1 //===- LoopRotation.cpp - Loop Rotation 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 file implements Loop Rotation Pass.
12 //===----------------------------------------------------------------------===//
14 #define DEBUG_TYPE "loop-rotate"
15 #include "llvm/Transforms/Scalar.h"
16 #include "llvm/Function.h"
17 #include "llvm/Instructions.h"
18 #include "llvm/Analysis/LoopInfo.h"
19 #include "llvm/Analysis/LoopPass.h"
20 #include "llvm/Analysis/Dominators.h"
21 #include "llvm/Analysis/ScalarEvolution.h"
22 #include "llvm/Transforms/Utils/Local.h"
23 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
24 #include "llvm/Support/CommandLine.h"
25 #include "llvm/Support/Debug.h"
26 #include "llvm/ADT/Statistic.h"
27 #include "llvm/ADT/SmallVector.h"
30 #define MAX_HEADER_SIZE 16
32 STATISTIC(NumRotated, "Number of loops rotated");
35 class VISIBILITY_HIDDEN RenameData {
37 RenameData(Instruction *O, Value *P, Instruction *H)
38 : Original(O), PreHeader(P), Header(H) { }
40 Instruction *Original; // Original instruction
41 Value *PreHeader; // Original pre-header replacement
42 Instruction *Header; // New header replacement
45 class VISIBILITY_HIDDEN LoopRotate : public LoopPass {
48 static char ID; // Pass ID, replacement for typeid
49 LoopRotate() : LoopPass(&ID) {}
51 // Rotate Loop L as many times as possible. Return true if
52 // loop is rotated at least once.
53 bool runOnLoop(Loop *L, LPPassManager &LPM);
55 // LCSSA form makes instruction renaming easier.
56 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
57 AU.addRequiredID(LoopSimplifyID);
58 AU.addPreservedID(LoopSimplifyID);
59 AU.addRequiredID(LCSSAID);
60 AU.addPreservedID(LCSSAID);
61 AU.addPreserved<ScalarEvolution>();
62 AU.addPreserved<LoopInfo>();
63 AU.addPreserved<DominatorTree>();
64 AU.addPreserved<DominanceFrontier>();
70 bool rotateLoop(Loop *L, LPPassManager &LPM);
72 /// Initialize local data
75 /// Make sure all Exit block PHINodes have required incoming values.
76 /// If incoming value is constant or defined outside the loop then
77 /// PHINode may not have an entry for original pre-header.
78 void updateExitBlock();
80 /// Return true if this instruction is used outside original header.
81 bool usedOutsideOriginalHeader(Instruction *In);
83 /// Find Replacement information for instruction. Return NULL if it is
85 const RenameData *findReplacementData(Instruction *I);
87 /// After loop rotation, loop pre-header has multiple sucessors.
88 /// Insert one forwarding basic block to ensure that loop pre-header
89 /// has only one successor.
90 void preserveCanonicalLoopForm(LPPassManager &LPM);
95 BasicBlock *OrigHeader;
96 BasicBlock *OrigPreHeader;
97 BasicBlock *OrigLatch;
98 BasicBlock *NewHeader;
100 LPPassManager *LPM_Ptr;
101 SmallVector<RenameData, MAX_HEADER_SIZE> LoopHeaderInfo;
105 char LoopRotate::ID = 0;
106 static RegisterPass<LoopRotate> X("loop-rotate", "Rotate Loops");
108 Pass *llvm::createLoopRotatePass() { return new LoopRotate(); }
110 /// Rotate Loop L as many times as possible. Return true if
111 /// loop is rotated at least once.
112 bool LoopRotate::runOnLoop(Loop *Lp, LPPassManager &LPM) {
114 bool RotatedOneLoop = false;
118 // One loop can be rotated multiple times.
119 while (rotateLoop(Lp,LPM)) {
120 RotatedOneLoop = true;
124 return RotatedOneLoop;
127 /// Rotate loop LP. Return true if the loop is rotated.
128 bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) {
131 OrigHeader = L->getHeader();
132 OrigPreHeader = L->getLoopPreheader();
133 OrigLatch = L->getLoopLatch();
135 // If loop has only one block then there is not much to rotate.
136 if (L->getBlocks().size() == 1)
139 assert(OrigHeader && OrigLatch && OrigPreHeader &&
140 "Loop is not in canonical form");
142 // If loop header is not one of the loop exit block then
143 // either this loop is already rotated or it is not
144 // suitable for loop rotation transformations.
145 if (!L->isLoopExit(OrigHeader))
148 BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
151 assert(BI->isConditional() && "Branch Instruction is not conditional");
153 // Updating PHInodes in loops with multiple exits adds complexity.
154 // Keep it simple, and restrict loop rotation to loops with one exit only.
155 // In future, lift this restriction and support for multiple exits if
157 SmallVector<BasicBlock*, 8> ExitBlocks;
158 L->getExitBlocks(ExitBlocks);
159 if (ExitBlocks.size() > 1)
162 // Check size of original header and reject
163 // loop if it is very big.
164 if (OrigHeader->size() > MAX_HEADER_SIZE)
167 // Now, this loop is suitable for rotation.
169 // Find new Loop header. NewHeader is a Header's one and only successor
170 // that is inside loop. Header's other successor is outside the
171 // loop. Otherwise loop is not suitable for rotation.
172 Exit = BI->getSuccessor(0);
173 NewHeader = BI->getSuccessor(1);
174 if (L->contains(Exit))
175 std::swap(Exit, NewHeader);
176 assert(NewHeader && "Unable to determine new loop header");
177 assert(L->contains(NewHeader) && !L->contains(Exit) &&
178 "Unable to determine loop header and exit blocks");
180 // This code assumes that new header has exactly one predecessor. Remove any
181 // single entry PHI nodes in it.
182 assert(NewHeader->getSinglePredecessor() &&
183 "New header doesn't have one pred!");
184 FoldSingleEntryPHINodes(NewHeader);
186 // Copy PHI nodes and other instructions from original header
187 // into original pre-header. Unlike original header, original pre-header is
188 // not a member of loop.
190 // New loop header is one and only successor of original header that
191 // is inside the loop. All other original header successors are outside
192 // the loop. Copy PHI Nodes from original header into new loop header.
193 // Add second incoming value, from original loop pre-header into these phi
194 // nodes. If a value defined in original header is used outside original
195 // header then new loop header will need new phi nodes with two incoming
196 // values, one definition from original header and second definition is
197 // from original loop pre-header.
199 // Remove terminator from Original pre-header. Original pre-header will
200 // receive a clone of original header terminator as a new terminator.
201 OrigPreHeader->getInstList().pop_back();
202 BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
204 for (; (PN = dyn_cast<PHINode>(I)); ++I) {
205 // PHI nodes are not copied into original pre-header. Instead their values
206 // are directly propagated.
207 Value *NPV = PN->getIncomingValueForBlock(OrigPreHeader);
209 // Create new PHI node with two incoming values for NewHeader.
210 // One incoming value is from OrigLatch (through OrigHeader) and
211 // second incoming value is from original pre-header.
212 PHINode *NH = PHINode::Create(PN->getType(), PN->getName(),
214 NH->addIncoming(PN->getIncomingValueForBlock(OrigLatch), OrigHeader);
215 NH->addIncoming(NPV, OrigPreHeader);
217 // "In" can be replaced by NH at various places.
218 LoopHeaderInfo.push_back(RenameData(PN, NPV, NH));
221 // Now, handle non-phi instructions.
222 for (; I != E; ++I) {
224 assert(!isa<PHINode>(In) && "PHINode is not expected here");
226 // This is not a PHI instruction. Insert its clone into original pre-header.
227 // If this instruction is using a value from same basic block then
228 // update it to use value from cloned instruction.
229 Instruction *C = In->clone();
230 C->setName(In->getName());
231 OrigPreHeader->getInstList().push_back(C);
233 for (unsigned opi = 0, e = In->getNumOperands(); opi != e; ++opi) {
234 Instruction *OpInsn = dyn_cast<Instruction>(In->getOperand(opi));
235 if (!OpInsn) continue; // Ignore non-instruction values.
236 if (const RenameData *D = findReplacementData(OpInsn))
237 C->setOperand(opi, D->PreHeader);
240 // If this instruction is used outside this basic block then
241 // create new PHINode for this instruction.
242 Instruction *NewHeaderReplacement = NULL;
243 if (usedOutsideOriginalHeader(In)) {
244 PHINode *PN = PHINode::Create(In->getType(), In->getName(),
246 PN->addIncoming(In, OrigHeader);
247 PN->addIncoming(C, OrigPreHeader);
248 NewHeaderReplacement = PN;
250 LoopHeaderInfo.push_back(RenameData(In, C, NewHeaderReplacement));
253 // Rename uses of original header instructions to reflect their new
254 // definitions (either from original pre-header node or from newly created
255 // new header PHINodes.
257 // Original header instructions are used in
258 // 1) Original header:
260 // If instruction is used in non-phi instructions then it is using
261 // defintion from original heder iteself. Do not replace this use
262 // with definition from new header or original pre-header.
264 // If instruction is used in phi node then it is an incoming
265 // value. Rename its use to reflect new definition from new-preheader
268 // 2) Inside loop but not in original header
270 // Replace this use to reflect definition from new header.
271 for (unsigned LHI = 0, LHI_E = LoopHeaderInfo.size(); LHI != LHI_E; ++LHI) {
272 const RenameData &ILoopHeaderInfo = LoopHeaderInfo[LHI];
274 if (!ILoopHeaderInfo.Header)
277 Instruction *OldPhi = ILoopHeaderInfo.Original;
278 Instruction *NewPhi = ILoopHeaderInfo.Header;
280 // Before replacing uses, collect them first, so that iterator is
282 SmallVector<Instruction *, 16> AllUses;
283 for (Value::use_iterator UI = OldPhi->use_begin(), UE = OldPhi->use_end();
285 AllUses.push_back(cast<Instruction>(UI));
287 for (SmallVector<Instruction *, 16>::iterator UI = AllUses.begin(),
288 UE = AllUses.end(); UI != UE; ++UI) {
289 Instruction *U = *UI;
290 BasicBlock *Parent = U->getParent();
292 // Used inside original header
293 if (Parent == OrigHeader) {
294 // Do not rename uses inside original header non-phi instructions.
295 PHINode *PU = dyn_cast<PHINode>(U);
299 // Do not rename uses inside original header phi nodes, if the
300 // incoming value is for new header.
301 if (PU->getBasicBlockIndex(NewHeader) != -1
302 && PU->getIncomingValueForBlock(NewHeader) == U)
305 U->replaceUsesOfWith(OldPhi, NewPhi);
309 // Used inside loop, but not in original header.
310 if (L->contains(U->getParent())) {
312 U->replaceUsesOfWith(OldPhi, NewPhi);
316 // Used inside Exit Block. Since we are in LCSSA form, U must be PHINode.
317 if (U->getParent() == Exit) {
318 assert(isa<PHINode>(U) && "Use in Exit Block that is not PHINode");
320 PHINode *UPhi = cast<PHINode>(U);
321 // UPhi already has one incoming argument from original header.
322 // Add second incoming argument from new Pre header.
323 UPhi->addIncoming(ILoopHeaderInfo.PreHeader, OrigPreHeader);
325 // Used outside Exit block. Create a new PHI node from exit block
326 // to receive value from ne new header ane pre header.
327 PHINode *PN = PHINode::Create(U->getType(), U->getName(),
329 PN->addIncoming(ILoopHeaderInfo.PreHeader, OrigPreHeader);
330 PN->addIncoming(OldPhi, OrigHeader);
331 U->replaceUsesOfWith(OldPhi, PN);
336 /// Make sure all Exit block PHINodes have required incoming values.
341 // Removing incoming branch from loop preheader to original header.
342 // Now original header is inside the loop.
343 for (BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
345 if (PHINode *PN = dyn_cast<PHINode>(I))
346 PN->removeIncomingValue(OrigPreHeader);
348 // Make NewHeader as the new header for the loop.
349 L->moveToHeader(NewHeader);
351 preserveCanonicalLoopForm(LPM);
357 /// Make sure all Exit block PHINodes have required incoming values.
358 /// If incoming value is constant or defined outside the loop then
359 /// PHINode may not have an entry for original pre-header.
360 void LoopRotate::updateExitBlock() {
362 for (BasicBlock::iterator I = Exit->begin(), E = Exit->end();
365 PHINode *PN = dyn_cast<PHINode>(I);
369 // There is already one incoming value from original pre-header block.
370 if (PN->getBasicBlockIndex(OrigPreHeader) != -1)
373 const RenameData *ILoopHeaderInfo;
374 Value *V = PN->getIncomingValueForBlock(OrigHeader);
375 if (isa<Instruction>(V) &&
376 (ILoopHeaderInfo = findReplacementData(cast<Instruction>(V)))) {
377 assert(ILoopHeaderInfo->PreHeader && "Missing New Preheader Instruction");
378 PN->addIncoming(ILoopHeaderInfo->PreHeader, OrigPreHeader);
380 PN->addIncoming(V, OrigPreHeader);
385 /// Initialize local data
386 void LoopRotate::initialize() {
389 OrigPreHeader = NULL;
393 LoopHeaderInfo.clear();
396 /// Return true if this instruction is used by any instructions in the loop that
397 /// aren't in original header.
398 bool LoopRotate::usedOutsideOriginalHeader(Instruction *In) {
399 for (Value::use_iterator UI = In->use_begin(), UE = In->use_end();
401 BasicBlock *UserBB = cast<Instruction>(UI)->getParent();
402 if (UserBB != OrigHeader && L->contains(UserBB))
409 /// Find Replacement information for instruction. Return NULL if it is
411 const RenameData *LoopRotate::findReplacementData(Instruction *In) {
413 // Since LoopHeaderInfo is small, linear walk is OK.
414 for (unsigned LHI = 0, LHI_E = LoopHeaderInfo.size(); LHI != LHI_E; ++LHI) {
415 const RenameData &ILoopHeaderInfo = LoopHeaderInfo[LHI];
416 if (ILoopHeaderInfo.Original == In)
417 return &ILoopHeaderInfo;
422 /// After loop rotation, loop pre-header has multiple sucessors.
423 /// Insert one forwarding basic block to ensure that loop pre-header
424 /// has only one successor.
425 void LoopRotate::preserveCanonicalLoopForm(LPPassManager &LPM) {
427 // Right now original pre-header has two successors, new header and
428 // exit block. Insert new block between original pre-header and
429 // new header such that loop's new pre-header has only one successor.
430 BasicBlock *NewPreHeader = BasicBlock::Create("bb.nph",
431 OrigHeader->getParent(),
433 LoopInfo &LI = LPM.getAnalysis<LoopInfo>();
434 if (Loop *PL = LI.getLoopFor(OrigPreHeader))
435 PL->addBasicBlockToLoop(NewPreHeader, LI.getBase());
436 BranchInst::Create(NewHeader, NewPreHeader);
438 BranchInst *OrigPH_BI = cast<BranchInst>(OrigPreHeader->getTerminator());
439 if (OrigPH_BI->getSuccessor(0) == NewHeader)
440 OrigPH_BI->setSuccessor(0, NewPreHeader);
442 assert(OrigPH_BI->getSuccessor(1) == NewHeader &&
443 "Unexpected original pre-header terminator");
444 OrigPH_BI->setSuccessor(1, NewPreHeader);
447 for (BasicBlock::iterator I = NewHeader->begin(), E = NewHeader->end();
449 PHINode *PN = dyn_cast<PHINode>(I);
453 int index = PN->getBasicBlockIndex(OrigPreHeader);
454 assert(index != -1 && "Expected incoming value from Original PreHeader");
455 PN->setIncomingBlock(index, NewPreHeader);
456 assert(PN->getBasicBlockIndex(OrigPreHeader) == -1 &&
457 "Expected only one incoming value from Original PreHeader");
460 if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) {
461 DT->addNewBlock(NewPreHeader, OrigPreHeader);
462 DT->changeImmediateDominator(L->getHeader(), NewPreHeader);
463 DT->changeImmediateDominator(Exit, OrigPreHeader);
464 for (Loop::block_iterator BI = L->block_begin(), BE = L->block_end();
467 if (L->getHeader() != B) {
468 DomTreeNode *Node = DT->getNode(B);
469 if (Node && Node->getBlock() == OrigHeader)
470 DT->changeImmediateDominator(*BI, L->getHeader());
473 DT->changeImmediateDominator(OrigHeader, OrigLatch);
476 if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>()) {
477 // New Preheader's dominance frontier is Exit block.
478 DominanceFrontier::DomSetType NewPHSet;
479 NewPHSet.insert(Exit);
480 DF->addBasicBlock(NewPreHeader, NewPHSet);
482 // New Header's dominance frontier now includes itself and Exit block
483 DominanceFrontier::iterator HeadI = DF->find(L->getHeader());
484 if (HeadI != DF->end()) {
485 DominanceFrontier::DomSetType & HeaderSet = HeadI->second;
487 HeaderSet.insert(L->getHeader());
488 HeaderSet.insert(Exit);
490 DominanceFrontier::DomSetType HeaderSet;
491 HeaderSet.insert(L->getHeader());
492 HeaderSet.insert(Exit);
493 DF->addBasicBlock(L->getHeader(), HeaderSet);
496 // Original header (new Loop Latch)'s dominance frontier is Exit.
497 DominanceFrontier::iterator LatchI = DF->find(L->getLoopLatch());
498 if (LatchI != DF->end()) {
499 DominanceFrontier::DomSetType &LatchSet = LatchI->second;
500 LatchSet = LatchI->second;
502 LatchSet.insert(Exit);
504 DominanceFrontier::DomSetType LatchSet;
505 LatchSet.insert(Exit);
506 DF->addBasicBlock(L->getHeader(), LatchSet);
509 // If a loop block dominates new loop latch then its frontier is
510 // new header and Exit.
511 BasicBlock *NewLatch = L->getLoopLatch();
512 DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>();
513 for (Loop::block_iterator BI = L->block_begin(), BE = L->block_end();
516 if (DT->dominates(B, NewLatch)) {
517 DominanceFrontier::iterator BDFI = DF->find(B);
518 if (BDFI != DF->end()) {
519 DominanceFrontier::DomSetType &BSet = BDFI->second;
522 BSet.insert(L->getHeader());
525 DominanceFrontier::DomSetType BSet;
526 BSet.insert(L->getHeader());
528 DF->addBasicBlock(B, BSet);
534 // Preserve canonical loop form, which means Exit block should
535 // have only one predecessor.
536 BasicBlock *NExit = SplitEdge(L->getLoopLatch(), Exit, this);
539 BasicBlock::iterator I = Exit->begin(), E = Exit->end();
541 for (; (PN = dyn_cast<PHINode>(I)); ++I) {
542 unsigned N = PN->getNumIncomingValues();
543 for (unsigned index = 0; index < N; ++index)
544 if (PN->getIncomingBlock(index) == NExit) {
545 PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName(),
547 NewPN->addIncoming(PN->getIncomingValue(index), L->getLoopLatch());
548 PN->setIncomingValue(index, NewPN);
549 PN->setIncomingBlock(index, NExit);
554 assert(NewHeader && L->getHeader() == NewHeader &&
555 "Invalid loop header after loop rotation");
556 assert(NewPreHeader && L->getLoopPreheader() == NewPreHeader &&
557 "Invalid loop preheader after loop rotation");
558 assert(L->getLoopLatch() &&
559 "Invalid loop latch after loop rotation");