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 #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"
32 static RegisterPass<IVUsers>
33 X("iv-users", "Induction Variable Users", false, true);
35 Pass *llvm::createIVUsersPass() {
39 /// CollectSubexprs - Split S into subexpressions which can be pulled out into
40 /// separate registers.
41 static void CollectSubexprs(const SCEV *S,
42 SmallVectorImpl<const SCEV *> &Ops,
43 ScalarEvolution &SE) {
44 if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {
45 // Break out add operands.
46 for (SCEVAddExpr::op_iterator I = Add->op_begin(), E = Add->op_end();
48 CollectSubexprs(*I, Ops, SE);
50 } else if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {
51 // Split a non-zero base out of an addrec.
52 if (!AR->getStart()->isZero()) {
53 CollectSubexprs(AR->getStart(), Ops, SE);
54 CollectSubexprs(SE.getAddRecExpr(SE.getIntegerSCEV(0, AR->getType()),
55 AR->getStepRecurrence(SE),
56 AR->getLoop()), Ops, SE);
61 // Otherwise use the value itself.
65 /// getSCEVStartAndStride - Compute the start and stride of this expression,
66 /// returning false if the expression is not a start/stride pair, or true if it
67 /// is. The stride must be a loop invariant expression, but the start may be
68 /// a mix of loop invariant and loop variant expressions. The start cannot,
69 /// however, contain an AddRec from a different loop, unless that loop is an
70 /// outer loop of the current loop.
71 static bool getSCEVStartAndStride(const SCEV *&SH, Loop *L, Loop *UseLoop,
72 const SCEV *&Start, const SCEV *&Stride,
73 ScalarEvolution *SE, DominatorTree *DT) {
74 const SCEV *TheAddRec = Start; // Initialize to zero.
76 // If the outer level is an AddExpr, the operands are all start values except
77 // for a nested AddRecExpr.
78 if (const SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(SH)) {
79 for (unsigned i = 0, e = AE->getNumOperands(); i != e; ++i)
80 if (const SCEVAddRecExpr *AddRec =
81 dyn_cast<SCEVAddRecExpr>(AE->getOperand(i)))
82 TheAddRec = SE->getAddExpr(AddRec, TheAddRec);
84 Start = SE->getAddExpr(Start, AE->getOperand(i));
85 } else if (isa<SCEVAddRecExpr>(SH)) {
88 return false; // not analyzable.
91 // Break down TheAddRec into its component parts.
92 SmallVector<const SCEV *, 4> Subexprs;
93 CollectSubexprs(TheAddRec, Subexprs, *SE);
95 // Look for an addrec on the current loop among the parts.
96 const SCEV *AddRecStride = 0;
97 for (SmallVectorImpl<const SCEV *>::iterator I = Subexprs.begin(),
98 E = Subexprs.end(); I != E; ++I) {
100 if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S))
101 if (AR->getLoop() == L) {
103 AddRecStride = AR->getStepRecurrence(*SE);
110 // Add up everything else into a start value (which may not be
112 const SCEV *AddRecStart = SE->getAddExpr(Subexprs);
114 // Use getSCEVAtScope to attempt to simplify other loops out of
116 AddRecStart = SE->getSCEVAtScope(AddRecStart, UseLoop);
118 Start = SE->getAddExpr(Start, AddRecStart);
120 // If stride is an instruction, make sure it properly dominates the header.
121 // Otherwise we could end up with a use before def situation.
122 if (!isa<SCEVConstant>(AddRecStride)) {
123 BasicBlock *Header = L->getHeader();
124 if (!AddRecStride->properlyDominates(Header, DT))
128 WriteAsOperand(dbgs(), L->getHeader(), /*PrintType=*/false);
129 dbgs() << "] Variable stride: " << *AddRecStride << "\n");
132 Stride = AddRecStride;
136 /// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression
137 /// and now we need to decide whether the user should use the preinc or post-inc
138 /// value. If this user should use the post-inc version of the IV, return true.
140 /// Choosing wrong here can break dominance properties (if we choose to use the
141 /// post-inc value when we cannot) or it can end up adding extra live-ranges to
142 /// the loop, resulting in reg-reg copies (if we use the pre-inc value when we
143 /// should use the post-inc value).
144 static bool IVUseShouldUsePostIncValue(Instruction *User, Instruction *IV,
145 Loop *L, LoopInfo *LI, DominatorTree *DT,
147 // If the user is in the loop, use the preinc value.
148 if (L->contains(User)) return false;
150 BasicBlock *LatchBlock = L->getLoopLatch();
154 // Ok, the user is outside of the loop. If it is dominated by the latch
155 // block, use the post-inc value.
156 if (DT->dominates(LatchBlock, User->getParent()))
159 // There is one case we have to be careful of: PHI nodes. These little guys
160 // can live in blocks that are not dominated by the latch block, but (since
161 // their uses occur in the predecessor block, not the block the PHI lives in)
162 // should still use the post-inc value. Check for this case now.
163 PHINode *PN = dyn_cast<PHINode>(User);
164 if (!PN) return false; // not a phi, not dominated by latch block.
166 // Look at all of the uses of IV by the PHI node. If any use corresponds to
167 // a block that is not dominated by the latch block, give up and use the
168 // preincremented value.
169 unsigned NumUses = 0;
170 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
171 if (PN->getIncomingValue(i) == IV) {
173 if (!DT->dominates(LatchBlock, PN->getIncomingBlock(i)))
177 // Okay, all uses of IV by PN are in predecessor blocks that really are
178 // dominated by the latch block. Use the post-incremented value.
182 /// AddUsersIfInteresting - Inspect the specified instruction. If it is a
183 /// reducible SCEV, recursively add its users to the IVUsesByStride set and
184 /// return true. Otherwise, return false.
185 bool IVUsers::AddUsersIfInteresting(Instruction *I) {
186 if (!SE->isSCEVable(I->getType()))
187 return false; // Void and FP expressions cannot be reduced.
189 // LSR is not APInt clean, do not touch integers bigger than 64-bits.
190 if (SE->getTypeSizeInBits(I->getType()) > 64)
193 if (!Processed.insert(I))
194 return true; // Instruction already handled.
196 // Get the symbolic expression for this instruction.
197 const SCEV *ISE = SE->getSCEV(I);
198 if (isa<SCEVCouldNotCompute>(ISE)) return false;
200 // Get the start and stride for this expression.
201 Loop *UseLoop = LI->getLoopFor(I->getParent());
202 const SCEV *Start = SE->getIntegerSCEV(0, ISE->getType());
203 const SCEV *Stride = Start;
205 if (!getSCEVStartAndStride(ISE, L, UseLoop, Start, Stride, SE, DT))
206 return false; // Non-reducible symbolic expression, bail out.
208 // Keep things simple. Don't touch loop-variant strides.
209 if (!Stride->isLoopInvariant(L) && L->contains(I))
212 SmallPtrSet<Instruction *, 4> UniqueUsers;
213 for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
215 Instruction *User = cast<Instruction>(*UI);
216 if (!UniqueUsers.insert(User))
219 // Do not infinitely recurse on PHI nodes.
220 if (isa<PHINode>(User) && Processed.count(User))
223 // Descend recursively, but not into PHI nodes outside the current loop.
224 // It's important to see the entire expression outside the loop to get
225 // choices that depend on addressing mode use right, although we won't
226 // consider references ouside the loop in all cases.
227 // If User is already in Processed, we don't want to recurse into it again,
228 // but do want to record a second reference in the same instruction.
229 bool AddUserToIVUsers = false;
230 if (LI->getLoopFor(User->getParent()) != L) {
231 if (isa<PHINode>(User) || Processed.count(User) ||
232 !AddUsersIfInteresting(User)) {
233 DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'
234 << " OF SCEV: " << *ISE << '\n');
235 AddUserToIVUsers = true;
237 } else if (Processed.count(User) ||
238 !AddUsersIfInteresting(User)) {
239 DEBUG(dbgs() << "FOUND USER: " << *User << '\n'
240 << " OF SCEV: " << *ISE << '\n');
241 AddUserToIVUsers = true;
244 if (AddUserToIVUsers) {
245 // Okay, we found a user that we cannot reduce. Analyze the instruction
246 // and decide what to do with it. If we are a use inside of the loop, use
247 // the value before incrementation, otherwise use it after incrementation.
248 if (IVUseShouldUsePostIncValue(User, I, L, LI, DT, this)) {
249 // The value used will be incremented by the stride more than we are
250 // expecting, so subtract this off.
251 const SCEV *NewStart = SE->getMinusSCEV(Start, Stride);
252 IVUses.push_back(new IVStrideUse(this, Stride, NewStart, User, I));
253 IVUses.back().setIsUseOfPostIncrementedValue(true);
254 DEBUG(dbgs() << " USING POSTINC SCEV, START=" << *NewStart<< "\n");
256 IVUses.push_back(new IVStrideUse(this, Stride, Start, User, I));
263 IVStrideUse &IVUsers::AddUser(const SCEV *Stride, const SCEV *Offset,
264 Instruction *User, Value *Operand) {
265 IVUses.push_back(new IVStrideUse(this, Stride, Offset, User, Operand));
266 return IVUses.back();
273 void IVUsers::getAnalysisUsage(AnalysisUsage &AU) const {
274 AU.addRequired<LoopInfo>();
275 AU.addRequired<DominatorTree>();
276 AU.addRequired<ScalarEvolution>();
277 AU.setPreservesAll();
280 bool IVUsers::runOnLoop(Loop *l, LPPassManager &LPM) {
283 LI = &getAnalysis<LoopInfo>();
284 DT = &getAnalysis<DominatorTree>();
285 SE = &getAnalysis<ScalarEvolution>();
287 // Find all uses of induction variables in this loop, and categorize
288 // them by stride. Start by finding all of the PHI nodes in the header for
289 // this loop. If they are induction variables, inspect their uses.
290 for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)
291 AddUsersIfInteresting(I);
296 /// getReplacementExpr - Return a SCEV expression which computes the
297 /// value of the OperandValToReplace of the given IVStrideUse.
298 const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &U) const {
300 const SCEV *RetVal = SE->getIntegerSCEV(0, U.getStride()->getType());
301 // Create the basic add recurrence.
302 RetVal = SE->getAddRecExpr(RetVal, U.getStride(), L);
303 // Add the offset in a separate step, because it may be loop-variant.
304 RetVal = SE->getAddExpr(RetVal, U.getOffset());
305 // For uses of post-incremented values, add an extra stride to compute
306 // the actual replacement value.
307 if (U.isUseOfPostIncrementedValue())
308 RetVal = SE->getAddExpr(RetVal, U.getStride());
312 /// getCanonicalExpr - Return a SCEV expression which computes the
313 /// value of the SCEV of the given IVStrideUse, ignoring the
314 /// isUseOfPostIncrementedValue flag.
315 const SCEV *IVUsers::getCanonicalExpr(const IVStrideUse &U) const {
317 const SCEV *RetVal = SE->getIntegerSCEV(0, U.getStride()->getType());
318 // Create the basic add recurrence.
319 RetVal = SE->getAddRecExpr(RetVal, U.getStride(), L);
320 // Add the offset in a separate step, because it may be loop-variant.
321 RetVal = SE->getAddExpr(RetVal, U.getOffset());
325 void IVUsers::print(raw_ostream &OS, const Module *M) const {
326 OS << "IV Users for loop ";
327 WriteAsOperand(OS, L->getHeader(), false);
328 if (SE->hasLoopInvariantBackedgeTakenCount(L)) {
329 OS << " with backedge-taken count "
330 << *SE->getBackedgeTakenCount(L);
334 // Use a defualt AssemblyAnnotationWriter to suppress the default info
335 // comments, which aren't relevant here.
336 AssemblyAnnotationWriter Annotator;
337 for (ilist<IVStrideUse>::const_iterator UI = IVUses.begin(),
338 E = IVUses.end(); UI != E; ++UI) {
340 WriteAsOperand(OS, UI->getOperandValToReplace(), false);
342 << *getReplacementExpr(*UI);
343 if (UI->isUseOfPostIncrementedValue())
346 UI->getUser()->print(OS, &Annotator);
351 void IVUsers::dump() const {
355 void IVUsers::releaseMemory() {
360 void IVStrideUse::deleted() {
361 // Remove this user from the list.
362 Parent->IVUses.erase(this);