LTO Visibility — Clang 4 documentation

# Clang 4 documentation

## LTO Visibility

LTO visibility is a property of an entity that specifies whether it can be referenced from outside the current LTO unit. A linkage unit is a set of translation units linked together into an executable or DSO, and a linkage unit’s LTO unit is the subset of the linkage unit that is linked together using link-time optimization; in the case where LTO is not being used, the linkage unit’s LTO unit is empty. Each linkage unit has only a single LTO unit.

The LTO visibility of a class is used by the compiler to determine which classes the virtual function call optimization and control flow integrity features apply to. These features use whole-program information, so they require the entire class hierarchy to be visible in order to work correctly.

If any translation unit in the program uses either of the virtual function call optimization or control flow integrity features, it is effectively an ODR violation to define a class with hidden LTO visibility in multiple linkage units. A class with public LTO visibility may be defined in multiple linkage units, but the tradeoff is that the virtual function call optimization and control flow integrity features can only be applied to classes with hidden LTO visibility. A class’s LTO visibility is treated as an ODR-relevant property of its definition, so it must be consistent between translation units.

In translation units built with LTO, LTO visibility is based on the class’s symbol visibility as expressed at the source level (i.e. the `__attribute__((visibility("...")))` attribute, or the `-fvisibility=` flag) or, on the Windows platform, the dllimport and dllexport attributes. When targeting non-Windows platforms, classes with a visibility other than hidden visibility receive public LTO visibility. When targeting Windows, classes with dllimport or dllexport attributes receive public LTO visibility. All other classes receive hidden LTO visibility. Classes with internal linkage (e.g. classes declared in unnamed namespaces) also receive hidden LTO visibility.

A class defined in a translation unit built without LTO receives public LTO visibility regardless of its object file visibility, linkage or other attributes.

This mechanism will produce the correct result in most cases, but there are two cases where it may wrongly infer hidden LTO visibility.

1. As a corollary of the above rules, if a linkage unit is produced from a combination of LTO object files and non-LTO object files, any hidden visibility class defined in both a translation unit built with LTO and a translation unit built without LTO must be defined with public LTO visibility in order to avoid an ODR violation.
2. Some ABIs provide the ability to define an abstract base class without visibility attributes in multiple linkage units and have virtual calls to derived classes in other linkage units work correctly. One example of this is COM on Windows platforms. If the ABI allows this, any base class used in this way must be defined with public LTO visibility.

Classes that fall into either of these categories can be marked up with the `[[clang::lto_visibility_public]]` attribute. To specifically handle the COM case, classes with the `__declspec(uuid())` attribute receive public LTO visibility. On Windows platforms, clang-cl’s `/MT` and `/MTd` flags statically link the program against a prebuilt standard library; these flags imply public LTO visibility for every class declared in the `std` and `stdext` namespaces.

## Example

The following example shows how LTO visibility works in practice in several cases involving two linkage units, `main` and `dso.so`.

    +-----------------------------------------------------------+  +----------------------------------------------------+
    | main (clang++ -fvisibility=hidden):                       |  | dso.so (clang++ -fvisibility=hidden):              |
    |                                                           |  |                                                    |
    |  +-----------------------------------------------------+  |  |  struct __attribute__((visibility("default"))) C { |
    |  | LTO unit (clang++ -fvisibility=hidden -flto):       |  |  |    virtual void f();                               |
    |  |                                                     |  |  |  }                                                 |
    |  |  struct A { ... };                                  |  |  |  void C::f() {}                                    |
    |  |  struct [[clang::lto_visibility_public]] B { ... }; |  |  |  struct D {                                        |
    |  |  struct __attribute__((visibility("default"))) C {  |  |  |    virtual void g() = 0;                           |
    |  |    virtual void f();                                |  |  |  };                                                |
    |  |  };                                                 |  |  |  struct E : D {                                    |
    |  |  struct [[clang::lto_visibility_public]] D {        |  |  |    virtual void g() { ... }                        |
    |  |    virtual void g() = 0;                            |  |  |  };                                                |
    |  |  };                                                 |  |  |  __attribute__(visibility("default"))) D *mkE() {  |
    |  |                                                     |  |  |    return new E;                                   |
    |  +-----------------------------------------------------+  |  |  }                                                 |
    |                                                           |  |                                                    |
    |  struct B { ... };                                        |  +----------------------------------------------------+
    |                                                           |
    +-----------------------------------------------------------+

We will now describe the LTO visibility of each of the classes defined in these linkage units.

Class `A` is not defined outside of `main`’s LTO unit, so it can have hidden LTO visibility. This is inferred from the object file visibility specified on the command line.

Class `B` is defined in `main`, both inside and outside its LTO unit. The definition outside the LTO unit has public LTO visibility, so the definition inside the LTO unit must also have public LTO visibility in order to avoid an ODR violation.

Class `C` is defined in both `main` and `dso.so` and therefore must have public LTO visibility. This is correctly inferred from the `visibility` attribute.

Class `D` is an abstract base class with a derived class `E` defined in `dso.so`. This is an example of the COM scenario; the definition of `D` in `main`’s LTO unit must have public LTO visibility in order to be compatible with the definition of `D` in `dso.so`, which is observable by calling the function `mkE`.