DIP45
| Title: | making export an attribute |
|---|---|
| DIP: | 45 |
| Version: | 2 |
| Status: | Draft |
| Created: | 2013-08-27 |
| Last Modified: | 2013-09-06 |
| Author: | Benjamin Thaut, Martin Nowak |
| Links: |
Abstract
Export and its behavior need to be changed in serveral ways to make it work on Windows and allow better code generation for other plattforms. The Rationale section explains the problems and shows how this DIP solves them.
Description
- The export protection level should be turned into a export attribute.
- If a module contains a single symbol annotated with the 'export' attribute all compiler internal symbols of this module should recieve the 'export' attribute too (e.g. module info).
- If a class is annotated with the 'export' attribute, all of its public and protected functions and members will automatically recieve the 'export' attribute. Also all its hidden compiler specific symbols will recieve the 'export' attribute.
- There should be only one meaning of 'export'.
- It should be possible to access TLS variables across DLL / shared library boundaries.
- On *nix systems default symbol visibility is changed to hidden, and only symbols marked with export become visible.
Rationale
Turning export into an attribute
Currently export is a protection level, the highest level of visibility actually. This however conflicts with the need to export 'protected' symbols. Consider a Base class in a shared library.
module sharedLib;
class Base {
protected final void doSomething() { ... }
}
module executable; import sharedLib;
class Derived : Base {
public void func()
{
doSomething();
}
}
In the above example 'doSomething' should only be visible to derived classes but it still needs to be exportable from a shared library. Therefor export should become a normal attribute which behaves orthogonal to protection.
Implicitly exporting compiler internal symbols
All compiler internal symbols need to be treated as exported if using an exported symbol might implicitly reference them to avoid link errors. The most prominent example is the ModuleInfo which needs linkage if the module has a static this().
export attribute inference
Currently export has to be specified in a lot of places to export all neccessary functions and data symbols. Export should be transitive in such a sense that it only needs to be specified once in a module to export all of its functions / data members including classes and their members / data symbols. Consider the following example:
module sharedLib:
export:
__gshared int g_Var; // should be exported
void globalFunc() { ... } // should be exported
class A // compiler internal members should be exported
{
private:
int m_a;
static int s_b; // should not be exported
void internalFunc() { ... } // should not be exported
protected:
void interalFunc2() { ... } // should be exported
public:
class Inner // compiler internal members should be exported
{
static s_inner; // should be exported
void innerMethod() { ... } // should be exported
}
void method() { ... } // should be exported
}
private class C // should not be exported
{
public void method() {... } // should not be exported
}
A single meaning of export
The classical solution to handle dllexport/dllimport attributes on Windows is to define a macro that depending on the current build setting expands to __declspec(dllexport) or to __declspec(dllimport). This complicates the build setup and means that object files for a static library can't be mixed well with object files for a DLL. Instead we propose that export symbols are accompanied with weak import aliases and always accessed through them. See the implementation detail section for how this will work for data symbols and function symbols. That way a compiled object file can be used for a DLL or a static library. And vice versa an object file can be linked against an import library or a static library.
Access TLS variables
Currently it is not possible to access TLS variables across shared library boundaries on Windows. This should be implemented (see implementation details for a proposal).
Change symbol visibility on *nix systems
When building shared libraries on *nix systems all symbols are visible by default. This is a main reason for the performance impact of PIC because every data access and every function call go through the GOT or PLT indirection. It also leads to long loading time because an excessive number of relocations have to be processed. Making all symbols hidden by default significantly reduces the size of the dynamic symbol table (faster lookup and smaller libraries). See http://gcc.gnu.org/wiki/Visibility and http://people.redhat.com/drepper/dsohowto.pdf for more details.
Also making every symbol accessible can inadvertently cause ABI dependencies making it harder to maintain libraries.
Implementation Details
Windows
Data Symbols
For data symbols the 'export' attribute always means 'dllexport' when defining a symbol and 'dllimport' when accessing a symbol. That is accessing an exported variable is done through dereferencing it's corresponding import symbols. When defining an exported variable the compiler will emit a corresponding import symbol that is initialized with address of the variable. The import symbol can be located in the read only data segment. The mangling of the import symbol consists of the '_imp_'/'__imp_' (Win32/Win64) prefix followed by the mangled name of the variable. Import symbols itself are not exported. When an exported variable of the same module is accessed the compiler might avoid the indirection and perform a direct access.
module a;
export __gshared int var = 5;
__gshared int* _imp__D1a3vari = &var; // import symbol generated by the compiler
void func()
{
var = 3; // accesses var directly, because in the same module
}
module b;
import a;
void bar()
{
var = 5; // accesses through indirection because var is marked as export and in a different module
// *_imp__D1a3vari = 5; // code generated by the compiler
}
Function Symbols
For function symbols the 'export' attribute always means 'dllexport' when defining a function and 'dllimport' when calling a function. That is calling an exported function is done through it's corresponding import symbol. When defining an exported function the compiler will emit a corresponding import symbols that is an alias to the function (See COFF weak externals and OMF ALIAS record on how to implement aliases). Thus calling an exported function becomes compatible with both import libraries and static libraries, in the later case without indirection.
TLS variables
For each exported TLS variable the compiler should generate a function that returns the address of the TLS variable in the current thread. These internal methods should have some kind of unified prefix to mark them as TLS import helpers. I propose "__tlsstub_". These internal methods are also exported. So when accessing an exported TLS variable the compiler will insert a call to '_imp__D1a15__tlsstub_g_tlsFZPi' instead. As an optimization accesses to exported TLS variables within the same module can be performed directly.
module a;
export int g_tls = 5; // thread local storage
export int* __tlsstub__g_tls() // generated by the compiler
{
return &g_tls;
}
alias _imp___tlsstub__g_tls = __tlsstub__g_tls; // also generated by the compiler
void func()
{
g_tls = 3; // direct access because marked as export and in the same module
}
module b;
import a;
void bar()
{
g_tls = 10; // access through _imp___tlsstub__g_tls function because marked as export and in a different module
// *_imp___tlsstub__g_tls() = 10; // code generated by the compiler
}
*nix
On *nix systems the default symbols visibility should be changed to hidden, i.e. -fvisibility=hidden argument of gcc. Only symbols marked with export should get the attribute visible.
Copyright
This document has been placed in the Public Domain.
