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