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