1==============
2LTO Visibility
3==============
4
5*LTO visibility* is a property of an entity that specifies whether it can be
6referenced from outside the current LTO unit. A *linkage unit* is a set of
7translation units linked together into an executable or DSO, and a linkage
8unit's *LTO unit* is the subset of the linkage unit that is linked together
9using link-time optimization; in the case where LTO is not being used, the
10linkage unit's LTO unit is empty. Each linkage unit has only a single LTO unit.
11
12The LTO visibility of a class is used by the compiler to determine which
13classes the whole-program devirtualization (``-fwhole-program-vtables``) and
14control flow integrity (``-fsanitize=cfi-vcall`` and ``-fsanitize=cfi-mfcall``)
15features apply to. These features use whole-program information, so they
16require the entire class hierarchy to be visible in order to work correctly.
17
18If any translation unit in the program uses either of the whole-program
19devirtualization or control flow integrity features, it is effectively an ODR
20violation to define a class with hidden LTO visibility in multiple linkage
21units. A class with public LTO visibility may be defined in multiple linkage
22units, but the tradeoff is that the whole-program devirtualization and
23control flow integrity features can only be applied to classes with hidden LTO
24visibility. A class's LTO visibility is treated as an ODR-relevant property
25of its definition, so it must be consistent between translation units.
26
27In translation units built with LTO, LTO visibility is based on the
28class's symbol visibility as expressed at the source level (i.e. the
29``__attribute__((visibility("...")))`` attribute, or the ``-fvisibility=``
30flag) or, on the Windows platform, the dllimport and dllexport attributes. When
31targeting non-Windows platforms, classes with a visibility other than hidden
32visibility receive public LTO visibility. When targeting Windows, classes
33with dllimport or dllexport attributes receive public LTO visibility. All
34other classes receive hidden LTO visibility. Classes with internal linkage
35(e.g. classes declared in unnamed namespaces) also receive hidden LTO
36visibility.
37
38A class defined in a translation unit built without LTO receives public
39LTO visibility regardless of its object file visibility, linkage or other
40attributes.
41
42This mechanism will produce the correct result in most cases, but there are
43two cases where it may wrongly infer hidden LTO visibility.
44
451. As a corollary of the above rules, if a linkage unit is produced from a
46   combination of LTO object files and non-LTO object files, any hidden
47   visibility class defined in both a translation unit built with LTO and
48   a translation unit built without LTO must be defined with public LTO
49   visibility in order to avoid an ODR violation.
50
512. Some ABIs provide the ability to define an abstract base class without
52   visibility attributes in multiple linkage units and have virtual calls
53   to derived classes in other linkage units work correctly. One example of
54   this is COM on Windows platforms. If the ABI allows this, any base class
55   used in this way must be defined with public LTO visibility.
56
57Classes that fall into either of these categories can be marked up with the
58``[[clang::lto_visibility_public]]`` attribute. To specifically handle the
59COM case, classes with the ``__declspec(uuid())`` attribute receive public
60LTO visibility. On Windows platforms, clang-cl's ``/MT`` and ``/MTd``
61flags statically link the program against a prebuilt standard library;
62these flags imply public LTO visibility for every class declared in the
63``std`` and ``stdext`` namespaces.
64
65Example
66=======
67
68The following example shows how LTO visibility works in practice in several
69cases involving two linkage units, ``main`` and ``dso.so``.
70
71.. code-block:: none
72
73    +-----------------------------------------------------------+  +----------------------------------------------------+
74    | main (clang++ -fvisibility=hidden):                       |  | dso.so (clang++ -fvisibility=hidden):              |
75    |                                                           |  |                                                    |
76    |  +-----------------------------------------------------+  |  |  struct __attribute__((visibility("default"))) C { |
77    |  | LTO unit (clang++ -fvisibility=hidden -flto):       |  |  |    virtual void f();                               |
78    |  |                                                     |  |  |  }                                                 |
79    |  |  struct A { ... };                                  |  |  |  void C::f() {}                                    |
80    |  |  struct [[clang::lto_visibility_public]] B { ... }; |  |  |  struct D {                                        |
81    |  |  struct __attribute__((visibility("default"))) C {  |  |  |    virtual void g() = 0;                           |
82    |  |    virtual void f();                                |  |  |  };                                                |
83    |  |  };                                                 |  |  |  struct E : D {                                    |
84    |  |  struct [[clang::lto_visibility_public]] D {        |  |  |    virtual void g() { ... }                        |
85    |  |    virtual void g() = 0;                            |  |  |  };                                                |
86    |  |  };                                                 |  |  |  __attribute__((visibility("default"))) D *mkE() { |
87    |  |                                                     |  |  |    return new E;                                   |
88    |  +-----------------------------------------------------+  |  |  }                                                 |
89    |                                                           |  |                                                    |
90    |  struct B { ... };                                        |  +----------------------------------------------------+
91    |                                                           |
92    +-----------------------------------------------------------+
93
94We will now describe the LTO visibility of each of the classes defined in
95these linkage units.
96
97Class ``A`` is not defined outside of ``main``'s LTO unit, so it can have
98hidden LTO visibility. This is inferred from the object file visibility
99specified on the command line.
100
101Class ``B`` is defined in ``main``, both inside and outside its LTO unit. The
102definition outside the LTO unit has public LTO visibility, so the definition
103inside the LTO unit must also have public LTO visibility in order to avoid
104an ODR violation.
105
106Class ``C`` is defined in both ``main`` and ``dso.so`` and therefore must
107have public LTO visibility. This is correctly inferred from the ``visibility``
108attribute.
109
110Class ``D`` is an abstract base class with a derived class ``E`` defined
111in ``dso.so``.  This is an example of the COM scenario; the definition of
112``D`` in ``main``'s LTO unit must have public LTO visibility in order to be
113compatible with the definition of ``D`` in ``dso.so``, which is observable
114by calling the function ``mkE``.
115