1 //===- BasicAliasAnalysis.cpp - Local Alias Analysis Impl -----------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // This file defines the default implementation of the Alias Analysis interface
11 // that simply implements a few identities (two different globals cannot alias,
12 // etc), but otherwise does no analysis.
14 // FIXME: This could be extended for a very simple form of mod/ref information.
15 // If a pointer is locally allocated (either malloc or alloca) and never passed
16 // into a call or stored to memory, then we know that calls will not mod/ref the
17 // memory. This can be important for tailcallelim.
19 //===----------------------------------------------------------------------===//
21 #include "llvm/Analysis/AliasAnalysis.h"
22 #include "llvm/Pass.h"
23 #include "llvm/Argument.h"
24 #include "llvm/iOther.h"
25 #include "llvm/Constants.h"
26 #include "llvm/GlobalValue.h"
27 #include "llvm/DerivedTypes.h"
28 #include "llvm/Target/TargetData.h"
29 #include "llvm/Support/GetElementPtrTypeIterator.h"
32 // Make sure that anything that uses AliasAnalysis pulls in this file...
33 void llvm::BasicAAStub() {}
36 struct BasicAliasAnalysis : public ImmutablePass, public AliasAnalysis {
38 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
39 AliasAnalysis::getAnalysisUsage(AU);
42 virtual void initializePass();
44 // alias - This is the only method here that does anything interesting...
46 AliasResult alias(const Value *V1, unsigned V1Size,
47 const Value *V2, unsigned V2Size);
49 // CheckGEPInstructions - Check two GEP instructions with known
50 // must-aliasing base pointers. This checks to see if the index expressions
51 // preclude the pointers from aliasing...
53 CheckGEPInstructions(const Type* BasePtr1Ty, std::vector<Value*> &GEP1Ops,
55 const Type *BasePtr2Ty, std::vector<Value*> &GEP2Ops,
59 // Register this pass...
60 RegisterOpt<BasicAliasAnalysis>
61 X("basicaa", "Basic Alias Analysis (default AA impl)");
63 // Declare that we implement the AliasAnalysis interface
64 RegisterAnalysisGroup<AliasAnalysis, BasicAliasAnalysis, true> Y;
65 } // End of anonymous namespace
67 void BasicAliasAnalysis::initializePass() {
68 InitializeAliasAnalysis(this);
71 // hasUniqueAddress - Return true if the specified value points to something
72 // with a unique, discernable, address.
73 static inline bool hasUniqueAddress(const Value *V) {
74 return isa<GlobalValue>(V) || isa<AllocationInst>(V);
77 // getUnderlyingObject - This traverses the use chain to figure out what object
78 // the specified value points to. If the value points to, or is derived from, a
79 // unique object or an argument, return it.
80 static const Value *getUnderlyingObject(const Value *V) {
81 if (!isa<PointerType>(V->getType())) return 0;
83 // If we are at some type of object... return it.
84 if (hasUniqueAddress(V) || isa<Argument>(V)) return V;
86 // Traverse through different addressing mechanisms...
87 if (const Instruction *I = dyn_cast<Instruction>(V)) {
88 if (isa<CastInst>(I) || isa<GetElementPtrInst>(I))
89 return getUnderlyingObject(I->getOperand(0));
90 } else if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {
91 if (CE->getOpcode() == Instruction::Cast ||
92 CE->getOpcode() == Instruction::GetElementPtr)
93 return getUnderlyingObject(CE->getOperand(0));
94 } else if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V)) {
95 return CPR->getValue();
100 static const User *isGEP(const Value *V) {
101 if (isa<GetElementPtrInst>(V) ||
102 (isa<ConstantExpr>(V) &&
103 cast<ConstantExpr>(V)->getOpcode() == Instruction::GetElementPtr))
104 return cast<User>(V);
108 static const Value *GetGEPOperands(const Value *V, std::vector<Value*> &GEPOps){
109 assert(GEPOps.empty() && "Expect empty list to populate!");
110 GEPOps.insert(GEPOps.end(), cast<User>(V)->op_begin()+1,
111 cast<User>(V)->op_end());
113 // Accumulate all of the chained indexes into the operand array
114 V = cast<User>(V)->getOperand(0);
116 while (const User *G = isGEP(V)) {
117 if (!isa<Constant>(GEPOps[0]) ||
118 !cast<Constant>(GEPOps[0])->isNullValue())
119 break; // Don't handle folding arbitrary pointer offsets yet...
120 GEPOps.erase(GEPOps.begin()); // Drop the zero index
121 GEPOps.insert(GEPOps.begin(), G->op_begin()+1, G->op_end());
122 V = G->getOperand(0);
128 // alias - Provide a bunch of ad-hoc rules to disambiguate in common cases, such
129 // as array references. Note that this function is heavily tail recursive.
130 // Hopefully we have a smart C++ compiler. :)
132 AliasAnalysis::AliasResult
133 BasicAliasAnalysis::alias(const Value *V1, unsigned V1Size,
134 const Value *V2, unsigned V2Size) {
135 // Strip off any constant expression casts if they exist
136 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V1))
137 if (CE->getOpcode() == Instruction::Cast)
138 V1 = CE->getOperand(0);
139 if (const ConstantExpr *CE = dyn_cast<ConstantExpr>(V2))
140 if (CE->getOpcode() == Instruction::Cast)
141 V2 = CE->getOperand(0);
143 // Strip off constant pointer refs if they exist
144 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V1))
145 V1 = CPR->getValue();
146 if (const ConstantPointerRef *CPR = dyn_cast<ConstantPointerRef>(V2))
147 V2 = CPR->getValue();
149 // Are we checking for alias of the same value?
150 if (V1 == V2) return MustAlias;
152 if ((!isa<PointerType>(V1->getType()) || !isa<PointerType>(V2->getType())) &&
153 V1->getType() != Type::LongTy && V2->getType() != Type::LongTy)
154 return NoAlias; // Scalars cannot alias each other
156 // Strip off cast instructions...
157 if (const Instruction *I = dyn_cast<CastInst>(V1))
158 return alias(I->getOperand(0), V1Size, V2, V2Size);
159 if (const Instruction *I = dyn_cast<CastInst>(V2))
160 return alias(V1, V1Size, I->getOperand(0), V2Size);
162 // Figure out what objects these things are pointing to if we can...
163 const Value *O1 = getUnderlyingObject(V1);
164 const Value *O2 = getUnderlyingObject(V2);
166 // Pointing at a discernible object?
168 if (isa<Argument>(O1)) {
169 // Incoming argument cannot alias locally allocated object!
170 if (isa<AllocationInst>(O2)) return NoAlias;
171 // Otherwise, nothing is known...
172 } else if (isa<Argument>(O2)) {
173 // Incoming argument cannot alias locally allocated object!
174 if (isa<AllocationInst>(O1)) return NoAlias;
175 // Otherwise, nothing is known...
177 // If they are two different objects, we know that we have no alias...
178 if (O1 != O2) return NoAlias;
181 // If they are the same object, they we can look at the indexes. If they
182 // index off of the object is the same for both pointers, they must alias.
183 // If they are provably different, they must not alias. Otherwise, we can't
185 } else if (O1 && !isa<Argument>(O1) && isa<ConstantPointerNull>(V2)) {
186 return NoAlias; // Unique values don't alias null
187 } else if (O2 && !isa<Argument>(O2) && isa<ConstantPointerNull>(V1)) {
188 return NoAlias; // Unique values don't alias null
191 // If we have two gep instructions with must-alias'ing base pointers, figure
192 // out if the indexes to the GEP tell us anything about the derived pointer.
193 // Note that we also handle chains of getelementptr instructions as well as
194 // constant expression getelementptrs here.
196 if (isGEP(V1) && isGEP(V2)) {
197 // Drill down into the first non-gep value, to test for must-aliasing of
198 // the base pointers.
199 const Value *BasePtr1 = V1, *BasePtr2 = V2;
201 BasePtr1 = cast<User>(BasePtr1)->getOperand(0);
202 } while (isGEP(BasePtr1) &&
203 cast<User>(BasePtr1)->getOperand(1) ==
204 Constant::getNullValue(cast<User>(BasePtr1)->getOperand(1)->getType()));
206 BasePtr2 = cast<User>(BasePtr2)->getOperand(0);
207 } while (isGEP(BasePtr2) &&
208 cast<User>(BasePtr2)->getOperand(1) ==
209 Constant::getNullValue(cast<User>(BasePtr2)->getOperand(1)->getType()));
211 // Do the base pointers alias?
212 AliasResult BaseAlias = alias(BasePtr1, V1Size, BasePtr2, V2Size);
213 if (BaseAlias == NoAlias) return NoAlias;
214 if (BaseAlias == MustAlias) {
215 // If the base pointers alias each other exactly, check to see if we can
216 // figure out anything about the resultant pointers, to try to prove
219 // Collect all of the chained GEP operands together into one simple place
220 std::vector<Value*> GEP1Ops, GEP2Ops;
221 BasePtr1 = GetGEPOperands(V1, GEP1Ops);
222 BasePtr2 = GetGEPOperands(V2, GEP2Ops);
225 CheckGEPInstructions(BasePtr1->getType(), GEP1Ops, V1Size,
226 BasePtr2->getType(), GEP2Ops, V2Size);
227 if (GAlias != MayAlias)
232 // Check to see if these two pointers are related by a getelementptr
233 // instruction. If one pointer is a GEP with a non-zero index of the other
234 // pointer, we know they cannot alias.
238 std::swap(V1Size, V2Size);
241 if (V1Size != ~0U && V2Size != ~0U)
242 if (const User *GEP = isGEP(V1)) {
243 std::vector<Value*> GEPOperands;
244 const Value *BasePtr = GetGEPOperands(V1, GEPOperands);
246 AliasResult R = alias(BasePtr, V1Size, V2, V2Size);
247 if (R == MustAlias) {
248 // If there is at least one non-zero constant index, we know they cannot
250 bool ConstantFound = false;
251 bool AllZerosFound = true;
252 for (unsigned i = 0, e = GEPOperands.size(); i != e; ++i)
253 if (const Constant *C = dyn_cast<Constant>(GEPOperands[i])) {
254 if (!C->isNullValue()) {
255 ConstantFound = true;
256 AllZerosFound = false;
260 AllZerosFound = false;
263 // If we have getelementptr <ptr>, 0, 0, 0, 0, ... and V2 must aliases
264 // the ptr, the end result is a must alias also.
269 if (V2Size <= 1 && V1Size <= 1) // Just pointer check?
272 // Otherwise we have to check to see that the distance is more than
273 // the size of the argument... build an index vector that is equal to
274 // the arguments provided, except substitute 0's for any variable
275 // indexes we find...
276 for (unsigned i = 0; i != GEPOperands.size(); ++i)
277 if (!isa<Constant>(GEPOperands[i]) ||
278 isa<ConstantExpr>(GEPOperands[i]))
279 GEPOperands[i] =Constant::getNullValue(GEPOperands[i]->getType());
280 int64_t Offset = getTargetData().getIndexedOffset(BasePtr->getType(),
282 if (Offset >= (int64_t)V2Size || Offset <= -(int64_t)V1Size)
291 /// CheckGEPInstructions - Check two GEP instructions with known must-aliasing
292 /// base pointers. This checks to see if the index expressions preclude the
293 /// pointers from aliasing...
294 AliasAnalysis::AliasResult BasicAliasAnalysis::
295 CheckGEPInstructions(const Type* BasePtr1Ty, std::vector<Value*> &GEP1Ops,
297 const Type *BasePtr2Ty, std::vector<Value*> &GEP2Ops,
299 // We currently can't handle the case when the base pointers have different
300 // primitive types. Since this is uncommon anyway, we are happy being
301 // extremely conservative.
302 if (BasePtr1Ty != BasePtr2Ty)
305 const Type *GEPPointerTy = BasePtr1Ty;
307 // Find the (possibly empty) initial sequence of equal values... which are not
308 // necessarily constants.
309 unsigned NumGEP1Operands = GEP1Ops.size(), NumGEP2Operands = GEP2Ops.size();
310 unsigned MinOperands = std::min(NumGEP1Operands, NumGEP2Operands);
311 unsigned MaxOperands = std::max(NumGEP1Operands, NumGEP2Operands);
312 unsigned UnequalOper = 0;
313 while (UnequalOper != MinOperands &&
314 GEP1Ops[UnequalOper] == GEP2Ops[UnequalOper]) {
315 // Advance through the type as we go...
317 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
318 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[UnequalOper-1]);
320 // If all operands equal each other, then the derived pointers must
321 // alias each other...
323 assert(UnequalOper == NumGEP1Operands && UnequalOper == NumGEP2Operands &&
324 "Ran out of type nesting, but not out of operands?");
329 // If we have seen all constant operands, and run out of indexes on one of the
330 // getelementptrs, check to see if the tail of the leftover one is all zeros.
331 // If so, return mustalias.
332 if (UnequalOper == MinOperands) {
333 if (GEP1Ops.size() < GEP2Ops.size()) std::swap(GEP1Ops, GEP2Ops);
335 bool AllAreZeros = true;
336 for (unsigned i = UnequalOper; i != MaxOperands; ++i)
337 if (!isa<Constant>(GEP1Ops[i]) ||
338 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
342 if (AllAreZeros) return MustAlias;
346 // So now we know that the indexes derived from the base pointers,
347 // which are known to alias, are different. We can still determine a
348 // no-alias result if there are differing constant pairs in the index
349 // chain. For example:
350 // A[i][0] != A[j][1] iff (&A[0][1]-&A[0][0] >= std::max(G1S, G2S))
352 unsigned SizeMax = std::max(G1S, G2S);
353 if (SizeMax == ~0U) return MayAlias; // Avoid frivolous work...
355 // Scan for the first operand that is constant and unequal in the
356 // two getelemenptrs...
357 unsigned FirstConstantOper = UnequalOper;
358 for (; FirstConstantOper != MinOperands; ++FirstConstantOper) {
359 const Value *G1Oper = GEP1Ops[FirstConstantOper];
360 const Value *G2Oper = GEP2Ops[FirstConstantOper];
362 if (G1Oper != G2Oper) // Found non-equal constant indexes...
363 if (Constant *G1OC = dyn_cast<Constant>(const_cast<Value*>(G1Oper)))
364 if (Constant *G2OC = dyn_cast<Constant>(const_cast<Value*>(G2Oper))) {
365 // Make sure they are comparable (ie, not constant expressions)...
366 // and make sure the GEP with the smaller leading constant is GEP1.
367 Constant *Compare = ConstantExpr::get(Instruction::SetGT, G1OC, G2OC);
368 if (ConstantBool *CV = dyn_cast<ConstantBool>(Compare)) {
369 if (CV->getValue()) // If they are comparable and G2 > G1
370 std::swap(GEP1Ops, GEP2Ops); // Make GEP1 < GEP2
374 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(G1Oper);
377 // No shared constant operands, and we ran out of common operands. At this
378 // point, the GEP instructions have run through all of their operands, and we
379 // haven't found evidence that there are any deltas between the GEP's.
380 // However, one GEP may have more operands than the other. If this is the
381 // case, there may still be hope. This this now.
382 if (FirstConstantOper == MinOperands) {
383 // Make GEP1Ops be the longer one if there is a longer one.
384 if (GEP1Ops.size() < GEP2Ops.size())
385 std::swap(GEP1Ops, GEP2Ops);
387 // Is there anything to check?
388 if (GEP1Ops.size() > MinOperands) {
389 for (unsigned i = FirstConstantOper; i != MaxOperands; ++i)
390 if (isa<Constant>(GEP1Ops[i]) && !isa<ConstantExpr>(GEP1Ops[i]) &&
391 !cast<Constant>(GEP1Ops[i])->isNullValue()) {
392 // Yup, there's a constant in the tail. Set all variables to
393 // constants in the GEP instruction to make it suiteable for
394 // TargetData::getIndexedOffset.
395 for (i = 0; i != MaxOperands; ++i)
396 if (!isa<Constant>(GEP1Ops[i]) || isa<ConstantExpr>(GEP1Ops[i]))
397 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
398 // Okay, now get the offset. This is the relative offset for the full
400 const TargetData &TD = getTargetData();
401 int64_t Offset1 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops);
403 // Now crop off any constants from the end...
404 GEP1Ops.resize(MinOperands);
405 int64_t Offset2 = TD.getIndexedOffset(GEPPointerTy, GEP1Ops);
407 // If the tail provided a bit enough offset, return noalias!
408 if ((uint64_t)(Offset2-Offset1) >= SizeMax)
413 // Couldn't find anything useful.
417 // If there are non-equal constants arguments, then we can figure
418 // out a minimum known delta between the two index expressions... at
419 // this point we know that the first constant index of GEP1 is less
420 // than the first constant index of GEP2.
422 // Advance BasePtr[12]Ty over this first differing constant operand.
423 BasePtr2Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(GEP2Ops[FirstConstantOper]);
424 BasePtr1Ty = cast<CompositeType>(BasePtr1Ty)->getTypeAtIndex(GEP1Ops[FirstConstantOper]);
426 // We are going to be using TargetData::getIndexedOffset to determine the
427 // offset that each of the GEP's is reaching. To do this, we have to convert
428 // all variable references to constant references. To do this, we convert the
429 // initial equal sequence of variables into constant zeros to start with.
430 for (unsigned i = 0; i != FirstConstantOper; ++i) {
431 if (!isa<Constant>(GEP1Ops[i]) || isa<ConstantExpr>(GEP1Ops[i]) ||
432 !isa<Constant>(GEP2Ops[i]) || isa<ConstantExpr>(GEP2Ops[i])) {
433 GEP1Ops[i] = Constant::getNullValue(GEP1Ops[i]->getType());
434 GEP2Ops[i] = Constant::getNullValue(GEP2Ops[i]->getType());
438 // We know that GEP1Ops[FirstConstantOper] & GEP2Ops[FirstConstantOper] are ok
440 // Loop over the rest of the operands...
441 for (unsigned i = FirstConstantOper+1; i != MaxOperands; ++i) {
442 const Value *Op1 = i < GEP1Ops.size() ? GEP1Ops[i] : 0;
443 const Value *Op2 = i < GEP2Ops.size() ? GEP2Ops[i] : 0;
444 // If they are equal, use a zero index...
445 if (Op1 == Op2 && BasePtr1Ty == BasePtr2Ty) {
446 if (!isa<Constant>(Op1) || isa<ConstantExpr>(Op1))
447 GEP1Ops[i] = GEP2Ops[i] = Constant::getNullValue(Op1->getType());
448 // Otherwise, just keep the constants we have.
451 if (const ConstantInt *Op1C = dyn_cast<ConstantInt>(Op1)) {
452 // If this is an array index, make sure the array element is in range.
453 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
454 if (Op1C->getRawValue() >= AT->getNumElements())
455 return MayAlias; // Be conservative with out-of-range accesses
458 // GEP1 is known to produce a value less than GEP2. To be
459 // conservatively correct, we must assume the largest possible
460 // constant is used in this position. This cannot be the initial
461 // index to the GEP instructions (because we know we have at least one
462 // element before this one with the different constant arguments), so
463 // we know that the current index must be into either a struct or
464 // array. Because we know it's not constant, this cannot be a
465 // structure index. Because of this, we can calculate the maximum
468 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
469 GEP1Ops[i] = ConstantSInt::get(Type::LongTy,AT->getNumElements()-1);
474 if (const ConstantInt *Op2C = dyn_cast<ConstantInt>(Op2)) {
475 // If this is an array index, make sure the array element is in range.
476 if (const ArrayType *AT = dyn_cast<ArrayType>(BasePtr1Ty))
477 if (Op2C->getRawValue() >= AT->getNumElements())
478 return MayAlias; // Be conservative with out-of-range accesses
479 } else { // Conservatively assume the minimum value for this index
480 GEP2Ops[i] = Constant::getNullValue(Op2->getType());
485 if (BasePtr1Ty && Op1) {
486 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr1Ty))
487 BasePtr1Ty = CT->getTypeAtIndex(GEP1Ops[i]);
492 if (BasePtr2Ty && Op2) {
493 if (const CompositeType *CT = dyn_cast<CompositeType>(BasePtr2Ty))
494 BasePtr2Ty = CT->getTypeAtIndex(GEP2Ops[i]);
500 int64_t Offset1 = getTargetData().getIndexedOffset(GEPPointerTy, GEP1Ops);
501 int64_t Offset2 = getTargetData().getIndexedOffset(GEPPointerTy, GEP2Ops);
502 assert(Offset1 < Offset2 &&"There is at least one different constant here!");
504 if ((uint64_t)(Offset2-Offset1) >= SizeMax) {
505 //std::cerr << "Determined that these two GEP's don't alias ["
506 // << SizeMax << " bytes]: \n" << *GEP1 << *GEP2;