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/ADT/STLExtras.h"
16#include "llvm/Analysis/AssumptionCache.h"
17#include "llvm/Analysis/CodeMetrics.h"
18#include "llvm/Analysis/IVUsers.h"
19#include "llvm/Analysis/LoopPass.h"
20#include "llvm/Analysis/ScalarEvolutionExpressions.h"
21#include "llvm/Analysis/ValueTracking.h"
22#include "llvm/IR/Constants.h"
23#include "llvm/IR/DataLayout.h"
24#include "llvm/IR/DerivedTypes.h"
25#include "llvm/IR/Dominators.h"
26#include "llvm/IR/Instructions.h"
27#include "llvm/IR/Module.h"
28#include "llvm/IR/Type.h"
29#include "llvm/Support/Debug.h"
30#include "llvm/Support/raw_ostream.h"
31#include <algorithm>
32using namespace llvm;
33
34#define DEBUG_TYPE "iv-users"
35
36char IVUsers::ID = 0;
37INITIALIZE_PASS_BEGIN(IVUsers, "iv-users",
38                      "Induction Variable Users", false, true)
39INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
40INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
41INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
42INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
43INITIALIZE_PASS_END(IVUsers, "iv-users",
44                      "Induction Variable Users", false, true)
45
46Pass *llvm::createIVUsersPass() {
47  return new IVUsers();
48}
49
50/// isInteresting - Test whether the given expression is "interesting" when
51/// used by the given expression, within the context of analyzing the
52/// given loop.
53static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
54                          ScalarEvolution *SE, LoopInfo *LI) {
55  // An addrec is interesting if it's affine or if it has an interesting start.
56  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
57    // Keep things simple. Don't touch loop-variant strides unless they're
58    // only used outside the loop and we can simplify them.
59    if (AR->getLoop() == L)
60      return AR->isAffine() ||
61             (!L->contains(I) &&
62              SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
63    // Otherwise recurse to see if the start value is interesting, and that
64    // the step value is not interesting, since we don't yet know how to
65    // do effective SCEV expansions for addrecs with interesting steps.
66    return isInteresting(AR->getStart(), I, L, SE, LI) &&
67          !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
68  }
69
70  // An add is interesting if exactly one of its operands is interesting.
71  if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
72    bool AnyInterestingYet = false;
73    for (SCEVAddExpr::op_iterator OI = Add->op_begin(), OE = Add->op_end();
74         OI != OE; ++OI)
75      if (isInteresting(*OI, I, L, SE, LI)) {
76        if (AnyInterestingYet)
77          return false;
78        AnyInterestingYet = true;
79      }
80    return AnyInterestingYet;
81  }
82
83  // Nothing else is interesting here.
84  return false;
85}
86
87/// Return true if all loop headers that dominate this block are in simplified
88/// form.
89static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
90                                 const LoopInfo *LI,
91                                 SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
92  Loop *NearestLoop = nullptr;
93  for (DomTreeNode *Rung = DT->getNode(BB);
94       Rung; Rung = Rung->getIDom()) {
95    BasicBlock *DomBB = Rung->getBlock();
96    Loop *DomLoop = LI->getLoopFor(DomBB);
97    if (DomLoop && DomLoop->getHeader() == DomBB) {
98      // If the domtree walk reaches a loop with no preheader, return false.
99      if (!DomLoop->isLoopSimplifyForm())
100        return false;
101      // If we have already checked this loop nest, stop checking.
102      if (SimpleLoopNests.count(DomLoop))
103        break;
104      // If we have not already checked this loop nest, remember the loop
105      // header nearest to BB. The nearest loop may not contain BB.
106      if (!NearestLoop)
107        NearestLoop = DomLoop;
108    }
109  }
110  if (NearestLoop)
111    SimpleLoopNests.insert(NearestLoop);
112  return true;
113}
114
115/// AddUsersImpl - Inspect the specified instruction.  If it is a
116/// reducible SCEV, recursively add its users to the IVUsesByStride set and
117/// return true.  Otherwise, return false.
118bool IVUsers::AddUsersImpl(Instruction *I,
119                           SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
120  const DataLayout &DL = I->getModule()->getDataLayout();
121
122  // Add this IV user to the Processed set before returning false to ensure that
123  // all IV users are members of the set. See IVUsers::isIVUserOrOperand.
124  if (!Processed.insert(I).second)
125    return true;    // Instruction already handled.
126
127  if (!SE->isSCEVable(I->getType()))
128    return false;   // Void and FP expressions cannot be reduced.
129
130  // IVUsers is used by LSR which assumes that all SCEV expressions are safe to
131  // pass to SCEVExpander. Expressions are not safe to expand if they represent
132  // operations that are not safe to speculate, namely integer division.
133  if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I))
134    return false;
135
136  // LSR is not APInt clean, do not touch integers bigger than 64-bits.
137  // Also avoid creating IVs of non-native types. For example, we don't want a
138  // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
139  uint64_t Width = SE->getTypeSizeInBits(I->getType());
140  if (Width > 64 || !DL.isLegalInteger(Width))
141    return false;
142
143  // Don't attempt to promote ephemeral values to indvars. They will be removed
144  // later anyway.
145  if (EphValues.count(I))
146    return false;
147
148  // Get the symbolic expression for this instruction.
149  const SCEV *ISE = SE->getSCEV(I);
150
151  // If we've come to an uninteresting expression, stop the traversal and
152  // call this a user.
153  if (!isInteresting(ISE, I, L, SE, LI))
154    return false;
155
156  SmallPtrSet<Instruction *, 4> UniqueUsers;
157  for (Use &U : I->uses()) {
158    Instruction *User = cast<Instruction>(U.getUser());
159    if (!UniqueUsers.insert(User).second)
160      continue;
161
162    // Do not infinitely recurse on PHI nodes.
163    if (isa<PHINode>(User) && Processed.count(User))
164      continue;
165
166    // Only consider IVUsers that are dominated by simplified loop
167    // headers. Otherwise, SCEVExpander will crash.
168    BasicBlock *UseBB = User->getParent();
169    // A phi's use is live out of its predecessor block.
170    if (PHINode *PHI = dyn_cast<PHINode>(User)) {
171      unsigned OperandNo = U.getOperandNo();
172      unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
173      UseBB = PHI->getIncomingBlock(ValNo);
174    }
175    if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
176      return false;
177
178    // Descend recursively, but not into PHI nodes outside the current loop.
179    // It's important to see the entire expression outside the loop to get
180    // choices that depend on addressing mode use right, although we won't
181    // consider references outside the loop in all cases.
182    // If User is already in Processed, we don't want to recurse into it again,
183    // but do want to record a second reference in the same instruction.
184    bool AddUserToIVUsers = false;
185    if (LI->getLoopFor(User->getParent()) != L) {
186      if (isa<PHINode>(User) || Processed.count(User) ||
187          !AddUsersImpl(User, SimpleLoopNests)) {
188        DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
189                     << "   OF SCEV: " << *ISE << '\n');
190        AddUserToIVUsers = true;
191      }
192    } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
193      DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
194                   << "   OF SCEV: " << *ISE << '\n');
195      AddUserToIVUsers = true;
196    }
197
198    if (AddUserToIVUsers) {
199      // Okay, we found a user that we cannot reduce.
200      IVStrideUse &NewUse = AddUser(User, I);
201      // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
202      // The regular return value here is discarded; instead of recording
203      // it, we just recompute it when we need it.
204      const SCEV *OriginalISE = ISE;
205      ISE = TransformForPostIncUse(NormalizeAutodetect,
206                                   ISE, User, I,
207                                   NewUse.PostIncLoops,
208                                   *SE, *DT);
209
210      // PostIncNormalization effectively simplifies the expression under
211      // pre-increment assumptions. Those assumptions (no wrapping) might not
212      // hold for the post-inc value. Catch such cases by making sure the
213      // transformation is invertible.
214      if (OriginalISE != ISE) {
215        const SCEV *DenormalizedISE =
216          TransformForPostIncUse(Denormalize, ISE, User, I,
217              NewUse.PostIncLoops, *SE, *DT);
218
219        // If we normalized the expression, but denormalization doesn't give the
220        // original one, discard this user.
221        if (OriginalISE != DenormalizedISE) {
222          DEBUG(dbgs() << "   DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "
223                       << *ISE << '\n');
224          IVUses.pop_back();
225          return false;
226        }
227      }
228      DEBUG(if (SE->getSCEV(I) != ISE)
229              dbgs() << "   NORMALIZED TO: " << *ISE << '\n');
230    }
231  }
232  return true;
233}
234
235bool IVUsers::AddUsersIfInteresting(Instruction *I) {
236  // SCEVExpander can only handle users that are dominated by simplified loop
237  // entries. Keep track of all loops that are only dominated by other simple
238  // loops so we don't traverse the domtree for each user.
239  SmallPtrSet<Loop*,16> SimpleLoopNests;
240
241  return AddUsersImpl(I, SimpleLoopNests);
242}
243
244IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
245  IVUses.push_back(new IVStrideUse(this, User, Operand));
246  return IVUses.back();
247}
248
249IVUsers::IVUsers()
250    : LoopPass(ID) {
251  initializeIVUsersPass(*PassRegistry::getPassRegistry());
252}
253
254void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
255  AU.addRequired<AssumptionCacheTracker>();
256  AU.addRequired<LoopInfoWrapperPass>();
257  AU.addRequired<DominatorTreeWrapperPass>();
258  AU.addRequired<ScalarEvolutionWrapperPass>();
259  AU.setPreservesAll();
260}
261
262bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
263
264  L = l;
265  AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
266      *L->getHeader()->getParent());
267  LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
268  DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
269  SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
270
271  // Collect ephemeral values so that AddUsersIfInteresting skips them.
272  EphValues.clear();
273  CodeMetrics::collectEphemeralValues(L, AC, EphValues);
274
275  // Find all uses of induction variables in this loop, and categorize
276  // them by stride.  Start by finding all of the PHI nodes in the header for
277  // this loop.  If they are induction variables, inspect their uses.
278  for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
279    (void)AddUsersIfInteresting(&*I);
280
281  return false;
282}
283
284void IVUsers::print(raw_ostream &OS, const Module *M) const {
285  OS << "IV Users for loop ";
286  L->getHeader()->printAsOperand(OS, false);
287  if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
288    OS << " with backedge-taken count "
289       << *SE->getBackedgeTakenCount(L);
290  }
291  OS << ":\n";
292
293  for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
294       E = IVUses.end(); UI != E; ++UI) {
295    OS << "  ";
296    UI->getOperandValToReplace()->printAsOperand(OS, false);
297    OS << " = " << *getReplacementExpr(*UI);
298    for (PostIncLoopSet::const_iterator
299         I = UI->PostIncLoops.begin(),
300         E = UI->PostIncLoops.end(); I != E; ++I) {
301      OS << " (post-inc with loop ";
302      (*I)->getHeader()->printAsOperand(OS, false);
303      OS << ")";
304    }
305    OS << " in  ";
306    if (UI->getUser())
307      UI->getUser()->print(OS);
308    else
309      OS << "Printing <null> User";
310    OS << '\n';
311  }
312}
313
314#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
315void IVUsers::dump() const {
316  print(dbgs());
317}
318#endif
319
320void IVUsers::releaseMemory() {
321  Processed.clear();
322  IVUses.clear();
323}
324
325/// getReplacementExpr - Return a SCEV expression which computes the
326/// value of the OperandValToReplace.
327const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
328  return SE->getSCEV(IU.getOperandValToReplace());
329}
330
331/// getExpr - Return the expression for the use.
332const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
333  return
334    TransformForPostIncUse(Normalize, getReplacementExpr(IU),
335                           IU.getUser(), IU.getOperandValToReplace(),
336                           const_cast<PostIncLoopSet &>(IU.getPostIncLoops()),
337                           *SE, *DT);
338}
339
340static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
341  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
342    if (AR->getLoop() == L)
343      return AR;
344    return findAddRecForLoop(AR->getStart(), L);
345  }
346
347  if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
348    for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
349         I != E; ++I)
350      if (const SCEVAddRecExpr *AR = findAddRecForLoop(*I, L))
351        return AR;
352    return nullptr;
353  }
354
355  return nullptr;
356}
357
358const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
359  if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
360    return AR->getStepRecurrence(*SE);
361  return nullptr;
362}
363
364void IVStrideUse::transformToPostInc(const Loop *L) {
365  PostIncLoops.insert(L);
366}
367
368void IVStrideUse::deleted() {
369  // Remove this user from the list.
370  Parent->Processed.erase(this->getUser());
371  Parent->IVUses.erase(this);
372  // this now dangles!
373}
374