1 //===- TypeBasedAliasAnalysis.cpp - Type-Based Alias Analysis -------------===//
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 defines the TypeBasedAliasAnalysis pass, which implements
11 // metadata-based TBAA.
13 // In LLVM IR, memory does not have types, so LLVM's own type system is not
14 // suitable for doing TBAA. Instead, metadata is added to the IR to describe
15 // a type system of a higher level language. This can be used to implement
16 // typical C/C++ TBAA, but it can also be used to implement custom alias
17 // analysis behavior for other languages.
19 // We now support two types of metadata format: scalar TBAA and struct-path
20 // aware TBAA. After all testing cases are upgraded to use struct-path aware
21 // TBAA and we can auto-upgrade existing bc files, the support for scalar TBAA
24 // The scalar TBAA metadata format is very simple. TBAA MDNodes have up to
25 // three fields, e.g.:
26 // !0 = metadata !{ metadata !"an example type tree" }
27 // !1 = metadata !{ metadata !"int", metadata !0 }
28 // !2 = metadata !{ metadata !"float", metadata !0 }
29 // !3 = metadata !{ metadata !"const float", metadata !2, i64 1 }
31 // The first field is an identity field. It can be any value, usually
32 // an MDString, which uniquely identifies the type. The most important
33 // name in the tree is the name of the root node. Two trees with
34 // different root node names are entirely disjoint, even if they
35 // have leaves with common names.
37 // The second field identifies the type's parent node in the tree, or
38 // is null or omitted for a root node. A type is considered to alias
39 // all of its descendants and all of its ancestors in the tree. Also,
40 // a type is considered to alias all types in other trees, so that
41 // bitcode produced from multiple front-ends is handled conservatively.
43 // If the third field is present, it's an integer which if equal to 1
44 // indicates that the type is "constant" (meaning pointsToConstantMemory
45 // should return true; see
46 // http://llvm.org/docs/AliasAnalysis.html#OtherItfs).
48 // With struct-path aware TBAA, the MDNodes attached to an instruction using
49 // "!tbaa" are called path tag nodes.
51 // The path tag node has 4 fields with the last field being optional.
53 // The first field is the base type node, it can be a struct type node
54 // or a scalar type node. The second field is the access type node, it
55 // must be a scalar type node. The third field is the offset into the base type.
56 // The last field has the same meaning as the last field of our scalar TBAA:
57 // it's an integer which if equal to 1 indicates that the access is "constant".
59 // The struct type node has a name and a list of pairs, one pair for each member
60 // of the struct. The first element of each pair is a type node (a struct type
61 // node or a sclar type node), specifying the type of the member, the second
62 // element of each pair is the offset of the member.
73 // For an acess to B.a.s, we attach !5 (a path tag node) to the load/store
74 // instruction. The base type is !4 (struct B), the access type is !2 (scalar
75 // type short) and the offset is 4.
77 // !0 = metadata !{metadata !"Simple C/C++ TBAA"}
78 // !1 = metadata !{metadata !"omnipotent char", metadata !0} // Scalar type node
79 // !2 = metadata !{metadata !"short", metadata !1} // Scalar type node
80 // !3 = metadata !{metadata !"A", metadata !2, i64 0} // Struct type node
81 // !4 = metadata !{metadata !"B", metadata !2, i64 0, metadata !3, i64 4}
82 // // Struct type node
83 // !5 = metadata !{metadata !4, metadata !2, i64 4} // Path tag node
85 // The struct type nodes and the scalar type nodes form a type DAG.
87 // char (!1) -- edge to Root
88 // short (!2) -- edge to char
89 // A (!3) -- edge with offset 0 to short
90 // B (!4) -- edge with offset 0 to short and edge with offset 4 to A
92 // To check if two tags (tagX and tagY) can alias, we start from the base type
93 // of tagX, follow the edge with the correct offset in the type DAG and adjust
94 // the offset until we reach the base type of tagY or until we reach the Root
96 // If we reach the base type of tagY, compare the adjusted offset with
97 // offset of tagY, return Alias if the offsets are the same, return NoAlias
99 // If we reach the Root node, perform the above starting from base type of tagY
100 // to see if we reach base type of tagX.
102 // If they have different roots, they're part of different potentially
103 // unrelated type systems, so we return Alias to be conservative.
104 // If neither node is an ancestor of the other and they have the same root,
105 // then we say NoAlias.
107 // TODO: The current metadata format doesn't support struct
108 // fields. For example:
113 // void foo(struct X *x, struct X *y, double *p) {
117 // Struct X has a double member, so the store to *x can alias the store to *p.
118 // Currently it's not possible to precisely describe all the things struct X
119 // aliases, so struct assignments must use conservative TBAA nodes. There's
120 // no scheme for attaching metadata to @llvm.memcpy yet either.
122 //===----------------------------------------------------------------------===//
124 #include "llvm/Analysis/Passes.h"
125 #include "llvm/Analysis/AliasAnalysis.h"
126 #include "llvm/IR/Constants.h"
127 #include "llvm/IR/LLVMContext.h"
128 #include "llvm/IR/Metadata.h"
129 #include "llvm/IR/Module.h"
130 #include "llvm/Pass.h"
131 #include "llvm/Support/CommandLine.h"
132 using namespace llvm;
134 // A handy option for disabling TBAA functionality. The same effect can also be
135 // achieved by stripping the !tbaa tags from IR, but this option is sometimes
137 static cl::opt<bool> EnableTBAA("enable-tbaa", cl::init(true));
140 /// TBAANode - This is a simple wrapper around an MDNode which provides a
141 /// higher-level interface by hiding the details of how alias analysis
142 /// information is encoded in its operands.
147 TBAANode() : Node(0) {}
148 explicit TBAANode(const MDNode *N) : Node(N) {}
150 /// getNode - Get the MDNode for this TBAANode.
151 const MDNode *getNode() const { return Node; }
153 /// getParent - Get this TBAANode's Alias tree parent.
154 TBAANode getParent() const {
155 if (Node->getNumOperands() < 2)
157 MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(1));
160 // Ok, this node has a valid parent. Return it.
164 /// TypeIsImmutable - Test if this TBAANode represents a type for objects
165 /// which are not modified (by any means) in the context where this
166 /// AliasAnalysis is relevant.
167 bool TypeIsImmutable() const {
168 if (Node->getNumOperands() < 3)
170 ConstantInt *CI = dyn_cast<ConstantInt>(Node->getOperand(2));
173 return CI->getValue()[0];
177 /// This is a simple wrapper around an MDNode which provides a
178 /// higher-level interface by hiding the details of how alias analysis
179 /// information is encoded in its operands.
180 class TBAAStructTagNode {
181 /// This node should be created with createTBAAStructTagNode.
185 TBAAStructTagNode() : Node(0) {}
186 explicit TBAAStructTagNode(const MDNode *N) : Node(N) {}
188 /// Get the MDNode for this TBAAStructTagNode.
189 const MDNode *getNode() const { return Node; }
191 const MDNode *getBaseType() const {
192 return dyn_cast_or_null<MDNode>(Node->getOperand(0));
194 const MDNode *getAccessType() const {
195 return dyn_cast_or_null<MDNode>(Node->getOperand(1));
197 uint64_t getOffset() const {
198 return cast<ConstantInt>(Node->getOperand(2))->getZExtValue();
200 /// TypeIsImmutable - Test if this TBAAStructTagNode represents a type for
201 /// objects which are not modified (by any means) in the context where this
202 /// AliasAnalysis is relevant.
203 bool TypeIsImmutable() const {
204 if (Node->getNumOperands() < 4)
206 ConstantInt *CI = dyn_cast<ConstantInt>(Node->getOperand(3));
209 return CI->getValue()[0];
213 /// This is a simple wrapper around an MDNode which provides a
214 /// higher-level interface by hiding the details of how alias analysis
215 /// information is encoded in its operands.
216 class TBAAStructTypeNode {
217 /// This node should be created with createTBAAStructTypeNode.
221 TBAAStructTypeNode() : Node(0) {}
222 explicit TBAAStructTypeNode(const MDNode *N) : Node(N) {}
224 /// Get the MDNode for this TBAAStructTypeNode.
225 const MDNode *getNode() const { return Node; }
227 /// Get this TBAAStructTypeNode's field in the type DAG with
228 /// given offset. Update the offset to be relative to the field type.
229 TBAAStructTypeNode getParent(uint64_t &Offset) const {
230 // Parent can be omitted for the root node.
231 if (Node->getNumOperands() < 2)
232 return TBAAStructTypeNode();
234 // Special handling for a scalar type node.
235 if (Node->getNumOperands() <= 3) {
236 MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(1));
238 return TBAAStructTypeNode();
239 return TBAAStructTypeNode(P);
242 // Assume the offsets are in order. We return the previous field if
243 // the current offset is bigger than the given offset.
245 for (unsigned Idx = 1; Idx < Node->getNumOperands(); Idx += 2) {
246 uint64_t Cur = cast<ConstantInt>(Node->getOperand(Idx + 1))->
250 "TBAAStructTypeNode::getParent should have an offset match!");
255 // Move along the last field.
257 TheIdx = Node->getNumOperands() - 2;
258 uint64_t Cur = cast<ConstantInt>(Node->getOperand(TheIdx + 1))->
261 MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(TheIdx));
263 return TBAAStructTypeNode();
264 return TBAAStructTypeNode(P);
270 /// TypeBasedAliasAnalysis - This is a simple alias analysis
271 /// implementation that uses TypeBased to answer queries.
272 class TypeBasedAliasAnalysis : public ImmutablePass,
273 public AliasAnalysis {
275 static char ID; // Class identification, replacement for typeinfo
276 TypeBasedAliasAnalysis() : ImmutablePass(ID) {
277 initializeTypeBasedAliasAnalysisPass(*PassRegistry::getPassRegistry());
280 virtual void initializePass() {
281 InitializeAliasAnalysis(this);
284 /// getAdjustedAnalysisPointer - This method is used when a pass implements
285 /// an analysis interface through multiple inheritance. If needed, it
286 /// should override this to adjust the this pointer as needed for the
287 /// specified pass info.
288 virtual void *getAdjustedAnalysisPointer(const void *PI) {
289 if (PI == &AliasAnalysis::ID)
290 return (AliasAnalysis*)this;
294 bool Aliases(const MDNode *A, const MDNode *B) const;
295 bool PathAliases(const MDNode *A, const MDNode *B) const;
298 virtual void getAnalysisUsage(AnalysisUsage &AU) const;
299 virtual AliasResult alias(const Location &LocA, const Location &LocB);
300 virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal);
301 virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
302 virtual ModRefBehavior getModRefBehavior(const Function *F);
303 virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
304 const Location &Loc);
305 virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
306 ImmutableCallSite CS2);
308 } // End of anonymous namespace
310 // Register this pass...
311 char TypeBasedAliasAnalysis::ID = 0;
312 INITIALIZE_AG_PASS(TypeBasedAliasAnalysis, AliasAnalysis, "tbaa",
313 "Type-Based Alias Analysis", false, true, false)
315 ImmutablePass *llvm::createTypeBasedAliasAnalysisPass() {
316 return new TypeBasedAliasAnalysis();
320 TypeBasedAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
321 AU.setPreservesAll();
322 AliasAnalysis::getAnalysisUsage(AU);
325 /// Check the first operand of the tbaa tag node, if it is a MDNode, we treat
326 /// it as struct-path aware TBAA format, otherwise, we treat it as scalar TBAA
328 static bool isStructPathTBAA(const MDNode *MD) {
329 return isa<MDNode>(MD->getOperand(0));
332 /// Aliases - Test whether the type represented by A may alias the
333 /// type represented by B.
335 TypeBasedAliasAnalysis::Aliases(const MDNode *A,
336 const MDNode *B) const {
337 if (isStructPathTBAA(A))
338 return PathAliases(A, B);
340 // Keep track of the root node for A and B.
341 TBAANode RootA, RootB;
343 // Climb the tree from A to see if we reach B.
344 for (TBAANode T(A); ; ) {
345 if (T.getNode() == B)
346 // B is an ancestor of A.
355 // Climb the tree from B to see if we reach A.
356 for (TBAANode T(B); ; ) {
357 if (T.getNode() == A)
358 // A is an ancestor of B.
367 // Neither node is an ancestor of the other.
369 // If they have different roots, they're part of different potentially
370 // unrelated type systems, so we must be conservative.
371 if (RootA.getNode() != RootB.getNode())
374 // If they have the same root, then we've proved there's no alias.
378 /// Test whether the struct-path tag represented by A may alias the
379 /// struct-path tag represented by B.
381 TypeBasedAliasAnalysis::PathAliases(const MDNode *A,
382 const MDNode *B) const {
383 // Keep track of the root node for A and B.
384 TBAAStructTypeNode RootA, RootB;
385 TBAAStructTagNode TagA(A), TagB(B);
387 // TODO: We need to check if AccessType of TagA encloses AccessType of
388 // TagB to support aggregate AccessType. If yes, return true.
390 // Start from the base type of A, follow the edge with the correct offset in
391 // the type DAG and adjust the offset until we reach the base type of B or
392 // until we reach the Root node.
393 // Compare the adjusted offset once we have the same base.
395 // Climb the type DAG from base type of A to see if we reach base type of B.
396 const MDNode *BaseA = TagA.getBaseType();
397 const MDNode *BaseB = TagB.getBaseType();
398 uint64_t OffsetA = TagA.getOffset(), OffsetB = TagB.getOffset();
399 for (TBAAStructTypeNode T(BaseA); ; ) {
400 if (T.getNode() == BaseB)
401 // Base type of A encloses base type of B, check if the offsets match.
402 return OffsetA == OffsetB;
405 // Follow the edge with the correct offset, OffsetA will be adjusted to
406 // be relative to the field type.
407 T = T.getParent(OffsetA);
412 // Reset OffsetA and climb the type DAG from base type of B to see if we reach
414 OffsetA = TagA.getOffset();
415 for (TBAAStructTypeNode T(BaseB); ; ) {
416 if (T.getNode() == BaseA)
417 // Base type of B encloses base type of A, check if the offsets match.
418 return OffsetA == OffsetB;
421 // Follow the edge with the correct offset, OffsetB will be adjusted to
422 // be relative to the field type.
423 T = T.getParent(OffsetB);
428 // Neither node is an ancestor of the other.
430 // If they have different roots, they're part of different potentially
431 // unrelated type systems, so we must be conservative.
432 if (RootA.getNode() != RootB.getNode())
435 // If they have the same root, then we've proved there's no alias.
439 AliasAnalysis::AliasResult
440 TypeBasedAliasAnalysis::alias(const Location &LocA,
441 const Location &LocB) {
443 return AliasAnalysis::alias(LocA, LocB);
445 // Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
447 const MDNode *AM = LocA.TBAATag;
448 if (!AM) return AliasAnalysis::alias(LocA, LocB);
449 const MDNode *BM = LocB.TBAATag;
450 if (!BM) return AliasAnalysis::alias(LocA, LocB);
452 // If they may alias, chain to the next AliasAnalysis.
454 return AliasAnalysis::alias(LocA, LocB);
456 // Otherwise return a definitive result.
460 bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Location &Loc,
463 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
465 const MDNode *M = Loc.TBAATag;
466 if (!M) return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
468 // If this is an "immutable" type, we can assume the pointer is pointing
469 // to constant memory.
470 if ((!isStructPathTBAA(M) && TBAANode(M).TypeIsImmutable()) ||
471 (isStructPathTBAA(M) && TBAAStructTagNode(M).TypeIsImmutable()))
474 return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
477 AliasAnalysis::ModRefBehavior
478 TypeBasedAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
480 return AliasAnalysis::getModRefBehavior(CS);
482 ModRefBehavior Min = UnknownModRefBehavior;
484 // If this is an "immutable" type, we can assume the call doesn't write
486 if (const MDNode *M = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
487 if ((!isStructPathTBAA(M) && TBAANode(M).TypeIsImmutable()) ||
488 (isStructPathTBAA(M) && TBAAStructTagNode(M).TypeIsImmutable()))
489 Min = OnlyReadsMemory;
491 return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
494 AliasAnalysis::ModRefBehavior
495 TypeBasedAliasAnalysis::getModRefBehavior(const Function *F) {
496 // Functions don't have metadata. Just chain to the next implementation.
497 return AliasAnalysis::getModRefBehavior(F);
500 AliasAnalysis::ModRefResult
501 TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
502 const Location &Loc) {
504 return AliasAnalysis::getModRefInfo(CS, Loc);
506 if (const MDNode *L = Loc.TBAATag)
507 if (const MDNode *M =
508 CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
512 return AliasAnalysis::getModRefInfo(CS, Loc);
515 AliasAnalysis::ModRefResult
516 TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
517 ImmutableCallSite CS2) {
519 return AliasAnalysis::getModRefInfo(CS1, CS2);
521 if (const MDNode *M1 =
522 CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
523 if (const MDNode *M2 =
524 CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
525 if (!Aliases(M1, M2))
528 return AliasAnalysis::getModRefInfo(CS1, CS2);
531 bool MDNode::isTBAAVtableAccess() const {
532 if (!isStructPathTBAA(this)) {
533 if (getNumOperands() < 1) return false;
534 if (MDString *Tag1 = dyn_cast<MDString>(getOperand(0))) {
535 if (Tag1->getString() == "vtable pointer") return true;
540 // For struct-path aware TBAA, we use the access type of the tag.
541 if (getNumOperands() < 2) return false;
542 MDNode *Tag = cast_or_null<MDNode>(getOperand(1));
543 if (!Tag) return false;
544 if (MDString *Tag1 = dyn_cast<MDString>(Tag->getOperand(0))) {
545 if (Tag1->getString() == "vtable pointer") return true;
550 MDNode *MDNode::getMostGenericTBAA(MDNode *A, MDNode *B) {
557 // For struct-path aware TBAA, we use the access type of the tag.
558 bool StructPath = isStructPathTBAA(A);
560 A = cast_or_null<MDNode>(A->getOperand(1));
562 B = cast_or_null<MDNode>(B->getOperand(1));
566 SmallVector<MDNode *, 4> PathA;
570 T = T->getNumOperands() >= 2 ? cast_or_null<MDNode>(T->getOperand(1)) : 0;
573 SmallVector<MDNode *, 4> PathB;
577 T = T->getNumOperands() >= 2 ? cast_or_null<MDNode>(T->getOperand(1)) : 0;
580 int IA = PathA.size() - 1;
581 int IB = PathB.size() - 1;
584 while (IA >= 0 && IB >=0) {
585 if (PathA[IA] == PathB[IB])
597 // We need to convert from a type node to a tag node.
598 Type *Int64 = IntegerType::get(A->getContext(), 64);
599 Value *Ops[3] = { Ret, Ret, ConstantInt::get(Int64, 0) };
600 return MDNode::get(A->getContext(), Ops);