1 //===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file implements bookkeeping for "interesting" users of expressions
11 // computed from induction variables.
13 //===----------------------------------------------------------------------===//
15 #include "llvm/ADT/STLExtras.h"
16 #include "llvm/Analysis/AssumptionCache.h"
17 #include "llvm/Analysis/CodeMetrics.h"
18 #include "llvm/Analysis/IVUsers.h"
19 #include "llvm/Analysis/LoopPass.h"
20 #include "llvm/Analysis/ScalarEvolutionExpressions.h"
21 #include "llvm/Analysis/ValueTracking.h"
22 #include "llvm/IR/Constants.h"
23 #include "llvm/IR/DataLayout.h"
24 #include "llvm/IR/DerivedTypes.h"
25 #include "llvm/IR/Dominators.h"
26 #include "llvm/IR/Instructions.h"
27 #include "llvm/IR/Module.h"
28 #include "llvm/IR/Type.h"
29 #include "llvm/Support/Debug.h"
30 #include "llvm/Support/raw_ostream.h"
34 #define DEBUG_TYPE "iv-users"
37 INITIALIZE_PASS_BEGIN(IVUsers, "iv-users",
38 "Induction Variable Users", false, true)
39 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
40 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
41 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
42 INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
43 INITIALIZE_PASS_END(IVUsers, "iv-users",
44 "Induction Variable Users", false, true)
46 Pass *llvm::createIVUsersPass() {
50 /// isInteresting - Test whether the given expression is "interesting" when
51 /// used by the given expression, within the context of analyzing the
53 static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,
54 ScalarEvolution *SE, LoopInfo *LI) {
55 // An addrec is interesting if it's affine or if it has an interesting start.
56 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
57 // Keep things simple. Don't touch loop-variant strides unless they're
58 // only used outside the loop and we can simplify them.
59 if (AR->getLoop() == L)
60 return AR->isAffine() ||
62 SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);
63 // Otherwise recurse to see if the start value is interesting, and that
64 // the step value is not interesting, since we don't yet know how to
65 // do effective SCEV expansions for addrecs with interesting steps.
66 return isInteresting(AR->getStart(), I, L, SE, LI) &&
67 !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);
70 // An add is interesting if exactly one of its operands is interesting.
71 if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
72 bool AnyInterestingYet = false;
73 for (SCEVAddExpr::op_iterator OI = Add->op_begin(), OE = Add->op_end();
75 if (isInteresting(*OI, I, L, SE, LI)) {
76 if (AnyInterestingYet)
78 AnyInterestingYet = true;
80 return AnyInterestingYet;
83 // Nothing else is interesting here.
87 /// Return true if all loop headers that dominate this block are in simplified
89 static bool isSimplifiedLoopNest(BasicBlock *BB, const DominatorTree *DT,
91 SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
92 Loop *NearestLoop = nullptr;
93 for (DomTreeNode *Rung = DT->getNode(BB);
94 Rung; Rung = Rung->getIDom()) {
95 BasicBlock *DomBB = Rung->getBlock();
96 Loop *DomLoop = LI->getLoopFor(DomBB);
97 if (DomLoop && DomLoop->getHeader() == DomBB) {
98 // If the domtree walk reaches a loop with no preheader, return false.
99 if (!DomLoop->isLoopSimplifyForm())
101 // If we have already checked this loop nest, stop checking.
102 if (SimpleLoopNests.count(DomLoop))
104 // If we have not already checked this loop nest, remember the loop
105 // header nearest to BB. The nearest loop may not contain BB.
107 NearestLoop = DomLoop;
111 SimpleLoopNests.insert(NearestLoop);
115 /// AddUsersImpl - Inspect the specified instruction. If it is a
116 /// reducible SCEV, recursively add its users to the IVUsesByStride set and
117 /// return true. Otherwise, return false.
118 bool IVUsers::AddUsersImpl(Instruction *I,
119 SmallPtrSetImpl<Loop*> &SimpleLoopNests) {
120 const DataLayout &DL = I->getModule()->getDataLayout();
122 // Add this IV user to the Processed set before returning false to ensure that
123 // all IV users are members of the set. See IVUsers::isIVUserOrOperand.
124 if (!Processed.insert(I).second)
125 return true; // Instruction already handled.
127 if (!SE->isSCEVable(I->getType()))
128 return false; // Void and FP expressions cannot be reduced.
130 // IVUsers is used by LSR which assumes that all SCEV expressions are safe to
131 // pass to SCEVExpander. Expressions are not safe to expand if they represent
132 // operations that are not safe to speculate, namely integer division.
133 if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I))
136 // LSR is not APInt clean, do not touch integers bigger than 64-bits.
137 // Also avoid creating IVs of non-native types. For example, we don't want a
138 // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.
139 uint64_t Width = SE->getTypeSizeInBits(I->getType());
140 if (Width > 64 || !DL.isLegalInteger(Width))
143 // Don't attempt to promote ephemeral values to indvars. They will be removed
145 if (EphValues.count(I))
148 // Get the symbolic expression for this instruction.
149 const SCEV *ISE = SE->getSCEV(I);
151 // If we've come to an uninteresting expression, stop the traversal and
153 if (!isInteresting(ISE, I, L, SE, LI))
156 SmallPtrSet<Instruction *, 4> UniqueUsers;
157 for (Use &U : I->uses()) {
158 Instruction *User = cast<Instruction>(U.getUser());
159 if (!UniqueUsers.insert(User).second)
162 // Do not infinitely recurse on PHI nodes.
163 if (isa<PHINode>(User) && Processed.count(User))
166 // Only consider IVUsers that are dominated by simplified loop
167 // headers. Otherwise, SCEVExpander will crash.
168 BasicBlock *UseBB = User->getParent();
169 // A phi's use is live out of its predecessor block.
170 if (PHINode *PHI = dyn_cast<PHINode>(User)) {
171 unsigned OperandNo = U.getOperandNo();
172 unsigned ValNo = PHINode::getIncomingValueNumForOperand(OperandNo);
173 UseBB = PHI->getIncomingBlock(ValNo);
175 if (!isSimplifiedLoopNest(UseBB, DT, LI, SimpleLoopNests))
178 // Descend recursively, but not into PHI nodes outside the current loop.
179 // It's important to see the entire expression outside the loop to get
180 // choices that depend on addressing mode use right, although we won't
181 // consider references outside the loop in all cases.
182 // If User is already in Processed, we don't want to recurse into it again,
183 // but do want to record a second reference in the same instruction.
184 bool AddUserToIVUsers = false;
185 if (LI->getLoopFor(User->getParent()) != L) {
186 if (isa<PHINode>(User) || Processed.count(User) ||
187 !AddUsersImpl(User, SimpleLoopNests)) {
188 DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
189 << " OF SCEV: " << *ISE << '\n');
190 AddUserToIVUsers = true;
192 } else if (Processed.count(User) || !AddUsersImpl(User, SimpleLoopNests)) {
193 DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
194 << " OF SCEV: " << *ISE << '\n');
195 AddUserToIVUsers = true;
198 if (AddUserToIVUsers) {
199 // Okay, we found a user that we cannot reduce.
200 IVStrideUse &NewUse = AddUser(User, I);
201 // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.
202 // The regular return value here is discarded; instead of recording
203 // it, we just recompute it when we need it.
204 const SCEV *OriginalISE = ISE;
205 ISE = TransformForPostIncUse(NormalizeAutodetect,
210 // PostIncNormalization effectively simplifies the expression under
211 // pre-increment assumptions. Those assumptions (no wrapping) might not
212 // hold for the post-inc value. Catch such cases by making sure the
213 // transformation is invertible.
214 if (OriginalISE != ISE) {
215 const SCEV *DenormalizedISE =
216 TransformForPostIncUse(Denormalize, ISE, User, I,
217 NewUse.PostIncLoops, *SE, *DT);
219 // If we normalized the expression, but denormalization doesn't give the
220 // original one, discard this user.
221 if (OriginalISE != DenormalizedISE) {
222 DEBUG(dbgs() << " DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "
228 DEBUG(if (SE->getSCEV(I) != ISE)
229 dbgs() << " NORMALIZED TO: " << *ISE << '\n');
235 bool IVUsers::AddUsersIfInteresting(Instruction *I) {
236 // SCEVExpander can only handle users that are dominated by simplified loop
237 // entries. Keep track of all loops that are only dominated by other simple
238 // loops so we don't traverse the domtree for each user.
239 SmallPtrSet<Loop*,16> SimpleLoopNests;
241 return AddUsersImpl(I, SimpleLoopNests);
244 IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {
245 IVUses.push_back(new IVStrideUse(this, User, Operand));
246 return IVUses.back();
251 initializeIVUsersPass(*PassRegistry::getPassRegistry());
254 void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
255 AU.addRequired<AssumptionCacheTracker>();
256 AU.addRequired<LoopInfoWrapperPass>();
257 AU.addRequired<DominatorTreeWrapperPass>();
258 AU.addRequired<ScalarEvolution>();
259 AU.setPreservesAll();
262 bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
265 AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
266 *L->getHeader()->getParent());
267 LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
268 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
269 SE = &getAnalysis<ScalarEvolution>();
271 // Collect ephemeral values so that AddUsersIfInteresting skips them.
273 CodeMetrics::collectEphemeralValues(L, AC, EphValues);
275 // Find all uses of induction variables in this loop, and categorize
276 // them by stride. Start by finding all of the PHI nodes in the header for
277 // this loop. If they are induction variables, inspect their uses.
278 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
279 (void)AddUsersIfInteresting(I);
284 void IVUsers::print(raw_ostream &OS, const Module *M) const {
285 OS << "IV Users for loop ";
286 L->getHeader()->printAsOperand(OS, false);
287 if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
288 OS << " with backedge-taken count "
289 << *SE->getBackedgeTakenCount(L);
293 for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
294 E = IVUses.end(); UI != E; ++UI) {
296 UI->getOperandValToReplace()->printAsOperand(OS, false);
297 OS << " = " << *getReplacementExpr(*UI);
298 for (PostIncLoopSet::const_iterator
299 I = UI->PostIncLoops.begin(),
300 E = UI->PostIncLoops.end(); I != E; ++I) {
301 OS << " (post-inc with loop ";
302 (*I)->getHeader()->printAsOperand(OS, false);
307 UI->getUser()->print(OS);
309 OS << "Printing <null> User";
314 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
315 void IVUsers::dump() const {
320 void IVUsers::releaseMemory() {
325 /// getReplacementExpr - Return a SCEV expression which computes the
326 /// value of the OperandValToReplace.
327 const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {
328 return SE->getSCEV(IU.getOperandValToReplace());
331 /// getExpr - Return the expression for the use.
332 const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {
334 TransformForPostIncUse(Normalize, getReplacementExpr(IU),
335 IU.getUser(), IU.getOperandValToReplace(),
336 const_cast<PostIncLoopSet &>(IU.getPostIncLoops()),
340 static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {
341 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
342 if (AR->getLoop() == L)
344 return findAddRecForLoop(AR->getStart(), L);
347 if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
348 for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
350 if (const SCEVAddRecExpr *AR = findAddRecForLoop(*I, L))
358 const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {
359 if (const SCEVAddRecExpr *AR = findAddRecForLoop(getExpr(IU), L))
360 return AR->getStepRecurrence(*SE);
364 void IVStrideUse::transformToPostInc(const Loop *L) {
365 PostIncLoops.insert(L);
368 void IVStrideUse::deleted() {
369 // Remove this user from the list.
370 Parent->Processed.erase(this->getUser());
371 Parent->IVUses.erase(this);