diff options
| -rw-r--r-- | tools/lto/LTOModule.cpp | 22 |
1 files changed, 20 insertions, 2 deletions
diff --git a/tools/lto/LTOModule.cpp b/tools/lto/LTOModule.cpp index c769af1e2e..939d0eacd1 100644 --- a/tools/lto/LTOModule.cpp +++ b/tools/lto/LTOModule.cpp @@ -270,8 +270,26 @@ void LTOModule::addDefinedDataSymbol(GlobalValue* v, Mangler& mangler) // add to list of defined symbols addDefinedSymbol(v, mangler, false); - // special case i386/ppc ObjC data structures in magic sections - if ( v->hasSection() ) { + // Special case i386/ppc ObjC data structures in magic sections: + // The issue is that the old ObjC object format did some strange + // contortions to avoid real linker symbols. For instance, the + // ObjC class data structure is allocated statically in the executable + // that defines that class. That data structures contains a pointer to + // its superclass. But instead of just initializing that part of the + // struct to the address of its superclass, and letting the static and + // dynamic linkers do the rest, the runtime works by having that field + // instead point to a C-string that is the name of the superclass. + // At runtime the objc initialization updates that pointer and sets + // it to point to the actual super class. As far as the linker + // knows it is just a pointer to a string. But then someone wanted the + // linker to issue errors at build time if the superclass was not found. + // So they figured out a way in mach-o object format to use an absolute + // symbols (.objc_class_name_Foo = 0) and a floating reference + // (.reference .objc_class_name_Bar) to cause the linker into erroring when + // a class was missing. + // The following synthesizes the implicit .objc_* symbols for the linker + // from the ObjC data structures generated by the front end. + if ( v->hasSection() /* && isTargetDarwin */ ) { // special case if this data blob is an ObjC class definition if ( v->getSection().compare(0, 15, "__OBJC,__class,") == 0 ) { if (GlobalVariable* gv = dyn_cast<GlobalVariable>(v)) { |