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191 lines
6.0 KiB
C++
191 lines
6.0 KiB
C++
//===- TypeBasedAliasAnalysis.cpp - Type-Based Alias Analysis -------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file defines the TypeBasedAliasAnalysis pass, which implements
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// metadata-based TBAA.
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//
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// In LLVM IR, memory does not have types, so LLVM's own type system is not
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// suitable for doing TBAA. Instead, metadata is added to the IR to describe
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// a type system of a higher level language.
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//
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// This pass is language-independent. The type system is encoded in
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// metadata. This allows this pass to support typical C and C++ TBAA, but
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// it can also support custom aliasing behavior for other languages.
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//
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// This is a work-in-progress. It doesn't work yet, and the metadata
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// format isn't stable.
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//
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// TODO: getModRefBehavior. The AliasAnalysis infrastructure will need to
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// be extended.
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// TODO: struct fields
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/Passes.h"
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#include "llvm/Module.h"
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#include "llvm/Metadata.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/CommandLine.h"
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using namespace llvm;
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// For testing purposes, enable TBAA only via a special option.
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static cl::opt<bool> EnableTBAA("enable-tbaa");
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namespace {
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/// TBAANode - This is a simple wrapper around an MDNode which provides a
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/// higher-level interface by hiding the details of how alias analysis
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/// information is encoded in its operands.
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class TBAANode {
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const MDNode *Node;
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public:
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TBAANode() : Node(0) {}
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explicit TBAANode(const MDNode *N) : Node(N) {}
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/// getNode - Get the MDNode for this TBAANode.
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const MDNode *getNode() const { return Node; }
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/// getParent - Get this TBAANode's Alias DAG parent.
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TBAANode getParent() const {
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if (Node->getNumOperands() < 2)
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return TBAANode();
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MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(1));
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if (!P)
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return TBAANode();
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// Ok, this node has a valid parent. Return it.
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return TBAANode(P);
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}
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/// TypeIsImmutable - Test if this TBAANode represents a type for objects
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/// which are not modified (by any means) in the context where this
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/// AliasAnalysis is relevant.
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bool TypeIsImmutable() const {
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if (Node->getNumOperands() < 3)
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return false;
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ConstantInt *CI = dyn_cast<ConstantInt>(Node->getOperand(2));
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if (!CI)
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return false;
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// TODO: Think about the encoding.
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return CI->isOne();
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}
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};
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}
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namespace {
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/// TypeBasedAliasAnalysis - This is a simple alias analysis
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/// implementation that uses TypeBased to answer queries.
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class TypeBasedAliasAnalysis : public ImmutablePass,
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public AliasAnalysis {
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public:
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static char ID; // Class identification, replacement for typeinfo
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TypeBasedAliasAnalysis() : ImmutablePass(ID) {}
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virtual void initializePass() {
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InitializeAliasAnalysis(this);
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}
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/// getAdjustedAnalysisPointer - This method is used when a pass implements
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/// an analysis interface through multiple inheritance. If needed, it
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/// should override this to adjust the this pointer as needed for the
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/// specified pass info.
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virtual void *getAdjustedAnalysisPointer(const void *PI) {
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if (PI == &AliasAnalysis::ID)
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return (AliasAnalysis*)this;
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return this;
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}
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private:
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virtual void getAnalysisUsage(AnalysisUsage &AU) const;
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virtual AliasResult alias(const Location &LocA, const Location &LocB);
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virtual bool pointsToConstantMemory(const Location &Loc);
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};
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} // End of anonymous namespace
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// Register this pass...
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char TypeBasedAliasAnalysis::ID = 0;
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INITIALIZE_AG_PASS(TypeBasedAliasAnalysis, AliasAnalysis, "tbaa",
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"Type-Based Alias Analysis", false, true, false)
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ImmutablePass *llvm::createTypeBasedAliasAnalysisPass() {
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return new TypeBasedAliasAnalysis();
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}
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void
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TypeBasedAliasAnalysis::getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesAll();
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AliasAnalysis::getAnalysisUsage(AU);
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}
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AliasAnalysis::AliasResult
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TypeBasedAliasAnalysis::alias(const Location &LocA,
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const Location &LocB) {
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if (!EnableTBAA)
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return AliasAnalysis::alias(LocA, LocB);
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// Get the attached MDNodes. If either value lacks a tbaa MDNode, we must
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// be conservative.
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const MDNode *AM = LocA.TBAATag;
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if (!AM) return AliasAnalysis::alias(LocA, LocB);
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const MDNode *BM = LocB.TBAATag;
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if (!BM) return AliasAnalysis::alias(LocA, LocB);
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// Keep track of the root node for A and B.
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TBAANode RootA, RootB;
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// Climb the DAG from A to see if we reach B.
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for (TBAANode T(AM); ; ) {
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if (T.getNode() == BM)
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// B is an ancestor of A.
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return AliasAnalysis::alias(LocA, LocB);
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RootA = T;
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T = T.getParent();
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if (!T.getNode())
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break;
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}
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// Climb the DAG from B to see if we reach A.
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for (TBAANode T(BM); ; ) {
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if (T.getNode() == AM)
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// A is an ancestor of B.
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return AliasAnalysis::alias(LocA, LocB);
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RootB = T;
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T = T.getParent();
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if (!T.getNode())
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break;
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}
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// Neither node is an ancestor of the other.
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// If they have the same root, then we've proved there's no alias.
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if (RootA.getNode() == RootB.getNode())
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return NoAlias;
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// If they have different roots, they're part of different potentially
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// unrelated type systems, so we must be conservative.
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return AliasAnalysis::alias(LocA, LocB);
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}
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bool TypeBasedAliasAnalysis::pointsToConstantMemory(const Location &Loc) {
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if (!EnableTBAA)
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return AliasAnalysis::pointsToConstantMemory(Loc);
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const MDNode *M = Loc.TBAATag;
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if (!M) return false;
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// If this is an "immutable" type, we can assume the pointer is pointing
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// to constant memory.
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if (TBAANode(M).TypeIsImmutable())
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return true;
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return AliasAnalysis::pointsToConstantMemory(Loc);
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}
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