Reference.h revision 314564 
1//===- Core/References.h - A Reference to Another Atom ----------*- C++ -*-===//
2//
3//                             The LLVM Linker
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9
10#ifndef LLD_CORE_REFERENCES_H
11#define LLD_CORE_REFERENCES_H
12
13#include <cstdint>
14
15namespace lld {
16
17class Atom;
18
19///
20/// The linker has a Graph Theory model of linking. An object file is seen
21/// as a set of Atoms with References to other Atoms.  Each Atom is a node
22/// and each Reference is an edge.
23///
24/// For example if a function contains a call site to "malloc" 40 bytes into
25/// the Atom, then the function Atom will have a Reference of: offsetInAtom=40,
26/// kind=callsite, target=malloc, addend=0.
27///
28/// Besides supporting traditional "relocations", references are also used
29/// forcing layout (one atom must follow another), marking data-in-code
30/// (jump tables or ARM constants), etc.
31///
32/// The "kind" of a reference is a tuple of <namespace, arch, value>.  This
33/// enable us to re-use existing relocation types definded for various
34/// file formats and architectures.
35///
36/// References and atoms form a directed graph. The dead-stripping pass
37/// traverses them starting from dead-strip root atoms to garbage collect
38/// unreachable ones.
39///
40/// References of any kind are considered as directed edges. In addition to
41/// that, references of some kind is considered as bidirected edges.
42class Reference {
43public:
44  /// Which universe defines the kindValue().
45  enum class KindNamespace {
46    all     = 0,
47    testing = 1,
48    mach_o  = 2,
49  };
50
51  KindNamespace kindNamespace() const { return (KindNamespace)_kindNamespace; }
52  void setKindNamespace(KindNamespace ns) { _kindNamespace = (uint8_t)ns; }
53
54  // Which architecture the kind value is for.
55  enum class KindArch { all, AArch64, ARM, x86, x86_64};
56
57  KindArch kindArch() const { return (KindArch)_kindArch; }
58  void setKindArch(KindArch a) { _kindArch = (uint8_t)a; }
59
60  typedef uint16_t KindValue;
61
62  KindValue kindValue() const { return _kindValue; }
63
64  /// setKindValue() is needed because during linking, some optimizations may
65  /// change the codegen and hence the reference kind.
66  void setKindValue(KindValue value) {
67    _kindValue = value;
68  }
69
70  /// KindValues used with KindNamespace::all and KindArch::all.
71  enum {
72    // kindLayoutAfter is treated as a bidirected edge by the dead-stripping
73    // pass.
74    kindLayoutAfter = 1,
75    kindAssociate,
76  };
77
78  // A value to be added to the value of a target
79  typedef int64_t Addend;
80
81  /// If the reference is a fixup in the Atom, then this returns the
82  /// byte offset into the Atom's content to do the fix up.
83  virtual uint64_t offsetInAtom() const = 0;
84
85  /// Returns the atom this reference refers to.
86  virtual const Atom *target() const = 0;
87
88  /// During linking, the linker may merge graphs which coalesces some nodes
89  /// (i.e. Atoms).  To switch the target of a reference, this method is called.
90  virtual void setTarget(const Atom *) = 0;
91
92  /// Some relocations require a symbol and a value (e.g. foo + 4).
93  virtual Addend addend() const = 0;
94
95  /// During linking, some optimzations may change addend value.
96  virtual void setAddend(Addend) = 0;
97
98  /// Returns target specific attributes of the reference.
99  virtual uint32_t tag() const { return 0; }
100
101protected:
102  /// Reference is an abstract base class.  Only subclasses can use constructor.
103  Reference(KindNamespace ns, KindArch a, KindValue value)
104      : _kindValue(value), _kindNamespace((uint8_t)ns), _kindArch((uint8_t)a) {}
105
106  /// The memory for Reference objects is always managed by the owning File
107  /// object.  Therefore, no one but the owning File object should call
108  /// delete on an Reference.  In fact, some File objects may bulk allocate
109  /// an array of References, so they cannot be individually deleted by anyone.
110  virtual ~Reference() = default;
111
112  KindValue  _kindValue;
113  uint8_t    _kindNamespace;
114  uint8_t    _kindArch;
115};
116
117} // end namespace lld
118
119#endif // LLD_CORE_REFERENCES_H
120