[llvm] 3b91030 - [llvm] add documentation for public interface annotations (LLVM_ABI, etc)
via llvm-commits
llvm-commits at lists.llvm.org
Tue Apr 15 10:51:05 PDT 2025
Author: Andrew Rogers
Date: 2025-04-15T10:51:02-07:00
New Revision: 3b9103044361094a8fde16a877f2e8cb0f96ce24
URL: https://github.com/llvm/llvm-project/commit/3b9103044361094a8fde16a877f2e8cb0f96ce24
DIFF: https://github.com/llvm/llvm-project/commit/3b9103044361094a8fde16a877f2e8cb0f96ce24.diff
LOG: [llvm] add documentation for public interface annotations (LLVM_ABI, etc)
## Purpose
Add documentation for the existing family of `LLVM_ABI` annotation
macros defined in llvm/Support/Compiler.h. These annotations are used to
describe LLVM's public interface.
## Background
This documentation is in support of the annotation effort described
[here](https://discourse.llvm.org/t/psa-annotating-llvm-public-interface/85307/).
## Validation
Manually inspected rendered ReST document on GitHub.
Co-authored-by: Saleem Abdulrasool <compnerd at compnerd.org>
Added:
llvm/docs/InterfaceExportAnnotations.rst
Modified:
llvm/docs/Reference.rst
Removed:
################################################################################
diff --git a/llvm/docs/InterfaceExportAnnotations.rst b/llvm/docs/InterfaceExportAnnotations.rst
new file mode 100644
index 0000000000000..eecf6ffe6eaca
--- /dev/null
+++ b/llvm/docs/InterfaceExportAnnotations.rst
@@ -0,0 +1,376 @@
+LLVM Interface Export Annotations
+=================================
+Symbols that are part of LLVM's public interface must be explicitly annotated
+to support shared library builds with hidden default symbol visibility. This
+document provides background and guidelines for annotating the codebase.
+
+LLVM Shared Library
+-------------------
+LLVM builds as a static library by default, but it can also be built as a shared
+library with the following configuration:
+
+::
+
+ LLVM_BUILD_LLVM_DYLIB=On
+ LLVM_LINK_LLVM_DYLIB=On
+
+There are three shared library executable formats we're interested in: PE
+Dynamic Link Library (.dll) on Windows, Mach-O Shared Object (.dylib) on Apple
+systems, and ELF Shared Object (.so) on Linux, BSD and other Unix-like systems.
+
+ELF and Mach-O Shared Object files can be built with no additional setup or
+configuration. This is because all global symbols in the library are exported by
+default -- the same as when building a static library. However, when building a
+DLL for Windows, the situation is more complex:
+
+- Symbols are not exported from a DLL by default. Symbols must be annotated with
+ ``__declspec(dllexport)`` when building the library to be externally visible.
+
+- Symbols imported from a Windows DLL should generally be annotated with
+ ``__declspec(dllimport)`` when compiling clients.
+
+- A single Windows DLL can export a maximum of 65,535 symbols.
+
+Because of the requirements for Windows DLLs, additional work must be done to
+ensure the proper set of public symbols is exported and visible to clients.
+
+Annotation Macros
+-----------------
+The distinct DLL import and export annotations required for Windows DLLs
+typically lead developers to define a preprocessor macro for annotating exported
+symbols in header public files. The custom macro resolves to the _export_
+annotation when building the library and the _import_ annotation when building
+the client.
+
+We have defined the `LLVM_ABI` macro in `llvm/Support/Compiler.h
+<https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Support/Compiler.h#L152>`__
+for this purpose:
+
+.. code:: cpp
+
+ #if defined(LLVM_EXPORTS)
+ #define LLVM_ABI __declspec(dllexport)
+ #else
+ #define LLVM_ABI __declspec(dllimport)
+ #endif
+
+Windows DLL symbol visibility requirements are approximated on ELF and Mach-O
+shared library builds by setting default symbol visibility to hidden
+(``-fvisibility-default=hidden``) when building with the following
+configuration:
+
+::
+
+ LLVM_BUILD_LLVM_DYLIB_VIS=On
+
+For an ELF or Mach-O platform with this setting, the ``LLVM_ABI`` macro is
+defined to override the default hidden symbol visibility:
+
+.. code:: cpp
+
+ #define LLVM_ABI __attribute__((visibility("default")))
+
+In addition to ``LLVM_ABI``, there are a few other macros for use in less
+common cases described below.
+
+Export macros are used to annotate symbols only within their intended shared
+library. This is necessary because of the way Windows handles import/export
+annotations.
+
+For example, ``LLVM_ABI`` resolves to ``__declspec(dllexport)`` only when
+building source that is part of the LLVM shared library (e.g. source under
+``llvm-project/llvm``). If ``LLVM_ABI`` were incorrectly used to annotate a
+symbol from a
diff erent LLVM project (such as Clang) it would always resolve to
+``__declspec(dllimport)`` and the symbol would not be properly exported.
+
+Annotating Symbols
+------------------
+Functions
+~~~~~~~~~
+Exported function declarations in header files must be annotated with
+``LLVM_ABI``.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ LLVM_ABI void exported_function(int a, int b);
+
+Global Variables
+~~~~~~~~~~~~~~~~
+Exported global variables must be annotated with ``LLVM_ABI`` at their
+``extern`` declarations.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ LLVM_ABI extern int exported_global_variable;
+
+Classes, Structs, and Unions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Classes, structs, and unions can be annotated with ``LLVM_ABI`` at their
+declaration, but this option is generally discouraged because it will
+export every class member, vtable, and type information. Instead, ``LLVM_ABI``
+should be applied to individual class members that require export.
+
+In the most common case, public and protected methods without a body in the
+class declaration must be annotated with ``LLVM_ABI``.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ class ExampleClass {
+ public:
+ // Public methods defined externally must be annotatated.
+ LLVM_ABI int sourceDefinedPublicMethod(int a, int b);
+
+ // Methods defined in the class definition do not need annotation.
+ int headerDefinedPublicMethod(int a, int b) {
+ return a + b;
+ }
+
+ // Constructors and destructors must be annotated if defined externally.
+ ExampleClass() {}
+ LLVM_ABI ~ExampleClass();
+
+ // Public static methods defined externally must be annotatated.
+ LLVM_ABI static int sourceDefinedPublicStaticMethod(int a, int b);
+ };
+
+Additionally, public and protected static fields that are not initialized at
+declaration must be annotated with ``LLVM_ABI``.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ class ExampleClass {
+ public:
+ // Public static fields defined externally must be annotated.
+ LLVM_ABI static int mutableStaticField;
+ LLVM_ABI static const int constStaticField;
+
+ // Static members initialized at declaration do not need to be annotated.
+ static const int initializedConstStaticField = 0;
+ static constexpr int initializedConstexprStaticField = 0;
+ };
+
+Private methods may also require ``LLVM_ABI`` annotation in certain cases. This
+situation occurs when a method defined in a header calls the private method. The
+private method call may be from within the class, a parent class, or a friend
+class.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ class ExampleClass {
+ private:
+ // Private methods must be annotated if referenced by a public method defined a
+ // header file.
+ LLVM_ABI int privateMethod(int a, int b);
+
+ public:
+ // Inlineable method defined in the class definition calls a private method
+ // defined externally. If the private method is not annotated for export, this
+ // method will fail to link.
+ int publicMethod(int a, int b) {
+ return privateMethod(a, b);
+ }
+ };
+
+There are less common cases where you may also need to annotate an inline
+function even though it is fully defined in a header. Annotating an inline
+function for export does not prevent it being inlined into client code. However,
+it does ensure there is a single, stable address for the function exported from
+the shared library.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ // Annotate the function so it is exported from the library at a fixed
+ // address.
+ LLVM_ABI inline int inlineFunction(int a, int b) {
+ return a + b;
+ }
+
+Similarly, if a stable pointer-to-member function address is required for a
+method in a C++ class, it may be annotated for export.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ class ExampleClass {
+ public:
+ // Annotate the method so it is exported from the library at a fixed
+ // address.
+ LLVM_ABI inline int inlineMethod(int a, int b) {
+ return a + b;
+ }
+ };
+
+.. note::
+
+ When an inline function is annotated for export, the header containing the
+ function definition **must** be included by at least one of the library's
+ source files or the function will never be compiled with the export
+ annotation.
+
+Friend Functions
+~~~~~~~~~~~~~~~~
+Friend functions declared in a class, struct or union must be annotated with
+``LLVM_ABI`` if the corresponding function declaration is also annotated. This
+requirement applies even when the class itself is annotated with ``LLVM_ABI``.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ // An exported function that has friend access to ExampleClass internals.
+ LLVM_ABI int friend_function(ExampleClass &obj);
+
+ class ExampleClass {
+ // Friend declaration of a function must be annotated the same as the actual
+ // function declaration.
+ LLVM_ABI friend int friend_function(ExampleClass &obj);
+ };
+
+.. note::
+
+ Annotating the friend declaration avoids an “inconsistent dll linkage”
+ compiler error when building for Windows. This annotation is harmless but not
+ required when building ELF or Mach-O shared libraries.
+
+Virtual Table and Type Info
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Classes and structs with exported virtual methods, or child classes that export
+overridden virtual methods, must also export their vtable for ELF and Mach-O
+builds. This can be achieved by annotating the class rather than individual
+class members.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ class ParentClass {
+ public:
+ virtual int virtualMethod(int a, int b);
+ virtual int anotherVirtualMethod(int a, int b);
+ virtual ~ParentClass();
+ };
+
+ // Annotating the class exports vtable and type information as well as all
+ // class members.
+ class LLVM_ABI ChildClass : public ParentClass {
+ public:
+ // Inline method override does not require the class be annotated.
+ int virtualMethod(int a, int b) override {
+ return ParentClass::virtualMethod(a, b);
+ }
+
+ // Overriding a virtual method from the parent requires the class be
+ // annotated. The parent class may require annotation as well.
+ int pureVirtualMethod(int a, int b) override;
+ ~ChildClass();
+ };
+
+If annotating a type with ``LLVM_ABI`` causes compilation issues such as those
+described
+`here <https://devblogs.microsoft.com/oldnewthing/20190927-00/?p=102932>`__,
+the class may require modification. Often, explicitly deleting the copy
+constructor and copy assignment operator will resolve the issue.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ #include <vector>
+
+ class LLVM_ABI ExportedClass {
+ public:
+ // Explicitly delete the copy constructor and assignment operator.
+ ExportedClass(ExportedClass const&) = delete;
+ ExportedClass& operator=(ExportedClass const&) = delete;
+ };
+
+Templates
+~~~~~~~~~
+Most template classes are entirely header-defined and do not need to be exported
+because they will be instantiated and compiled into the client as needed. Such
+template classes require no export annotations. However, there are some less
+common cases where annotations are required for templates.
+
+Specialized Template Functions
+++++++++++++++++++++++++++++++
+As with any other exported function, an exported specialization of a template
+function not defined in a header file must have its declaration annotated with
+``LLVM_ABI``.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ template <typename T> T templateMethod(T a, T b) {
+ return a + b;
+ }
+
+ // The explicitly specialized definition of templateMethod for int is located in
+ // a source file. This declaration must be annotated with LLVM_ABI to export it.
+ template <> LLVM_ABI int templateMethod(int a, int b);
+
+Similarly, an exported specialization of a method in a template class must have
+its declaration annotated with ``LLVM_ABI``.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ template <typename T> class TemplateClass {
+ public:
+ int method(int a, int b) {
+ return a + b;
+ }
+ };
+
+ // The explicitly specialized definition of method for int is defined in a
+ // source file. The declaration must be annotated with LLVM_ABI to export it.
+ template <> LLVM_ABI int TemplateStruct<int>::method(int a, int b);
+
+Explicitly Instantiated Template Classes
+++++++++++++++++++++++++++++++++++++++++
+Explicitly instantiated template classes must be annotated with
+template-specific annotations at both declaration and definition.
+
+An extern template instantiation in a header file must be annotated with
+``LLVM_TEMPLATE_ABI``. This will typically be located in a header file.
+
+.. code:: cpp
+
+ #include "llvm/Support/Compiler.h"
+
+ template <typename T> class TemplateClass {
+ public:
+ TemplateClass(T val) : val_(val) {}
+
+ T get() const { return val_; }
+
+ private:
+ const T val_;
+ };
+
+ // Explicitly instantiate and export TempalateClass for int type.
+ extern template class LLVM_TEMPLATE_ABI TemplateClass<int>;
+
+The corresponding definition of the template instantiation must be annotated
+with ``LLVM_EXPORT_TEMPLATE``. This will typically be located in a source file.
+
+.. code:: cpp
+
+ #include "TemplateClass.h"
+
+ // Explicitly instantiate and export TempalateClass for int type.
+ template class LLVM_EXPORT_TEMPLATE TemplateClass<int>;
diff --git a/llvm/docs/Reference.rst b/llvm/docs/Reference.rst
index 470b6bd024b89..e1f46b00f2b30 100644
--- a/llvm/docs/Reference.rst
+++ b/llvm/docs/Reference.rst
@@ -31,6 +31,7 @@ LLVM and API reference documentation.
HowToSetUpLLVMStyleRTTI
HowToUseAttributes
InAlloca
+ InterfaceExportAnnotations
LangRef
LibFuzzer
MarkedUpDisassembly
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