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1 : //===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===//
2 : //
3 : // The LLVM Compiler Infrastructure
4 : //
5 : // This file is distributed under the University of Illinois Open Source
6 : // License. See LICENSE.TXT for details.
7 : //
8 : //===----------------------------------------------------------------------===//
9 : //
10 : // This file implements bookkeeping for "interesting" users of expressions
11 : // computed from induction variables.
12 : //
13 : //===----------------------------------------------------------------------===//
14 :
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/Config/llvm-config.h"
24 : #include "llvm/IR/Constants.h"
25 : #include "llvm/IR/DataLayout.h"
26 : #include "llvm/IR/DerivedTypes.h"
27 : #include "llvm/IR/Dominators.h"
28 : #include "llvm/IR/Instructions.h"
29 : #include "llvm/IR/Module.h"
30 : #include "llvm/IR/Type.h"
31 : #include "llvm/Support/Debug.h"
32 : #include "llvm/Support/raw_ostream.h"
33 : #include <algorithm>
34 : using namespace llvm;
35 :
36 : #define DEBUG_TYPE "iv-users"
37 :
38 : AnalysisKey IVUsersAnalysis::Key;
39 :
40 12 : IVUsers IVUsersAnalysis::run(Loop &L, LoopAnalysisManager &AM,
41 : LoopStandardAnalysisResults &AR) {
42 12 : return IVUsers(&L, &AR.AC, &AR.LI, &AR.DT, &AR.SE);
43 : }
44 :
45 : char IVUsersWrapperPass::ID = 0;
46 33048 : INITIALIZE_PASS_BEGIN(IVUsersWrapperPass, "iv-users",
47 : "Induction Variable Users", false, true)
48 33048 : INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
49 33048 : INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
50 33048 : INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
51 33048 : INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
52 97035 : INITIALIZE_PASS_END(IVUsersWrapperPass, "iv-users", "Induction Variable Users",
53 : false, true)
54 :
55 0 : Pass *llvm::createIVUsersPass() { return new IVUsersWrapperPass(); }
56 :
57 : /// isInteresting - Test whether the given expression is "interesting" when
58 : /// used by the given expression, within the context of analyzing the
59 : /// given loop.
60 33840 : static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
61 : ScalarEvolution *SE, LoopInfo *LI) {
62 : // An addrec is interesting if it's affine or if it has an interesting start.
63 : if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
64 : // Keep things simple. Don't touch loop-variant strides unless they're
65 : // only used outside the loop and we can simplify them.
66 26896 : if (AR->getLoop() == L)
67 26774 : return AR->isAffine() ||
68 7 : (!L->contains(I) &&
69 14 : SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
70 : // Otherwise recurse to see if the start value is interesting, and that
71 : // the step value is not interesting, since we don't yet know how to
72 : // do effective SCEV expansions for addrecs with interesting steps.
73 455 : return isInteresting(AR->getStart(), I, L, SE, LI) &&
74 151 : !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
75 : }
76 :
77 : // An add is interesting if exactly one of its operands is interesting.
78 : if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
79 : bool AnyInterestingYet = false;
80 3041 : for (const auto *Op : Add->operands())
81 2191 : if (isInteresting(Op, I, L, SE, LI)) {
82 810 : if (AnyInterestingYet)
83 : return false;
84 : AnyInterestingYet = true;
85 : }
86 850 : return AnyInterestingYet;
87 : }
88 :
89 : // Nothing else is interesting here.
90 : return false;
91 : }
92 :
93 : /// Return true if all loop headers that dominate this block are in simplified
94 : /// form.
95 40162 : static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
96 : const LoopInfo *LI,
97 : SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
98 : Loop *NearestLoop = nullptr;
99 79005 : for (DomTreeNode *Rung = DT->getNode(BB);
100 119167 : Rung; Rung = Rung->getIDom()) {
101 112971 : BasicBlock *DomBB = Rung->getBlock();
102 : Loop *DomLoop = LI->getLoopFor(DomBB);
103 85448 : if (DomLoop && DomLoop->getHeader() == DomBB) {
104 : // If the domtree walk reaches a loop with no preheader, return false.
105 44486 : if (!DomLoop->isLoopSimplifyForm())
106 : return false;
107 : // If we have already checked this loop nest, stop checking.
108 44452 : if (SimpleLoopNests.count(DomLoop))
109 : break;
110 : // If we have not already checked this loop nest, remember the loop
111 : // header nearest to BB. The nearest loop may not contain BB.
112 10520 : if (!NearestLoop)
113 : NearestLoop = DomLoop;
114 : }
115 : }
116 40128 : if (NearestLoop)
117 6396 : SimpleLoopNests.insert(NearestLoop);
118 : return true;
119 : }
120 :
121 : /// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression
122 : /// and now we need to decide whether the user should use the preinc or post-inc
123 : /// value. If this user should use the post-inc version of the IV, return true.
124 : ///
125 : /// Choosing wrong here can break dominance properties (if we choose to use the
126 : /// post-inc value when we cannot) or it can end up adding extra live-ranges to
127 : /// the loop, resulting in reg-reg copies (if we use the pre-inc value when we
128 : /// should use the post-inc value).
129 20407 : static bool IVUseShouldUsePostIncValue(Instruction *User, Value *Operand,
130 : const Loop *L, DominatorTree *DT) {
131 : // If the user is in the loop, use the preinc value.
132 20407 : if (L->contains(User))
133 : return false;
134 :
135 2281 : BasicBlock *LatchBlock = L->getLoopLatch();
136 2281 : if (!LatchBlock)
137 : return false;
138 :
139 : // Ok, the user is outside of the loop. If it is dominated by the latch
140 : // block, use the post-inc value.
141 2281 : if (DT->dominates(LatchBlock, User->getParent()))
142 : return true;
143 :
144 : // There is one case we have to be careful of: PHI nodes. These little guys
145 : // can live in blocks that are not dominated by the latch block, but (since
146 : // their uses occur in the predecessor block, not the block the PHI lives in)
147 : // should still use the post-inc value. Check for this case now.
148 : PHINode *PN = dyn_cast<PHINode>(User);
149 1784 : if (!PN || !Operand)
150 : return false; // not a phi, not dominated by latch block.
151 :
152 : // Look at all of the uses of Operand by the PHI node. If any use corresponds
153 : // to a block that is not dominated by the latch block, give up and use the
154 : // preincremented value.
155 3004 : for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
156 3445 : if (PN->getIncomingValue(i) == Operand &&
157 1096 : !DT->dominates(LatchBlock, PN->getIncomingBlock(i)))
158 : return false;
159 :
160 : // Okay, all uses of Operand by PN are in predecessor blocks that really are
161 : // dominated by the latch block. Use the post-incremented value.
162 : return true;
163 : }
164 :
165 : /// AddUsersImpl - Inspect the specified instruction. If it is a
166 : /// reducible SCEV, recursively add its users to the IVUsesByStride set and
167 : /// return true. Otherwise, return false.
168 50758 : bool IVUsers::AddUsersImpl(Instruction *I,
169 : SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
170 50758 : const DataLayout &DL = I->getModule()->getDataLayout();
171 :
172 : // Add this IV user to the Processed set before returning false to ensure that
173 : // all IV users are members of the set. See IVUsers::isIVUserOrOperand.
174 50758 : if (!Processed.insert(I).second)
175 : return true; // Instruction already handled.
176 :
177 50718 : if (!SE->isSCEVable(I->getType()))
178 : return false; // Void and FP expressions cannot be reduced.
179 :
180 : // IVUsers is used by LSR which assumes that all SCEV expressions are safe to
181 : // pass to SCEVExpander. Expressions are not safe to expand if they represent
182 : // operations that are not safe to speculate, namely integer division.
183 83686 : if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I))
184 : return false;
185 :
186 : // LSR is not APInt clean, do not touch integers bigger than 64-bits.
187 : // Also avoid creating IVs of non-native types. For example, we don't want a
188 : // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
189 36991 : uint64_t Width = SE->getTypeSizeInBits(I->getType());
190 73960 : if (Width > 64 || !DL.isLegalInteger(Width))
191 : return false;
192 :
193 : // Don't attempt to promote ephemeral values to indvars. They will be removed
194 : // later anyway.
195 31347 : if (EphValues.count(I))
196 : return false;
197 :
198 : // Get the symbolic expression for this instruction.
199 31346 : const SCEV *ISE = SE->getSCEV(I);
200 :
201 : // If we've come to an uninteresting expression, stop the traversal and
202 : // call this a user.
203 31346 : if (!isInteresting(ISE, I, L, SE, LI))
204 : return false;
205 :
206 : SmallPtrSet<Instruction *, 4> UniqueUsers;
207 72989 : for (Use &U : I->uses()) {
208 46315 : Instruction *User = cast<Instruction>(U.getUser());
209 46315 : if (!UniqueUsers.insert(User).second)
210 6153 : continue;
211 :
212 : // Do not infinitely recurse on PHI nodes.
213 92434 : if (isa<PHINode>(User) && Processed.count(User))
214 : continue;
215 :
216 : // Only consider IVUsers that are dominated by simplified loop
217 : // headers. Otherwise, SCEVExpander will crash.
218 40162 : BasicBlock *UseBB = User->getParent();
219 : // A phi's use is live out of its predecessor block.
220 : if (PHINode *PHI = dyn_cast<PHINode>(User)) {
221 1485 : unsigned OperandNo = U.getOperandNo();
222 : unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
223 : UseBB = PHI->getIncomingBlock(ValNo);
224 : }
225 40162 : if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
226 40 : return false;
227 :
228 : // Descend recursively, but not into PHI nodes outside the current loop.
229 : // It's important to see the entire expression outside the loop to get
230 : // choices that depend on addressing mode use right, although we won't
231 : // consider references outside the loop in all cases.
232 : // If User is already in Processed, we don't want to recurse into it again,
233 : // but do want to record a second reference in the same instruction.
234 : bool AddUserToIVUsers = false;
235 80256 : if (LI->getLoopFor(User->getParent()) != L) {
236 9036 : if (isa<PHINode>(User) || Processed.count(User) ||
237 2138 : !AddUsersImpl(User, SimpleLoopNests)) {
238 : LLVM_DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
239 : << " OF SCEV: " << *ISE << '\n');
240 : AddUserToIVUsers = true;
241 : }
242 36679 : } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
243 : LLVM_DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
244 : << " OF SCEV: " << *ISE << '\n');
245 : AddUserToIVUsers = true;
246 : }
247 :
248 : if (AddUserToIVUsers) {
249 : // Okay, we found a user that we cannot reduce.
250 19458 : IVStrideUse &NewUse = AddUser(User, I);
251 : // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
252 : // The regular return value here is discarded; instead of recording
253 : // it, we just recompute it when we need it.
254 : const SCEV *OriginalISE = ISE;
255 :
256 : auto NormalizePred = [&](const SCEVAddRecExpr *AR) {
257 : auto *L = AR->getLoop();
258 : bool Result = IVUseShouldUsePostIncValue(User, I, L, DT);
259 : if (Result)
260 : NewUse.PostIncLoops.insert(L);
261 : return Result;
262 19458 : };
263 :
264 38916 : ISE = normalizeForPostIncUseIf(ISE, NormalizePred, *SE);
265 :
266 : // PostIncNormalization effectively simplifies the expression under
267 : // pre-increment assumptions. Those assumptions (no wrapping) might not
268 : // hold for the post-inc value. Catch such cases by making sure the
269 : // transformation is invertible.
270 19458 : if (OriginalISE != ISE) {
271 : const SCEV *DenormalizedISE =
272 1123 : denormalizeForPostIncUse(ISE, NewUse.PostIncLoops, *SE);
273 :
274 : // If we normalized the expression, but denormalization doesn't give the
275 : // original one, discard this user.
276 1123 : if (OriginalISE != DenormalizedISE) {
277 : LLVM_DEBUG(dbgs()
278 : << " DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "
279 : << *ISE << '\n');
280 : IVUses.pop_back();
281 6 : return false;
282 : }
283 : }
284 : LLVM_DEBUG(if (SE->getSCEV(I) != ISE) dbgs()
285 : << " NORMALIZED TO: " << *ISE << '\n');
286 : }
287 : }
288 : return true;
289 : }
290 :
291 12317 : bool IVUsers::AddUsersIfInteresting(Instruction *I) {
292 : // SCEVExpander can only handle users that are dominated by simplified loop
293 : // entries. Keep track of all loops that are only dominated by other simple
294 : // loops so we don't traverse the domtree for each user.
295 : SmallPtrSet<Loop*,16> SimpleLoopNests;
296 :
297 12317 : return AddUsersImpl(I, SimpleLoopNests);
298 : }
299 :
300 19471 : IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
301 19471 : IVUses.push_back(new IVStrideUse(this, User, Operand));
302 19471 : return IVUses.back();
303 : }
304 :
305 8078 : IVUsers::IVUsers(Loop *L, AssumptionCache *AC, LoopInfo *LI, DominatorTree *DT,
306 8078 : ScalarEvolution *SE)
307 8078 : : L(L), AC(AC), LI(LI), DT(DT), SE(SE), IVUses() {
308 : // Collect ephemeral values so that AddUsersIfInteresting skips them.
309 8078 : EphValues.clear();
310 8078 : CodeMetrics::collectEphemeralValues(L, AC, EphValues);
311 :
312 : // Find all uses of induction variables in this loop, and categorize
313 : // them by stride. Start by finding all of the PHI nodes in the header for
314 : // this loop. If they are induction variables, inspect their uses.
315 20395 : for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
316 12317 : (void)AddUsersIfInteresting(&*I);
317 8078 : }
318 :
319 23 : void IVUsers::print(raw_ostream &OS, const Module *M) const {
320 23 : OS << "IV Users for loop ";
321 46 : L->getHeader()->printAsOperand(OS, false);
322 23 : if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
323 19 : OS << " with backedge-taken count " << *SE->getBackedgeTakenCount(L);
324 : }
325 23 : OS << ":\n";
326 :
327 43 : for (const IVStrideUse &IVUse : IVUses) {
328 20 : OS << " ";
329 20 : IVUse.getOperandValToReplace()->printAsOperand(OS, false);
330 20 : OS << " = " << *getReplacementExpr(IVUse);
331 33 : for (auto PostIncLoop : IVUse.PostIncLoops) {
332 13 : OS << " (post-inc with loop ";
333 13 : PostIncLoop->getHeader()->printAsOperand(OS, false);
334 13 : OS << ")";
335 : }
336 20 : OS << " in ";
337 20 : if (IVUse.getUser())
338 20 : IVUse.getUser()->print(OS);
339 : else
340 0 : OS << "Printing <null> User";
341 : OS << '\n';
342 : }
343 23 : }
344 :
345 : #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
346 : LLVM_DUMP_METHOD void IVUsers::dump() const { print(dbgs()); }
347 : #endif
348 :
349 8066 : void IVUsers::releaseMemory() {
350 8066 : Processed.clear();
351 : IVUses.clear();
352 8066 : }
353 :
354 40366 : IVUsersWrapperPass::IVUsersWrapperPass() : LoopPass(ID) {
355 20183 : initializeIVUsersWrapperPassPass(*PassRegistry::getPassRegistry());
356 20183 : }
357 :
358 20183 : void IVUsersWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
359 : AU.addRequired<AssumptionCacheTracker>();
360 : AU.addRequired<LoopInfoWrapperPass>();
361 : AU.addRequired<DominatorTreeWrapperPass>();
362 : AU.addRequired<ScalarEvolutionWrapperPass>();
363 : AU.setPreservesAll();
364 20183 : }
365 :
366 8066 : bool IVUsersWrapperPass::runOnLoop(Loop *L, LPPassManager &LPM) {
367 8066 : auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
368 8066 : *L->getHeader()->getParent());
369 8066 : auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
370 8066 : auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
371 8066 : auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
372 :
373 8066 : IU.reset(new IVUsers(L, AC, LI, DT, SE));
374 8066 : return false;
375 : }
376 :
377 18 : void IVUsersWrapperPass::print(raw_ostream &OS, const Module *M) const {
378 18 : IU->print(OS, M);
379 18 : }
380 :
381 16132 : void IVUsersWrapperPass::releaseMemory() { IU->releaseMemory(); }
382 :
383 : /// getReplacementExpr - Return a SCEV expression which computes the
384 : /// value of the OperandValToReplace.
385 33614 : const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
386 67228 : return SE->getSCEV(IU.getOperandValToReplace());
387 : }
388 :
389 : /// getExpr - Return the expression for the use.
390 33594 : const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
391 33594 : return normalizeForPostIncUse(getReplacementExpr(IU), IU.getPostIncLoops(),
392 33594 : *SE);
393 : }
394 :
395 491 : static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
396 : if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
397 489 : if (AR->getLoop() == L)
398 : return AR;
399 6 : return findAddRecForLoop(AR->getStart(), L);
400 : }
401 :
402 : if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
403 5 : for (const auto *Op : Add->operands())
404 5 : if (const SCEVAddRecExpr *AR = findAddRecForLoop(Op, L))
405 5 : return AR;
406 : return nullptr;
407 : }
408 :
409 : return nullptr;
410 : }
411 :
412 486 : const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
413 486 : if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
414 486 : return AR->getStepRecurrence(*SE);
415 : return nullptr;
416 : }
417 :
418 3508 : void IVStrideUse::transformToPostInc(const Loop *L) {
419 3508 : PostIncLoops.insert(L);
420 3508 : }
421 :
422 39 : void IVStrideUse::deleted() {
423 : // Remove this user from the list.
424 39 : Parent->Processed.erase(this->getUser());
425 : Parent->IVUses.erase(this);
426 : // this now dangles!
427 39 : }
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