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//===- 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
// 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;
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 {
static char ID; // Class identification, replacement for typeinfo
TypeBasedAliasAnalysis() : ImmutablePass(ID) {
virtual void initializePass() {
/// 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;
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();
TypeBasedAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
/// Aliases - Test whether the type represented by A may alias the
/// type represented by B.
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())
// 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())
// 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;
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);
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);
TypeBasedAliasAnalysis::getModRefBehavior(const Function *F) {
// Functions don't have metadata. Just chain to the next implementation.
return AliasAnalysis::getModRefBehavior(F);
TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
const Location &Loc) {
if (!EnableTBAA)
return AliasAnalysis::getModRefInfo(CS, Loc);
if (const MDNode *L = Loc.TBAATag)
if (const MDNode *M =
if (!Aliases(L, M))
return NoModRef;
return AliasAnalysis::getModRefInfo(CS, Loc);
TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
ImmutableCallSite CS2) {
if (!EnableTBAA)
return AliasAnalysis::getModRefInfo(CS1, CS2);
if (const MDNode *M1 =
if (const MDNode *M2 =
if (!Aliases(M1, M2))
return NoModRef;
return AliasAnalysis::getModRefInfo(CS1, CS2);