// 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
+// We now support two types of metadata format: scalar TBAA and struct-path
+// aware TBAA. After all testing cases are upgraded to use struct-path aware
+// TBAA and we can auto-upgrade existing bc files, the support for scalar TBAA
+// can be dropped.
+//
+// The scalar TBAA 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 }
// should return true; see
// http://llvm.org/docs/AliasAnalysis.html#OtherItfs).
//
+// With struct-path aware TBAA, the MDNodes attached to an instruction using
+// "!tbaa" are called path tag nodes.
+//
+// The path tag node has 4 fields with the last field being optional.
+//
+// The first field is the base type node, it can be a struct type node
+// or a scalar type node. The second field is the access type node, it
+// must be a scalar type node. The third field is the offset into the base type.
+// The last field has the same meaning as the last field of our scalar TBAA:
+// it's an integer which if equal to 1 indicates that the access is "constant".
+//
+// The struct type node has a name and a list of pairs, one pair for each member
+// of the struct. The first element of each pair is a type node (a struct type
+// node or a sclar type node), specifying the type of the member, the second
+// element of each pair is the offset of the member.
+//
+// Given an example
+// typedef struct {
+// short s;
+// } A;
+// typedef struct {
+// uint16_t s;
+// A a;
+// } B;
+//
+// For an acess to B.a.s, we attach !5 (a path tag node) to the load/store
+// instruction. The base type is !4 (struct B), the access type is !2 (scalar
+// type short) and the offset is 4.
+//
+// !0 = metadata !{metadata !"Simple C/C++ TBAA"}
+// !1 = metadata !{metadata !"omnipotent char", metadata !0} // Scalar type node
+// !2 = metadata !{metadata !"short", metadata !1} // Scalar type node
+// !3 = metadata !{metadata !"A", metadata !2, i64 0} // Struct type node
+// !4 = metadata !{metadata !"B", metadata !2, i64 0, metadata !3, i64 4}
+// // Struct type node
+// !5 = metadata !{metadata !4, metadata !2, i64 4} // Path tag node
+//
+// The struct type nodes and the scalar type nodes form a type DAG.
+// Root (!0)
+// char (!1) -- edge to Root
+// short (!2) -- edge to char
+// A (!3) -- edge with offset 0 to short
+// B (!4) -- edge with offset 0 to short and edge with offset 4 to A
+//
+// To check if two tags (tagX and tagY) can alias, we start from the base type
+// of tagX, follow the edge with the correct offset in the type DAG and adjust
+// the offset until we reach the base type of tagY or until we reach the Root
+// node.
+// If we reach the base type of tagY, compare the adjusted offset with
+// offset of tagY, return Alias if the offsets are the same, return NoAlias
+// otherwise.
+// If we reach the Root node, perform the above starting from base type of tagY
+// to see if we reach base type of tagX.
+//
+// If they have different roots, they're part of different potentially
+// unrelated type systems, so we return Alias to be conservative.
+// If neither node is an ancestor of the other and they have the same root,
+// then we say NoAlias.
+//
// TODO: The current metadata format doesn't support struct
// fields. For example:
// struct X {
// 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));
-static cl::opt<bool> EnableStructPathTBAA("struct-path-tbaa", cl::init(false));
namespace {
/// TBAANode - This is a simple wrapper around an MDNode which provides a
if (Node->getNumOperands() < 2)
return TBAAStructTypeNode();
- // Special handling for a scalar type node.
+ // Fast path for a scalar type node and a struct type node with a single
+ // field.
if (Node->getNumOperands() <= 3) {
+ uint64_t Cur = Node->getNumOperands() == 2 ? 0 :
+ cast<ConstantInt>(Node->getOperand(2))->getZExtValue();
+ Offset -= Cur;
MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(1));
if (!P)
return TBAAStructTypeNode();
initializeTypeBasedAliasAnalysisPass(*PassRegistry::getPassRegistry());
}
- virtual void initializePass() {
+ void initializePass() override {
InitializeAliasAnalysis(this);
}
/// 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) {
+ void *getAdjustedAnalysisPointer(const void *PI) override {
if (PI == &AliasAnalysis::ID)
return (AliasAnalysis*)this;
return this;
bool PathAliases(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);
+ void getAnalysisUsage(AnalysisUsage &AU) const override;
+ AliasResult alias(const Location &LocA, const Location &LocB) override;
+ bool pointsToConstantMemory(const Location &Loc, bool OrLocal) override;
+ ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override;
+ ModRefBehavior getModRefBehavior(const Function *F) override;
+ ModRefResult getModRefInfo(ImmutableCallSite CS,
+ const Location &Loc) override;
+ ModRefResult getModRefInfo(ImmutableCallSite CS1,
+ ImmutableCallSite CS2) override;
};
} // End of anonymous namespace
AliasAnalysis::getAnalysisUsage(AU);
}
+/// Check the first operand of the tbaa tag node, if it is a MDNode, we treat
+/// it as struct-path aware TBAA format, otherwise, we treat it as scalar TBAA
+/// format.
+static bool isStructPathTBAA(const MDNode *MD) {
+ // Anonymous TBAA root starts with a MDNode and dragonegg uses it as
+ // a TBAA tag.
+ return isa<MDNode>(MD->getOperand(0)) && MD->getNumOperands() >= 3;
+}
+
/// 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 {
- if (EnableStructPathTBAA)
+ if (isStructPathTBAA(A))
return PathAliases(A, B);
// Keep track of the root node for A and B.
// If this is an "immutable" type, we can assume the pointer is pointing
// to constant memory.
- if ((!EnableStructPathTBAA && TBAANode(M).TypeIsImmutable()) ||
- (EnableStructPathTBAA && TBAAStructTagNode(M).TypeIsImmutable()))
+ if ((!isStructPathTBAA(M) && TBAANode(M).TypeIsImmutable()) ||
+ (isStructPathTBAA(M) && TBAAStructTagNode(M).TypeIsImmutable()))
return true;
return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
// 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 ((!EnableStructPathTBAA && TBAANode(M).TypeIsImmutable()) ||
- (EnableStructPathTBAA && TBAAStructTagNode(M).TypeIsImmutable()))
+ if ((!isStructPathTBAA(M) && TBAANode(M).TypeIsImmutable()) ||
+ (isStructPathTBAA(M) && TBAAStructTagNode(M).TypeIsImmutable()))
Min = OnlyReadsMemory;
return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
}
bool MDNode::isTBAAVtableAccess() const {
- if (!EnableStructPathTBAA) {
+ if (!isStructPathTBAA(this)) {
if (getNumOperands() < 1) return false;
if (MDString *Tag1 = dyn_cast<MDString>(getOperand(0))) {
if (Tag1->getString() == "vtable pointer") return true;
return A;
// For struct-path aware TBAA, we use the access type of the tag.
- if (EnableStructPathTBAA) {
+ bool StructPath = isStructPathTBAA(A);
+ if (StructPath) {
A = cast_or_null<MDNode>(A->getOperand(1));
if (!A) return 0;
B = cast_or_null<MDNode>(B->getOperand(1));
--IA;
--IB;
}
- if (!EnableStructPathTBAA)
+ if (!StructPath)
return Ret;
if (!Ret)
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