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llvm/lib/Analysis/TypeBasedAliasAnalysis.cpp
Alexander Kornienko cd52a7a381 Revert r240137 (Fixed/added namespace ending comments using clang-tidy. NFC) 
Apparently, the style needs to be agreed upon first.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@240390 91177308-0d34-0410-b5e6-96231b3b80d8
2015-06-23 09:49:53 +00:00

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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.
//
// 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 }
// !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).
//
// 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 {
// 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/Passes.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/ADT/SetVector.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(nullptr) {}
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 = mdconst::dyn_extract<ConstantInt>(Node->getOperand(2));
if (!CI)
return false;
return CI->getValue()[0];
}
};
/// 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 TBAAStructTagNode {
/// This node should be created with createTBAAStructTagNode.
const MDNode *Node;
public:
explicit TBAAStructTagNode(const MDNode *N) : Node(N) {}
/// Get the MDNode for this TBAAStructTagNode.
const MDNode *getNode() const { return Node; }
const MDNode *getBaseType() const {
return dyn_cast_or_null<MDNode>(Node->getOperand(0));
}
const MDNode *getAccessType() const {
return dyn_cast_or_null<MDNode>(Node->getOperand(1));
}
uint64_t getOffset() const {
return mdconst::extract<ConstantInt>(Node->getOperand(2))->getZExtValue();
}
/// TypeIsImmutable - Test if this TBAAStructTagNode 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() < 4)
return false;
ConstantInt *CI = mdconst::dyn_extract<ConstantInt>(Node->getOperand(3));
if (!CI)
return false;
return CI->getValue()[0];
}
};
/// 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 TBAAStructTypeNode {
/// This node should be created with createTBAAStructTypeNode.
const MDNode *Node;
public:
TBAAStructTypeNode() : Node(nullptr) {}
explicit TBAAStructTypeNode(const MDNode *N) : Node(N) {}
/// Get the MDNode for this TBAAStructTypeNode.
const MDNode *getNode() const { return Node; }
/// Get this TBAAStructTypeNode's field in the type DAG with
/// given offset. Update the offset to be relative to the field type.
TBAAStructTypeNode getParent(uint64_t &Offset) const {
// Parent can be omitted for the root node.
if (Node->getNumOperands() < 2)
return TBAAStructTypeNode();
// 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
: mdconst::extract<ConstantInt>(Node->getOperand(2))
->getZExtValue();
Offset -= Cur;
MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(1));
if (!P)
return TBAAStructTypeNode();
return TBAAStructTypeNode(P);
}
// Assume the offsets are in order. We return the previous field if
// the current offset is bigger than the given offset.
unsigned TheIdx = 0;
for (unsigned Idx = 1; Idx < Node->getNumOperands(); Idx += 2) {
uint64_t Cur = mdconst::extract<ConstantInt>(Node->getOperand(Idx + 1))
->getZExtValue();
if (Cur > Offset) {
assert(Idx >= 3 &&
"TBAAStructTypeNode::getParent should have an offset match!");
TheIdx = Idx - 2;
break;
}
}
// Move along the last field.
if (TheIdx == 0)
TheIdx = Node->getNumOperands() - 2;
uint64_t Cur = mdconst::extract<ConstantInt>(Node->getOperand(TheIdx + 1))
->getZExtValue();
Offset -= Cur;
MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(TheIdx));
if (!P)
return TBAAStructTypeNode();
return TBAAStructTypeNode(P);
}
};
}
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());
}
bool doInitialization(Module &M) override;
/// 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.
void *getAdjustedAnalysisPointer(const void *PI) override {
if (PI == &AliasAnalysis::ID)
return (AliasAnalysis*)this;
return this;
}
bool Aliases(const MDNode *A, const MDNode *B) const;
bool PathAliases(const MDNode *A, const MDNode *B) const;
private:
void getAnalysisUsage(AnalysisUsage &AU) const override;
AliasResult alias(const MemoryLocation &LocA,
const MemoryLocation &LocB) override;
bool pointsToConstantMemory(const MemoryLocation &Loc,
bool OrLocal) override;
ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override;
ModRefBehavior getModRefBehavior(const Function *F) override;
ModRefResult getModRefInfo(ImmutableCallSite CS,
const MemoryLocation &Loc) override;
ModRefResult getModRefInfo(ImmutableCallSite CS1,
ImmutableCallSite CS2) override;
};
} // 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();
}
bool TypeBasedAliasAnalysis::doInitialization(Module &M) {
InitializeAliasAnalysis(this, &M.getDataLayout());
return true;
}
void
TypeBasedAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesAll();
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 {
// Make sure that both MDNodes are struct-path aware.
if (isStructPathTBAA(A) && isStructPathTBAA(B))
return PathAliases(A, B);
// 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;
}
/// Test whether the struct-path tag represented by A may alias the
/// struct-path tag represented by B.
bool
TypeBasedAliasAnalysis::PathAliases(const MDNode *A,
const MDNode *B) const {
// Verify that both input nodes are struct-path aware.
assert(isStructPathTBAA(A) && "MDNode A is not struct-path aware.");
assert(isStructPathTBAA(B) && "MDNode B is not struct-path aware.");
// Keep track of the root node for A and B.
TBAAStructTypeNode RootA, RootB;
TBAAStructTagNode TagA(A), TagB(B);
// TODO: We need to check if AccessType of TagA encloses AccessType of
// TagB to support aggregate AccessType. If yes, return true.
// Start from the base type of A, follow the edge with the correct offset in
// the type DAG and adjust the offset until we reach the base type of B or
// until we reach the Root node.
// Compare the adjusted offset once we have the same base.
// Climb the type DAG from base type of A to see if we reach base type of B.
const MDNode *BaseA = TagA.getBaseType();
const MDNode *BaseB = TagB.getBaseType();
uint64_t OffsetA = TagA.getOffset(), OffsetB = TagB.getOffset();
for (TBAAStructTypeNode T(BaseA); ; ) {
if (T.getNode() == BaseB)
// Base type of A encloses base type of B, check if the offsets match.
return OffsetA == OffsetB;
RootA = T;
// Follow the edge with the correct offset, OffsetA will be adjusted to
// be relative to the field type.
T = T.getParent(OffsetA);
if (!T.getNode())
break;
}
// Reset OffsetA and climb the type DAG from base type of B to see if we reach
// base type of A.
OffsetA = TagA.getOffset();
for (TBAAStructTypeNode T(BaseB); ; ) {
if (T.getNode() == BaseA)
// Base type of B encloses base type of A, check if the offsets match.
return OffsetA == OffsetB;
RootB = T;
// Follow the edge with the correct offset, OffsetB will be adjusted to
// be relative to the field type.
T = T.getParent(OffsetB);
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;
}
AliasResult TypeBasedAliasAnalysis::alias(const MemoryLocation &LocA,
const MemoryLocation &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.AATags.TBAA;
if (!AM) return AliasAnalysis::alias(LocA, LocB);
const MDNode *BM = LocB.AATags.TBAA;
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 MemoryLocation &Loc,
bool OrLocal) {
if (!EnableTBAA)
return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
const MDNode *M = Loc.AATags.TBAA;
if (!M) return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
// If this is an "immutable" type, we can assume the pointer is pointing
// to constant memory.
if ((!isStructPathTBAA(M) && TBAANode(M).TypeIsImmutable()) ||
(isStructPathTBAA(M) && TBAAStructTagNode(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 ((!isStructPathTBAA(M) && TBAANode(M).TypeIsImmutable()) ||
(isStructPathTBAA(M) && TBAAStructTagNode(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 MemoryLocation &Loc) {
if (!EnableTBAA)
return AliasAnalysis::getModRefInfo(CS, Loc);
if (const MDNode *L = Loc.AATags.TBAA)
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);
}
bool MDNode::isTBAAVtableAccess() const {
if (!isStructPathTBAA(this)) {
if (getNumOperands() < 1) return false;
if (MDString *Tag1 = dyn_cast<MDString>(getOperand(0))) {
if (Tag1->getString() == "vtable pointer") return true;
}
return false;
}
// For struct-path aware TBAA, we use the access type of the tag.
if (getNumOperands() < 2) return false;
MDNode *Tag = cast_or_null<MDNode>(getOperand(1));
if (!Tag) return false;
if (MDString *Tag1 = dyn_cast<MDString>(Tag->getOperand(0))) {
if (Tag1->getString() == "vtable pointer") return true;
}
return false;
}
MDNode *MDNode::getMostGenericTBAA(MDNode *A, MDNode *B) {
if (!A || !B)
return nullptr;
if (A == B)
return A;
// For struct-path aware TBAA, we use the access type of the tag.
bool StructPath = isStructPathTBAA(A) && isStructPathTBAA(B);
if (StructPath) {
A = cast_or_null<MDNode>(A->getOperand(1));
if (!A) return nullptr;
B = cast_or_null<MDNode>(B->getOperand(1));
if (!B) return nullptr;
}
SmallSetVector<MDNode *, 4> PathA;
MDNode *T = A;
while (T) {
if (PathA.count(T))
report_fatal_error("Cycle found in TBAA metadata.");
PathA.insert(T);
T = T->getNumOperands() >= 2 ? cast_or_null<MDNode>(T->getOperand(1))
: nullptr;
}
SmallSetVector<MDNode *, 4> PathB;
T = B;
while (T) {
if (PathB.count(T))
report_fatal_error("Cycle found in TBAA metadata.");
PathB.insert(T);
T = T->getNumOperands() >= 2 ? cast_or_null<MDNode>(T->getOperand(1))
: nullptr;
}
int IA = PathA.size() - 1;
int IB = PathB.size() - 1;
MDNode *Ret = nullptr;
while (IA >= 0 && IB >=0) {
if (PathA[IA] == PathB[IB])
Ret = PathA[IA];
else
break;
--IA;
--IB;
}
if (!StructPath)
return Ret;
if (!Ret)
return nullptr;
// We need to convert from a type node to a tag node.
Type *Int64 = IntegerType::get(A->getContext(), 64);
Metadata *Ops[3] = {Ret, Ret,
ConstantAsMetadata::get(ConstantInt::get(Int64, 0))};
return MDNode::get(A->getContext(), Ops);
}
void Instruction::getAAMetadata(AAMDNodes &N, bool Merge) const {
if (Merge)
N.TBAA =
MDNode::getMostGenericTBAA(N.TBAA, getMetadata(LLVMContext::MD_tbaa));
else
N.TBAA = getMetadata(LLVMContext::MD_tbaa);
if (Merge)
N.Scope = MDNode::getMostGenericAliasScope(
N.Scope, getMetadata(LLVMContext::MD_alias_scope));
else
N.Scope = getMetadata(LLVMContext::MD_alias_scope);
if (Merge)
N.NoAlias =
MDNode::intersect(N.NoAlias, getMetadata(LLVMContext::MD_noalias));
else
N.NoAlias = getMetadata(LLVMContext::MD_noalias);
}
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