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 #include "llvm/Analysis/IVUsers.h"
16 #include "llvm/ADT/STLExtras.h"
17 #include "llvm/Analysis/LoopPass.h"
18 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
19 #include "llvm/Analysis/ValueTracking.h"
20 #include "llvm/IR/Constants.h"
21 #include "llvm/IR/DataLayout.h"
22 #include "llvm/IR/DerivedTypes.h"
23 #include "llvm/IR/Dominators.h"
24 #include "llvm/IR/Instructions.h"
25 #include "llvm/IR/Type.h"
26 #include "llvm/Support/Debug.h"
27 #include "llvm/Support/raw_ostream.h"
28 #include <algorithm>
29 using namespace llvm;
30
31 #define DEBUG_TYPE "iv-users"
32
33 char IVUsers::ID = 0;
34 INITIALIZE_PASS_BEGIN(IVUsers, "iv-users",
35 "Induction Variable Users", false, true)
INITIALIZE_PASS_DEPENDENCY(LoopInfo)36 INITIALIZE_PASS_DEPENDENCY(LoopInfo)
37 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
38 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
39 INITIALIZE_PASS_END(IVUsers, "iv-users",
40 "Induction Variable Users", false, true)
41
42 Pass *llvm::createIVUsersPass() {
43 return new IVUsers();
44 }
45
46 /// isInteresting - Test whether the given expression is "interesting" when
47 /// used by the given expression, within the context of analyzing the
48 /// given loop.
isInteresting(const SCEV * S,const Instruction * I,const Loop * L,ScalarEvolution * SE,LoopInfo * LI)49 static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
50 ScalarEvolution *SE, LoopInfo *LI) {
51 // An addrec is interesting if it's affine or if it has an interesting start.
52 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
53 // Keep things simple. Don't touch loop-variant strides unless they're
54 // only used outside the loop and we can simplify them.
55 if (AR->getLoop() == L)
56 return AR->isAffine() ||
57 (!L->contains(I) &&
58 SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
59 // Otherwise recurse to see if the start value is interesting, and that
60 // the step value is not interesting, since we don't yet know how to
61 // do effective SCEV expansions for addrecs with interesting steps.
62 return isInteresting(AR->getStart(), I, L, SE, LI) &&
63 !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
64 }
65
66 // An add is interesting if exactly one of its operands is interesting.
67 if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
68 bool AnyInterestingYet = false;
69 for (SCEVAddExpr::op_iterator OI = Add->op_begin(), OE = Add->op_end();
70 OI != OE; ++OI)
71 if (isInteresting(*OI, I, L, SE, LI)) {
72 if (AnyInterestingYet)
73 return false;
74 AnyInterestingYet = true;
75 }
76 return AnyInterestingYet;
77 }
78
79 // Nothing else is interesting here.
80 return false;
81 }
82
83 /// Return true if all loop headers that dominate this block are in simplified
84 /// form.
isSimplifiedLoopNest(BasicBlock * BB,const DominatorTree * DT,const LoopInfo * LI,SmallPtrSet<Loop *,16> & SimpleLoopNests)85 static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
86 const LoopInfo *LI,
87 SmallPtrSet<Loop*,16> &SimpleLoopNests) {
88 Loop *NearestLoop = nullptr;
89 for (DomTreeNode *Rung = DT->getNode(BB);
90 Rung; Rung = Rung->getIDom()) {
91 BasicBlock *DomBB = Rung->getBlock();
92 Loop *DomLoop = LI->getLoopFor(DomBB);
93 if (DomLoop && DomLoop->getHeader() == DomBB) {
94 // If the domtree walk reaches a loop with no preheader, return false.
95 if (!DomLoop->isLoopSimplifyForm())
96 return false;
97 // If we have already checked this loop nest, stop checking.
98 if (SimpleLoopNests.count(DomLoop))
99 break;
100 // If we have not already checked this loop nest, remember the loop
101 // header nearest to BB. The nearest loop may not contain BB.
102 if (!NearestLoop)
103 NearestLoop = DomLoop;
104 }
105 }
106 if (NearestLoop)
107 SimpleLoopNests.insert(NearestLoop);
108 return true;
109 }
110
111 /// AddUsersImpl - Inspect the specified instruction. If it is a
112 /// reducible SCEV, recursively add its users to the IVUsesByStride set and
113 /// return true. Otherwise, return false.
AddUsersImpl(Instruction * I,SmallPtrSet<Loop *,16> & SimpleLoopNests)114 bool IVUsers::AddUsersImpl(Instruction *I,
115 SmallPtrSet<Loop*,16> &SimpleLoopNests) {
116 // Add this IV user to the Processed set before returning false to ensure that
117 // all IV users are members of the set. See IVUsers::isIVUserOrOperand.
118 if (!Processed.insert(I))
119 return true; // Instruction already handled.
120
121 if (!SE->isSCEVable(I->getType()))
122 return false; // Void and FP expressions cannot be reduced.
123
124 // IVUsers is used by LSR which assumes that all SCEV expressions are safe to
125 // pass to SCEVExpander. Expressions are not safe to expand if they represent
126 // operations that are not safe to speculate, namely integer division.
127 if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I, DL))
128 return false;
129
130 // LSR is not APInt clean, do not touch integers bigger than 64-bits.
131 // Also avoid creating IVs of non-native types. For example, we don't want a
132 // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
133 uint64_t Width = SE->getTypeSizeInBits(I->getType());
134 if (Width > 64 || (DL && !DL->isLegalInteger(Width)))
135 return false;
136
137 // Get the symbolic expression for this instruction.
138 const SCEV *ISE = SE->getSCEV(I);
139
140 // If we've come to an uninteresting expression, stop the traversal and
141 // call this a user.
142 if (!isInteresting(ISE, I, L, SE, LI))
143 return false;
144
145 SmallPtrSet<Instruction *, 4> UniqueUsers;
146 for (Use &U : I->uses()) {
147 Instruction *User = cast<Instruction>(U.getUser());
148 if (!UniqueUsers.insert(User))
149 continue;
150
151 // Do not infinitely recurse on PHI nodes.
152 if (isa<PHINode>(User) && Processed.count(User))
153 continue;
154
155 // Only consider IVUsers that are dominated by simplified loop
156 // headers. Otherwise, SCEVExpander will crash.
157 BasicBlock *UseBB = User->getParent();
158 // A phi's use is live out of its predecessor block.
159 if (PHINode *PHI = dyn_cast<PHINode>(User)) {
160 unsigned OperandNo = U.getOperandNo();
161 unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
162 UseBB = PHI->getIncomingBlock(ValNo);
163 }
164 if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
165 return false;
166
167 // Descend recursively, but not into PHI nodes outside the current loop.
168 // It's important to see the entire expression outside the loop to get
169 // choices that depend on addressing mode use right, although we won't
170 // consider references outside the loop in all cases.
171 // If User is already in Processed, we don't want to recurse into it again,
172 // but do want to record a second reference in the same instruction.
173 bool AddUserToIVUsers = false;
174 if (LI->getLoopFor(User->getParent()) != L) {
175 if (isa<PHINode>(User) || Processed.count(User) ||
176 !AddUsersImpl(User, SimpleLoopNests)) {
177 DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
178 << " OF SCEV: " << *ISE << '\n');
179 AddUserToIVUsers = true;
180 }
181 } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
182 DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
183 << " OF SCEV: " << *ISE << '\n');
184 AddUserToIVUsers = true;
185 }
186
187 if (AddUserToIVUsers) {
188 // Okay, we found a user that we cannot reduce.
189 IVStrideUse &NewUse = AddUser(User, I);
190 // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
191 // The regular return value here is discarded; instead of recording
192 // it, we just recompute it when we need it.
193 const SCEV *OriginalISE = ISE;
194 ISE = TransformForPostIncUse(NormalizeAutodetect,
195 ISE, User, I,
196 NewUse.PostIncLoops,
197 *SE, *DT);
198
199 // PostIncNormalization effectively simplifies the expression under
200 // pre-increment assumptions. Those assumptions (no wrapping) might not
201 // hold for the post-inc value. Catch such cases by making sure the
202 // transformation is invertible.
203 if (OriginalISE != ISE) {
204 const SCEV *DenormalizedISE =
205 TransformForPostIncUse(Denormalize, ISE, User, I,
206 NewUse.PostIncLoops, *SE, *DT);
207
208 // If we normalized the expression, but denormalization doesn't give the
209 // original one, discard this user.
210 if (OriginalISE != DenormalizedISE) {
211 DEBUG(dbgs() << " DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "
212 << *ISE << '\n');
213 IVUses.pop_back();
214 return false;
215 }
216 }
217 DEBUG(if (SE->getSCEV(I) != ISE)
218 dbgs() << " NORMALIZED TO: " << *ISE << '\n');
219 }
220 }
221 return true;
222 }
223
AddUsersIfInteresting(Instruction * I)224 bool IVUsers::AddUsersIfInteresting(Instruction *I) {
225 // SCEVExpander can only handle users that are dominated by simplified loop
226 // entries. Keep track of all loops that are only dominated by other simple
227 // loops so we don't traverse the domtree for each user.
228 SmallPtrSet<Loop*,16> SimpleLoopNests;
229
230 return AddUsersImpl(I, SimpleLoopNests);
231 }
232
AddUser(Instruction * User,Value * Operand)233 IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
234 IVUses.push_back(new IVStrideUse(this, User, Operand));
235 return IVUses.back();
236 }
237
IVUsers()238 IVUsers::IVUsers()
239 : LoopPass(ID) {
240 initializeIVUsersPass(*PassRegistry::getPassRegistry());
241 }
242
getAnalysisUsage(AnalysisUsage & AU) const243 void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
244 AU.addRequired<LoopInfo>();
245 AU.addRequired<DominatorTreeWrapperPass>();
246 AU.addRequired<ScalarEvolution>();
247 AU.setPreservesAll();
248 }
249
runOnLoop(Loop * l,LPPassManager & LPM)250 bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
251
252 L = l;
253 LI = &getAnalysis<LoopInfo>();
254 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
255 SE = &getAnalysis<ScalarEvolution>();
256 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
257 DL = DLP ? &DLP->getDataLayout() : nullptr;
258
259 // Find all uses of induction variables in this loop, and categorize
260 // them by stride. Start by finding all of the PHI nodes in the header for
261 // this loop. If they are induction variables, inspect their uses.
262 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
263 (void)AddUsersIfInteresting(I);
264
265 return false;
266 }
267
print(raw_ostream & OS,const Module * M) const268 void IVUsers::print(raw_ostream &OS, const Module *M) const {
269 OS << "IV Users for loop ";
270 L->getHeader()->printAsOperand(OS, false);
271 if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
272 OS << " with backedge-taken count "
273 << *SE->getBackedgeTakenCount(L);
274 }
275 OS << ":\n";
276
277 for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
278 E = IVUses.end(); UI != E; ++UI) {
279 OS << " ";
280 UI->getOperandValToReplace()->printAsOperand(OS, false);
281 OS << " = " << *getReplacementExpr(*UI);
282 for (PostIncLoopSet::const_iterator
283 I = UI->PostIncLoops.begin(),
284 E = UI->PostIncLoops.end(); I != E; ++I) {
285 OS << " (post-inc with loop ";
286 (*I)->getHeader()->printAsOperand(OS, false);
287 OS << ")";
288 }
289 OS << " in ";
290 if (UI->getUser())
291 UI->getUser()->print(OS);
292 else
293 OS << "Printing <null> User";
294 OS << '\n';
295 }
296 }
297
298 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
dump() const299 void IVUsers::dump() const {
300 print(dbgs());
301 }
302 #endif
303
releaseMemory()304 void IVUsers::releaseMemory() {
305 Processed.clear();
306 IVUses.clear();
307 }
308
309 /// getReplacementExpr - Return a SCEV expression which computes the
310 /// value of the OperandValToReplace.
getReplacementExpr(const IVStrideUse & IU) const311 const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
312 return SE->getSCEV(IU.getOperandValToReplace());
313 }
314
315 /// getExpr - Return the expression for the use.
getExpr(const IVStrideUse & IU) const316 const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
317 return
318 TransformForPostIncUse(Normalize, getReplacementExpr(IU),
319 IU.getUser(), IU.getOperandValToReplace(),
320 const_cast<PostIncLoopSet &>(IU.getPostIncLoops()),
321 *SE, *DT);
322 }
323
findAddRecForLoop(const SCEV * S,const Loop * L)324 static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
325 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
326 if (AR->getLoop() == L)
327 return AR;
328 return findAddRecForLoop(AR->getStart(), L);
329 }
330
331 if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
332 for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
333 I != E; ++I)
334 if (const SCEVAddRecExpr *AR = findAddRecForLoop(*I, L))
335 return AR;
336 return nullptr;
337 }
338
339 return nullptr;
340 }
341
getStride(const IVStrideUse & IU,const Loop * L) const342 const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
343 if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
344 return AR->getStepRecurrence(*SE);
345 return nullptr;
346 }
347
transformToPostInc(const Loop * L)348 void IVStrideUse::transformToPostInc(const Loop *L) {
349 PostIncLoops.insert(L);
350 }
351
deleted()352 void IVStrideUse::deleted() {
353 // Remove this user from the list.
354 Parent->Processed.erase(this->getUser());
355 Parent->IVUses.erase(this);
356 // this now dangles!
357 }
358