IVUsers.cpp revision 75ae20366fd1b480f4cc38400bb075c43c9f4f7f 
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/Constants.h"
18#include "llvm/Instructions.h"
19#include "llvm/Type.h"
20#include "llvm/DerivedTypes.h"
21#include "llvm/Analysis/Dominators.h"
22#include "llvm/Analysis/LoopPass.h"
23#include "llvm/Analysis/ScalarEvolutionExpressions.h"
24#include "llvm/Target/TargetData.h"
25#include "llvm/Assembly/Writer.h"
26#include "llvm/ADT/STLExtras.h"
27#include "llvm/Support/Debug.h"
28#include "llvm/Support/raw_ostream.h"
29#include <algorithm>
30using namespace llvm;
31
32char IVUsers::ID = 0;
33INITIALIZE_PASS_BEGIN(IVUsers, "iv-users",
34                      "Induction Variable Users", false, true)
35INITIALIZE_PASS_DEPENDENCY(LoopInfo)
36INITIALIZE_PASS_DEPENDENCY(DominatorTree)
37INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
38INITIALIZE_PASS_END(IVUsers, "iv-users",
39                      "Induction Variable Users", false, true)
40
41Pass *llvm::createIVUsersPass() {
42  return new IVUsers();
43}
44
45/// isInteresting - Test whether the given expression is "interesting" when
46/// used by the given expression, within the context of analyzing the
47/// given loop.
48static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
49                          ScalarEvolution *SE, LoopInfo *LI) {
50  // An addrec is interesting if it's affine or if it has an interesting start.
51  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
52    // Keep things simple. Don't touch loop-variant strides unless they're
53    // only used outside the loop and we can simplify them.
54    if (AR->getLoop() == L)
55      return AR->isAffine() ||
56             (!L->contains(I) &&
57              SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
58    // Otherwise recurse to see if the start value is interesting, and that
59    // the step value is not interesting, since we don't yet know how to
60    // do effective SCEV expansions for addrecs with interesting steps.
61    return isInteresting(AR->getStart(), I, L, SE, LI) &&
62          !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
63  }
64
65  // An add is interesting if exactly one of its operands is interesting.
66  if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
67    bool AnyInterestingYet = false;
68    for (SCEVAddExpr::op_iterator OI = Add->op_begin(), OE = Add->op_end();
69         OI != OE; ++OI)
70      if (isInteresting(*OI, I, L, SE, LI)) {
71        if (AnyInterestingYet)
72          return false;
73        AnyInterestingYet = true;
74      }
75    return AnyInterestingYet;
76  }
77
78  // Nothing else is interesting here.
79  return false;
80}
81
82/// Return true if this loop and all loop headers that dominate it are in
83/// simplified form.
84static bool isSimplifiedLoopNest(Loop *L, const DominatorTree *DT,
85                                 const LoopInfo *LI) {
86  if (!L->isLoopSimplifyForm())
87    return false;
88
89  for (DomTreeNode *Rung = DT->getNode(L->getLoopPreheader());
90       Rung; Rung = Rung->getIDom()) {
91    BasicBlock *BB = Rung->getBlock();
92    const Loop *DomLoop = LI->getLoopFor(BB);
93    if (DomLoop && DomLoop->getHeader() == BB) {
94      if (!DomLoop->isLoopSimplifyForm())
95        return false;
96    }
97  }
98  return true;
99}
100
101/// AddUsersIfInteresting - Inspect the specified instruction.  If it is a
102/// reducible SCEV, recursively add its users to the IVUsesByStride set and
103/// return true.  Otherwise, return false.
104bool IVUsers::AddUsersIfInteresting(Instruction *I,
105                                    SmallPtrSet<Loop*,16> &SimpleLoopNests) {
106  // Add this IV user to the Processed set before returning false to ensure that
107  // all IV users are members of the set. See IVUsers::isIVUserOrOperand.
108  if (!Processed.insert(I))
109    return true;    // Instruction already handled.
110
111  if (!SE->isSCEVable(I->getType()))
112    return false;   // Void and FP expressions cannot be reduced.
113
114  // LSR is not APInt clean, do not touch integers bigger than 64-bits.
115  // Also avoid creating IVs of non-native types. For example, we don't want a
116  // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
117  uint64_t Width = SE->getTypeSizeInBits(I->getType());
118  if (Width > 64 || (TD && !TD->isLegalInteger(Width)))
119    return false;
120
121  // Get the symbolic expression for this instruction.
122  const SCEV *ISE = SE->getSCEV(I);
123
124  // If we've come to an uninteresting expression, stop the traversal and
125  // call this a user.
126  if (!isInteresting(ISE, I, L, SE, LI))
127    return false;
128
129  SmallPtrSet<Instruction *, 4> UniqueUsers;
130  for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
131       UI != E; ++UI) {
132    Instruction *User = cast<Instruction>(*UI);
133    if (!UniqueUsers.insert(User))
134      continue;
135
136    // Do not infinitely recurse on PHI nodes.
137    if (isa<PHINode>(User) && Processed.count(User))
138      continue;
139
140    Loop *UserLoop = LI->getLoopFor(User->getParent());
141
142    // Only consider IVUsers that are dominated by simplified loop
143    // headers. Otherwise, SCEVExpander will crash.
144    if (UserLoop && !SimpleLoopNests.count(UserLoop)) {
145      if (!isSimplifiedLoopNest(UserLoop, DT, LI))
146        return false;
147      SimpleLoopNests.insert(UserLoop);
148    }
149
150    // Descend recursively, but not into PHI nodes outside the current loop.
151    // It's important to see the entire expression outside the loop to get
152    // choices that depend on addressing mode use right, although we won't
153    // consider references outside the loop in all cases.
154    // If User is already in Processed, we don't want to recurse into it again,
155    // but do want to record a second reference in the same instruction.
156    bool AddUserToIVUsers = false;
157    if (UserLoop != L) {
158      if (isa<PHINode>(User) || Processed.count(User) ||
159          !AddUsersIfInteresting(User, SimpleLoopNests)) {
160        DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
161                     << "   OF SCEV: " << *ISE << '\n');
162        AddUserToIVUsers = true;
163      }
164    } else if (Processed.count(User)
165               || !AddUsersIfInteresting(User, SimpleLoopNests)) {
166      DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
167                   << "   OF SCEV: " << *ISE << '\n');
168      AddUserToIVUsers = true;
169    }
170
171    if (AddUserToIVUsers) {
172      // Okay, we found a user that we cannot reduce.
173      IVUses.push_back(new IVStrideUse(this, User, I));
174      IVStrideUse &NewUse = IVUses.back();
175      // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
176      // The regular return value here is discarded; instead of recording
177      // it, we just recompute it when we need it.
178      ISE = TransformForPostIncUse(NormalizeAutodetect,
179                                   ISE, User, I,
180                                   NewUse.PostIncLoops,
181                                   *SE, *DT);
182      DEBUG(if (SE->getSCEV(I) != ISE)
183              dbgs() << "   NORMALIZED TO: " << *ISE << '\n');
184    }
185  }
186  return true;
187}
188
189IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
190  IVUses.push_back(new IVStrideUse(this, User, Operand));
191  return IVUses.back();
192}
193
194IVUsers::IVUsers()
195    : LoopPass(ID) {
196  initializeIVUsersPass(*PassRegistry::getPassRegistry());
197}
198
199void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
200  AU.addRequired<LoopInfo>();
201  AU.addRequired<DominatorTree>();
202  AU.addRequired<ScalarEvolution>();
203  AU.setPreservesAll();
204}
205
206bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
207
208  L = l;
209  LI = &getAnalysis<LoopInfo>();
210  DT = &getAnalysis<DominatorTree>();
211  SE = &getAnalysis<ScalarEvolution>();
212  TD = getAnalysisIfAvailable<TargetData>();
213
214  // SCEVExpander can only handle users that are dominated by simplified loop
215  // entries. Keep track of all loops that are only dominated by other simple
216  // loops so we don't traverse the domtree for each user.
217  SmallPtrSet<Loop*,16> SimpleLoopNests;
218
219  // Find all uses of induction variables in this loop, and categorize
220  // them by stride.  Start by finding all of the PHI nodes in the header for
221  // this loop.  If they are induction variables, inspect their uses.
222  for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
223    (void)AddUsersIfInteresting(I, SimpleLoopNests);
224
225  return false;
226}
227
228void IVUsers::print(raw_ostream &OS, const Module *M) const {
229  OS << "IV Users for loop ";
230  WriteAsOperand(OS, L->getHeader(), false);
231  if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
232    OS << " with backedge-taken count "
233       << *SE->getBackedgeTakenCount(L);
234  }
235  OS << ":\n";
236
237  for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
238       E = IVUses.end(); UI != E; ++UI) {
239    OS << "  ";
240    WriteAsOperand(OS, UI->getOperandValToReplace(), false);
241    OS << " = " << *getReplacementExpr(*UI);
242    for (PostIncLoopSet::const_iterator
243         I = UI->PostIncLoops.begin(),
244         E = UI->PostIncLoops.end(); I != E; ++I) {
245      OS << " (post-inc with loop ";
246      WriteAsOperand(OS, (*I)->getHeader(), false);
247      OS << ")";
248    }
249    OS << " in  ";
250    UI->getUser()->print(OS);
251    OS << '\n';
252  }
253}
254
255void IVUsers::dump() const {
256  print(dbgs());
257}
258
259void IVUsers::releaseMemory() {
260  Processed.clear();
261  IVUses.clear();
262}
263
264/// getReplacementExpr - Return a SCEV expression which computes the
265/// value of the OperandValToReplace.
266const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
267  return SE->getSCEV(IU.getOperandValToReplace());
268}
269
270/// getExpr - Return the expression for the use.
271const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
272  return
273    TransformForPostIncUse(Normalize, getReplacementExpr(IU),
274                           IU.getUser(), IU.getOperandValToReplace(),
275                           const_cast<PostIncLoopSet &>(IU.getPostIncLoops()),
276                           *SE, *DT);
277}
278
279static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
280  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
281    if (AR->getLoop() == L)
282      return AR;
283    return findAddRecForLoop(AR->getStart(), L);
284  }
285
286  if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
287    for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
288         I != E; ++I)
289      if (const SCEVAddRecExpr *AR = findAddRecForLoop(*I, L))
290        return AR;
291    return 0;
292  }
293
294  return 0;
295}
296
297const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
298  if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
299    return AR->getStepRecurrence(*SE);
300  return 0;
301}
302
303void IVStrideUse::transformToPostInc(const Loop *L) {
304  PostIncLoops.insert(L);
305}
306
307void IVStrideUse::deleted() {
308  // Remove this user from the list.
309  Parent->Processed.erase(this->getUser());
310  Parent->IVUses.erase(this);
311  // this now dangles!
312}
313