1 //===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===//
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 bookkeeping for "interesting" users of expressions
11 // computed from induction variables.
13 //===----------------------------------------------------------------------===//
15 #include "llvm/Analysis/IVUsers.h"
16 #include "llvm/ADT/STLExtras.h"
17 #include "llvm/Analysis/AssumptionCache.h"
18 #include "llvm/Analysis/CodeMetrics.h"
19 #include "llvm/Analysis/LoopAnalysisManager.h"
20 #include "llvm/Analysis/LoopPass.h"
21 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
22 #include "llvm/Analysis/ValueTracking.h"
23 #include "llvm/IR/Constants.h"
24 #include "llvm/IR/DataLayout.h"
25 #include "llvm/IR/DerivedTypes.h"
26 #include "llvm/IR/Dominators.h"
27 #include "llvm/IR/Instructions.h"
28 #include "llvm/IR/Module.h"
29 #include "llvm/IR/Type.h"
30 #include "llvm/Support/Debug.h"
31 #include "llvm/Support/raw_ostream.h"
35 #define DEBUG_TYPE "iv-users"
37 AnalysisKey IVUsersAnalysis::Key;
39 IVUsers IVUsersAnalysis::run(Loop &L, LoopAnalysisManager &AM,
40 LoopStandardAnalysisResults &AR) {
41 return IVUsers(&L, &AR.AC, &AR.LI, &AR.DT, &AR.SE);
44 char IVUsersWrapperPass::ID = 0;
45 INITIALIZE_PASS_BEGIN(IVUsersWrapperPass, "iv-users",
46 "Induction Variable Users", false, true)
47 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
48 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
49 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
50 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
51 INITIALIZE_PASS_END(IVUsersWrapperPass, "iv-users", "Induction Variable Users",
54 Pass *llvm::createIVUsersPass() { return new IVUsersWrapperPass(); }
56 /// isInteresting - Test whether the given expression is "interesting" when
57 /// used by the given expression, within the context of analyzing the
59 static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
60 ScalarEvolution *SE, LoopInfo *LI) {
61 // An addrec is interesting if it's affine or if it has an interesting start.
62 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
63 // Keep things simple. Don't touch loop-variant strides unless they're
64 // only used outside the loop and we can simplify them.
65 if (AR->getLoop() == L)
66 return AR->isAffine() ||
68 SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
69 // Otherwise recurse to see if the start value is interesting, and that
70 // the step value is not interesting, since we don't yet know how to
71 // do effective SCEV expansions for addrecs with interesting steps.
72 return isInteresting(AR->getStart(), I, L, SE, LI) &&
73 !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
76 // An add is interesting if exactly one of its operands is interesting.
77 if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
78 bool AnyInterestingYet = false;
79 for (const auto *Op : Add->operands())
80 if (isInteresting(Op, I, L, SE, LI)) {
81 if (AnyInterestingYet)
83 AnyInterestingYet = true;
85 return AnyInterestingYet;
88 // Nothing else is interesting here.
92 /// Return true if all loop headers that dominate this block are in simplified
94 static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
96 SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
97 Loop *NearestLoop = nullptr;
98 for (DomTreeNode *Rung = DT->getNode(BB);
99 Rung; Rung = Rung->getIDom()) {
100 BasicBlock *DomBB = Rung->getBlock();
101 Loop *DomLoop = LI->getLoopFor(DomBB);
102 if (DomLoop && DomLoop->getHeader() == DomBB) {
103 // If the domtree walk reaches a loop with no preheader, return false.
104 if (!DomLoop->isLoopSimplifyForm())
106 // If we have already checked this loop nest, stop checking.
107 if (SimpleLoopNests.count(DomLoop))
109 // If we have not already checked this loop nest, remember the loop
110 // header nearest to BB. The nearest loop may not contain BB.
112 NearestLoop = DomLoop;
116 SimpleLoopNests.insert(NearestLoop);
120 /// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression
121 /// and now we need to decide whether the user should use the preinc or post-inc
122 /// value. If this user should use the post-inc version of the IV, return true.
124 /// Choosing wrong here can break dominance properties (if we choose to use the
125 /// post-inc value when we cannot) or it can end up adding extra live-ranges to
126 /// the loop, resulting in reg-reg copies (if we use the pre-inc value when we
127 /// should use the post-inc value).
128 static bool IVUseShouldUsePostIncValue(Instruction *User, Value *Operand,
129 const Loop *L, DominatorTree *DT) {
130 // If the user is in the loop, use the preinc value.
131 if (L->contains(User))
134 BasicBlock *LatchBlock = L->getLoopLatch();
138 // Ok, the user is outside of the loop. If it is dominated by the latch
139 // block, use the post-inc value.
140 if (DT->dominates(LatchBlock, User->getParent()))
143 // There is one case we have to be careful of: PHI nodes. These little guys
144 // can live in blocks that are not dominated by the latch block, but (since
145 // their uses occur in the predecessor block, not the block the PHI lives in)
146 // should still use the post-inc value. Check for this case now.
147 PHINode *PN = dyn_cast<PHINode>(User);
149 return false; // not a phi, not dominated by latch block.
151 // Look at all of the uses of Operand by the PHI node. If any use corresponds
152 // to a block that is not dominated by the latch block, give up and use the
153 // preincremented value.
154 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
155 if (PN->getIncomingValue(i) == Operand &&
156 !DT->dominates(LatchBlock, PN->getIncomingBlock(i)))
159 // Okay, all uses of Operand by PN are in predecessor blocks that really are
160 // dominated by the latch block. Use the post-incremented value.
164 /// AddUsersImpl - Inspect the specified instruction. If it is a
165 /// reducible SCEV, recursively add its users to the IVUsesByStride set and
166 /// return true. Otherwise, return false.
167 bool IVUsers::AddUsersImpl(Instruction *I,
168 SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
169 const DataLayout &DL = I->getModule()->getDataLayout();
171 // Add this IV user to the Processed set before returning false to ensure that
172 // all IV users are members of the set. See IVUsers::isIVUserOrOperand.
173 if (!Processed.insert(I).second)
174 return true; // Instruction already handled.
176 if (!SE->isSCEVable(I->getType()))
177 return false; // Void and FP expressions cannot be reduced.
179 // IVUsers is used by LSR which assumes that all SCEV expressions are safe to
180 // pass to SCEVExpander. Expressions are not safe to expand if they represent
181 // operations that are not safe to speculate, namely integer division.
182 if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I))
185 // LSR is not APInt clean, do not touch integers bigger than 64-bits.
186 // Also avoid creating IVs of non-native types. For example, we don't want a
187 // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
188 uint64_t Width = SE->getTypeSizeInBits(I->getType());
189 if (Width > 64 || !DL.isLegalInteger(Width))
192 // Don't attempt to promote ephemeral values to indvars. They will be removed
194 if (EphValues.count(I))
197 // Get the symbolic expression for this instruction.
198 const SCEV *ISE = SE->getSCEV(I);
200 // If we've come to an uninteresting expression, stop the traversal and
202 if (!isInteresting(ISE, I, L, SE, LI))
205 SmallPtrSet<Instruction *, 4> UniqueUsers;
206 for (Use &U : I->uses()) {
207 Instruction *User = cast<Instruction>(U.getUser());
208 if (!UniqueUsers.insert(User).second)
211 // Do not infinitely recurse on PHI nodes.
212 if (isa<PHINode>(User) && Processed.count(User))
215 // Only consider IVUsers that are dominated by simplified loop
216 // headers. Otherwise, SCEVExpander will crash.
217 BasicBlock *UseBB = User->getParent();
218 // A phi's use is live out of its predecessor block.
219 if (PHINode *PHI = dyn_cast<PHINode>(User)) {
220 unsigned OperandNo = U.getOperandNo();
221 unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
222 UseBB = PHI->getIncomingBlock(ValNo);
224 if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
227 // Descend recursively, but not into PHI nodes outside the current loop.
228 // It's important to see the entire expression outside the loop to get
229 // choices that depend on addressing mode use right, although we won't
230 // consider references outside the loop in all cases.
231 // If User is already in Processed, we don't want to recurse into it again,
232 // but do want to record a second reference in the same instruction.
233 bool AddUserToIVUsers = false;
234 if (LI->getLoopFor(User->getParent()) != L) {
235 if (isa<PHINode>(User) || Processed.count(User) ||
236 !AddUsersImpl(User, SimpleLoopNests)) {
237 DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
238 << " OF SCEV: " << *ISE << '\n');
239 AddUserToIVUsers = true;
241 } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
242 DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
243 << " OF SCEV: " << *ISE << '\n');
244 AddUserToIVUsers = true;
247 if (AddUserToIVUsers) {
248 // Okay, we found a user that we cannot reduce.
249 IVStrideUse &NewUse = AddUser(User, I);
250 // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
251 // The regular return value here is discarded; instead of recording
252 // it, we just recompute it when we need it.
253 const SCEV *OriginalISE = ISE;
255 auto NormalizePred = [&](const SCEVAddRecExpr *AR) {
256 auto *L = AR->getLoop();
257 bool Result = IVUseShouldUsePostIncValue(User, I, L, DT);
259 NewUse.PostIncLoops.insert(L);
263 ISE = normalizeForPostIncUseIf(ISE, NormalizePred, *SE);
265 // PostIncNormalization effectively simplifies the expression under
266 // pre-increment assumptions. Those assumptions (no wrapping) might not
267 // hold for the post-inc value. Catch such cases by making sure the
268 // transformation is invertible.
269 if (OriginalISE != ISE) {
270 const SCEV *DenormalizedISE =
271 denormalizeForPostIncUse(ISE, NewUse.PostIncLoops, *SE);
273 // If we normalized the expression, but denormalization doesn't give the
274 // original one, discard this user.
275 if (OriginalISE != DenormalizedISE) {
276 DEBUG(dbgs() << " DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "
282 DEBUG(if (SE->getSCEV(I) != ISE)
283 dbgs() << " NORMALIZED TO: " << *ISE << '\n');
289 bool IVUsers::AddUsersIfInteresting(Instruction *I) {
290 // SCEVExpander can only handle users that are dominated by simplified loop
291 // entries. Keep track of all loops that are only dominated by other simple
292 // loops so we don't traverse the domtree for each user.
293 SmallPtrSet<Loop*,16> SimpleLoopNests;
295 return AddUsersImpl(I, SimpleLoopNests);
298 IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
299 IVUses.push_back(new IVStrideUse(this, User, Operand));
300 return IVUses.back();
303 IVUsers::IVUsers(Loop *L, AssumptionCache *AC, LoopInfo *LI, DominatorTree *DT,
305 : L(L), AC(AC), LI(LI), DT(DT), SE(SE), IVUses() {
306 // Collect ephemeral values so that AddUsersIfInteresting skips them.
308 CodeMetrics::collectEphemeralValues(L, AC, EphValues);
310 // Find all uses of induction variables in this loop, and categorize
311 // them by stride. Start by finding all of the PHI nodes in the header for
312 // this loop. If they are induction variables, inspect their uses.
313 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
314 (void)AddUsersIfInteresting(&*I);
317 void IVUsers::print(raw_ostream &OS, const Module *M) const {
318 OS << "IV Users for loop ";
319 L->getHeader()->printAsOperand(OS, false);
320 if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
321 OS << " with backedge-taken count " << *SE->getBackedgeTakenCount(L);
325 for (const IVStrideUse &IVUse : IVUses) {
327 IVUse.getOperandValToReplace()->printAsOperand(OS, false);
328 OS << " = " << *getReplacementExpr(IVUse);
329 for (auto PostIncLoop : IVUse.PostIncLoops) {
330 OS << " (post-inc with loop ";
331 PostIncLoop->getHeader()->printAsOperand(OS, false);
336 IVUse.getUser()->print(OS);
338 OS << "Printing <null> User";
343 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
344 LLVM_DUMP_METHOD void IVUsers::dump() const { print(dbgs()); }
347 void IVUsers::releaseMemory() {
352 IVUsersWrapperPass::IVUsersWrapperPass() : LoopPass(ID) {
353 initializeIVUsersWrapperPassPass(*PassRegistry::getPassRegistry());
356 void IVUsersWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
357 AU.addRequired<AssumptionCacheTracker>();
358 AU.addRequired<LoopInfoWrapperPass>();
359 AU.addRequired<DominatorTreeWrapperPass>();
360 AU.addRequired<ScalarEvolutionWrapperPass>();
361 AU.setPreservesAll();
364 bool IVUsersWrapperPass::runOnLoop(Loop *L, LPPassManager &LPM) {
365 auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
366 *L->getHeader()->getParent());
367 auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
368 auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
369 auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
371 IU.reset(new IVUsers(L, AC, LI, DT, SE));
375 void IVUsersWrapperPass::print(raw_ostream &OS, const Module *M) const {
379 void IVUsersWrapperPass::releaseMemory() { IU->releaseMemory(); }
381 /// getReplacementExpr - Return a SCEV expression which computes the
382 /// value of the OperandValToReplace.
383 const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
384 return SE->getSCEV(IU.getOperandValToReplace());
387 /// getExpr - Return the expression for the use.
388 const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
389 return normalizeForPostIncUse(getReplacementExpr(IU), IU.getPostIncLoops(),
393 static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
394 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
395 if (AR->getLoop() == L)
397 return findAddRecForLoop(AR->getStart(), L);
400 if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
401 for (const auto *Op : Add->operands())
402 if (const SCEVAddRecExpr *AR = findAddRecForLoop(Op, L))
410 const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
411 if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
412 return AR->getStepRecurrence(*SE);
416 void IVStrideUse::transformToPostInc(const Loop *L) {
417 PostIncLoops.insert(L);
420 void IVStrideUse::deleted() {
421 // Remove this user from the list.
422 Parent->Processed.erase(this->getUser());
423 Parent->IVUses.erase(this);