forked from OSchip/llvm-project
219 lines
11 KiB
ReStructuredText
219 lines
11 KiB
ReStructuredText
========================
|
|
Symbol Visibility Macros
|
|
========================
|
|
|
|
.. contents::
|
|
:local:
|
|
|
|
.. _visibility-macros:
|
|
|
|
Overview
|
|
========
|
|
|
|
Libc++ uses various "visibility" macros in order to provide a stable ABI in
|
|
both the library and the headers. These macros work by changing the
|
|
visibility and inlining characteristics of the symbols they are applied to.
|
|
|
|
Visibility Macros
|
|
=================
|
|
|
|
**_LIBCPP_HIDDEN**
|
|
Mark a symbol as hidden so it will not be exported from shared libraries.
|
|
|
|
**_LIBCPP_FUNC_VIS**
|
|
Mark a symbol as being exported by the libc++ library. This attribute must
|
|
be applied to the declaration of all functions exported by the libc++ dylib.
|
|
|
|
**_LIBCPP_EXPORTED_FROM_ABI**
|
|
Mark a symbol as being exported by the libc++ library. This attribute may
|
|
only be applied to objects defined in the libc++ runtime library. On Windows,
|
|
this macro applies `dllimport`/`dllexport` to the symbol, and on other
|
|
platforms it gives the symbol default visibility.
|
|
|
|
**_LIBCPP_OVERRIDABLE_FUNC_VIS**
|
|
Mark a symbol as being exported by the libc++ library, but allow it to be
|
|
overridden locally. On non-Windows, this is equivalent to `_LIBCPP_FUNC_VIS`.
|
|
This macro is applied to all `operator new` and `operator delete` overloads.
|
|
|
|
**Windows Behavior**: Any symbol marked `dllimport` cannot be overridden
|
|
locally, since `dllimport` indicates the symbol should be bound to a separate
|
|
DLL. All `operator new` and `operator delete` overloads are required to be
|
|
locally overridable, and therefore must not be marked `dllimport`. On Windows,
|
|
this macro therefore expands to `__declspec(dllexport)` when building the
|
|
library and has an empty definition otherwise.
|
|
|
|
**_LIBCPP_HIDE_FROM_ABI**
|
|
Mark a function as not being part of the ABI of any final linked image that
|
|
uses it.
|
|
|
|
**_LIBCPP_INLINE_VISIBILITY**
|
|
Historical predecessor of ``_LIBCPP_HIDE_FROM_ABI`` -- please use
|
|
``_LIBCPP_HIDE_FROM_ABI`` instead.
|
|
|
|
**_LIBCPP_HIDE_FROM_ABI_AFTER_V1**
|
|
Mark a function as being hidden from the ABI (per `_LIBCPP_HIDE_FROM_ABI`)
|
|
when libc++ is built with an ABI version after ABI v1. This macro is used to
|
|
maintain ABI compatibility for symbols that have been historically exported
|
|
by libc++ in v1 of the ABI, but that we don't want to export in the future.
|
|
|
|
This macro works as follows. When we build libc++, we either hide the symbol
|
|
from the ABI (if the symbol is not part of the ABI in the version we're
|
|
building), or we leave it included. From user code (i.e. when we're not
|
|
building libc++), the macro always marks symbols as internal so that programs
|
|
built using new libc++ headers stop relying on symbols that are removed from
|
|
the ABI in a future version. Each time we release a new stable version of the
|
|
ABI, we should create a new _LIBCPP_HIDE_FROM_ABI_AFTER_XXX macro, and we can
|
|
use it to start removing symbols from the ABI after that stable version.
|
|
|
|
**_LIBCPP_HIDE_FROM_ABI_PER_TU**
|
|
This macro controls whether symbols hidden from the ABI with `_LIBCPP_HIDE_FROM_ABI`
|
|
are local to each translation unit in addition to being local to each final
|
|
linked image. This macro is defined to either 0 or 1. When it is defined to
|
|
1, translation units compiled with different versions of libc++ can be linked
|
|
together, since all non ABI-facing functions are local to each translation unit.
|
|
This allows static archives built with different versions of libc++ to be linked
|
|
together. This also means that functions marked with `_LIBCPP_HIDE_FROM_ABI`
|
|
are not guaranteed to have the same address across translation unit boundaries.
|
|
|
|
When the macro is defined to 0, there is no guarantee that translation units
|
|
compiled with different versions of libc++ can interoperate. However, this
|
|
leads to code size improvements, since non ABI-facing functions can be
|
|
deduplicated across translation unit boundaries.
|
|
|
|
This macro can be defined by users to control the behavior they want from
|
|
libc++. The default value of this macro (0 or 1) is controlled by whether
|
|
`_LIBCPP_HIDE_FROM_ABI_PER_TU_BY_DEFAULT` is defined, which is intended to
|
|
be used by vendors only (see below).
|
|
|
|
**_LIBCPP_HIDE_FROM_ABI_PER_TU_BY_DEFAULT**
|
|
This macro controls the default value for `_LIBCPP_HIDE_FROM_ABI_PER_TU`.
|
|
When the macro is defined, per TU ABI insulation is enabled by default, and
|
|
`_LIBCPP_HIDE_FROM_ABI_PER_TU` is defined to 1 unless overridden by users.
|
|
Otherwise, per TU ABI insulation is disabled by default, and
|
|
`_LIBCPP_HIDE_FROM_ABI_PER_TU` is defined to 0 unless overridden by users.
|
|
|
|
This macro is intended for vendors to control whether they want to ship
|
|
libc++ with per TU ABI insulation enabled by default. Users can always
|
|
control the behavior they want by defining `_LIBCPP_HIDE_FROM_ABI_PER_TU`
|
|
appropriately.
|
|
|
|
By default, this macro is not defined, which means that per TU ABI insulation
|
|
is not provided unless explicitly overridden by users.
|
|
|
|
**_LIBCPP_TYPE_VIS**
|
|
Mark a type's typeinfo, vtable and members as having default visibility.
|
|
This attribute cannot be used on class templates.
|
|
|
|
**_LIBCPP_TEMPLATE_VIS**
|
|
Mark a type's typeinfo and vtable as having default visibility.
|
|
This macro has no effect on the visibility of the type's member functions.
|
|
|
|
**GCC Behavior**: GCC does not support Clang's `type_visibility(...)`
|
|
attribute. With GCC the `visibility(...)` attribute is used and member
|
|
functions are affected.
|
|
|
|
**Windows Behavior**: DLLs do not support dllimport/export on class templates.
|
|
The macro has an empty definition on this platform.
|
|
|
|
|
|
**_LIBCPP_ENUM_VIS**
|
|
Mark the typeinfo of an enum as having default visibility. This attribute
|
|
should be applied to all enum declarations.
|
|
|
|
**Windows Behavior**: DLLs do not support importing or exporting enumeration
|
|
typeinfo. The macro has an empty definition on this platform.
|
|
|
|
**GCC Behavior**: GCC un-hides the typeinfo for enumerations by default, even
|
|
if `-fvisibility=hidden` is specified. Additionally applying a visibility
|
|
attribute to an enum class results in a warning. The macro has an empty
|
|
definition with GCC.
|
|
|
|
**_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS**
|
|
Mark the member functions, typeinfo, and vtable of the type named in
|
|
a `_LIBCPP_EXTERN_TEMPLATE` declaration as being exported by the libc++ library.
|
|
This attribute must be specified on all extern class template declarations.
|
|
|
|
This macro is used to override the `_LIBCPP_TEMPLATE_VIS` attribute
|
|
specified on the primary template and to export the member functions produced
|
|
by the explicit instantiation in the dylib.
|
|
|
|
**Windows Behavior**: `extern template` and `dllexport` are fundamentally
|
|
incompatible *on a class template* on Windows; the former suppresses
|
|
instantiation, while the latter forces it. Specifying both on the same
|
|
declaration makes the class template be instantiated, which is not desirable
|
|
inside headers. This macro therefore expands to `dllimport` outside of libc++
|
|
but nothing inside of it (rather than expanding to `dllexport`); instead, the
|
|
explicit instantiations themselves are marked as exported. Note that this
|
|
applies *only* to extern *class* templates. Extern *function* templates obey
|
|
regular import/export semantics, and applying `dllexport` directly to the
|
|
extern template declaration (i.e. using `_LIBCPP_FUNC_VIS`) is the correct
|
|
thing to do for them.
|
|
|
|
**_LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS**
|
|
Mark the member functions, typeinfo, and vtable of an explicit instantiation
|
|
of a class template as being exported by the libc++ library. This attribute
|
|
must be specified on all class template explicit instantiations.
|
|
|
|
It is only necessary to mark the explicit instantiation itself (as opposed to
|
|
the extern template declaration) as exported on Windows, as discussed above.
|
|
On all other platforms, this macro has an empty definition.
|
|
|
|
**_LIBCPP_METHOD_TEMPLATE_IMPLICIT_INSTANTIATION_VIS**
|
|
Mark a symbol as hidden so it will not be exported from shared libraries. This
|
|
is intended specifically for method templates of either classes marked with
|
|
`_LIBCPP_TYPE_VIS` or classes with an extern template instantiation
|
|
declaration marked with `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS`.
|
|
|
|
When building libc++ with hidden visibility, we want explicit template
|
|
instantiations to export members, which is consistent with existing Windows
|
|
behavior. We also want classes annotated with `_LIBCPP_TYPE_VIS` to export
|
|
their members, which is again consistent with existing Windows behavior.
|
|
Both these changes are necessary for clients to be able to link against a
|
|
libc++ DSO built with hidden visibility without encountering missing symbols.
|
|
|
|
An unfortunate side effect, however, is that method templates of classes
|
|
either marked `_LIBCPP_TYPE_VIS` or with extern template instantiation
|
|
declarations marked with `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` also get default
|
|
visibility when instantiated. These methods are often implicitly instantiated
|
|
inside other libraries which use the libc++ headers, and will therefore end up
|
|
being exported from those libraries, since those implicit instantiations will
|
|
receive default visibility. This is not acceptable for libraries that wish to
|
|
control their visibility, and led to PR30642.
|
|
|
|
Consequently, all such problematic method templates are explicitly marked
|
|
either hidden (via this macro) or inline, so that they don't leak into client
|
|
libraries. The problematic methods were found by running
|
|
`bad-visibility-finder <https://github.com/smeenai/bad-visibility-finder>`_
|
|
against the libc++ headers after making `_LIBCPP_TYPE_VIS` and
|
|
`_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` expand to default visibility.
|
|
|
|
**_LIBCPP_EXCEPTION_ABI**
|
|
Mark the member functions, typeinfo, and vtable of the type as being exported
|
|
by the libc++ library. This macro must be applied to all *exception types*.
|
|
Exception types should be defined directly in namespace `std` and not the
|
|
versioning namespace. This allows throwing and catching some exception types
|
|
between libc++ and libstdc++.
|
|
|
|
**_LIBCPP_INTERNAL_LINKAGE**
|
|
Mark the affected entity as having internal linkage (i.e. the `static`
|
|
keyword in C). This is only a best effort: when the `internal_linkage`
|
|
attribute is not available, we fall back to forcing the function to be
|
|
inlined, which approximates internal linkage since an externally visible
|
|
symbol is never generated for that function. This is an internal macro
|
|
used as an implementation detail by other visibility macros. Never mark
|
|
a function or a class with this macro directly.
|
|
|
|
**_LIBCPP_ALWAYS_INLINE**
|
|
Forces inlining of the function it is applied to. For visibility purposes,
|
|
this macro is used to make sure that an externally visible symbol is never
|
|
generated in an object file when the `internal_linkage` attribute is not
|
|
available. This is an internal macro used by other visibility macros, and
|
|
it should not be used directly.
|
|
|
|
Links
|
|
=====
|
|
|
|
* `[cfe-dev] Visibility in libc++ - 1 <http://lists.llvm.org/pipermail/cfe-dev/2013-July/030610.html>`_
|
|
* `[cfe-dev] Visibility in libc++ - 2 <http://lists.llvm.org/pipermail/cfe-dev/2013-August/031195.html>`_
|
|
* `[libcxx] Visibility fixes for Windows <http://lists.llvm.org/pipermail/cfe-commits/Week-of-Mon-20130805/085461.html>`_
|