1 //===- AliasAnalysis.cpp - Generic Alias Analysis Interface Implementation -==//
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 the generic AliasAnalysis interface which is used as the
11 // common interface used by all clients and implementations of alias analysis.
12 //
13 // This file also implements the default version of the AliasAnalysis interface
14 // that is to be used when no other implementation is specified. This does some
15 // simple tests that detect obvious cases: two different global pointers cannot
16 // alias, a global cannot alias a malloc, two different mallocs cannot alias,
17 // etc.
18 //
19 // This alias analysis implementation really isn't very good for anything, but
20 // it is very fast, and makes a nice clean default implementation. Because it
21 // handles lots of little corner cases, other, more complex, alias analysis
22 // implementations may choose to rely on this pass to resolve these simple and
23 // easy cases.
24 //
25 //===----------------------------------------------------------------------===//
26
27 #include "llvm/Analysis/AliasAnalysis.h"
28 #include "llvm/Pass.h"
29 #include "llvm/BasicBlock.h"
30 #include "llvm/Function.h"
31 #include "llvm/IntrinsicInst.h"
32 #include "llvm/Instructions.h"
33 #include "llvm/Type.h"
34 #include "llvm/Target/TargetData.h"
35 using namespace llvm;
36
37 // Register the AliasAnalysis interface, providing a nice name to refer to.
38 static RegisterAnalysisGroup<AliasAnalysis> Z("Alias Analysis");
39 char AliasAnalysis::ID = 0;
40
41 //===----------------------------------------------------------------------===//
42 // Default chaining methods
43 //===----------------------------------------------------------------------===//
44
45 AliasAnalysis::AliasResult
alias(const Value * V1,unsigned V1Size,const Value * V2,unsigned V2Size)46 AliasAnalysis::alias(const Value *V1, unsigned V1Size,
47 const Value *V2, unsigned V2Size) {
48 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
49 return AA->alias(V1, V1Size, V2, V2Size);
50 }
51
pointsToConstantMemory(const Value * P)52 bool AliasAnalysis::pointsToConstantMemory(const Value *P) {
53 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
54 return AA->pointsToConstantMemory(P);
55 }
56
deleteValue(Value * V)57 void AliasAnalysis::deleteValue(Value *V) {
58 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
59 AA->deleteValue(V);
60 }
61
copyValue(Value * From,Value * To)62 void AliasAnalysis::copyValue(Value *From, Value *To) {
63 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
64 AA->copyValue(From, To);
65 }
66
67 AliasAnalysis::ModRefResult
getModRefInfo(ImmutableCallSite CS,const Value * P,unsigned Size)68 AliasAnalysis::getModRefInfo(ImmutableCallSite CS,
69 const Value *P, unsigned Size) {
70 // Don't assert AA because BasicAA calls us in order to make use of the
71 // logic here.
72
73 ModRefBehavior MRB = getModRefBehavior(CS);
74 if (MRB == DoesNotAccessMemory)
75 return NoModRef;
76
77 ModRefResult Mask = ModRef;
78 if (MRB == OnlyReadsMemory)
79 Mask = Ref;
80 else if (MRB == AliasAnalysis::AccessesArguments) {
81 bool doesAlias = false;
82 for (ImmutableCallSite::arg_iterator AI = CS.arg_begin(), AE = CS.arg_end();
83 AI != AE; ++AI)
84 if (!isNoAlias(*AI, ~0U, P, Size)) {
85 doesAlias = true;
86 break;
87 }
88
89 if (!doesAlias)
90 return NoModRef;
91 }
92
93 // If P points to a constant memory location, the call definitely could not
94 // modify the memory location.
95 if ((Mask & Mod) && pointsToConstantMemory(P))
96 Mask = ModRefResult(Mask & ~Mod);
97
98 // If this is BasicAA, don't forward.
99 if (!AA) return Mask;
100
101 // Otherwise, fall back to the next AA in the chain. But we can merge
102 // in any mask we've managed to compute.
103 return ModRefResult(AA->getModRefInfo(CS, P, Size) & Mask);
104 }
105
106 AliasAnalysis::ModRefResult
getModRefInfo(ImmutableCallSite CS1,ImmutableCallSite CS2)107 AliasAnalysis::getModRefInfo(ImmutableCallSite CS1, ImmutableCallSite CS2) {
108 // Don't assert AA because BasicAA calls us in order to make use of the
109 // logic here.
110
111 // If CS1 or CS2 are readnone, they don't interact.
112 ModRefBehavior CS1B = getModRefBehavior(CS1);
113 if (CS1B == DoesNotAccessMemory) return NoModRef;
114
115 ModRefBehavior CS2B = getModRefBehavior(CS2);
116 if (CS2B == DoesNotAccessMemory) return NoModRef;
117
118 // If they both only read from memory, there is no dependence.
119 if (CS1B == OnlyReadsMemory && CS2B == OnlyReadsMemory)
120 return NoModRef;
121
122 AliasAnalysis::ModRefResult Mask = ModRef;
123
124 // If CS1 only reads memory, the only dependence on CS2 can be
125 // from CS1 reading memory written by CS2.
126 if (CS1B == OnlyReadsMemory)
127 Mask = ModRefResult(Mask & Ref);
128
129 // If CS2 only access memory through arguments, accumulate the mod/ref
130 // information from CS1's references to the memory referenced by
131 // CS2's arguments.
132 if (CS2B == AccessesArguments) {
133 AliasAnalysis::ModRefResult R = NoModRef;
134 for (ImmutableCallSite::arg_iterator
135 I = CS2.arg_begin(), E = CS2.arg_end(); I != E; ++I) {
136 R = ModRefResult((R | getModRefInfo(CS1, *I, UnknownSize)) & Mask);
137 if (R == Mask)
138 break;
139 }
140 return R;
141 }
142
143 // If CS1 only accesses memory through arguments, check if CS2 references
144 // any of the memory referenced by CS1's arguments. If not, return NoModRef.
145 if (CS1B == AccessesArguments) {
146 AliasAnalysis::ModRefResult R = NoModRef;
147 for (ImmutableCallSite::arg_iterator
148 I = CS1.arg_begin(), E = CS1.arg_end(); I != E; ++I)
149 if (getModRefInfo(CS2, *I, UnknownSize) != NoModRef) {
150 R = Mask;
151 break;
152 }
153 if (R == NoModRef)
154 return R;
155 }
156
157 // If this is BasicAA, don't forward.
158 if (!AA) return Mask;
159
160 // Otherwise, fall back to the next AA in the chain. But we can merge
161 // in any mask we've managed to compute.
162 return ModRefResult(AA->getModRefInfo(CS1, CS2) & Mask);
163 }
164
165 AliasAnalysis::ModRefBehavior
getModRefBehavior(ImmutableCallSite CS)166 AliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
167 // Don't assert AA because BasicAA calls us in order to make use of the
168 // logic here.
169
170 ModRefBehavior Min = UnknownModRefBehavior;
171
172 // Call back into the alias analysis with the other form of getModRefBehavior
173 // to see if it can give a better response.
174 if (const Function *F = CS.getCalledFunction())
175 Min = getModRefBehavior(F);
176
177 // If this is BasicAA, don't forward.
178 if (!AA) return Min;
179
180 // Otherwise, fall back to the next AA in the chain. But we can merge
181 // in any result we've managed to compute.
182 return std::min(AA->getModRefBehavior(CS), Min);
183 }
184
185 AliasAnalysis::ModRefBehavior
getModRefBehavior(const Function * F)186 AliasAnalysis::getModRefBehavior(const Function *F) {
187 assert(AA && "AA didn't call InitializeAliasAnalysis in its run method!");
188 return AA->getModRefBehavior(F);
189 }
190
191
192 //===----------------------------------------------------------------------===//
193 // AliasAnalysis non-virtual helper method implementation
194 //===----------------------------------------------------------------------===//
195
196 AliasAnalysis::ModRefResult
getModRefInfo(const LoadInst * L,const Value * P,unsigned Size)197 AliasAnalysis::getModRefInfo(const LoadInst *L, const Value *P, unsigned Size) {
198 // Be conservative in the face of volatile.
199 if (L->isVolatile())
200 return ModRef;
201
202 // If the load address doesn't alias the given address, it doesn't read
203 // or write the specified memory.
204 if (!alias(L->getOperand(0), getTypeStoreSize(L->getType()), P, Size))
205 return NoModRef;
206
207 // Otherwise, a load just reads.
208 return Ref;
209 }
210
211 AliasAnalysis::ModRefResult
getModRefInfo(const StoreInst * S,const Value * P,unsigned Size)212 AliasAnalysis::getModRefInfo(const StoreInst *S, const Value *P, unsigned Size) {
213 // Be conservative in the face of volatile.
214 if (S->isVolatile())
215 return ModRef;
216
217 // If the store address cannot alias the pointer in question, then the
218 // specified memory cannot be modified by the store.
219 if (!alias(S->getOperand(1),
220 getTypeStoreSize(S->getOperand(0)->getType()), P, Size))
221 return NoModRef;
222
223 // If the pointer is a pointer to constant memory, then it could not have been
224 // modified by this store.
225 if (pointsToConstantMemory(P))
226 return NoModRef;
227
228 // Otherwise, a store just writes.
229 return Mod;
230 }
231
232 AliasAnalysis::ModRefResult
getModRefInfo(const VAArgInst * V,const Value * P,unsigned Size)233 AliasAnalysis::getModRefInfo(const VAArgInst *V, const Value *P, unsigned Size) {
234 // If the va_arg address cannot alias the pointer in question, then the
235 // specified memory cannot be accessed by the va_arg.
236 if (!alias(V->getOperand(0), UnknownSize, P, Size))
237 return NoModRef;
238
239 // If the pointer is a pointer to constant memory, then it could not have been
240 // modified by this va_arg.
241 if (pointsToConstantMemory(P))
242 return NoModRef;
243
244 // Otherwise, a va_arg reads and writes.
245 return ModRef;
246 }
247
248
249 AliasAnalysis::ModRefBehavior
getIntrinsicModRefBehavior(unsigned iid)250 AliasAnalysis::getIntrinsicModRefBehavior(unsigned iid) {
251 #define GET_INTRINSIC_MODREF_BEHAVIOR
252 #include "llvm/Intrinsics.gen"
253 #undef GET_INTRINSIC_MODREF_BEHAVIOR
254 }
255
256 // AliasAnalysis destructor: DO NOT move this to the header file for
257 // AliasAnalysis or else clients of the AliasAnalysis class may not depend on
258 // the AliasAnalysis.o file in the current .a file, causing alias analysis
259 // support to not be included in the tool correctly!
260 //
~AliasAnalysis()261 AliasAnalysis::~AliasAnalysis() {}
262
263 /// InitializeAliasAnalysis - Subclasses must call this method to initialize the
264 /// AliasAnalysis interface before any other methods are called.
265 ///
InitializeAliasAnalysis(Pass * P)266 void AliasAnalysis::InitializeAliasAnalysis(Pass *P) {
267 TD = P->getAnalysisIfAvailable<TargetData>();
268 AA = &P->getAnalysis<AliasAnalysis>();
269 }
270
271 // getAnalysisUsage - All alias analysis implementations should invoke this
272 // directly (using AliasAnalysis::getAnalysisUsage(AU)).
getAnalysisUsage(AnalysisUsage & AU) const273 void AliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
274 AU.addRequired<AliasAnalysis>(); // All AA's chain
275 }
276
277 /// getTypeStoreSize - Return the TargetData store size for the given type,
278 /// if known, or a conservative value otherwise.
279 ///
getTypeStoreSize(const Type * Ty)280 unsigned AliasAnalysis::getTypeStoreSize(const Type *Ty) {
281 return TD ? TD->getTypeStoreSize(Ty) : ~0u;
282 }
283
284 /// canBasicBlockModify - Return true if it is possible for execution of the
285 /// specified basic block to modify the value pointed to by Ptr.
286 ///
canBasicBlockModify(const BasicBlock & BB,const Value * Ptr,unsigned Size)287 bool AliasAnalysis::canBasicBlockModify(const BasicBlock &BB,
288 const Value *Ptr, unsigned Size) {
289 return canInstructionRangeModify(BB.front(), BB.back(), Ptr, Size);
290 }
291
292 /// canInstructionRangeModify - Return true if it is possible for the execution
293 /// of the specified instructions to modify the value pointed to by Ptr. The
294 /// instructions to consider are all of the instructions in the range of [I1,I2]
295 /// INCLUSIVE. I1 and I2 must be in the same basic block.
296 ///
canInstructionRangeModify(const Instruction & I1,const Instruction & I2,const Value * Ptr,unsigned Size)297 bool AliasAnalysis::canInstructionRangeModify(const Instruction &I1,
298 const Instruction &I2,
299 const Value *Ptr, unsigned Size) {
300 assert(I1.getParent() == I2.getParent() &&
301 "Instructions not in same basic block!");
302 BasicBlock::const_iterator I = &I1;
303 BasicBlock::const_iterator E = &I2;
304 ++E; // Convert from inclusive to exclusive range.
305
306 for (; I != E; ++I) // Check every instruction in range
307 if (getModRefInfo(I, Ptr, Size) & Mod)
308 return true;
309 return false;
310 }
311
312 /// isNoAliasCall - Return true if this pointer is returned by a noalias
313 /// function.
isNoAliasCall(const Value * V)314 bool llvm::isNoAliasCall(const Value *V) {
315 if (isa<CallInst>(V) || isa<InvokeInst>(V))
316 return ImmutableCallSite(cast<Instruction>(V))
317 .paramHasAttr(0, Attribute::NoAlias);
318 return false;
319 }
320
321 /// isIdentifiedObject - Return true if this pointer refers to a distinct and
322 /// identifiable object. This returns true for:
323 /// Global Variables and Functions (but not Global Aliases)
324 /// Allocas and Mallocs
325 /// ByVal and NoAlias Arguments
326 /// NoAlias returns
327 ///
isIdentifiedObject(const Value * V)328 bool llvm::isIdentifiedObject(const Value *V) {
329 if (isa<AllocaInst>(V))
330 return true;
331 if (isa<GlobalValue>(V) && !isa<GlobalAlias>(V))
332 return true;
333 if (isNoAliasCall(V))
334 return true;
335 if (const Argument *A = dyn_cast<Argument>(V))
336 return A->hasNoAliasAttr() || A->hasByValAttr();
337 return false;
338 }
339
340 // Because of the way .a files work, we must force the BasicAA implementation to
341 // be pulled in if the AliasAnalysis classes are pulled in. Otherwise we run
342 // the risk of AliasAnalysis being used, but the default implementation not
343 // being linked into the tool that uses it.
344 DEFINING_FILE_FOR(AliasAnalysis)
345