| //===- TypeBasedAliasAnalysis.cpp - Type-Based Alias Analysis -------------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file defines the TypeBasedAliasAnalysis pass, which implements |
| // metadata-based TBAA. |
| // |
| // In LLVM IR, memory does not have types, so LLVM's own type system is not |
| // suitable for doing TBAA. Instead, metadata is added to the IR to describe |
| // a type system of a higher level language. This can be used to implement |
| // typical C/C++ TBAA, but it can also be used to implement custom alias |
| // analysis behavior for other languages. |
| // |
| // The current metadata format is very simple. TBAA MDNodes have up to |
| // three fields, e.g.: |
| // !0 = metadata !{ metadata !"an example type tree" } |
| // !1 = metadata !{ metadata !"int", metadata !0 } |
| // !2 = metadata !{ metadata !"float", metadata !0 } |
| // !3 = metadata !{ metadata !"const float", metadata !2, i64 1 } |
| // |
| // The first field is an identity field. It can be any value, usually |
| // an MDString, which uniquely identifies the type. The most important |
| // name in the tree is the name of the root node. Two trees with |
| // different root node names are entirely disjoint, even if they |
| // have leaves with common names. |
| // |
| // The second field identifies the type's parent node in the tree, or |
| // is null or omitted for a root node. A type is considered to alias |
| // all of its descendants and all of its ancestors in the tree. Also, |
| // a type is considered to alias all types in other trees, so that |
| // bitcode produced from multiple front-ends is handled conservatively. |
| // |
| // If the third field is present, it's an integer which if equal to 1 |
| // indicates that the type is "constant" (meaning pointsToConstantMemory |
| // should return true; see |
| // http://llvm.org/docs/AliasAnalysis.html#OtherItfs). |
| // |
| // TODO: The current metadata format doesn't support struct |
| // fields. For example: |
| // struct X { |
| // double d; |
| // int i; |
| // }; |
| // void foo(struct X *x, struct X *y, double *p) { |
| // *x = *y; |
| // *p = 0.0; |
| // } |
| // Struct X has a double member, so the store to *x can alias the store to *p. |
| // Currently it's not possible to precisely describe all the things struct X |
| // aliases, so struct assignments must use conservative TBAA nodes. There's |
| // no scheme for attaching metadata to @llvm.memcpy yet either. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #include "llvm/Analysis/AliasAnalysis.h" |
| #include "llvm/Analysis/Passes.h" |
| #include "llvm/Constants.h" |
| #include "llvm/LLVMContext.h" |
| #include "llvm/Module.h" |
| #include "llvm/Metadata.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Support/CommandLine.h" |
| using namespace llvm; |
| |
| // A handy option for disabling TBAA functionality. The same effect can also be |
| // achieved by stripping the !tbaa tags from IR, but this option is sometimes |
| // more convenient. |
| static cl::opt<bool> EnableTBAA("enable-tbaa", cl::init(true)); |
| |
| namespace { |
| /// TBAANode - This is a simple wrapper around an MDNode which provides a |
| /// higher-level interface by hiding the details of how alias analysis |
| /// information is encoded in its operands. |
| class TBAANode { |
| const MDNode *Node; |
| |
| public: |
| TBAANode() : Node(0) {} |
| explicit TBAANode(const MDNode *N) : Node(N) {} |
| |
| /// getNode - Get the MDNode for this TBAANode. |
| const MDNode *getNode() const { return Node; } |
| |
| /// getParent - Get this TBAANode's Alias tree parent. |
| TBAANode getParent() const { |
| if (Node->getNumOperands() < 2) |
| return TBAANode(); |
| MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(1)); |
| if (!P) |
| return TBAANode(); |
| // Ok, this node has a valid parent. Return it. |
| return TBAANode(P); |
| } |
| |
| /// TypeIsImmutable - Test if this TBAANode represents a type for objects |
| /// which are not modified (by any means) in the context where this |
| /// AliasAnalysis is relevant. |
| bool TypeIsImmutable() const { |
| if (Node->getNumOperands() < 3) |
| return false; |
| ConstantInt *CI = dyn_cast<ConstantInt>(Node->getOperand(2)); |
| if (!CI) |
| return false; |
| return CI->getValue()[0]; |
| } |
| }; |
| } |
| |
| namespace { |
| /// TypeBasedAliasAnalysis - This is a simple alias analysis |
| /// implementation that uses TypeBased to answer queries. |
| class TypeBasedAliasAnalysis : public ImmutablePass, |
| public AliasAnalysis { |
| public: |
| static char ID; // Class identification, replacement for typeinfo |
| TypeBasedAliasAnalysis() : ImmutablePass(ID) { |
| initializeTypeBasedAliasAnalysisPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| virtual void initializePass() { |
| InitializeAliasAnalysis(this); |
| } |
| |
| /// getAdjustedAnalysisPointer - This method is used when a pass implements |
| /// an analysis interface through multiple inheritance. If needed, it |
| /// should override this to adjust the this pointer as needed for the |
| /// specified pass info. |
| virtual void *getAdjustedAnalysisPointer(const void *PI) { |
| if (PI == &AliasAnalysis::ID) |
| return (AliasAnalysis*)this; |
| return this; |
| } |
| |
| bool Aliases(const MDNode *A, const MDNode *B) const; |
| |
| private: |
| virtual void getAnalysisUsage(AnalysisUsage &AU) const; |
| virtual AliasResult alias(const Location &LocA, const Location &LocB); |
| virtual bool pointsToConstantMemory(const Location &Loc, bool OrLocal); |
| virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS); |
| virtual ModRefBehavior getModRefBehavior(const Function *F); |
| virtual ModRefResult getModRefInfo(ImmutableCallSite CS, |
| const Location &Loc); |
| virtual ModRefResult getModRefInfo(ImmutableCallSite CS1, |
| ImmutableCallSite CS2); |
| }; |
| } // End of anonymous namespace |
| |
| // Register this pass... |
| char TypeBasedAliasAnalysis::ID = 0; |
| INITIALIZE_AG_PASS(TypeBasedAliasAnalysis, AliasAnalysis, "tbaa", |
| "Type-Based Alias Analysis", false, true, false) |
| |
| ImmutablePass *llvm::createTypeBasedAliasAnalysisPass() { |
| return new TypeBasedAliasAnalysis(); |
| } |
| |
| void |
| TypeBasedAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.setPreservesAll(); |
| AliasAnalysis::getAnalysisUsage(AU); |
| } |
| |
| /// Aliases - Test whether the type represented by A may alias the |
| /// type represented by B. |
| bool |
| TypeBasedAliasAnalysis::Aliases(const MDNode *A, |
| const MDNode *B) const { |
| // Keep track of the root node for A and B. |
| TBAANode RootA, RootB; |
| |
| // Climb the tree from A to see if we reach B. |
| for (TBAANode T(A); ; ) { |
| if (T.getNode() == B) |
| // B is an ancestor of A. |
| return true; |
| |
| RootA = T; |
| T = T.getParent(); |
| if (!T.getNode()) |
| break; |
| } |
| |
| // Climb the tree from B to see if we reach A. |
| for (TBAANode T(B); ; ) { |
| if (T.getNode() == A) |
| // A is an ancestor of B. |
| return true; |
| |
| RootB = T; |
| T = T.getParent(); |
| if (!T.getNode()) |
| break; |
| } |
| |
| // Neither node is an ancestor of the other. |
| |
| // If they have different roots, they're part of different potentially |
| // unrelated type systems, so we must be conservative. |
| if (RootA.getNode() != RootB.getNode()) |
| return true; |
| |
| // If they have the same root, then we've proved there's no alias. |
| return false; |
| } |
| |
| AliasAnalysis::AliasResult |
| TypeBasedAliasAnalysis::alias(const Location &LocA, |
| const Location &LocB) { |
| if (!EnableTBAA) |
| return AliasAnalysis::alias(LocA, LocB); |
| |
| // Get the attached MDNodes. If either value lacks a tbaa MDNode, we must |
| // be conservative. |
| const MDNode *AM = LocA.TBAATag; |
| if (!AM) return AliasAnalysis::alias(LocA, LocB); |
| const MDNode *BM = LocB.TBAATag; |
| if (!BM) return AliasAnalysis::alias(LocA, LocB); |
| |
| // If they may alias, chain to the next AliasAnalysis. |
| if (Aliases(AM, BM)) |
| return AliasAnalysis::alias(LocA, LocB); |
| |
| // Otherwise return a definitive result. |
| return NoAlias; |
| } |
| |
| bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Location &Loc, |
| bool OrLocal) { |
| if (!EnableTBAA) |
| return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); |
| |
| const MDNode *M = Loc.TBAATag; |
| if (!M) return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); |
| |
| // If this is an "immutable" type, we can assume the pointer is pointing |
| // to constant memory. |
| if (TBAANode(M).TypeIsImmutable()) |
| return true; |
| |
| return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal); |
| } |
| |
| AliasAnalysis::ModRefBehavior |
| TypeBasedAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) { |
| if (!EnableTBAA) |
| return AliasAnalysis::getModRefBehavior(CS); |
| |
| ModRefBehavior Min = UnknownModRefBehavior; |
| |
| // If this is an "immutable" type, we can assume the call doesn't write |
| // to memory. |
| if (const MDNode *M = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa)) |
| if (TBAANode(M).TypeIsImmutable()) |
| Min = OnlyReadsMemory; |
| |
| return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min); |
| } |
| |
| AliasAnalysis::ModRefBehavior |
| TypeBasedAliasAnalysis::getModRefBehavior(const Function *F) { |
| // Functions don't have metadata. Just chain to the next implementation. |
| return AliasAnalysis::getModRefBehavior(F); |
| } |
| |
| AliasAnalysis::ModRefResult |
| TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS, |
| const Location &Loc) { |
| if (!EnableTBAA) |
| return AliasAnalysis::getModRefInfo(CS, Loc); |
| |
| if (const MDNode *L = Loc.TBAATag) |
| if (const MDNode *M = |
| CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa)) |
| if (!Aliases(L, M)) |
| return NoModRef; |
| |
| return AliasAnalysis::getModRefInfo(CS, Loc); |
| } |
| |
| AliasAnalysis::ModRefResult |
| TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS1, |
| ImmutableCallSite CS2) { |
| if (!EnableTBAA) |
| return AliasAnalysis::getModRefInfo(CS1, CS2); |
| |
| if (const MDNode *M1 = |
| CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa)) |
| if (const MDNode *M2 = |
| CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa)) |
| if (!Aliases(M1, M2)) |
| return NoModRef; |
| |
| return AliasAnalysis::getModRefInfo(CS1, CS2); |
| } |