1 //===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements bookkeeping for "interesting" users of expressions
10 // computed from induction variables.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/Analysis/IVUsers.h"
15 #include "llvm/ADT/STLExtras.h"
16 #include "llvm/Analysis/AssumptionCache.h"
17 #include "llvm/Analysis/CodeMetrics.h"
18 #include "llvm/Analysis/LoopAnalysisManager.h"
19 #include "llvm/Analysis/LoopPass.h"
20 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
21 #include "llvm/Analysis/ValueTracking.h"
22 #include "llvm/Config/llvm-config.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/InitializePasses.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 IVUsers IVUsersAnalysis::run(Loop &L, LoopAnalysisManager &AM,
41                              LoopStandardAnalysisResults &AR) {
42   return IVUsers(&L, &AR.AC, &AR.LI, &AR.DT, &AR.SE);
43 }
44 
45 char IVUsersWrapperPass::ID = 0;
46 INITIALIZE_PASS_BEGIN(IVUsersWrapperPass, "iv-users",
47                       "Induction Variable Users", false, true)
48 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
49 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
50 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
51 INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
52 INITIALIZE_PASS_END(IVUsersWrapperPass, "iv-users", "Induction Variable Users",
53                     false, true)
54 
55 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 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     if (AR->getLoop() == L)
67       return AR->isAffine() ||
68              (!L->contains(I) &&
69               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     return isInteresting(AR->getStart(), I, L, SE, LI) &&
74           !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     for (const auto *Op : Add->operands())
81       if (isInteresting(Op, I, L, SE, LI)) {
82         if (AnyInterestingYet)
83           return false;
84         AnyInterestingYet = true;
85       }
86     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 static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
96                                  const LoopInfo *LI,
97                                  SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
98   Loop *NearestLoop = nullptr;
99   for (DomTreeNode *Rung = DT->getNode(BB);
100        Rung; Rung = Rung->getIDom()) {
101     BasicBlock *DomBB = Rung->getBlock();
102     Loop *DomLoop = LI->getLoopFor(DomBB);
103     if (DomLoop && DomLoop->getHeader() == DomBB) {
104       // If the domtree walk reaches a loop with no preheader, return false.
105       if (!DomLoop->isLoopSimplifyForm())
106         return false;
107       // If we have already checked this loop nest, stop checking.
108       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       if (!NearestLoop)
113         NearestLoop = DomLoop;
114     }
115   }
116   if (NearestLoop)
117     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 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   if (L->contains(User))
133     return false;
134 
135   BasicBlock *LatchBlock = L->getLoopLatch();
136   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   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   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   for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
156     if (PN->getIncomingValue(i) == Operand &&
157         !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 bool IVUsers::AddUsersImpl(Instruction *I,
169                            SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
170   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   if (!Processed.insert(I).second)
175     return true;    // Instruction already handled.
176 
177   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   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   uint64_t Width = SE->getTypeSizeInBits(I->getType());
190   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   if (EphValues.count(I))
196     return false;
197 
198   // Get the symbolic expression for this instruction.
199   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   if (!isInteresting(ISE, I, L, SE, LI))
204     return false;
205 
206   SmallPtrSet<Instruction *, 4> UniqueUsers;
207   for (Use &U : I->uses()) {
208     Instruction *User = cast<Instruction>(U.getUser());
209     if (!UniqueUsers.insert(User).second)
210       continue;
211 
212     // Do not infinitely recurse on PHI nodes.
213     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     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       unsigned OperandNo = U.getOperandNo();
222       unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
223       UseBB = PHI->getIncomingBlock(ValNo);
224     }
225     if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
226       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     if (LI->getLoopFor(User->getParent()) != L) {
236       if (isa<PHINode>(User) || Processed.count(User) ||
237           !AddUsersImpl(User, SimpleLoopNests)) {
238         LLVM_DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
239                           << "   OF SCEV: " << *ISE << '\n');
240         AddUserToIVUsers = true;
241       }
242     } 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       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       };
263 
264       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       if (OriginalISE != ISE) {
271         const SCEV *DenormalizedISE =
272             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         if (OriginalISE != DenormalizedISE) {
277           LLVM_DEBUG(dbgs()
278                      << "   DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "
279                      << *ISE << '\n');
280           IVUses.pop_back();
281           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 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   return AddUsersImpl(I, SimpleLoopNests);
298 }
299 
300 IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
301   IVUses.push_back(new IVStrideUse(this, User, Operand));
302   return IVUses.back();
303 }
304 
305 IVUsers::IVUsers(Loop *L, AssumptionCache *AC, LoopInfo *LI, DominatorTree *DT,
306                  ScalarEvolution *SE)
307     : L(L), AC(AC), LI(LI), DT(DT), SE(SE), IVUses() {
308   // Collect ephemeral values so that AddUsersIfInteresting skips them.
309   EphValues.clear();
310   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   for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
316     (void)AddUsersIfInteresting(&*I);
317 }
318 
319 void IVUsers::print(raw_ostream &OS, const Module *M) const {
320   OS << "IV Users for loop ";
321   L->getHeader()->printAsOperand(OS, false);
322   if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
323     OS << " with backedge-taken count " << *SE->getBackedgeTakenCount(L);
324   }
325   OS << ":\n";
326 
327   for (const IVStrideUse &IVUse : IVUses) {
328     OS << "  ";
329     IVUse.getOperandValToReplace()->printAsOperand(OS, false);
330     OS << " = " << *getReplacementExpr(IVUse);
331     for (auto PostIncLoop : IVUse.PostIncLoops) {
332       OS << " (post-inc with loop ";
333       PostIncLoop->getHeader()->printAsOperand(OS, false);
334       OS << ")";
335     }
336     OS << " in  ";
337     if (IVUse.getUser())
338       IVUse.getUser()->print(OS);
339     else
340       OS << "Printing <null> User";
341     OS << '\n';
342   }
343 }
344 
345 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
346 LLVM_DUMP_METHOD void IVUsers::dump() const { print(dbgs()); }
347 #endif
348 
349 void IVUsers::releaseMemory() {
350   Processed.clear();
351   IVUses.clear();
352 }
353 
354 IVUsersWrapperPass::IVUsersWrapperPass() : LoopPass(ID) {
355   initializeIVUsersWrapperPassPass(*PassRegistry::getPassRegistry());
356 }
357 
358 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 }
365 
366 bool IVUsersWrapperPass::runOnLoop(Loop *L, LPPassManager &LPM) {
367   auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
368       *L->getHeader()->getParent());
369   auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
370   auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
371   auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
372 
373   IU.reset(new IVUsers(L, AC, LI, DT, SE));
374   return false;
375 }
376 
377 void IVUsersWrapperPass::print(raw_ostream &OS, const Module *M) const {
378   IU->print(OS, M);
379 }
380 
381 void IVUsersWrapperPass::releaseMemory() { IU->releaseMemory(); }
382 
383 /// getReplacementExpr - Return a SCEV expression which computes the
384 /// value of the OperandValToReplace.
385 const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
386   return SE->getSCEV(IU.getOperandValToReplace());
387 }
388 
389 /// getExpr - Return the expression for the use.
390 const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
391   return normalizeForPostIncUse(getReplacementExpr(IU), IU.getPostIncLoops(),
392                                 *SE);
393 }
394 
395 static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
396   if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
397     if (AR->getLoop() == L)
398       return AR;
399     return findAddRecForLoop(AR->getStart(), L);
400   }
401 
402   if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
403     for (const auto *Op : Add->operands())
404       if (const SCEVAddRecExpr *AR = findAddRecForLoop(Op, L))
405         return AR;
406     return nullptr;
407   }
408 
409   return nullptr;
410 }
411 
412 const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
413   if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
414     return AR->getStepRecurrence(*SE);
415   return nullptr;
416 }
417 
418 void IVStrideUse::transformToPostInc(const Loop *L) {
419   PostIncLoops.insert(L);
420 }
421 
422 void IVStrideUse::deleted() {
423   // Remove this user from the list.
424   Parent->Processed.erase(this->getUser());
425   Parent->IVUses.erase(this);
426   // this now dangles!
427 }
428