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