 for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
928 if (isa<IndirectBrInst>((ExitingBlocks[i])->getTerminator())) {
929 HasIndBrExiting = true;
934 assert(HasIndBrExiting &&
935 "LoopSimplify has no excuse for missing exit block info!");
936 (void)HasIndBrExiting;
941 void LoopSimplify::verifyAnalysis() const {
942 // FIXME: This routine is being called mid-way through the loop pass manager
943 // as loop passes destroy this analysis. That's actually fine, but we have no
944 // way of expressing that here. Once all of the passes that destroy this are
945 // hoisted out of the loop pass manager we can add back verification here.
947 for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)