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3a84a6377c
Change `Instruction::getMetadata()` to return `Value` as part of PR21433. Update most callers to use `Instruction::getMDNode()`, which wraps the result in a `cast_or_null<MDNode>`. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221024 91177308-0d34-0410-b5e6-96231b3b80d8
633 lines
22 KiB
C++
633 lines
22 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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// We now support two types of metadata format: scalar TBAA and struct-path
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// aware TBAA. After all testing cases are upgraded to use struct-path aware
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// TBAA and we can auto-upgrade existing bc files, the support for scalar TBAA
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// can be dropped.
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//
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// The scalar TBAA 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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// With struct-path aware TBAA, the MDNodes attached to an instruction using
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// "!tbaa" are called path tag nodes.
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//
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// The path tag node has 4 fields with the last field being optional.
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//
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// The first field is the base type node, it can be a struct type node
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// or a scalar type node. The second field is the access type node, it
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// must be a scalar type node. The third field is the offset into the base type.
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// The last field has the same meaning as the last field of our scalar TBAA:
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// it's an integer which if equal to 1 indicates that the access is "constant".
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//
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// The struct type node has a name and a list of pairs, one pair for each member
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// of the struct. The first element of each pair is a type node (a struct type
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// node or a sclar type node), specifying the type of the member, the second
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// element of each pair is the offset of the member.
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//
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// Given an example
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// typedef struct {
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// short s;
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// } A;
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// typedef struct {
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// uint16_t s;
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// A a;
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// } B;
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//
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// For an acess to B.a.s, we attach !5 (a path tag node) to the load/store
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// instruction. The base type is !4 (struct B), the access type is !2 (scalar
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// type short) and the offset is 4.
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//
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// !0 = metadata !{metadata !"Simple C/C++ TBAA"}
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// !1 = metadata !{metadata !"omnipotent char", metadata !0} // Scalar type node
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// !2 = metadata !{metadata !"short", metadata !1} // Scalar type node
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// !3 = metadata !{metadata !"A", metadata !2, i64 0} // Struct type node
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// !4 = metadata !{metadata !"B", metadata !2, i64 0, metadata !3, i64 4}
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// // Struct type node
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// !5 = metadata !{metadata !4, metadata !2, i64 4} // Path tag node
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//
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// The struct type nodes and the scalar type nodes form a type DAG.
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// Root (!0)
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// char (!1) -- edge to Root
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// short (!2) -- edge to char
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// A (!3) -- edge with offset 0 to short
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// B (!4) -- edge with offset 0 to short and edge with offset 4 to A
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//
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// To check if two tags (tagX and tagY) can alias, we start from the base type
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// of tagX, follow the edge with the correct offset in the type DAG and adjust
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// the offset until we reach the base type of tagY or until we reach the Root
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// node.
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// If we reach the base type of tagY, compare the adjusted offset with
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// offset of tagY, return Alias if the offsets are the same, return NoAlias
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// otherwise.
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// If we reach the Root node, perform the above starting from base type of tagY
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// to see if we reach base type of tagX.
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//
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// If they have different roots, they're part of different potentially
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// unrelated type systems, so we return Alias to be conservative.
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// If neither node is an ancestor of the other and they have the same root,
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// then we say NoAlias.
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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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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(nullptr) {}
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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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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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/// TypeIsImmutable - Test if this TBAAStructTagNode represents a type for
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/// objects 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() < 4)
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return false;
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ConstantInt *CI = dyn_cast<ConstantInt>(Node->getOperand(3));
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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 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(nullptr) {}
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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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// Parent can be omitted for the root node.
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if (Node->getNumOperands() < 2)
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return TBAAStructTypeNode();
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// Fast path for a scalar type node and a struct type node with a single
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// field.
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if (Node->getNumOperands() <= 3) {
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uint64_t Cur = Node->getNumOperands() == 2 ? 0 :
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cast<ConstantInt>(Node->getOperand(2))->getZExtValue();
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Offset -= Cur;
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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 TBAAStructTypeNode();
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return TBAAStructTypeNode(P);
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}
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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 + 1))->
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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 + 1))->
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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));
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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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void initializePass() override {
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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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void *getAdjustedAnalysisPointer(const void *PI) override {
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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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void getAnalysisUsage(AnalysisUsage &AU) const override;
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AliasResult alias(const Location &LocA, const Location &LocB) override;
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bool pointsToConstantMemory(const Location &Loc, bool OrLocal) override;
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ModRefBehavior getModRefBehavior(ImmutableCallSite CS) override;
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ModRefBehavior getModRefBehavior(const Function *F) override;
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ModRefResult getModRefInfo(ImmutableCallSite CS,
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const Location &Loc) override;
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ModRefResult getModRefInfo(ImmutableCallSite CS1,
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ImmutableCallSite CS2) override;
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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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/// Check the first operand of the tbaa tag node, if it is a MDNode, we treat
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/// it as struct-path aware TBAA format, otherwise, we treat it as scalar TBAA
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/// format.
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static bool isStructPathTBAA(const MDNode *MD) {
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// Anonymous TBAA root starts with a MDNode and dragonegg uses it as
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// a TBAA tag.
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return isa<MDNode>(MD->getOperand(0)) && MD->getNumOperands() >= 3;
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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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// Make sure that both MDNodes are struct-path aware.
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if (isStructPathTBAA(A) && isStructPathTBAA(B))
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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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// Verify that both input nodes are struct-path aware.
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assert(isStructPathTBAA(A) && "MDNode A is not struct-path aware.");
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assert(isStructPathTBAA(B) && "MDNode B is not struct-path aware.");
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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.AATags.TBAA;
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if (!AM) return AliasAnalysis::alias(LocA, LocB);
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const MDNode *BM = LocB.AATags.TBAA;
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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) {
|
|
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()->getMDNode(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 Location &Loc) {
|
|
if (!EnableTBAA)
|
|
return AliasAnalysis::getModRefInfo(CS, Loc);
|
|
|
|
if (const MDNode *L = Loc.AATags.TBAA)
|
|
if (const MDNode *M = CS.getInstruction()->getMDNode(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()->getMDNode(LLVMContext::MD_tbaa))
|
|
if (const MDNode *M2 =
|
|
CS2.getInstruction()->getMDNode(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;
|
|
}
|
|
|
|
SmallVector<MDNode *, 4> PathA;
|
|
MDNode *T = A;
|
|
while (T) {
|
|
PathA.push_back(T);
|
|
T = T->getNumOperands() >= 2 ? cast_or_null<MDNode>(T->getOperand(1))
|
|
: nullptr;
|
|
}
|
|
|
|
SmallVector<MDNode *, 4> PathB;
|
|
T = B;
|
|
while (T) {
|
|
PathB.push_back(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);
|
|
Value *Ops[3] = { Ret, Ret, 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, getMDNode(LLVMContext::MD_tbaa));
|
|
else
|
|
N.TBAA = getMDNode(LLVMContext::MD_tbaa);
|
|
|
|
if (Merge)
|
|
N.Scope =
|
|
MDNode::intersect(N.Scope, getMDNode(LLVMContext::MD_alias_scope));
|
|
else
|
|
N.Scope = getMDNode(LLVMContext::MD_alias_scope);
|
|
|
|
if (Merge)
|
|
N.NoAlias =
|
|
MDNode::intersect(N.NoAlias, getMDNode(LLVMContext::MD_noalias));
|
|
else
|
|
N.NoAlias = getMDNode(LLVMContext::MD_noalias);
|
|
}
|
|
|