Rework interface for bitset-using features to use a notion of LTO visibility.

Bitsets, and the compiler features they rely on (vtable opt, CFI),
only have visibility within the LTO'd part of the linkage unit. Therefore,
only enable these features for classes with hidden LTO visibility. This
notion is based on object file visibility or (on Windows)
dllimport/dllexport attributes.

We provide the [[clang::lto_visibility_public]] attribute to override the
compiler's LTO visibility inference in cases where the class is defined
in the non-LTO'd part of the linkage unit, or where the ABI supports
calling classes derived from abstract base classes with hidden visibility
in other linkage units (e.g. COM on Windows).

If the cross-DSO CFI mode is enabled, bitset checks are emitted even for
classes with public LTO visibility, as that mode uses a separate mechanism
to cause bitsets to be exported.

This mechanism replaces the whole-program-vtables blacklist, so remove the
-fwhole-program-vtables-blacklist flag.

Because __declspec(uuid()) now implies [[clang::lto_visibility_public]], the
support for the special attr:uuid blacklist entry is removed.

Differential Revision: http://reviews.llvm.org/D18635

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@267784 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 111 insertions, 0 deletions

diff --git a/docs/LTOVisibility.rst b/docs/LTOVisibility.rst
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+==============
+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 symbol
+visibility 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``.
+
+.. code-block:: none
+
+    +-----------------------------------------------------------+  +----------------------------------------------------+
+    | 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``.