1 //===- LoopRotation.cpp - Loop Rotation Pass ------------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by Devang Patel and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements Loop Rotation Pass.
12 //===----------------------------------------------------------------------===//
14 #define DEBUG_TYPE "loop-rotate"
16 #include "llvm/Transforms/Scalar.h"
17 #include "llvm/Function.h"
18 #include "llvm/Instructions.h"
19 #include "llvm/Analysis/LoopInfo.h"
20 #include "llvm/Analysis/LoopPass.h"
21 #include "llvm/Transforms/Utils/Local.h"
22 #include "llvm/Support/CommandLine.h"
23 #include "llvm/Support/Debug.h"
24 #include "llvm/ADT/Statistic.h"
25 #include "llvm/ADT/SmallVector.h"
29 #define MAX_HEADER_SIZE 16
31 STATISTIC(NumRotated, "Number of loops rotated");
34 class VISIBILITY_HIDDEN RenameData {
36 RenameData(Instruction *O, Instruction *P, Instruction *H)
37 : Original(O), PreHeader(P), Header(H) { }
39 Instruction *Original; // Original instruction
40 Instruction *PreHeader; // New pre-header replacement
41 Instruction *Header; // New header replacement
44 class VISIBILITY_HIDDEN LoopRotate : public LoopPass {
48 // Rotate Loop L as many times as possible. Return true if
49 // loop is rotated at least once.
50 bool runOnLoop(Loop *L, LPPassManager &LPM);
52 // LCSSA form makes instruction renaming easier.
53 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
54 AU.addRequiredID(LCSSAID);
55 AU.addPreservedID(LCSSAID);
61 bool rotateLoop(Loop *L, LPPassManager &LPM);
63 /// Initialize local data
66 /// Make sure all Exit block PHINodes have required incoming values.
67 /// If incoming value is constant or defined outside the loop then
68 /// PHINode may not have an entry for new pre-header.
69 void updateExitBlock();
71 /// Return true if this instruction is used outside original header.
72 bool usedOutsideOriginalHeader(Instruction *In);
74 /// Find Replacement information for instruction. Return NULL if it is
76 const RenameData *findReplacementData(Instruction *I);
81 BasicBlock *OrigHeader;
82 BasicBlock *OrigPreHeader;
83 BasicBlock *OrigLatch;
84 BasicBlock *NewHeader;
85 BasicBlock *NewPreHeader;
88 SmallVector<RenameData, MAX_HEADER_SIZE> LoopHeaderInfo;
91 RegisterPass<LoopRotate> X ("loop-rotate", "Rotate Loops");
94 LoopPass *llvm::createLoopRotatePass() { return new LoopRotate(); }
96 /// Rotate Loop L as many times as possible. Return true if
97 /// loop is rotated at least once.
98 bool LoopRotate::runOnLoop(Loop *Lp, LPPassManager &LPM) {
100 bool RotatedOneLoop = false;
103 // One loop can be rotated multiple times.
104 while (rotateLoop(Lp,LPM)) {
105 RotatedOneLoop = true;
109 return RotatedOneLoop;
112 /// Rotate loop LP. Return true if it loop is rotated.
113 bool LoopRotate::rotateLoop(Loop *Lp, LPPassManager &LPM) {
117 OrigHeader = L->getHeader();
118 OrigPreHeader = L->getLoopPreheader();
119 OrigLatch = L->getLoopLatch();
121 // If loop has only one block then there is not much to rotate.
122 if (L->getBlocks().size() == 1)
125 if (!OrigHeader || !OrigLatch || !OrigPreHeader)
128 // If loop header is not one of the loop exit block then
129 // either this loop is already rotated or it is not
130 // suitable for loop rotation transformations.
131 if (!L->isLoopExit(OrigHeader))
134 BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
137 assert (BI->isConditional() && "Branch Instruction is not condiitional");
139 // Updating PHInodes in loops with multiple exits adds complexity.
140 // Keep it simple, and restrict loop rotation to loops with one exit only.
141 // In future, lift this restriction and support for multiple exits if
143 std::vector<BasicBlock *> ExitBlocks;
144 L->getExitBlocks(ExitBlocks);
145 if (ExitBlocks.size() > 1)
148 // Find new Loop header. NewHeader is a Header's one and only successor
149 // that is inside loop. Header's other successor is out side the
150 // loop. Otherwise loop is not suitable for rotation.
151 Exit = BI->getSuccessor(0);
152 NewHeader = BI->getSuccessor(1);
153 if (L->contains(Exit))
154 std::swap(Exit, NewHeader);
155 assert (NewHeader && "Unable to determine new loop header");
156 assert(L->contains(NewHeader) && !L->contains(Exit) &&
157 "Unable to determine loop header and exit blocks");
159 // Check size of original header and reject
160 // loop if it is very big.
161 if (OrigHeader->getInstList().size() > MAX_HEADER_SIZE)
164 // Now, this loop is suitable for rotation.
166 // Copy PHI nodes and other instructions from original header
167 // into new pre-header. Unlike original header, new pre-header is
168 // not a member of loop. New pre-header has only one predecessor,
169 // that is original loop pre-header.
171 // New loop header is one and only successor of original header that
172 // is inside the loop. All other original header successors are outside
173 // the loop. Copy PHI Nodes from original header into new loop header.
174 // Add second incoming value, from new loop pre-header into these phi
175 // nodes. If a value defined in original header is used outside original
176 // header then new loop header will need new phi nodes with two incoming
177 // values, one definition from original header and second definition is
178 // from new loop pre-header (which is a clone of original header definition).
180 NewPreHeader = new BasicBlock("bb.nph", OrigHeader->getParent(), OrigHeader);
181 BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
183 for (; (PN = dyn_cast<PHINode>(I)); ++I) {
186 // Create new PHI node with one value incoming from OrigPreHeader.
187 // NewPreHeader has only one predecessor, OrigPreHeader.
188 PHINode *NPH = new PHINode(In->getType(), In->getName());
189 NPH->addIncoming(PN->getIncomingValueForBlock(OrigPreHeader),
191 NewPreHeader->getInstList().push_back(NPH);
193 // Create new PHI node with two incoming values for NewHeader.
194 // One incoming value is from OrigLatch (through OrigHeader) and
195 // second incoming value is from NewPreHeader.
196 PHINode *NH = new PHINode(In->getType(), In->getName());
197 NH->addIncoming(PN->getIncomingValueForBlock(OrigLatch), OrigHeader);
198 NH->addIncoming(NPH, NewPreHeader);
199 NewHeader->getInstList().push_front(NH);
201 // "In" can be replaced by NPH or NH at various places.
202 LoopHeaderInfo.push_back(RenameData(In, NPH, NH));
205 // Now, handle non-phi instructions.
206 for (; I != E; ++I) {
209 assert (!isa<PHINode>(In) && "PHINode is not expected here");
210 // This is not a PHI instruction. Insert its clone into NewPreHeader.
211 // If this instruction is using a value from same basic block then
212 // update it to use value from cloned instruction.
213 Instruction *C = In->clone();
214 C->setName(In->getName());
215 NewPreHeader->getInstList().push_back(C);
217 // If this instruction is used outside this basic block then
218 // create new PHINode for this instruction.
219 Instruction *NewHeaderReplacement = NULL;
220 if (usedOutsideOriginalHeader(In)) {
221 PHINode *PN = new PHINode(In->getType(), In->getName());
222 PN->addIncoming(In, OrigHeader);
223 PN->addIncoming(C, NewPreHeader);
224 NewHeader->getInstList().push_front(PN);
225 NewHeaderReplacement = PN;
228 // "In" can be replaced by NPH or NH at various places.
229 LoopHeaderInfo.push_back(RenameData(In, C, NewHeaderReplacement));
232 // Update new pre-header.
233 // Rename values that are defined in original header to reflects values
234 // defined in new pre-header.
235 for(unsigned LHI = 0, LHI_E = LoopHeaderInfo.size(); LHI != LHI_E; ++LHI) {
236 const RenameData &ILoopHeaderInfo = LoopHeaderInfo[LHI];
237 Instruction *In = ILoopHeaderInfo.Original;
238 Instruction *C = ILoopHeaderInfo.PreHeader;
240 // If this instruction is not from new pre-header then is not new
241 // pre-header then this instruction is not handled here.
242 if (C->getParent() != NewPreHeader)
245 // PHINodes uses value from pre-header predecessors.
246 if (isa<PHINode>(In))
249 for (unsigned opi = 0, e = In->getNumOperands(); opi != e; ++opi) {
250 if (Instruction *OpPhi = dyn_cast<PHINode>(In->getOperand(opi))) {
251 if (const RenameData *D = findReplacementData(OpPhi))
252 C->setOperand(opi, D->PreHeader);
254 else if (Instruction *OpInsn =
255 dyn_cast<Instruction>(In->getOperand(opi))) {
256 if (const RenameData *D = findReplacementData(OpInsn))
257 C->setOperand(opi, D->PreHeader);
262 // Rename uses of original header instructions to reflect their new
263 // definitions (either from new pre-header node or from newly created
264 // new header PHINodes.
266 // Original header instructions are used in
267 // 1) Original header:
269 // If instruction is used in non-phi instructions then it is using
270 // defintion from original heder iteself. Do not replace this use
271 // with definition from new header or new pre-header.
273 // If instruction is used in phi node then it is an incoming
274 // value. Rename its use to reflect new definition from new-preheader
277 // 2) Inside loop but not in original header
279 // Replace this use to reflect definition from new header.
280 for(unsigned LHI = 0, LHI_E = LoopHeaderInfo.size(); LHI != LHI_E; ++LHI) {
281 const RenameData &ILoopHeaderInfo = LoopHeaderInfo[LHI];
283 if (!ILoopHeaderInfo.Header)
286 Instruction *OldPhi = ILoopHeaderInfo.Original;
287 Instruction *NewPhi = ILoopHeaderInfo.Header;
289 // Before replacing uses, collect them first, so that iterator is
291 SmallVector<Instruction *, 16> AllUses;
292 for (Value::use_iterator UI = OldPhi->use_begin(), UE = OldPhi->use_end();
294 Instruction *U = cast<Instruction>(UI);
295 AllUses.push_back(U);
298 for (SmallVector<Instruction *, 16>::iterator UI = AllUses.begin(),
299 UE = AllUses.end(); UI != UE; ++UI) {
300 Instruction *U = *UI;
301 BasicBlock *Parent = U->getParent();
303 // Used inside original header
304 if (Parent == OrigHeader) {
305 // Do not rename uses inside original header non-phi instructions.
306 PHINode *PU = dyn_cast<PHINode>(U);
310 // Do not rename uses inside original header phi nodes, if the
311 // incoming value is for new header.
312 if (PU->getBasicBlockIndex(NewHeader) != -1
313 && PU->getIncomingValueForBlock(NewHeader) == U)
316 U->replaceUsesOfWith(OldPhi, NewPhi);
320 // Used inside loop, but not in original header.
321 if (L->contains(U->getParent())) {
323 U->replaceUsesOfWith(OldPhi, NewPhi);
327 // Used inside Exit Block. Since we are in LCSSA form, U must be PHINode.
328 assert (U->getParent() == Exit
329 && "Need to propagate new PHI into Exit blocks");
330 assert (isa<PHINode>(U) && "Use in Exit Block that is not PHINode");
332 PHINode *UPhi = cast<PHINode>(U);
334 // UPhi already has one incoming argument from original header.
335 // Add second incoming argument from new Pre header.
337 UPhi->addIncoming(ILoopHeaderInfo.PreHeader, NewPreHeader);
341 /// Make sure all Exit block PHINodes have required incoming values.
346 // Removing incoming branch from loop preheader to original header.
347 // Now original header is inside the loop.
348 OrigHeader->removePredecessor(OrigPreHeader);
350 // Establish NewPreHeader as loop preheader. Add unconditional branch
351 // from original loop pre-header to new loop pre-header. Add NewPreHEader
353 BranchInst *PH_BI = cast<BranchInst>(OrigPreHeader->getTerminator());
354 PH_BI->setSuccessor(0, NewPreHeader);
355 LoopInfo &LI = LPM.getAnalysis<LoopInfo>();
356 if (Loop *PL = LI.getLoopFor(OrigPreHeader))
357 PL->addBasicBlockToLoop(NewPreHeader, LI);
359 // Make NewHeader as the new header for the loop.
360 L->moveToHeader(NewHeader);
366 /// Make sure all Exit block PHINodes have required incoming values.
367 /// If incoming value is constant or defined outside the loop then
368 /// PHINode may not have an entry for new pre-header.
369 void LoopRotate::updateExitBlock() {
371 for (BasicBlock::iterator I = Exit->begin(), E = Exit->end();
374 PHINode *PN = dyn_cast<PHINode>(I);
378 // There is already one incoming value from new pre-header block.
379 if (PN->getBasicBlockIndex(NewPreHeader) != -1)
382 const RenameData *ILoopHeaderInfo;
383 Value *V = PN->getIncomingValueForBlock(OrigHeader);
384 if (isa<Instruction>(V) &&
385 (ILoopHeaderInfo = findReplacementData(cast<Instruction>(V)))) {
386 assert (ILoopHeaderInfo->PreHeader && "Missing New Preheader Instruction");
387 PN->addIncoming(ILoopHeaderInfo->PreHeader, NewPreHeader);
389 PN->addIncoming(V, NewPreHeader);
394 /// Initialize local data
395 void LoopRotate::initialize() {
398 OrigPreHeader = NULL;
403 LoopHeaderInfo.clear();
406 /// Return true if this instruction is used by any instructions in the loop that
407 /// aren't in original header.
408 bool LoopRotate::usedOutsideOriginalHeader(Instruction *In) {
410 for (Value::use_iterator UI = In->use_begin(), UE = In->use_end();
412 Instruction *U = cast<Instruction>(UI);
413 if (U->getParent() != OrigHeader) {
414 if (L->contains(U->getParent()))
422 /// Find Replacement information for instruction. Return NULL if it is
424 const RenameData *LoopRotate::findReplacementData(Instruction *In) {
426 // Since LoopHeaderInfo is small, linear walk is OK.
427 for(unsigned LHI = 0, LHI_E = LoopHeaderInfo.size(); LHI != LHI_E; ++LHI) {
428 const RenameData &ILoopHeaderInfo = LoopHeaderInfo[LHI];
429 if (ILoopHeaderInfo.Original == In)
430 return &ILoopHeaderInfo;