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273 lines
9.1 KiB
C++
273 lines
9.1 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. This can be used to implement
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// typical C/C++ TBAA, but it can also be used to implement custom alias
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// analysis behavior for other languages.
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//
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// The current metadata format is very simple. TBAA MDNodes have up to
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// three fields, e.g.:
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// !0 = metadata !{ metadata !"an example type tree" }
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// !1 = metadata !{ metadata !"int", metadata !0 }
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// !2 = metadata !{ metadata !"float", metadata !0 }
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// !3 = metadata !{ metadata !"const float", metadata !2, i64 1 }
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//
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// The first field is an identity field. It can be any value, usually
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// an MDString, which uniquely identifies the type. The most important
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// name in the tree is the name of the root node. Two trees with
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// different root node names are entirely disjoint, even if they
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// have leaves with common names.
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//
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// The second field identifies the type's parent node in the tree, or
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// is null or omitted for a root node. A type is considered to alias
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// all of its decendents and all of its ancestors in the tree. Also,
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// a type is considered to alias all types in other trees, so that
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// bitcode produced from multiple front-ends is handled conservatively.
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//
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// If the third field is present, it's an integer which if equal to 1
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// indicates that the type is "constant" (meaning pointsToConstantMemory
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// should return true; see
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// http://llvm.org/docs/AliasAnalysis.html#OtherItfs).
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//
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// TODO: The current metadata format doesn't support struct
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// fields. For example:
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// struct X {
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// double d;
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// int i;
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// };
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// void foo(struct X *x, struct X *y, double *p) {
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// *x = *y;
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// *p = 0.0;
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// }
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// Struct X has a double member, so the store to *x can alias the store to *p.
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// Currently it's not possible to precisely describe all the things struct X
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// aliases, so struct assignments must use conservative TBAA nodes. There's
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// no scheme for attaching metadata to @llvm.memcpy yet either.
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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/LLVMContext.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 tree 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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return CI->getValue()[0];
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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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initializeTypeBasedAliasAnalysisPass(*PassRegistry::getPassRegistry());
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}
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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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bool Aliases(const MDNode *A, const MDNode *B) const;
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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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virtual ModRefResult getModRefInfo(ImmutableCallSite CS,
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const Location &Loc);
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virtual ModRefResult getModRefInfo(ImmutableCallSite CS1,
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ImmutableCallSite CS2);
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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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/// Aliases - Test whether the type represented by A may alias the
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/// type represented by B.
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bool
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TypeBasedAliasAnalysis::Aliases(const MDNode *A,
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const MDNode *B) const {
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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 tree from A to see if we reach B.
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for (TBAANode T(A); ; ) {
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if (T.getNode() == B)
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// B is an ancestor of A.
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return true;
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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 tree from B to see if we reach A.
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for (TBAANode T(B); ; ) {
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if (T.getNode() == A)
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// A is an ancestor of B.
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return true;
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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 different roots, they're part of different potentially
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// unrelated type systems, so we must be conservative.
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if (RootA.getNode() != RootB.getNode())
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return true;
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// If they have the same root, then we've proved there's no alias.
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return false;
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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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// If they may alias, chain to the next AliasAnalysis.
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if (Aliases(AM, BM))
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return AliasAnalysis::alias(LocA, LocB);
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// Otherwise return a definitive result.
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return NoAlias;
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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 AliasAnalysis::pointsToConstantMemory(Loc);
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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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AliasAnalysis::ModRefResult
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TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS,
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const Location &Loc) {
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if (!EnableTBAA)
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return AliasAnalysis::getModRefInfo(CS, Loc);
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if (const MDNode *L = Loc.TBAATag)
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if (const MDNode *M =
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CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
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if (!Aliases(L, M))
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return NoModRef;
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return AliasAnalysis::getModRefInfo(CS, Loc);
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}
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AliasAnalysis::ModRefResult
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TypeBasedAliasAnalysis::getModRefInfo(ImmutableCallSite CS1,
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ImmutableCallSite CS2) {
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if (!EnableTBAA)
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return AliasAnalysis::getModRefInfo(CS1, CS2);
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if (const MDNode *M1 =
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CS1.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
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if (const MDNode *M2 =
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CS2.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
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if (!Aliases(M1, M2))
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return NoModRef;
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return AliasAnalysis::getModRefInfo(CS1, CS2);
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}
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