1 //===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This pass performs several transformations to transform natural loops into a
11 // simpler form, which makes subsequent analyses and transformations simpler and
14 // Loop pre-header insertion guarantees that there is a single, non-critical
15 // entry edge from outside of the loop to the loop header. This simplifies a
16 // number of analyses and transformations, such as LICM.
18 // Loop exit-block insertion guarantees that all exit blocks from the loop
19 // (blocks which are outside of the loop that have predecessors inside of the
20 // loop) are dominated by the loop header. This simplifies transformations such
21 // as store-sinking that are built into LICM.
23 // This pass also guarantees that loops will have exactly one backedge.
25 // Note that the simplifycfg pass will clean up blocks which are split out but
26 // end up being unnecessary, so usage of this pass should not pessimize
29 // This pass obviously modifies the CFG, but updates loop information and
30 // dominator information.
32 //===----------------------------------------------------------------------===//
34 #include "llvm/Transforms/Scalar.h"
35 #include "llvm/Analysis/Dominators.h"
36 #include "llvm/Analysis/LoopInfo.h"
37 #include "llvm/Function.h"
38 #include "llvm/iTerminators.h"
39 #include "llvm/iPHINode.h"
40 #include "llvm/Constant.h"
41 #include "llvm/Support/CFG.h"
42 #include "Support/SetOperations.h"
43 #include "Support/Statistic.h"
44 #include "Support/DepthFirstIterator.h"
50 NumInserted("loopsimplify", "Number of pre-header blocks inserted");
52 struct LoopSimplify : public FunctionPass {
53 virtual bool runOnFunction(Function &F);
55 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
56 // We need loop information to identify the loops...
57 AU.addRequired<LoopInfo>();
58 AU.addRequired<DominatorSet>();
60 AU.addPreserved<LoopInfo>();
61 AU.addPreserved<DominatorSet>();
62 AU.addPreserved<ImmediateDominators>();
63 AU.addPreserved<DominatorTree>();
64 AU.addPreserved<DominanceFrontier>();
65 AU.addPreservedID(BreakCriticalEdgesID); // No crit edges added....
68 bool ProcessLoop(Loop *L);
69 BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix,
70 const std::vector<BasicBlock*> &Preds);
71 void RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
72 void InsertPreheaderForLoop(Loop *L);
73 void InsertUniqueBackedgeBlock(Loop *L);
75 void UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB,
76 std::vector<BasicBlock*> &PredBlocks);
79 RegisterOpt<LoopSimplify>
80 X("loopsimplify", "Canonicalize natural loops", true);
83 // Publically exposed interface to pass...
84 const PassInfo *LoopSimplifyID = X.getPassInfo();
85 Pass *createLoopSimplifyPass() { return new LoopSimplify(); }
87 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
88 /// it in any convenient order) inserting preheaders...
90 bool LoopSimplify::runOnFunction(Function &F) {
92 LoopInfo &LI = getAnalysis<LoopInfo>();
94 for (unsigned i = 0, e = LI.getTopLevelLoops().size(); i != e; ++i)
95 Changed |= ProcessLoop(LI.getTopLevelLoops()[i]);
101 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
102 /// all loops have preheaders.
104 bool LoopSimplify::ProcessLoop(Loop *L) {
105 bool Changed = false;
107 // Does the loop already have a preheader? If so, don't modify the loop...
108 if (L->getLoopPreheader() == 0) {
109 InsertPreheaderForLoop(L);
114 // Regardless of whether or not we added a preheader to the loop we must
115 // guarantee that the preheader dominates all exit nodes. If there are any
116 // exit nodes not dominated, split them now.
117 DominatorSet &DS = getAnalysis<DominatorSet>();
118 BasicBlock *Header = L->getHeader();
119 for (unsigned i = 0, e = L->getExitBlocks().size(); i != e; ++i)
120 if (!DS.dominates(Header, L->getExitBlocks()[i])) {
121 RewriteLoopExitBlock(L, L->getExitBlocks()[i]);
122 assert(DS.dominates(Header, L->getExitBlocks()[i]) &&
123 "RewriteLoopExitBlock failed?");
128 // The preheader may have more than two predecessors at this point (from the
129 // preheader and from the backedges). To simplify the loop more, insert an
130 // extra back-edge block in the loop so that there is exactly one backedge.
131 if (L->getNumBackEdges() != 1) {
132 InsertUniqueBackedgeBlock(L);
137 const std::vector<Loop*> &SubLoops = L->getSubLoops();
138 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
139 Changed |= ProcessLoop(SubLoops[i]);
143 /// SplitBlockPredecessors - Split the specified block into two blocks. We want
144 /// to move the predecessors specified in the Preds list to point to the new
145 /// block, leaving the remaining predecessors pointing to BB. This method
146 /// updates the SSA PHINode's, but no other analyses.
148 BasicBlock *LoopSimplify::SplitBlockPredecessors(BasicBlock *BB,
150 const std::vector<BasicBlock*> &Preds) {
152 // Create new basic block, insert right before the original block...
153 BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB);
155 // The preheader first gets an unconditional branch to the loop header...
156 BranchInst *BI = new BranchInst(BB);
157 NewBB->getInstList().push_back(BI);
159 // For every PHI node in the block, insert a PHI node into NewBB where the
160 // incoming values from the out of loop edges are moved to NewBB. We have two
161 // possible cases here. If the loop is dead, we just insert dummy entries
162 // into the PHI nodes for the new edge. If the loop is not dead, we move the
163 // incoming edges in BB into new PHI nodes in NewBB.
165 if (!Preds.empty()) { // Is the loop not obviously dead?
166 for (BasicBlock::iterator I = BB->begin();
167 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
169 // Create the new PHI node, insert it into NewBB at the end of the block
170 PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);
172 // Move all of the edges from blocks outside the loop to the new PHI
173 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
174 Value *V = PN->removeIncomingValue(Preds[i]);
175 NewPHI->addIncoming(V, Preds[i]);
178 // Add an incoming value to the PHI node in the loop for the preheader
180 PN->addIncoming(NewPHI, NewBB);
183 // Now that the PHI nodes are updated, actually move the edges from
184 // Preds to point to NewBB instead of BB.
186 for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
187 TerminatorInst *TI = Preds[i]->getTerminator();
188 for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
189 if (TI->getSuccessor(s) == BB)
190 TI->setSuccessor(s, NewBB);
193 } else { // Otherwise the loop is dead...
194 for (BasicBlock::iterator I = BB->begin();
195 PHINode *PN = dyn_cast<PHINode>(I); ++I)
196 // Insert dummy values as the incoming value...
197 PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
202 // ChangeExitBlock - This recursive function is used to change any exit blocks
203 // that use OldExit to use NewExit instead. This is recursive because children
204 // may need to be processed as well.
206 static void ChangeExitBlock(Loop *L, BasicBlock *OldExit, BasicBlock *NewExit) {
207 if (L->hasExitBlock(OldExit)) {
208 L->changeExitBlock(OldExit, NewExit);
209 const std::vector<Loop*> &SubLoops = L->getSubLoops();
210 for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
211 ChangeExitBlock(SubLoops[i], OldExit, NewExit);
216 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
217 /// preheader, this method is called to insert one. This method has two phases:
218 /// preheader insertion and analysis updating.
220 void LoopSimplify::InsertPreheaderForLoop(Loop *L) {
221 BasicBlock *Header = L->getHeader();
223 // Compute the set of predecessors of the loop that are not in the loop.
224 std::vector<BasicBlock*> OutsideBlocks;
225 for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
227 if (!L->contains(*PI)) // Coming in from outside the loop?
228 OutsideBlocks.push_back(*PI); // Keep track of it...
230 // Split out the loop pre-header
232 SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);
234 //===--------------------------------------------------------------------===//
235 // Update analysis results now that we have performed the transformation
238 // We know that we have loop information to update... update it now.
239 if (Loop *Parent = L->getParentLoop())
240 Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
242 // If the header for the loop used to be an exit node for another loop, then
243 // we need to update this to know that the loop-preheader is now the exit
244 // node. Note that the only loop that could have our header as an exit node
245 // is a sibling loop, ie, one with the same parent loop, or one if it's
248 const std::vector<Loop*> *ParentSubLoops;
249 if (Loop *Parent = L->getParentLoop())
250 ParentSubLoops = &Parent->getSubLoops();
251 else // Must check top-level loops...
252 ParentSubLoops = &getAnalysis<LoopInfo>().getTopLevelLoops();
254 // Loop over all sibling loops, performing the substitution (recursively to
255 // include child loops)...
256 for (unsigned i = 0, e = ParentSubLoops->size(); i != e; ++i)
257 ChangeExitBlock((*ParentSubLoops)[i], Header, NewBB);
259 DominatorSet &DS = getAnalysis<DominatorSet>(); // Update dominator info
261 // The blocks that dominate NewBB are the blocks that dominate Header,
262 // minus Header, plus NewBB.
263 DominatorSet::DomSetType DomSet = DS.getDominators(Header);
264 DomSet.insert(NewBB); // We dominate ourself
265 DomSet.erase(Header); // Header does not dominate us...
266 DS.addBasicBlock(NewBB, DomSet);
268 // The newly created basic block dominates all nodes dominated by Header.
269 for (Function::iterator I = Header->getParent()->begin(),
270 E = Header->getParent()->end(); I != E; ++I)
271 if (DS.dominates(Header, I))
272 DS.addDominator(I, NewBB);
275 // Update immediate dominator information if we have it...
276 if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
277 // Whatever i-dominated the header node now immediately dominates NewBB
278 ID->addNewBlock(NewBB, ID->get(Header));
280 // The preheader now is the immediate dominator for the header node...
281 ID->setImmediateDominator(Header, NewBB);
284 // Update DominatorTree information if it is active.
285 if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
286 // The immediate dominator of the preheader is the immediate dominator of
289 DominatorTree::Node *HeaderNode = DT->getNode(Header);
290 DominatorTree::Node *PHNode = DT->createNewNode(NewBB,
291 HeaderNode->getIDom());
293 // Change the header node so that PNHode is the new immediate dominator
294 DT->changeImmediateDominator(HeaderNode, PHNode);
297 // Update dominance frontier information...
298 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
299 // The DF(NewBB) is just (DF(Header)-Header), because NewBB dominates
300 // everything that Header does, and it strictly dominates Header in
302 assert(DF->find(Header) != DF->end() && "Header node doesn't have DF set?");
303 DominanceFrontier::DomSetType NewDFSet = DF->find(Header)->second;
304 NewDFSet.erase(Header);
305 DF->addBasicBlock(NewBB, NewDFSet);
307 // Now we must loop over all of the dominance frontiers in the function,
308 // replacing occurrences of Header with NewBB in some cases. If a block
309 // dominates a (now) predecessor of NewBB, but did not strictly dominate
310 // Header, it will have Header in it's DF set, but should now have NewBB in
312 for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
313 // Get all of the dominators of the predecessor...
314 const DominatorSet::DomSetType &PredDoms =
315 DS.getDominators(OutsideBlocks[i]);
316 for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
317 PDE = PredDoms.end(); PDI != PDE; ++PDI) {
318 BasicBlock *PredDom = *PDI;
319 // If the loop header is in DF(PredDom), then PredDom didn't dominate
320 // the header but did dominate a predecessor outside of the loop. Now
321 // we change this entry to include the preheader in the DF instead of
323 DominanceFrontier::iterator DFI = DF->find(PredDom);
324 assert(DFI != DF->end() && "No dominance frontier for node?");
325 if (DFI->second.count(Header)) {
326 DF->removeFromFrontier(DFI, Header);
327 DF->addToFrontier(DFI, NewBB);
334 void LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
335 DominatorSet &DS = getAnalysis<DominatorSet>();
336 assert(!DS.dominates(L->getHeader(), Exit) &&
337 "Loop already dominates exit block??");
338 assert(std::find(L->getExitBlocks().begin(), L->getExitBlocks().end(), Exit)
339 != L->getExitBlocks().end() && "Not a current exit block!");
341 std::vector<BasicBlock*> LoopBlocks;
342 for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
344 LoopBlocks.push_back(*I);
346 assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
347 BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks);
349 // Update Loop Information - we know that the new block will be in the parent
351 if (Loop *Parent = L->getParentLoop())
352 Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());
354 // Replace any instances of Exit with NewBB in this and any nested loops...
355 for (df_iterator<Loop*> I = df_begin(L), E = df_end(L); I != E; ++I)
356 if (I->hasExitBlock(Exit))
357 I->changeExitBlock(Exit, NewBB); // Update exit block information
359 // Update dominator information (set, immdom, domtree, and domfrontier)
360 UpdateDomInfoForRevectoredPreds(NewBB, LoopBlocks);
363 /// InsertUniqueBackedgeBlock - This method is called when the specified loop
364 /// has more than one backedge in it. If this occurs, revector all of these
365 /// backedges to target a new basic block and have that block branch to the loop
366 /// header. This ensures that loops have exactly one backedge.
368 void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) {
369 assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
371 // Get information about the loop
372 BasicBlock *Preheader = L->getLoopPreheader();
373 BasicBlock *Header = L->getHeader();
374 Function *F = Header->getParent();
376 // Figure out which basic blocks contain back-edges to the loop header.
377 std::vector<BasicBlock*> BackedgeBlocks;
378 for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I)
379 if (*I != Preheader) BackedgeBlocks.push_back(*I);
381 // Create and insert the new backedge block...
382 BasicBlock *BEBlock = new BasicBlock(Header->getName()+".backedge", F);
383 Instruction *BETerminator = new BranchInst(Header);
384 BEBlock->getInstList().push_back(BETerminator);
386 // Move the new backedge block to right after the last backedge block.
387 Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
388 F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
390 // Now that the block has been inserted into the function, create PHI nodes in
391 // the backedge block which correspond to any PHI nodes in the header block.
392 for (BasicBlock::iterator I = Header->begin();
393 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
394 PHINode *NewPN = new PHINode(PN->getType(), PN->getName()+".be",
396 NewPN->op_reserve(2*BackedgeBlocks.size());
398 // Loop over the PHI node, moving all entries except the one for the
399 // preheader over to the new PHI node.
400 unsigned PreheaderIdx = ~0U;
401 bool HasUniqueIncomingValue = true;
402 Value *UniqueValue = 0;
403 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
404 BasicBlock *IBB = PN->getIncomingBlock(i);
405 Value *IV = PN->getIncomingValue(i);
406 if (IBB == Preheader) {
409 NewPN->addIncoming(IV, IBB);
410 if (HasUniqueIncomingValue) {
411 if (UniqueValue == 0)
413 else if (UniqueValue != IV)
414 HasUniqueIncomingValue = false;
419 // Delete all of the incoming values from the old PN except the preheader's
420 assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
421 if (PreheaderIdx != 0) {
422 PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
423 PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
425 PN->op_erase(PN->op_begin()+2, PN->op_end());
427 // Finally, add the newly constructed PHI node as the entry for the BEBlock.
428 PN->addIncoming(NewPN, BEBlock);
430 // As an optimization, if all incoming values in the new PhiNode (which is a
431 // subset of the incoming values of the old PHI node) have the same value,
432 // eliminate the PHI Node.
433 if (HasUniqueIncomingValue) {
434 NewPN->replaceAllUsesWith(UniqueValue);
435 BEBlock->getInstList().erase(NewPN);
439 // Now that all of the PHI nodes have been inserted and adjusted, modify the
440 // backedge blocks to just to the BEBlock instead of the header.
441 for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
442 TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
443 for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
444 if (TI->getSuccessor(Op) == Header)
445 TI->setSuccessor(Op, BEBlock);
448 //===--- Update all analyses which we must preserve now -----------------===//
450 // Update Loop Information - we know that this block is now in the current
451 // loop and all parent loops.
452 L->addBasicBlockToLoop(BEBlock, getAnalysis<LoopInfo>());
454 // Replace any instances of Exit with NewBB in this and any nested loops...
455 for (df_iterator<Loop*> I = df_begin(L), E = df_end(L); I != E; ++I)
456 if (I->hasExitBlock(Header))
457 I->changeExitBlock(Header, BEBlock); // Update exit block information
459 // Update dominator information (set, immdom, domtree, and domfrontier)
460 UpdateDomInfoForRevectoredPreds(BEBlock, BackedgeBlocks);
463 /// UpdateDomInfoForRevectoredPreds - This method is used to update the four
464 /// different kinds of dominator information (dominator sets, immediate
465 /// dominators, dominator trees, and dominance frontiers) after a new block has
466 /// been added to the CFG.
468 /// This only supports the case when an existing block (known as "Exit"), had
469 /// some of its predecessors factored into a new basic block. This
470 /// transformation inserts a new basic block ("NewBB"), with a single
471 /// unconditional branch to Exit, and moves some predecessors of "Exit" to now
472 /// branch to NewBB. These predecessors are listed in PredBlocks, even though
473 /// they are the same as pred_begin(NewBB)/pred_end(NewBB).
475 void LoopSimplify::UpdateDomInfoForRevectoredPreds(BasicBlock *NewBB,
476 std::vector<BasicBlock*> &PredBlocks) {
477 assert(succ_begin(NewBB) != succ_end(NewBB) &&
478 ++succ_begin(NewBB) == succ_end(NewBB) &&
479 "NewBB should have a single successor!");
480 DominatorSet &DS = getAnalysis<DominatorSet>();
482 // Update dominator information... The blocks that dominate NewBB are the
483 // intersection of the dominators of predecessors, plus the block itself.
484 // The newly created basic block does not dominate anything except itself.
486 DominatorSet::DomSetType NewBBDomSet = DS.getDominators(PredBlocks[0]);
487 for (unsigned i = 1, e = PredBlocks.size(); i != e; ++i)
488 set_intersect(NewBBDomSet, DS.getDominators(PredBlocks[i]));
489 NewBBDomSet.insert(NewBB); // All blocks dominate themselves...
490 DS.addBasicBlock(NewBB, NewBBDomSet);
492 // Update immediate dominator information if we have it...
493 BasicBlock *NewBBIDom = 0;
494 if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
495 // This block does not strictly dominate anything, so it is not an immediate
496 // dominator. To find the immediate dominator of the new exit node, we
497 // trace up the immediate dominators of a predecessor until we find a basic
498 // block that dominates the exit block.
500 BasicBlock *Dom = PredBlocks[0]; // Some random predecessor...
501 while (!NewBBDomSet.count(Dom)) { // Loop until we find a dominator...
502 assert(Dom != 0 && "No shared dominator found???");
506 // Set the immediate dominator now...
507 ID->addNewBlock(NewBB, Dom);
508 NewBBIDom = Dom; // Reuse this if calculating DominatorTree info...
511 // Update DominatorTree information if it is active.
512 if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
513 // NewBB doesn't dominate anything, so just create a node and link it into
514 // its immediate dominator. If we don't have ImmediateDominator info
515 // around, calculate the idom as above.
516 DominatorTree::Node *NewBBIDomNode;
518 NewBBIDomNode = DT->getNode(NewBBIDom);
520 NewBBIDomNode = DT->getNode(PredBlocks[0]); // Random pred
521 while (!NewBBDomSet.count(NewBBIDomNode->getBlock())) {
522 NewBBIDomNode = NewBBIDomNode->getIDom();
523 assert(NewBBIDomNode && "No shared dominator found??");
527 // Create the new dominator tree node...
528 DT->createNewNode(NewBB, NewBBIDomNode);
531 // Update dominance frontier information...
532 if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
533 // DF(NewBB) is {Exit} because NewBB does not strictly dominate Exit, but it
534 // does dominate itself (and there is an edge (NewBB -> Exit)). Exit is the
535 // single successor of NewBB.
536 DominanceFrontier::DomSetType NewDFSet;
537 BasicBlock *Exit = *succ_begin(NewBB);
538 NewDFSet.insert(Exit);
539 DF->addBasicBlock(NewBB, NewDFSet);
541 // Now we must loop over all of the dominance frontiers in the function,
542 // replacing occurrences of Exit with NewBB in some cases. All blocks that
543 // dominate a block in PredBlocks and contained Exit in their dominance
544 // frontier must be updated to contain NewBB instead. This only occurs if
545 // there is more than one block in PredBlocks.
547 if (PredBlocks.size() > 1) {
548 for (unsigned i = 0, e = PredBlocks.size(); i != e; ++i) {
549 BasicBlock *Pred = PredBlocks[i];
550 // Get all of the dominators of the predecessor...
551 const DominatorSet::DomSetType &PredDoms = DS.getDominators(Pred);
552 for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
553 PDE = PredDoms.end(); PDI != PDE; ++PDI) {
554 BasicBlock *PredDom = *PDI;
556 // If the Exit node is in DF(PredDom), then PredDom didn't dominate
557 // Exit but did dominate a predecessor of it. Now we change this
558 // entry to include NewBB in the DF instead of Exit.
559 DominanceFrontier::iterator DFI = DF->find(PredDom);
560 assert(DFI != DF->end() && "No dominance frontier for node?");
561 if (DFI->second.count(Exit)) {
562 DF->removeFromFrontier(DFI, Exit);
563 DF->addToFrontier(DFI, NewBB);
571 } // End llvm namespace