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The tag is of type TBAANode when flag EnableStructPathTBAA is off. Move implementation of MDNode::getMostGenericTBAA to TypeBasedAliasAnalysis.cpp since it depends on how to interprete the MDNodes for scalar TBAA and struct-path aware TBAA. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@180068 91177308-0d34-0410-b5e6-96231b3b80d8
495 lines
16 KiB
C++
495 lines
16 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 descendants 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/Passes.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/LLVMContext.h"
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#include "llvm/IR/Metadata.h"
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#include "llvm/IR/Module.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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// A handy option for disabling TBAA functionality. The same effect can also be
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// achieved by stripping the !tbaa tags from IR, but this option is sometimes
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// more convenient.
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static cl::opt<bool> EnableTBAA("enable-tbaa", cl::init(true));
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static cl::opt<bool> EnableStructPathTBAA("struct-path-tbaa", cl::init(false));
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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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/// 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 TBAAStructTagNode {
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/// This node should be created with createTBAAStructTagNode.
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const MDNode *Node;
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public:
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TBAAStructTagNode() : Node(0) {}
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explicit TBAAStructTagNode(const MDNode *N) : Node(N) {}
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/// Get the MDNode for this TBAAStructTagNode.
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const MDNode *getNode() const { return Node; }
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const MDNode *getBaseType() const {
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return dyn_cast_or_null<MDNode>(Node->getOperand(0));
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}
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const MDNode *getAccessType() const {
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return dyn_cast_or_null<MDNode>(Node->getOperand(1));
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}
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uint64_t getOffset() const {
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return cast<ConstantInt>(Node->getOperand(2))->getZExtValue();
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}
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};
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/// 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 TBAAStructTypeNode {
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/// This node should be created with createTBAAStructTypeNode.
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const MDNode *Node;
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public:
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TBAAStructTypeNode() : Node(0) {}
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explicit TBAAStructTypeNode(const MDNode *N) : Node(N) {}
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/// Get the MDNode for this TBAAStructTypeNode.
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const MDNode *getNode() const { return Node; }
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/// Get this TBAAStructTypeNode's field in the type DAG with
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/// given offset. Update the offset to be relative to the field type.
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TBAAStructTypeNode getParent(uint64_t &Offset) const {
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if (Node->getNumOperands() < 2)
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return TBAAStructTypeNode();
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// Assume the offsets are in order. We return the previous field if
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// the current offset is bigger than the given offset.
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unsigned TheIdx = 0;
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for (unsigned Idx = 1; Idx < Node->getNumOperands(); Idx += 2) {
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uint64_t Cur = cast<ConstantInt>(Node->getOperand(Idx))->getZExtValue();
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if (Cur > Offset) {
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assert(Idx >= 3 &&
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"TBAAStructTypeNode::getParent should have an offset match!");
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TheIdx = Idx - 2;
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break;
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}
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}
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// Move along the last field.
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if (TheIdx == 0)
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TheIdx = Node->getNumOperands() - 2;
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uint64_t Cur = cast<ConstantInt>(Node->getOperand(TheIdx))->
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getZExtValue();
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Offset -= Cur;
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MDNode *P = dyn_cast_or_null<MDNode>(Node->getOperand(TheIdx + 1));
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if (!P)
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return TBAAStructTypeNode();
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return TBAAStructTypeNode(P);
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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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bool PathAliases(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, bool OrLocal);
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virtual ModRefBehavior getModRefBehavior(ImmutableCallSite CS);
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virtual ModRefBehavior getModRefBehavior(const Function *F);
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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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if (EnableStructPathTBAA)
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return PathAliases(A, B);
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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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/// Test whether the struct-path tag represented by A may alias the
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/// struct-path tag represented by B.
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bool
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TypeBasedAliasAnalysis::PathAliases(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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TBAAStructTypeNode RootA, RootB;
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TBAAStructTagNode TagA(A), TagB(B);
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// TODO: We need to check if AccessType of TagA encloses AccessType of
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// TagB to support aggregate AccessType. If yes, return true.
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// Start from the base type of A, follow the edge with the correct offset in
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// the type DAG and adjust the offset until we reach the base type of B or
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// until we reach the Root node.
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// Compare the adjusted offset once we have the same base.
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// Climb the type DAG from base type of A to see if we reach base type of B.
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const MDNode *BaseA = TagA.getBaseType();
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const MDNode *BaseB = TagB.getBaseType();
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uint64_t OffsetA = TagA.getOffset(), OffsetB = TagB.getOffset();
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for (TBAAStructTypeNode T(BaseA); ; ) {
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if (T.getNode() == BaseB)
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// Base type of A encloses base type of B, check if the offsets match.
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return OffsetA == OffsetB;
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RootA = T;
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// Follow the edge with the correct offset, OffsetA will be adjusted to
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// be relative to the field type.
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T = T.getParent(OffsetA);
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if (!T.getNode())
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break;
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}
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// Reset OffsetA and climb the type DAG from base type of B to see if we reach
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// base type of A.
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OffsetA = TagA.getOffset();
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for (TBAAStructTypeNode T(BaseB); ; ) {
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if (T.getNode() == BaseA)
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// Base type of B encloses base type of A, check if the offsets match.
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return OffsetA == OffsetB;
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RootB = T;
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// Follow the edge with the correct offset, OffsetB will be adjusted to
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// be relative to the field type.
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T = T.getParent(OffsetB);
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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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bool OrLocal) {
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if (!EnableTBAA)
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return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
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const MDNode *M = Loc.TBAATag;
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if (!M) return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
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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 (!EnableStructPathTBAA && TBAANode(M).TypeIsImmutable())
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return true;
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return AliasAnalysis::pointsToConstantMemory(Loc, OrLocal);
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}
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AliasAnalysis::ModRefBehavior
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TypeBasedAliasAnalysis::getModRefBehavior(ImmutableCallSite CS) {
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if (!EnableTBAA)
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return AliasAnalysis::getModRefBehavior(CS);
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ModRefBehavior Min = UnknownModRefBehavior;
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// If this is an "immutable" type, we can assume the call doesn't write
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// to memory.
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if (const MDNode *M = CS.getInstruction()->getMetadata(LLVMContext::MD_tbaa))
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if (!EnableStructPathTBAA && TBAANode(M).TypeIsImmutable())
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Min = OnlyReadsMemory;
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return ModRefBehavior(AliasAnalysis::getModRefBehavior(CS) & Min);
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}
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AliasAnalysis::ModRefBehavior
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TypeBasedAliasAnalysis::getModRefBehavior(const Function *F) {
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// Functions don't have metadata. Just chain to the next implementation.
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return AliasAnalysis::getModRefBehavior(F);
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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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MDNode *MDNode::getMostGenericTBAA(MDNode *A, MDNode *B) {
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if (!A || !B)
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return NULL;
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if (A == B)
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return A;
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// For struct-path aware TBAA, we use the access type of the tag.
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if (EnableStructPathTBAA) {
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A = cast_or_null<MDNode>(A->getOperand(1));
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if (!A) return 0;
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B = cast_or_null<MDNode>(B->getOperand(1));
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if (!B) return 0;
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}
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SmallVector<MDNode *, 4> PathA;
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MDNode *T = A;
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while (T) {
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PathA.push_back(T);
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if (EnableStructPathTBAA)
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T = T->getNumOperands() >= 3 ? cast_or_null<MDNode>(T->getOperand(2)) : 0;
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else
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T = T->getNumOperands() >= 2 ? cast_or_null<MDNode>(T->getOperand(1)) : 0;
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}
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SmallVector<MDNode *, 4> PathB;
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T = B;
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while (T) {
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PathB.push_back(T);
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if (EnableStructPathTBAA)
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T = T->getNumOperands() >= 3 ? cast_or_null<MDNode>(T->getOperand(2)) : 0;
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else
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T = T->getNumOperands() >= 2 ? cast_or_null<MDNode>(T->getOperand(1)) : 0;
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}
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int IA = PathA.size() - 1;
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int IB = PathB.size() - 1;
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MDNode *Ret = 0;
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while (IA >= 0 && IB >=0) {
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if (PathA[IA] == PathB[IB])
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Ret = PathA[IA];
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else
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break;
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--IA;
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--IB;
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}
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if (!EnableStructPathTBAA)
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return Ret;
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if (!Ret)
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return 0;
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// We need to convert from a type node to a tag node.
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Type *Int64 = IntegerType::get(A->getContext(), 64);
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Value *Ops[3] = { Ret, Ret, ConstantInt::get(Int64, 0) };
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return MDNode::get(A->getContext(), Ops);
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}
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