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 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();
182 for (; I != E; ++I) {
185 PHINode *PN = dyn_cast<PHINode>(I);
189 // Create new PHI node with one value incoming from OrigPreHeader.
190 // NewPreHeader has only one predecessor, OrigPreHeader.
191 PHINode *NPH = new PHINode(In->getType(), In->getName());
192 NPH->addIncoming(PN->getIncomingValueForBlock(OrigPreHeader),
194 NewPreHeader->getInstList().push_back(NPH);
196 // Create new PHI node with two incoming values for NewHeader.
197 // One incoming value is from OrigLatch (through OrigHeader) and
198 // second incoming value is from NewPreHeader.
199 PHINode *NH = new PHINode(In->getType(), In->getName());
200 NH->addIncoming(PN->getIncomingValueForBlock(OrigLatch), OrigHeader);
201 NH->addIncoming(NPH, NewPreHeader);
202 NewHeader->getInstList().push_front(NH);
204 // "In" can be replaced by NPH or NH at various places.
205 LoopHeaderInfo.push_back(RenameData(In, NPH, NH));
208 // Now, handle non-phi instructions.
209 for (; I != E; ++I) {
212 assert (!isa<PHINode>(In) && "PHINode is not expected here");
213 // This is not a PHI instruction. Insert its clone into NewPreHeader.
214 // If this instruction is using a value from same basic block then
215 // update it to use value from cloned instruction.
216 Instruction *C = In->clone();
217 C->setName(In->getName());
218 NewPreHeader->getInstList().push_back(C);
220 // If this instruction is used outside this basic block then
221 // create new PHINode for this instruction.
222 Instruction *NewHeaderReplacement = NULL;
223 if (usedOutsideOriginalHeader(In)) {
224 PHINode *PN = new PHINode(In->getType(), In->getName());
225 PN->addIncoming(In, OrigHeader);
226 PN->addIncoming(C, NewPreHeader);
227 NewHeader->getInstList().push_front(PN);
228 NewHeaderReplacement = PN;
231 // "In" can be replaced by NPH or NH at various places.
232 LoopHeaderInfo.push_back(RenameData(In, C, NewHeaderReplacement));
235 // Update new pre-header.
236 // Rename values that are defined in original header to reflects values
237 // defined in new pre-header.
238 for (SmallVector<RenameData, MAX_HEADER_SIZE>::iterator
239 I = LoopHeaderInfo.begin(), E = LoopHeaderInfo.end(); I != E; ++I) {
241 const RenameData &ILoopHeaderInfo = *I;
242 Instruction *In = ILoopHeaderInfo.Original;
243 Instruction *C = ILoopHeaderInfo.PreHeader;
245 // If this instruction is not from new pre-header then is not new
246 // pre-header then this instruction is not handled here.
247 if (C->getParent() != NewPreHeader)
250 // PHINodes uses value from pre-header predecessors.
251 if (isa<PHINode>(In))
254 for (unsigned opi = 0, e = In->getNumOperands(); opi != e; ++opi) {
255 if (Instruction *OpPhi = dyn_cast<PHINode>(In->getOperand(opi))) {
256 if (RenameData *D = findReplacementData(OpPhi))
257 C->setOperand(opi, D->PreHeader);
259 else if (Instruction *OpInsn =
260 dyn_cast<Instruction>(In->getOperand(opi))) {
261 if (RenameData *D = findReplacementData(OpInsn))
262 C->setOperand(opi, D->PreHeader);
267 // Rename uses of original header instructions to reflect their new
268 // definitions (either from new pre-header node or from newly created
269 // new header PHINodes.
271 // Original header instructions are used in
272 // 1) Original header:
274 // If instruction is used in non-phi instructions then it is using
275 // defintion from original heder iteself. Do not replace this use
276 // with definition from new header or new pre-header.
278 // If instruction is used in phi node then it is an incoming
279 // value. Rename its use to reflect new definition from new-preheader
282 // 2) Inside loop but not in original header
284 // Replace this use to reflect definition from new header.
285 for (SmallVector<RenameData, MAX_HEADER_SIZE>::iterator
286 I = LoopHeaderInfo.begin(), E = LoopHeaderInfo.end(); I != E; ++I) {
288 const RenameData &ILoopHeaderInfo = *I;
289 if (!ILoopHeaderInfo.Header)
292 Instruction *OldPhi = ILoopHeaderInfo.Original;
293 Instruction *NewPhi = ILoopHeaderInfo.Header;
295 // Before replacing uses, collect them first, so that iterator is
297 SmallVector<Instruction *, 16> AllUses;
298 for (Value::use_iterator UI = OldPhi->use_begin(), UE = OldPhi->use_end();
300 Instruction *U = cast<Instruction>(UI);
301 AllUses.push_back(U);
304 for (SmallVector<Instruction *, 16>::iterator UI = AllUses.begin(),
305 UE = AllUses.end(); UI != UE; ++UI) {
306 Instruction *U = *UI;
307 BasicBlock *Parent = U->getParent();
309 // Used inside original header
310 if (Parent == OrigHeader) {
311 // Do not rename uses inside original header non-phi instructions.
312 PHINode *PU = dyn_cast<PHINode>(U);
316 // Do not rename uses inside original header phi nodes, if the
317 // incoming value is for new header.
318 if (PU->getBasicBlockIndex(NewHeader) != -1
319 && PU->getIncomingValueForBlock(NewHeader) == U)
322 U->replaceUsesOfWith(OldPhi, NewPhi);
326 // Used inside loop, but not in original header.
327 if (L->contains(U->getParent())) {
329 U->replaceUsesOfWith(OldPhi, NewPhi);
333 // Used inside Exit Block. Since we are in LCSSA form, U must be PHINode.
334 assert (U->getParent() == Exit
335 && "Need to propagate new PHI into Exit blocks");
336 assert (isa<PHINode>(U) && "Use in Exit Block that is not PHINode");
338 PHINode *UPhi = cast<PHINode>(U);
340 // UPhi already has one incoming argument from original header.
341 // Add second incoming argument from new Pre header.
343 UPhi->addIncoming(ILoopHeaderInfo.PreHeader, NewPreHeader);
347 /// Make sure all Exit block PHINodes have required incoming values.
352 // Removing incoming branch from loop preheader to original header.
353 // Now original header is inside the loop.
354 OrigHeader->removePredecessor(OrigPreHeader);
356 // Establish NewPreHeader as loop preheader. Add unconditional branch
357 // from original loop pre-header to new loop pre-header. Add NewPreHEader
359 BranchInst *PH_BI = cast<BranchInst>(OrigPreHeader->getTerminator());
360 PH_BI->setSuccessor(0, NewPreHeader);
361 LoopInfo &LI = LPM.getAnalysis<LoopInfo>();
362 if (Loop *PL = LI.getLoopFor(OrigPreHeader))
363 PL->addBasicBlockToLoop(NewPreHeader, LI);
365 // Make NewHeader as the new header for the loop.
366 L->moveToHeader(NewHeader);
373 /// Make sure all Exit block PHINodes have required incoming values.
374 /// If incoming value is constant or defined outside the loop then
375 /// PHINode may not have an entry for new pre-header.
376 void LoopRotate::updateExitBlock() {
378 for (BasicBlock::iterator I = Exit->begin(), E = Exit->end();
381 PHINode *PN = dyn_cast<PHINode>(I);
385 if (PN->getBasicBlockIndex(NewPreHeader) == -1) {
386 Value *V = PN->getIncomingValueForBlock(OrigHeader);
387 if (isa<Constant>(V))
388 PN->addIncoming(V, NewPreHeader);
390 RenameData *ILoopHeaderInfo = findReplacementData(cast<Instruction>(V));
391 assert (ILoopHeaderInfo && ILoopHeaderInfo->PreHeader && "Missing New Preheader Instruction");
392 PN->addIncoming(ILoopHeaderInfo->PreHeader, NewPreHeader);
399 /// Initialize local data
400 void LoopRotate::initialize() {
403 OrigPreHeader = NULL;
408 LoopHeaderInfo.clear();
411 /// Return true if this instruction is used by any instructions in the loop that
412 /// aren't in original header.
413 bool LoopRotate::usedOutsideOriginalHeader(Instruction *In) {
415 for (Value::use_iterator UI = In->use_begin(), UE = In->use_end();
417 Instruction *U = cast<Instruction>(UI);
418 if (U->getParent() != OrigHeader) {
419 if (L->contains(U->getParent()))
427 /// Find Replacement information for instruction. Return NULL if it is
429 RenameData *LoopRotate::findReplacementData(Instruction *In) {
431 // Since LoopHeaderInfo is small, linear walk is OK.
432 for (SmallVector<RenameData, MAX_HEADER_SIZE>::iterator
433 I = LoopHeaderInfo.begin(), E = LoopHeaderInfo.end(); I != E; ++I)
434 if (I->Original == In)