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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 pass is language-independent. The type system is encoded in
// metadata. This allows this pass to support typical C and C++ TBAA, but
// it can also support custom aliasing behavior for other languages.
//
// This is a work-in-progress. It doesn't work yet, and the metadata
// format isn't stable.
//
// TODO: getModRefBehavior. The AliasAnalysis infrastructure will need to
// be extended.
// TODO: AA chaining
// TODO: struct fields
//
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/Passes.h"
#include "llvm/Module.h"
#include "llvm/Metadata.h"
#include "llvm/Pass.h"
using namespace llvm;
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(MDNode *N) : Node(N) {}
/// getNode - Get the MDNode for this TBAANode.
const MDNode *getNode() const { return Node; }
/// getParent - Get this TBAANode's Alias DAG parent.
TBAANode getParent() const {
if (Node->getNumOperands() < 2)
return TBAANode();
MDNode *P = dyn_cast<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;
// TODO: Think about the encoding.
return CI->isOne();
}
};
}
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) {}
/// 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 PassInfo *PI) {
if (PI->isPassID(&AliasAnalysis::ID))
return (AliasAnalysis*)this;
return this;
}
private:
virtual void getAnalysisUsage(AnalysisUsage &AU) const;
virtual AliasResult alias(const Value *V1, unsigned V1Size,
const Value *V2, unsigned V2Size);
virtual bool pointsToConstantMemory(const Value *P);
};
} // 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);
}
AliasAnalysis::AliasResult
TypeBasedAliasAnalysis::alias(const Value *A, unsigned ASize,
const Value *B, unsigned BSize) {
// Currently, metadata can only be attached to Instructions.
const Instruction *AI = dyn_cast<Instruction>(A);
if (!AI) return MayAlias;
const Instruction *BI = dyn_cast<Instruction>(B);
if (!BI) return MayAlias;
// Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
// be conservative.
MDNode *AM =
AI->getMetadata(AI->getParent()->getParent()->getParent()
->getMDKindID("tbaa"));
if (!AM) return MayAlias;
MDNode *BM =
BI->getMetadata(BI->getParent()->getParent()->getParent()
->getMDKindID("tbaa"));
if (!BM) return MayAlias;
// Keep track of the root node for A and B.
TBAANode RootA, RootB;
// Climb the DAG from A to see if we reach B.
for (TBAANode T(AM); ; ) {
if (T.getNode() == BM)
// B is an ancestor of A.
return MayAlias;
RootA = T;
T = T.getParent();
if (!T.getNode())
break;
}
// Climb the DAG from B to see if we reach A.
for (TBAANode T(BM); ; ) {
if (T.getNode() == AM)
// A is an ancestor of B.
return MayAlias;
RootB = T;
T = T.getParent();
if (!T.getNode())
break;
}
// Neither node is an ancestor of the other.
// If they have the same root, then we've proved there's no alias.
if (RootA.getNode() == RootB.getNode())
return NoAlias;
// If they have different roots, they're part of different potentially
// unrelated type systems, so we must be conservative.
return MayAlias;
}
bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Value *P) {
// Currently, metadata can only be attached to Instructions.
const Instruction *I = dyn_cast<Instruction>(P);
if (!I) return false;
MDNode *M =
I->getMetadata(I->getParent()->getParent()->getParent()
->getMDKindID("tbaa"));
if (!M) return false;
// If this is an "immutable" type, we can assume the pointer is pointing
// to constant memory.
return TBAANode(M).TypeIsImmutable();
}
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