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/Analysis/CodeMetrics.h"
18 #include "llvm/Analysis/LoopPass.h"
19 #include "llvm/Analysis/InstructionSimplify.h"
20 #include "llvm/Analysis/ScalarEvolution.h"
21 #include "llvm/Transforms/Utils/Local.h"
22 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
23 #include "llvm/Transforms/Utils/SSAUpdater.h"
24 #include "llvm/Transforms/Utils/ValueMapper.h"
25 #include "llvm/Support/Debug.h"
26 #include "llvm/ADT/Statistic.h"
29 #define MAX_HEADER_SIZE 16
31 STATISTIC(NumRotated, "Number of loops rotated");
34 class LoopRotate : public LoopPass {
36 static char ID; // Pass ID, replacement for typeid
37 LoopRotate() : LoopPass(ID) {
38 initializeLoopRotatePass(*PassRegistry::getPassRegistry());
41 // Rotate Loop L as many times as possible. Return true if
42 // loop is rotated at least once.
43 bool runOnLoop(Loop *L, LPPassManager &LPM);
45 // LCSSA form makes instruction renaming easier.
46 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
47 AU.addPreserved<DominatorTree>();
48 AU.addRequired<LoopInfo>();
49 AU.addPreserved<LoopInfo>();
50 AU.addRequiredID(LoopSimplifyID);
51 AU.addPreservedID(LoopSimplifyID);
52 AU.addRequiredID(LCSSAID);
53 AU.addPreservedID(LCSSAID);
54 AU.addPreserved<ScalarEvolution>();
60 bool rotateLoop(Loop *L);
62 /// After loop rotation, loop pre-header has multiple sucessors.
63 /// Insert one forwarding basic block to ensure that loop pre-header
64 /// has only one successor.
65 void preserveCanonicalLoopForm(Loop *L, BasicBlock *OrigHeader,
66 BasicBlock *OrigPreHeader,
67 BasicBlock *OrigLatch, BasicBlock *NewHeader,
75 char LoopRotate::ID = 0;
76 INITIALIZE_PASS_BEGIN(LoopRotate, "loop-rotate", "Rotate Loops", false, false)
77 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
78 INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
79 INITIALIZE_PASS_DEPENDENCY(LCSSA)
80 INITIALIZE_PASS_END(LoopRotate, "loop-rotate", "Rotate Loops", false, false)
82 Pass *llvm::createLoopRotatePass() { return new LoopRotate(); }
84 /// Rotate Loop L as many times as possible. Return true if
85 /// the loop is rotated at least once.
86 bool LoopRotate::runOnLoop(Loop *L, LPPassManager &LPM) {
87 LI = &getAnalysis<LoopInfo>();
89 // One loop can be rotated multiple times.
90 bool MadeChange = false;
97 /// Rotate loop LP. Return true if the loop is rotated.
98 bool LoopRotate::rotateLoop(Loop *L) {
99 // If the loop has only one block then there is not much to rotate.
100 if (L->getBlocks().size() == 1)
103 BasicBlock *OrigHeader = L->getHeader();
105 BranchInst *BI = dyn_cast<BranchInst>(OrigHeader->getTerminator());
106 if (BI == 0 || BI->isUnconditional())
109 // If the loop header is not one of the loop exiting blocks then
110 // either this loop is already rotated or it is not
111 // suitable for loop rotation transformations.
112 if (!L->isLoopExiting(OrigHeader))
115 // Updating PHInodes in loops with multiple exits adds complexity.
116 // Keep it simple, and restrict loop rotation to loops with one exit only.
117 // In future, lift this restriction and support for multiple exits if
119 SmallVector<BasicBlock*, 8> ExitBlocks;
120 L->getExitBlocks(ExitBlocks);
121 if (ExitBlocks.size() > 1)
124 // Check size of original header and reject loop if it is very big.
127 Metrics.analyzeBasicBlock(OrigHeader);
128 if (Metrics.NumInsts > MAX_HEADER_SIZE)
132 // Now, this loop is suitable for rotation.
133 BasicBlock *OrigPreHeader = L->getLoopPreheader();
134 BasicBlock *OrigLatch = L->getLoopLatch();
135 assert(OrigPreHeader && OrigLatch && "Loop not in canonical form?");
137 // Anything ScalarEvolution may know about this loop or the PHI nodes
138 // in its header will soon be invalidated.
139 if (ScalarEvolution *SE = getAnalysisIfAvailable<ScalarEvolution>())
142 // Find new Loop header. NewHeader is a Header's one and only successor
143 // that is inside loop. Header's other successor is outside the
144 // loop. Otherwise loop is not suitable for rotation.
145 BasicBlock *Exit = BI->getSuccessor(0);
146 BasicBlock *NewHeader = BI->getSuccessor(1);
147 if (L->contains(Exit))
148 std::swap(Exit, NewHeader);
149 assert(NewHeader && "Unable to determine new loop header");
150 assert(L->contains(NewHeader) && !L->contains(Exit) &&
151 "Unable to determine loop header and exit blocks");
153 // This code assumes that the new header has exactly one predecessor.
154 // Remove any single-entry PHI nodes in it.
155 assert(NewHeader->getSinglePredecessor() &&
156 "New header doesn't have one pred!");
157 FoldSingleEntryPHINodes(NewHeader);
159 // Begin by walking OrigHeader and populating ValueMap with an entry for
161 BasicBlock::iterator I = OrigHeader->begin(), E = OrigHeader->end();
162 ValueToValueMapTy ValueMap;
164 // For PHI nodes, the value available in OldPreHeader is just the
165 // incoming value from OldPreHeader.
166 for (; PHINode *PN = dyn_cast<PHINode>(I); ++I)
167 ValueMap[PN] = PN->getIncomingValue(PN->getBasicBlockIndex(OrigPreHeader));
169 // For the rest of the instructions, either hoist to the OrigPreheader if
170 // possible or create a clone in the OldPreHeader if not.
171 TerminatorInst *LoopEntryBranch = OrigPreHeader->getTerminator();
173 Instruction *Inst = I++;
175 // If the instruction's operands are invariant and it doesn't read or write
176 // memory, then it is safe to hoist. Doing this doesn't change the order of
177 // execution in the preheader, but does prevent the instruction from
178 // executing in each iteration of the loop. This means it is safe to hoist
179 // something that might trap, but isn't safe to hoist something that reads
180 // memory (without proving that the loop doesn't write).
181 if (L->hasLoopInvariantOperands(Inst) &&
182 !Inst->mayReadFromMemory() && !Inst->mayWriteToMemory() &&
183 !isa<TerminatorInst>(Inst)) {
184 Inst->moveBefore(LoopEntryBranch);
188 // Otherwise, create a duplicate of the instruction.
189 Instruction *C = Inst->clone();
191 // Eagerly remap the operands of the instruction.
192 RemapInstruction(C, ValueMap,
193 RF_NoModuleLevelChanges|RF_IgnoreMissingEntries);
195 // With the operands remapped, see if the instruction constant folds or is
196 // otherwise simplifyable. This commonly occurs because the entry from PHI
197 // nodes allows icmps and other instructions to fold.
198 Value *V = SimplifyInstruction(C);
199 if (V && LI->replacementPreservesLCSSAForm(C, V)) {
200 // If so, then delete the temporary instruction and stick the folded value
205 // Otherwise, stick the new instruction into the new block!
206 C->setName(Inst->getName());
207 C->insertBefore(LoopEntryBranch);
212 // Along with all the other instructions, we just cloned OrigHeader's
213 // terminator into OrigPreHeader. Fix up the PHI nodes in each of OrigHeader's
214 // successors by duplicating their incoming values for OrigHeader.
215 TerminatorInst *TI = OrigHeader->getTerminator();
216 for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
217 for (BasicBlock::iterator BI = TI->getSuccessor(i)->begin();
218 PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
219 PN->addIncoming(PN->getIncomingValueForBlock(OrigHeader), OrigPreHeader);
221 // Now that OrigPreHeader has a clone of OrigHeader's terminator, remove
222 // OrigPreHeader's old terminator (the original branch into the loop), and
223 // remove the corresponding incoming values from the PHI nodes in OrigHeader.
224 LoopEntryBranch->eraseFromParent();
225 for (I = OrigHeader->begin(); PHINode *PN = dyn_cast<PHINode>(I); ++I)
226 PN->removeIncomingValue(PN->getBasicBlockIndex(OrigPreHeader));
228 // Now fix up users of the instructions in OrigHeader, inserting PHI nodes
231 for (I = OrigHeader->begin(); I != E; ++I) {
232 Value *OrigHeaderVal = I;
233 Value *OrigPreHeaderVal = ValueMap[OrigHeaderVal];
235 // If there are no uses of the value (e.g. because it returns void), there
236 // is nothing to rewrite.
237 if (OrigHeaderVal->use_empty() && OrigPreHeaderVal->use_empty())
240 // The value now exits in two versions: the initial value in the preheader
241 // and the loop "next" value in the original header.
242 SSA.Initialize(OrigHeaderVal->getType(), OrigHeaderVal->getName());
243 SSA.AddAvailableValue(OrigHeader, OrigHeaderVal);
244 SSA.AddAvailableValue(OrigPreHeader, OrigPreHeaderVal);
246 // Visit each use of the OrigHeader instruction.
247 for (Value::use_iterator UI = OrigHeaderVal->use_begin(),
248 UE = OrigHeaderVal->use_end(); UI != UE; ) {
249 // Grab the use before incrementing the iterator.
250 Use &U = UI.getUse();
252 // Increment the iterator before removing the use from the list.
255 // SSAUpdater can't handle a non-PHI use in the same block as an
256 // earlier def. We can easily handle those cases manually.
257 Instruction *UserInst = cast<Instruction>(U.getUser());
258 if (!isa<PHINode>(UserInst)) {
259 BasicBlock *UserBB = UserInst->getParent();
261 // The original users in the OrigHeader are already using the
262 // original definitions.
263 if (UserBB == OrigHeader)
266 // Users in the OrigPreHeader need to use the value to which the
267 // original definitions are mapped.
268 if (UserBB == OrigPreHeader) {
269 U = OrigPreHeaderVal;
274 // Anything else can be handled by SSAUpdater.
279 // NewHeader is now the header of the loop.
280 L->moveToHeader(NewHeader);
282 // Move the original header to the bottom of the loop, where it now more
283 // naturally belongs. This isn't necessary for correctness, and CodeGen can
284 // usually reorder blocks on its own to fix things like this up, but it's
285 // still nice to keep the IR readable.
287 // The original header should have only one predecessor at this point, since
288 // we checked that the loop had a proper preheader and unique backedge before
290 assert(OrigHeader->getSinglePredecessor() &&
291 "Original loop header has too many predecessors after loop rotation!");
292 OrigHeader->moveAfter(OrigHeader->getSinglePredecessor());
294 // Also, since this original header only has one predecessor, zap its
295 // PHI nodes, which are now trivial.
296 FoldSingleEntryPHINodes(OrigHeader);
298 // TODO: We could just go ahead and merge OrigHeader into its predecessor
299 // at this point, if we don't mind updating dominator info.
301 // Establish a new preheader, update dominators, etc.
302 preserveCanonicalLoopForm(L, OrigHeader, OrigPreHeader, OrigLatch,
310 /// Update LoopInfo, DominatorTree, and DomFrontiers to reflect the CFG change
311 /// we just made. Then split edges as necessary to preserve LoopSimplify form.
312 void LoopRotate::preserveCanonicalLoopForm(Loop *L, BasicBlock *OrigHeader,
313 BasicBlock *OrigPreHeader,
314 BasicBlock *OrigLatch,
315 BasicBlock *NewHeader,
317 assert(L->getHeader() == NewHeader && "Latch block is our new header");
319 if (DominatorTree *DT = getAnalysisIfAvailable<DominatorTree>()) {
320 // Since OrigPreheader now has the conditional branch to Exit block, it is
321 // the dominator of Exit.
322 DT->changeImmediateDominator(Exit, OrigPreHeader);
323 DT->changeImmediateDominator(NewHeader, OrigPreHeader);
325 // Update OrigHeader to be dominated by the new header block.
326 DT->changeImmediateDominator(OrigHeader, OrigLatch);
329 // Right now OrigPreHeader has two successors, NewHeader and ExitBlock, and
330 // thus is not a preheader anymore. Split the edge to form a real preheader.
331 BasicBlock *NewPH = SplitCriticalEdge(OrigPreHeader, NewHeader, this);
332 NewPH->setName(NewHeader->getName() + ".lr.ph");
334 // Preserve canonical loop form, which means Exit block should have only one
336 SplitCriticalEdge(L->getLoopLatch(), Exit, this);
338 assert(NewHeader && L->getHeader() == NewHeader &&
339 "Invalid loop header after loop rotation");
340 assert(L->getLoopPreheader() == NewPH &&
341 "Invalid loop preheader after loop rotation");
342 assert(L->getLoopLatch() && "Invalid loop latch after loop rotation");