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