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 38During the LTO link, all classes with public LTO visibility but not marked with 39``[[clang::lto_visibility_public]]`` (see below) will be refined to hidden LTO 40visibility when the ``--lto-whole-program-visibility`` lld linker option is 41applied (``-plugin-opt=whole-program-visibility`` for gold). This flag can be 42used to defer specifying whether classes have hidden LTO visibility until link 43time, to allow bitcode objects to be shared by different LTO links. Due to an 44implementation limitation, symbols associated with classes with hidden LTO 45visibility may still be exported from the binary when using this flag. It is 46unsafe to refer to these symbols, and their visibility may be relaxed to hidden 47in a future compiler release. 48 49A class defined in a translation unit built without LTO receives public 50LTO visibility regardless of its object file visibility, linkage or other 51attributes. 52 53This mechanism will produce the correct result in most cases, but there are 54two cases where it may wrongly infer hidden LTO visibility. 55 561. As a corollary of the above rules, if a linkage unit is produced from a 57 combination of LTO object files and non-LTO object files, any hidden 58 visibility class defined in both a translation unit built with LTO and 59 a translation unit built without LTO must be defined with public LTO 60 visibility in order to avoid an ODR violation. 61 622. Some ABIs provide the ability to define an abstract base class without 63 visibility attributes in multiple linkage units and have virtual calls 64 to derived classes in other linkage units work correctly. One example of 65 this is COM on Windows platforms. If the ABI allows this, any base class 66 used in this way must be defined with public LTO visibility. 67 68Classes that fall into either of these categories can be marked up with the 69``[[clang::lto_visibility_public]]`` attribute. To specifically handle the 70COM case, classes with the ``__declspec(uuid())`` attribute receive public 71LTO visibility. On Windows platforms, clang-cl's ``/MT`` and ``/MTd`` 72flags statically link the program against a prebuilt standard library; 73these flags imply public LTO visibility for every class declared in the 74``std`` and ``stdext`` namespaces. 75 76Example 77======= 78 79The following example shows how LTO visibility works in practice in several 80cases involving two linkage units, ``main`` and ``dso.so``. 81 82.. code-block:: none 83 84 +-----------------------------------------------------------+ +----------------------------------------------------+ 85 | main (clang++ -fvisibility=hidden): | | dso.so (clang++ -fvisibility=hidden): | 86 | | | | 87 | +-----------------------------------------------------+ | | struct __attribute__((visibility("default"))) C { | 88 | | LTO unit (clang++ -fvisibility=hidden -flto): | | | virtual void f(); | 89 | | | | | } | 90 | | struct A { ... }; | | | void C::f() {} | 91 | | struct [[clang::lto_visibility_public]] B { ... }; | | | struct D { | 92 | | struct __attribute__((visibility("default"))) C { | | | virtual void g() = 0; | 93 | | virtual void f(); | | | }; | 94 | | }; | | | struct E : D { | 95 | | struct [[clang::lto_visibility_public]] D { | | | virtual void g() { ... } | 96 | | virtual void g() = 0; | | | }; | 97 | | }; | | | __attribute__((visibility("default"))) D *mkE() { | 98 | | | | | return new E; | 99 | +-----------------------------------------------------+ | | } | 100 | | | | 101 | struct B { ... }; | +----------------------------------------------------+ 102 | | 103 +-----------------------------------------------------------+ 104 105We will now describe the LTO visibility of each of the classes defined in 106these linkage units. 107 108Class ``A`` is not defined outside of ``main``'s LTO unit, so it can have 109hidden LTO visibility. This is inferred from the object file visibility 110specified on the command line. 111 112Class ``B`` is defined in ``main``, both inside and outside its LTO unit. The 113definition outside the LTO unit has public LTO visibility, so the definition 114inside the LTO unit must also have public LTO visibility in order to avoid 115an ODR violation. 116 117Class ``C`` is defined in both ``main`` and ``dso.so`` and therefore must 118have public LTO visibility. This is correctly inferred from the ``visibility`` 119attribute. 120 121Class ``D`` is an abstract base class with a derived class ``E`` defined 122in ``dso.so``. This is an example of the COM scenario; the definition of 123``D`` in ``main``'s LTO unit must have public LTO visibility in order to be 124compatible with the definition of ``D`` in ``dso.so``, which is observable 125by calling the function ``mkE``. 126