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