wxWidgets/include/wx/cocoa/objc/objc_uniquifying.h

/////////////////////////////////////////////////////////////////////////////
// Name:        wx/cocoa/objc/objc_uniquifying.h
// Purpose:     Allows wxWidgets code to get a direct pointer to a compiled
//              Objective-C class and provides a method to fix up the
//              name to include a unique identifier (currently the address
//              of the objc_class structure).
// Author:      David Elliott <dfe@cox.net>
// Modified by:
// Created:     2007/05/15
// RCS-ID:      $Id$
// Copyright:   (c) 2007 Software 2000 Ltd.
// Licence:     wxWindows licence
/////////////////////////////////////////////////////////////////////////////

#ifndef __WX_COCOA_OBJC_CLASS_H__
#define __WX_COCOA_OBJC_CLASS_H__

#if wxUSE_OBJC_UNIQUIFYING

// objc_getClass and stuff
#include <objc/objc-runtime.h>

////////////// Objective-C uniquifying implementation //////////////

template <typename ObjcType>
class wxObjcClassInitializer;

template <typename ObjcType>
class UniquifiedName;

template <typename ObjcType>
class wxObjcCompilerInformation
{
    friend class wxObjcClassInitializer<ObjcType>;
    friend class UniquifiedName<ObjcType>;
private:
    // GetCompiledClass must be partially specialized for an ObjcType
    // If you're not using it, implement an inline returning NULL
    inline static struct objc_class * GetCompiledClass();

    // sm_theClassName must be partially specialized for each type
    static const char sm_theClassName[];

    // GetSuperclass must be specialized. Typically one of two ways:
    // 1. objc_getClass("SomeRealClassName")
    // 2. wxGetObjcClass_SomeWxClassName();
    inline static struct objc_class *GetSuperclass();
};


template <typename ObjcType>
struct UniquifiedName
{
    // We're going for OriginalClassName@ClassStructureAddress
    // Therefore our size is the sizeof the original class name constant string (which includes the terminating NULL)
    // plus the sizeof a pointer to struct objc_class times two (two hex digits for each byte) plus 3 for "@0x"
    typedef char Type[sizeof(wxObjcCompilerInformation<ObjcType>::sm_theClassName) + (sizeof(struct objc_class*)<<1) + 3];
    static void Init(Type m_theString, const objc_class *aClass)
    {
        snprintf(const_cast<char*>(m_theString), sizeof(Type), "%s@%p", wxObjcCompilerInformation<ObjcType>::sm_theClassName, aClass);
    }
};

template <typename ObjcType>
class wxObjcClassInitializer
{
public:
    static struct objc_class* Get()
    {
        static wxObjcClassInitializer<ObjcType> s_theInstance;
        s_theInstance.noop(); // Make the compiler think we need this instance
        return wxObjcCompilerInformation<ObjcType>::GetCompiledClass();
    }
private:
    void noop()
    {}
    // This "constructor" operates solely on static data
    // It exists so that we can take advantage of a function-static
    // "instance" of this class to do the static data initialization.
    wxObjcClassInitializer()
    {
        // Objective-C class initialization occurs before C++ static initialization because the
        // libobjc.dylib gets notified directly by dyld on Tiger.
        // Therefore, even though we change the name, the class is still registered with the
        // original name.  We unfortunately can't change that.

        // The first time the class is loaded, Objective-C will already have fixed up the super_class
        // and isa->isa and isa->super_class variables so much of this won't do anything.  But
        // the next time the class is loaded, Objective-C will ignore it and thus we need to
        // initialize the data structures appropriately.

        // Ideally we'd have some sort of lock here, but we depend on the fact that we get called
        // just before the first time someone wants to send a class message so it should be
        // reasonably safe to do this without any locks.

        struct objc_class &theClassData = *wxObjcCompilerInformation<ObjcType>::GetCompiledClass();
        // Initialize the uniquified class name
        UniquifiedName<ObjcType>::Init(sm_theUniquifiedClassName, &theClassData);

        //////// Class Initialization ////////
        // Use objc_getClass to fix up the superclass pointer
        theClassData.super_class = wxObjcCompilerInformation<ObjcType>::GetSuperclass();
        // Fix up the compiler generated class struct to use the new name
        theClassData.name = sm_theUniquifiedClassName;

        //////// Meta-Class Initialization ////////
        // theClassData.isa is the metaclass pointer
        // Globals on Darwin use PC-relative access (slow) so it's quicker to use theClassData.isa

        // In any object hierarchy a metaclass's metaclass is always the root class's metaclass
        // Therefore, our superclass's metaclass's metaclass should already be the root class's metaclass
        theClassData.isa->isa = theClassData.super_class->isa->isa;
        // A metaclass's superclass is always the superclass's metaclass.
        theClassData.isa->super_class = theClassData.super_class->isa;
        // Fix up the compiler generated metaclass struct to use the new name
        theClassData.isa->name = sm_theUniquifiedClassName;

        // We need to set the initialized flag because after we change the name, Objective-C can't
        // look us up by name because we're only registered with the original name.
        theClassData.isa->info |= CLS_INITIALIZED;
    }
    wxObjcClassInitializer(const wxObjcClassInitializer&); // NO COPY
    wxObjcClassInitializer& operator =(const wxObjcClassInitializer&); // NO ASSIGN
    static typename UniquifiedName<ObjcType>::Type sm_theUniquifiedClassName;
};

template<typename ObjcType>
typename UniquifiedName<ObjcType>::Type wxObjcClassInitializer<ObjcType>::sm_theUniquifiedClassName;

// WX_DECLARE_GET_OBJC_CLASS
// Declares a function to get a direct pointer to an objective-C class.
// The class is guaranteed to be usable.
// When wxCocoa is built into a Mach-O bundle this function allows the wxCocoa
// code to get a reference to the Objective-C class structure located in the
// same bundle.  This allows a static wxCocoa library to be built into
// two different Mach-O bundles without having one bundle's Objective-C
// classes trample on the other's.
// Right now we toss the ObjcSuperClass parameter, but we might use it later.
#define WX_DECLARE_GET_OBJC_CLASS(ObjcClass,ObjcSuperClass) \
struct objc_class* wx_GetObjcClass_ ## ObjcClass();

// WX_IMPLEMENT_OBJC_GET_COMPILED_CLASS(ObjcClass)
// Provides an architecture-dependent way to get the direct pointer to the
// objc_class structure in the __OBJC segment.
// This takes advantage of the fact that the Objective-C compiler uses guessable
// local assembler labels for the class structures.
// Those class structures are only available on the Objective-C file containing the
// @implementation block.

#if 1
// Generic implementation - Tested on i386 and PPC.  Should work in all cases.
// This is a hack that depends on GCC asm symbol names.
// The static variable winds up being initialized with a direct reference to the appropriate
// L_OBJC_CLASS and no global symbol reference is generated because nothing uses the global symbol
// except for the static initializer which does it directly.
// The generated assembler for s_objc_class_ptr is basically like this:
// _s_objc_class_ptr_ObjcClass:
//     .long L_OBJC_CLASS_ObjcClass
// Once that static symbol is defined, the function implementation is easy for GCC to generate.
// Do note that return &s_objc_class_data_ObjcClass won't work.  The code is wrong in the case.
#define WX_IMPLEMENT_OBJC_GET_COMPILED_CLASS(ObjcClass) \
extern "C" objc_class s_objc_class_data_ ## ObjcClass asm("L_OBJC_CLASS_" #ObjcClass); \
static objc_class * s_objc_class_ptr_ ## ObjcClass = &s_objc_class_data_ ## ObjcClass; \
template<> \
inline objc_class * wxObjcCompilerInformation<ObjcClass>::GetCompiledClass() \
{ \
    return s_objc_class_ptr_## ObjcClass; \
}

#elif defined(__i386__)
// Not used because the generic implementation seems to work fine.
// But this is here since it was written beforehand and it also works.

// This is based on the code GCC generates for accessing file-static data on i386.
// The i386 PC-relative addressing happens in this manner
// 1. The program counter is placed into ecx using the code that GCC should have
//    already generated.
// 2. A label is placed directly after the call to get the program counter.
// 3. The Load Effective Address instruction is used to add the offset of the
//    local assembler label we're interested in minus the local assembler label
//    from step 2 to the program counter register in ecx and place the result
//    into the result register (typically eax if not inlined).
#define WX_IMPLEMENT_OBJC_GET_COMPILED_CLASS(ObjcClass) \
template<> \
inline objc_class * wxObjcCompilerInformation<ObjcClass>::GetCompiledClass() \
{ \
    register struct objc_class *retval; \
    asm \
    (   "call ___i686.get_pc_thunk.cx\n" \
        "\"LPC_FOR_GET_CLASS_" #ObjcClass "\":\n\t" \
        "leal L_OBJC_CLASS_" #ObjcClass "-\"LPC_FOR_GET_CLASS_" #ObjcClass "\"(%%ecx), %0" \
    :   "=r"(retval) \
    :    \
    :   "ecx" \
    ); \
    return retval; \
}

#elif defined(__ppc__)
// Not used because the generic implementation seems to work fine.
// But this is here since it was written beforehand and it also works.

// This is based on the code GCC generates for accessing file-static data on PPC.
// The PowerPC PC-relative addressing happens in this manner
// 1. The link register is saved (mflr) to a temporary (we re-use the output register for this)
// 2. An unconditional branch instruction (bcl) "branches" to the following address (labeled)
// 3. The link register (filled in by bcl) is saved to r10 (a temporary)
// 4. The previous link register is restored (mtlr) (from the output register we were using as a temporary)
// 5. The address of the LPC label as executed is added to the high 16 bits of the offset between that label and the static data we want
//    and stored in a temporary register (r2)
// 6. That temporary register plus the low 16 bits of the offset are stored into the result register.
#define WX_IMPLEMENT_OBJC_GET_COMPILED_CLASS(ObjcClass) \
template<> \
inline objc_class * wxObjcCompilerInformation<ObjcClass>::GetCompiledClass() \
{ \
    register struct objc_class *retval; \
    asm \
    (   "mflr %0" \
        "\n\tbcl 20, 31, \"LPC_FOR_GET_CLASS_" #ObjcClass "\"" \
        "\n\"LPC_FOR_GET_CLASS_" #ObjcClass "\":" \
        "\n\tmflr r10" \
        "\n\tmtlr %0" \
        "\n\taddis r2,r10,ha16(L_OBJC_CLASS_" #ObjcClass "-\"LPC_FOR_GET_CLASS_" #ObjcClass "\")" \
        "\n\tla %0,lo16(L_OBJC_CLASS_" #ObjcClass "-\"LPC_FOR_GET_CLASS_" #ObjcClass "\")(r2)" \
    :   "=r" (retval) \
    : \
    :   "r10","r2" \
    ); \
    return retval; \
}

// TODO: __x86_64__, __ppc64__
#else // Can't wrie inline asm to bust into __OBJC segment
// This won't be used since the generic implementation takes precedence.

#warning "Don't know how to implement wxObjcCompilerInformation<ObjcClass>::GetCompiledClass on this platform"

#endif // platforms

// The WX_IMPLEMENT_OBJC_GET_SUPERCLASS macro implements the template specialization
// to get the superclass.  This only works if it's a real superclass.  If you are
// deriving from a class that's already being uniquified then you'd need to
// implement the specialization to call the appropriate get method instead.
#define WX_IMPLEMENT_OBJC_GET_SUPERCLASS(ObjcClass,ObjcSuperClass) \
    template <> \
    inline objc_class* wxObjcCompilerInformation<ObjcClass>::GetSuperclass() \
    { \
        return objc_getClass(#ObjcSuperClass); \
    }

// The WX_IMPLEMENT_OBJC_CLASS_NAME macro implements the template specialization
// of the sm_theClassName constant.  As soon as this specialization is in place
// sizeof(sm_theClassName) will return the number of bytes at compile time.
#define WX_IMPLEMENT_OBJC_CLASS_NAME(ObjcClass) \
    template <> \
    const char wxObjcCompilerInformation<ObjcClass>::sm_theClassName[] = #ObjcClass;

// The WX_IMPLEMENT_GET_OBJC_CLASS macro combines all of these together and adds
// a global wx_GetObjcClass_ObjcClass() function.
#define WX_IMPLEMENT_GET_OBJC_CLASS(ObjcClass,ObjcSuperClass) \
    WX_IMPLEMENT_OBJC_GET_COMPILED_CLASS(ObjcClass) \
    WX_IMPLEMENT_OBJC_GET_SUPERCLASS(ObjcClass,ObjcSuperClass) \
    WX_IMPLEMENT_OBJC_CLASS_NAME(ObjcClass) \
    objc_class* wx_GetObjcClass_ ## ObjcClass() \
    { \
        return wxObjcClassInitializer<ObjcClass>::Get(); \
    }

// The WX_GET_OBJC_CLASS macro is intended to wrap the class name when the class
// is used as a message receiver (e.g. for calling class methods).  When
// class name uniquifying is used, this calls the global function implemented
// in the Objective-C file containing the class @implementation.
#define WX_GET_OBJC_CLASS(ObjcClass) wx_GetObjcClass_ ## ObjcClass()

#else // wxUSE_OBJC_UNIQUIFYING

// Define WX_DECLARE_GET_OBJC_CLASS as nothing
#define WX_DECLARE_GET_OBJC_CLASS(ObjcClass,ObjcSuperClass)
// Define WX_IMPLEMENT_GET_OBJC_CLASS as nothing
#define WX_IMPLEMENT_GET_OBJC_CLASS(ObjcClass,ObjcSuperClass)

// Define WX_GET_OBJC_CLASS macro to output the class name and let the compiler do the normal thing
// The WX_GET_OBJC_CLASS macro is intended to wrap the class name when the class
// is used as a message receiver (e.g. for calling class methods).  When
// class name uniquifying is not used, this is simply defined to be the class
// name which will allow the compiler to do the normal thing.
#define WX_GET_OBJC_CLASS(ObjcClass) ObjcClass

#endif // wxUSE_OBJC_UNIQUIFYING

#endif //ndef __WX_COCOA_OBJC_CLASS_H__