1 //===- LoopInfo.cpp - Natural Loop Calculator -----------------------------===//
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 file defines the LoopInfo class that is used to identify natural loops
11 // and determine the loop depth of various nodes of the CFG. Note that the
12 // loops identified may actually be several natural loops that share the same
13 // header node... not just a single natural loop.
15 //===----------------------------------------------------------------------===//
17 #include "llvm/Analysis/LoopInfo.h"
18 #include "llvm/Constants.h"
19 #include "llvm/Instructions.h"
20 #include "llvm/Analysis/Dominators.h"
21 #include "llvm/Analysis/LoopInfoImpl.h"
22 #include "llvm/Analysis/LoopIterator.h"
23 #include "llvm/Analysis/ValueTracking.h"
24 #include "llvm/Assembly/Writer.h"
25 #include "llvm/Support/CFG.h"
26 #include "llvm/Support/CommandLine.h"
27 #include "llvm/Support/Debug.h"
28 #include "llvm/ADT/DepthFirstIterator.h"
29 #include "llvm/ADT/SmallPtrSet.h"
33 // Explicitly instantiate methods in LoopInfoImpl.h for IR-level Loops.
34 template class llvm::LoopBase<BasicBlock, Loop>;
35 template class llvm::LoopInfoBase<BasicBlock, Loop>;
37 // Always verify loopinfo if expensive checking is enabled.
39 static bool VerifyLoopInfo = true;
41 static bool VerifyLoopInfo = false;
43 static cl::opt<bool,true>
44 VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
45 cl::desc("Verify loop info (time consuming)"));
47 char LoopInfo::ID = 0;
48 INITIALIZE_PASS_BEGIN(LoopInfo, "loops", "Natural Loop Information", true, true)
49 INITIALIZE_PASS_DEPENDENCY(DominatorTree)
50 INITIALIZE_PASS_END(LoopInfo, "loops", "Natural Loop Information", true, true)
52 //===----------------------------------------------------------------------===//
53 // Loop implementation
56 /// isLoopInvariant - Return true if the specified value is loop invariant
58 bool Loop::isLoopInvariant(Value *V) const {
59 if (Instruction *I = dyn_cast<Instruction>(V))
61 return true; // All non-instructions are loop invariant
64 /// hasLoopInvariantOperands - Return true if all the operands of the
65 /// specified instruction are loop invariant.
66 bool Loop::hasLoopInvariantOperands(Instruction *I) const {
67 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
68 if (!isLoopInvariant(I->getOperand(i)))
74 /// makeLoopInvariant - If the given value is an instruciton inside of the
75 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
76 /// Return true if the value after any hoisting is loop invariant. This
77 /// function can be used as a slightly more aggressive replacement for
80 /// If InsertPt is specified, it is the point to hoist instructions to.
81 /// If null, the terminator of the loop preheader is used.
83 bool Loop::makeLoopInvariant(Value *V, bool &Changed,
84 Instruction *InsertPt) const {
85 if (Instruction *I = dyn_cast<Instruction>(V))
86 return makeLoopInvariant(I, Changed, InsertPt);
87 return true; // All non-instructions are loop-invariant.
90 /// makeLoopInvariant - If the given instruction is inside of the
91 /// loop and it can be hoisted, do so to make it trivially loop-invariant.
92 /// Return true if the instruction after any hoisting is loop invariant. This
93 /// function can be used as a slightly more aggressive replacement for
96 /// If InsertPt is specified, it is the point to hoist instructions to.
97 /// If null, the terminator of the loop preheader is used.
99 bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
100 Instruction *InsertPt) const {
101 // Test if the value is already loop-invariant.
102 if (isLoopInvariant(I))
104 if (!isSafeToSpeculativelyExecute(I))
106 if (I->mayReadFromMemory())
108 // The landingpad instruction is immobile.
109 if (isa<LandingPadInst>(I))
111 // Determine the insertion point, unless one was given.
113 BasicBlock *Preheader = getLoopPreheader();
114 // Without a preheader, hoisting is not feasible.
117 InsertPt = Preheader->getTerminator();
119 // Don't hoist instructions with loop-variant operands.
120 for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i)
121 if (!makeLoopInvariant(I->getOperand(i), Changed, InsertPt))
125 I->moveBefore(InsertPt);
130 /// getCanonicalInductionVariable - Check to see if the loop has a canonical
131 /// induction variable: an integer recurrence that starts at 0 and increments
132 /// by one each time through the loop. If so, return the phi node that
133 /// corresponds to it.
135 /// The IndVarSimplify pass transforms loops to have a canonical induction
138 PHINode *Loop::getCanonicalInductionVariable() const {
139 BasicBlock *H = getHeader();
141 BasicBlock *Incoming = 0, *Backedge = 0;
142 pred_iterator PI = pred_begin(H);
143 assert(PI != pred_end(H) &&
144 "Loop must have at least one backedge!");
146 if (PI == pred_end(H)) return 0; // dead loop
148 if (PI != pred_end(H)) return 0; // multiple backedges?
150 if (contains(Incoming)) {
151 if (contains(Backedge))
153 std::swap(Incoming, Backedge);
154 } else if (!contains(Backedge))
157 // Loop over all of the PHI nodes, looking for a canonical indvar.
158 for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
159 PHINode *PN = cast<PHINode>(I);
160 if (ConstantInt *CI =
161 dyn_cast<ConstantInt>(PN->getIncomingValueForBlock(Incoming)))
162 if (CI->isNullValue())
163 if (Instruction *Inc =
164 dyn_cast<Instruction>(PN->getIncomingValueForBlock(Backedge)))
165 if (Inc->getOpcode() == Instruction::Add &&
166 Inc->getOperand(0) == PN)
167 if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
168 if (CI->equalsInt(1))
174 /// isLCSSAForm - Return true if the Loop is in LCSSA form
175 bool Loop::isLCSSAForm(DominatorTree &DT) const {
176 // Sort the blocks vector so that we can use binary search to do quick
178 SmallPtrSet<BasicBlock*, 16> LoopBBs(block_begin(), block_end());
180 for (block_iterator BI = block_begin(), E = block_end(); BI != E; ++BI) {
181 BasicBlock *BB = *BI;
182 for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;++I)
183 for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
186 BasicBlock *UserBB = cast<Instruction>(U)->getParent();
187 if (PHINode *P = dyn_cast<PHINode>(U))
188 UserBB = P->getIncomingBlock(UI);
190 // Check the current block, as a fast-path, before checking whether
191 // the use is anywhere in the loop. Most values are used in the same
192 // block they are defined in. Also, blocks not reachable from the
193 // entry are special; uses in them don't need to go through PHIs.
195 !LoopBBs.count(UserBB) &&
196 DT.isReachableFromEntry(UserBB))
204 /// isLoopSimplifyForm - Return true if the Loop is in the form that
205 /// the LoopSimplify form transforms loops to, which is sometimes called
207 bool Loop::isLoopSimplifyForm() const {
208 // Normal-form loops have a preheader, a single backedge, and all of their
209 // exits have all their predecessors inside the loop.
210 return getLoopPreheader() && getLoopLatch() && hasDedicatedExits();
213 /// isSafeToClone - Return true if the loop body is safe to clone in practice.
214 /// Routines that reform the loop CFG and split edges often fail on indirectbr.
215 bool Loop::isSafeToClone() const {
216 // Return false if any loop blocks contain indirectbrs.
217 for (Loop::block_iterator I = block_begin(), E = block_end(); I != E; ++I) {
218 if (isa<IndirectBrInst>((*I)->getTerminator()))
224 /// hasDedicatedExits - Return true if no exit block for the loop
225 /// has a predecessor that is outside the loop.
226 bool Loop::hasDedicatedExits() const {
227 // Sort the blocks vector so that we can use binary search to do quick
229 SmallPtrSet<BasicBlock *, 16> LoopBBs(block_begin(), block_end());
230 // Each predecessor of each exit block of a normal loop is contained
232 SmallVector<BasicBlock *, 4> ExitBlocks;
233 getExitBlocks(ExitBlocks);
234 for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
235 for (pred_iterator PI = pred_begin(ExitBlocks[i]),
236 PE = pred_end(ExitBlocks[i]); PI != PE; ++PI)
237 if (!LoopBBs.count(*PI))
239 // All the requirements are met.
243 /// getUniqueExitBlocks - Return all unique successor blocks of this loop.
244 /// These are the blocks _outside of the current loop_ which are branched to.
245 /// This assumes that loop exits are in canonical form.
248 Loop::getUniqueExitBlocks(SmallVectorImpl<BasicBlock *> &ExitBlocks) const {
249 assert(hasDedicatedExits() &&
250 "getUniqueExitBlocks assumes the loop has canonical form exits!");
252 // Sort the blocks vector so that we can use binary search to do quick
254 SmallVector<BasicBlock *, 128> LoopBBs(block_begin(), block_end());
255 std::sort(LoopBBs.begin(), LoopBBs.end());
257 SmallVector<BasicBlock *, 32> switchExitBlocks;
259 for (block_iterator BI = block_begin(), BE = block_end(); BI != BE; ++BI) {
261 BasicBlock *current = *BI;
262 switchExitBlocks.clear();
264 for (succ_iterator I = succ_begin(*BI), E = succ_end(*BI); I != E; ++I) {
265 // If block is inside the loop then it is not a exit block.
266 if (std::binary_search(LoopBBs.begin(), LoopBBs.end(), *I))
269 pred_iterator PI = pred_begin(*I);
270 BasicBlock *firstPred = *PI;
272 // If current basic block is this exit block's first predecessor
273 // then only insert exit block in to the output ExitBlocks vector.
274 // This ensures that same exit block is not inserted twice into
275 // ExitBlocks vector.
276 if (current != firstPred)
279 // If a terminator has more then two successors, for example SwitchInst,
280 // then it is possible that there are multiple edges from current block
281 // to one exit block.
282 if (std::distance(succ_begin(current), succ_end(current)) <= 2) {
283 ExitBlocks.push_back(*I);
287 // In case of multiple edges from current block to exit block, collect
288 // only one edge in ExitBlocks. Use switchExitBlocks to keep track of
290 if (std::find(switchExitBlocks.begin(), switchExitBlocks.end(), *I)
291 == switchExitBlocks.end()) {
292 switchExitBlocks.push_back(*I);
293 ExitBlocks.push_back(*I);
299 /// getUniqueExitBlock - If getUniqueExitBlocks would return exactly one
300 /// block, return that block. Otherwise return null.
301 BasicBlock *Loop::getUniqueExitBlock() const {
302 SmallVector<BasicBlock *, 8> UniqueExitBlocks;
303 getUniqueExitBlocks(UniqueExitBlocks);
304 if (UniqueExitBlocks.size() == 1)
305 return UniqueExitBlocks[0];
309 void Loop::dump() const {
313 //===----------------------------------------------------------------------===//
314 // UnloopUpdater implementation
318 /// Find the new parent loop for all blocks within the "unloop" whose last
319 /// backedges has just been removed.
320 class UnloopUpdater {
326 // Map unloop's immediate subloops to their nearest reachable parents. Nested
327 // loops within these subloops will not change parents. However, an immediate
328 // subloop's new parent will be the nearest loop reachable from either its own
329 // exits *or* any of its nested loop's exits.
330 DenseMap<Loop*, Loop*> SubloopParents;
332 // Flag the presence of an irreducible backedge whose destination is a block
333 // directly contained by the original unloop.
337 UnloopUpdater(Loop *UL, LoopInfo *LInfo) :
338 Unloop(UL), LI(LInfo), DFS(UL), FoundIB(false) {}
340 void updateBlockParents();
342 void removeBlocksFromAncestors();
344 void updateSubloopParents();
347 Loop *getNearestLoop(BasicBlock *BB, Loop *BBLoop);
349 } // end anonymous namespace
351 /// updateBlockParents - Update the parent loop for all blocks that are directly
352 /// contained within the original "unloop".
353 void UnloopUpdater::updateBlockParents() {
354 if (Unloop->getNumBlocks()) {
355 // Perform a post order CFG traversal of all blocks within this loop,
356 // propagating the nearest loop from sucessors to predecessors.
357 LoopBlocksTraversal Traversal(DFS, LI);
358 for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
359 POE = Traversal.end(); POI != POE; ++POI) {
361 Loop *L = LI->getLoopFor(*POI);
362 Loop *NL = getNearestLoop(*POI, L);
365 // For reducible loops, NL is now an ancestor of Unloop.
366 assert((NL != Unloop && (!NL || NL->contains(Unloop))) &&
367 "uninitialized successor");
368 LI->changeLoopFor(*POI, NL);
371 // Or the current block is part of a subloop, in which case its parent
373 assert((FoundIB || Unloop->contains(L)) && "uninitialized successor");
377 // Each irreducible loop within the unloop induces a round of iteration using
378 // the DFS result cached by Traversal.
379 bool Changed = FoundIB;
380 for (unsigned NIters = 0; Changed; ++NIters) {
381 assert(NIters < Unloop->getNumBlocks() && "runaway iterative algorithm");
383 // Iterate over the postorder list of blocks, propagating the nearest loop
384 // from successors to predecessors as before.
386 for (LoopBlocksDFS::POIterator POI = DFS.beginPostorder(),
387 POE = DFS.endPostorder(); POI != POE; ++POI) {
389 Loop *L = LI->getLoopFor(*POI);
390 Loop *NL = getNearestLoop(*POI, L);
392 assert(NL != Unloop && (!NL || NL->contains(Unloop)) &&
393 "uninitialized successor");
394 LI->changeLoopFor(*POI, NL);
401 /// removeBlocksFromAncestors - Remove unloop's blocks from all ancestors below
402 /// their new parents.
403 void UnloopUpdater::removeBlocksFromAncestors() {
404 // Remove all unloop's blocks (including those in nested subloops) from
405 // ancestors below the new parent loop.
406 for (Loop::block_iterator BI = Unloop->block_begin(),
407 BE = Unloop->block_end(); BI != BE; ++BI) {
408 Loop *OuterParent = LI->getLoopFor(*BI);
409 if (Unloop->contains(OuterParent)) {
410 while (OuterParent->getParentLoop() != Unloop)
411 OuterParent = OuterParent->getParentLoop();
412 OuterParent = SubloopParents[OuterParent];
414 // Remove blocks from former Ancestors except Unloop itself which will be
416 for (Loop *OldParent = Unloop->getParentLoop(); OldParent != OuterParent;
417 OldParent = OldParent->getParentLoop()) {
418 assert(OldParent && "new loop is not an ancestor of the original");
419 OldParent->removeBlockFromLoop(*BI);
424 /// updateSubloopParents - Update the parent loop for all subloops directly
425 /// nested within unloop.
426 void UnloopUpdater::updateSubloopParents() {
427 while (!Unloop->empty()) {
428 Loop *Subloop = *llvm::prior(Unloop->end());
429 Unloop->removeChildLoop(llvm::prior(Unloop->end()));
431 assert(SubloopParents.count(Subloop) && "DFS failed to visit subloop");
432 if (SubloopParents[Subloop])
433 SubloopParents[Subloop]->addChildLoop(Subloop);
435 LI->addTopLevelLoop(Subloop);
439 /// getNearestLoop - Return the nearest parent loop among this block's
440 /// successors. If a successor is a subloop header, consider its parent to be
441 /// the nearest parent of the subloop's exits.
443 /// For subloop blocks, simply update SubloopParents and return NULL.
444 Loop *UnloopUpdater::getNearestLoop(BasicBlock *BB, Loop *BBLoop) {
446 // Initially for blocks directly contained by Unloop, NearLoop == Unloop and
447 // is considered uninitialized.
448 Loop *NearLoop = BBLoop;
451 if (NearLoop != Unloop && Unloop->contains(NearLoop)) {
453 // Find the subloop ancestor that is directly contained within Unloop.
454 while (Subloop->getParentLoop() != Unloop) {
455 Subloop = Subloop->getParentLoop();
456 assert(Subloop && "subloop is not an ancestor of the original loop");
458 // Get the current nearest parent of the Subloop exits, initially Unloop.
459 if (!SubloopParents.count(Subloop))
460 SubloopParents[Subloop] = Unloop;
461 NearLoop = SubloopParents[Subloop];
464 succ_iterator I = succ_begin(BB), E = succ_end(BB);
466 assert(!Subloop && "subloop blocks must have a successor");
467 NearLoop = 0; // unloop blocks may now exit the function.
469 for (; I != E; ++I) {
471 continue; // self loops are uninteresting
473 Loop *L = LI->getLoopFor(*I);
475 // This successor has not been processed. This path must lead to an
476 // irreducible backedge.
477 assert((FoundIB || !DFS.hasPostorder(*I)) && "should have seen IB");
480 if (L != Unloop && Unloop->contains(L)) {
481 // Successor is in a subloop.
483 continue; // Branching within subloops. Ignore it.
485 // BB branches from the original into a subloop header.
486 assert(L->getParentLoop() == Unloop && "cannot skip into nested loops");
488 // Get the current nearest parent of the Subloop's exits.
489 L = SubloopParents[L];
490 // L could be Unloop if the only exit was an irreducible backedge.
495 // Handle critical edges from Unloop into a sibling loop.
496 if (L && !L->contains(Unloop)) {
497 L = L->getParentLoop();
499 // Remember the nearest parent loop among successors or subloop exits.
500 if (NearLoop == Unloop || !NearLoop || NearLoop->contains(L))
504 SubloopParents[Subloop] = NearLoop;
510 //===----------------------------------------------------------------------===//
511 // LoopInfo implementation
513 bool LoopInfo::runOnFunction(Function &) {
515 LI.Calculate(getAnalysis<DominatorTree>().getBase()); // Update
519 /// updateUnloop - The last backedge has been removed from a loop--now the
520 /// "unloop". Find a new parent for the blocks contained within unloop and
521 /// update the loop tree. We don't necessarily have valid dominators at this
522 /// point, but LoopInfo is still valid except for the removal of this loop.
524 /// Note that Unloop may now be an empty loop. Calling Loop::getHeader without
525 /// checking first is illegal.
526 void LoopInfo::updateUnloop(Loop *Unloop) {
528 // First handle the special case of no parent loop to simplify the algorithm.
529 if (!Unloop->getParentLoop()) {
530 // Since BBLoop had no parent, Unloop blocks are no longer in a loop.
531 for (Loop::block_iterator I = Unloop->block_begin(),
532 E = Unloop->block_end(); I != E; ++I) {
534 // Don't reparent blocks in subloops.
535 if (getLoopFor(*I) != Unloop)
538 // Blocks no longer have a parent but are still referenced by Unloop until
539 // the Unloop object is deleted.
540 LI.changeLoopFor(*I, 0);
543 // Remove the loop from the top-level LoopInfo object.
544 for (LoopInfo::iterator I = LI.begin();; ++I) {
545 assert(I != LI.end() && "Couldn't find loop");
552 // Move all of the subloops to the top-level.
553 while (!Unloop->empty())
554 LI.addTopLevelLoop(Unloop->removeChildLoop(llvm::prior(Unloop->end())));
559 // Update the parent loop for all blocks within the loop. Blocks within
560 // subloops will not change parents.
561 UnloopUpdater Updater(Unloop, this);
562 Updater.updateBlockParents();
564 // Remove blocks from former ancestor loops.
565 Updater.removeBlocksFromAncestors();
567 // Add direct subloops as children in their new parent loop.
568 Updater.updateSubloopParents();
570 // Remove unloop from its parent loop.
571 Loop *ParentLoop = Unloop->getParentLoop();
572 for (Loop::iterator I = ParentLoop->begin();; ++I) {
573 assert(I != ParentLoop->end() && "Couldn't find loop");
575 ParentLoop->removeChildLoop(I);
581 void LoopInfo::verifyAnalysis() const {
582 // LoopInfo is a FunctionPass, but verifying every loop in the function
583 // each time verifyAnalysis is called is very expensive. The
584 // -verify-loop-info option can enable this. In order to perform some
585 // checking by default, LoopPass has been taught to call verifyLoop
586 // manually during loop pass sequences.
588 if (!VerifyLoopInfo) return;
590 DenseSet<const Loop*> Loops;
591 for (iterator I = begin(), E = end(); I != E; ++I) {
592 assert(!(*I)->getParentLoop() && "Top-level loop has a parent!");
593 (*I)->verifyLoopNest(&Loops);
596 // Verify that blocks are mapped to valid loops.
598 // FIXME: With an up-to-date DFS (see LoopIterator.h) and DominatorTree, we
599 // could also verify that the blocks are still in the correct loops.
600 for (DenseMap<BasicBlock*, Loop*>::const_iterator I = LI.BBMap.begin(),
601 E = LI.BBMap.end(); I != E; ++I) {
602 assert(Loops.count(I->second) && "orphaned loop");
603 assert(I->second->contains(I->first) && "orphaned block");
607 void LoopInfo::getAnalysisUsage(AnalysisUsage &AU) const {
608 AU.setPreservesAll();
609 AU.addRequired<DominatorTree>();
612 void LoopInfo::print(raw_ostream &OS, const Module*) const {
616 //===----------------------------------------------------------------------===//
617 // LoopBlocksDFS implementation
620 /// Traverse the loop blocks and store the DFS result.
621 /// Useful for clients that just want the final DFS result and don't need to
622 /// visit blocks during the initial traversal.
623 void LoopBlocksDFS::perform(LoopInfo *LI) {
624 LoopBlocksTraversal Traversal(*this, LI);
625 for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
626 POE = Traversal.end(); POI != POE; ++POI) ;