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#define DEBUG_TYPE "iv-users"
16#include "llvm/Analysis/IVUsers.h"
17#include "llvm/ADT/STLExtras.h"
18#include "llvm/Analysis/Dominators.h"
19#include "llvm/Analysis/LoopPass.h"
20#include "llvm/Analysis/ScalarEvolutionExpressions.h"
21#include "llvm/Analysis/ValueTracking.h"
22#include "llvm/Assembly/Writer.h"
23#include "llvm/IR/Constants.h"
24#include "llvm/IR/DataLayout.h"
25#include "llvm/IR/DerivedTypes.h"
26#include "llvm/IR/Instructions.h"
27#include "llvm/IR/Type.h"
28#include "llvm/Support/Debug.h"
29#include "llvm/Support/raw_ostream.h"
30#include <algorithm>
31using namespace llvm;
32
33char IVUsers::ID = 0;
34INITIALIZE_PASS_BEGIN(IVUsers, "iv-users",
35                      "Induction Variable Users", false, true)
36INITIALIZE_PASS_DEPENDENCY(LoopInfo)
37INITIALIZE_PASS_DEPENDENCY(DominatorTree)
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 = 0;
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, TD))
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 || (TD && !TD->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 (Value::use_iterator UI = I->use_begin(), E = I->use_end();
147       UI != E; ++UI) {
148    Instruction *User = cast<Instruction>(*UI);
149    if (!UniqueUsers.insert(User))
150      continue;
151
152    // Do not infinitely recurse on PHI nodes.
153    if (isa<PHINode>(User) && Processed.count(User))
154      continue;
155
156    // Only consider IVUsers that are dominated by simplified loop
157    // headers. Otherwise, SCEVExpander will crash.
158    BasicBlock *UseBB = User->getParent();
159    // A phi's use is live out of its predecessor block.
160    if (PHINode *PHI = dyn_cast<PHINode>(User)) {
161      unsigned OperandNo = UI.getOperandNo();
162      unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
163      UseBB = PHI->getIncomingBlock(ValNo);
164    }
165    if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
166      return false;
167
168    // Descend recursively, but not into PHI nodes outside the current loop.
169    // It's important to see the entire expression outside the loop to get
170    // choices that depend on addressing mode use right, although we won't
171    // consider references outside the loop in all cases.
172    // If User is already in Processed, we don't want to recurse into it again,
173    // but do want to record a second reference in the same instruction.
174    bool AddUserToIVUsers = false;
175    if (LI->getLoopFor(User->getParent()) != L) {
176      if (isa<PHINode>(User) || Processed.count(User) ||
177          !AddUsersImpl(User, SimpleLoopNests)) {
178        DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
179                     << "   OF SCEV: " << *ISE << '\n');
180        AddUserToIVUsers = true;
181      }
182    } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
183      DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
184                   << "   OF SCEV: " << *ISE << '\n');
185      AddUserToIVUsers = true;
186    }
187
188    if (AddUserToIVUsers) {
189      // Okay, we found a user that we cannot reduce.
190      IVUses.push_back(new IVStrideUse(this, User, I));
191      IVStrideUse &NewUse = IVUses.back();
192      // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
193      // The regular return value here is discarded; instead of recording
194      // it, we just recompute it when we need it.
195      ISE = TransformForPostIncUse(NormalizeAutodetect,
196                                   ISE, User, I,
197                                   NewUse.PostIncLoops,
198                                   *SE, *DT);
199      DEBUG(if (SE->getSCEV(I) != ISE)
200              dbgs() << "   NORMALIZED TO: " << *ISE << '\n');
201    }
202  }
203  return true;
204}
205
206bool IVUsers::AddUsersIfInteresting(Instruction *I) {
207  // SCEVExpander can only handle users that are dominated by simplified loop
208  // entries. Keep track of all loops that are only dominated by other simple
209  // loops so we don't traverse the domtree for each user.
210  SmallPtrSet<Loop*,16> SimpleLoopNests;
211
212  return AddUsersImpl(I, SimpleLoopNests);
213}
214
215IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
216  IVUses.push_back(new IVStrideUse(this, User, Operand));
217  return IVUses.back();
218}
219
220IVUsers::IVUsers()
221    : LoopPass(ID) {
222  initializeIVUsersPass(*PassRegistry::getPassRegistry());
223}
224
225void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
226  AU.addRequired<LoopInfo>();
227  AU.addRequired<DominatorTree>();
228  AU.addRequired<ScalarEvolution>();
229  AU.setPreservesAll();
230}
231
232bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
233
234  L = l;
235  LI = &getAnalysis<LoopInfo>();
236  DT = &getAnalysis<DominatorTree>();
237  SE = &getAnalysis<ScalarEvolution>();
238  TD = getAnalysisIfAvailable<DataLayout>();
239
240  // Find all uses of induction variables in this loop, and categorize
241  // them by stride.  Start by finding all of the PHI nodes in the header for
242  // this loop.  If they are induction variables, inspect their uses.
243  for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
244    (void)AddUsersIfInteresting(I);
245
246  return false;
247}
248
249void IVUsers::print(raw_ostream &OS, const Module *M) const {
250  OS << "IV Users for loop ";
251  WriteAsOperand(OS, L->getHeader(), false);
252  if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
253    OS << " with backedge-taken count "
254       << *SE->getBackedgeTakenCount(L);
255  }
256  OS << ":\n";
257
258  for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
259       E = IVUses.end(); UI != E; ++UI) {
260    OS << "  ";
261    WriteAsOperand(OS, UI->getOperandValToReplace(), false);
262    OS << " = " << *getReplacementExpr(*UI);
263    for (PostIncLoopSet::const_iterator
264         I = UI->PostIncLoops.begin(),
265         E = UI->PostIncLoops.end(); I != E; ++I) {
266      OS << " (post-inc with loop ";
267      WriteAsOperand(OS, (*I)->getHeader(), false);
268      OS << ")";
269    }
270    OS << " in  ";
271    UI->getUser()->print(OS);
272    OS << '\n';
273  }
274}
275
276#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
277void IVUsers::dump() const {
278  print(dbgs());
279}
280#endif
281
282void IVUsers::releaseMemory() {
283  Processed.clear();
284  IVUses.clear();
285}
286
287/// getReplacementExpr - Return a SCEV expression which computes the
288/// value of the OperandValToReplace.
289const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
290  return SE->getSCEV(IU.getOperandValToReplace());
291}
292
293/// getExpr - Return the expression for the use.
294const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
295  return
296    TransformForPostIncUse(Normalize, getReplacementExpr(IU),
297                           IU.getUser(), IU.getOperandValToReplace(),
298                           const_cast<PostIncLoopSet &>(IU.getPostIncLoops()),
299                           *SE, *DT);
300}
301
302static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
303  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
304    if (AR->getLoop() == L)
305      return AR;
306    return findAddRecForLoop(AR->getStart(), L);
307  }
308
309  if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
310    for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
311         I != E; ++I)
312      if (const SCEVAddRecExpr *AR = findAddRecForLoop(*I, L))
313        return AR;
314    return 0;
315  }
316
317  return 0;
318}
319
320const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
321  if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
322    return AR->getStepRecurrence(*SE);
323  return 0;
324}
325
326void IVStrideUse::transformToPostInc(const Loop *L) {
327  PostIncLoops.insert(L);
328}
329
330void IVStrideUse::deleted() {
331  // Remove this user from the list.
332  Parent->Processed.erase(this->getUser());
333  Parent->IVUses.erase(this);
334  // this now dangles!
335}
336