...
The following compiler options are invoked via /make/sw/flags.mk. They are presented here for reference.
-shared
The linker will expect each module to have a "dynamic section" and a hash table for those symbols with global visibility.
* -fno-pic
Although pic saves on relocations (one per function instead of one per function call) it requires the dynamic linker to know the special relocation types needed for the Global Offset Table and the Procedure Linkage Table. The latter is especially tricky to get right. It's simpler to use non-pic and just deal with the regular relocations.
The other usual reason to use pic (that most of the code can be mapped into multiple address spaces at the same time) doesn't apply here since rtems only offers one global address space.
* -nostdlib
We supply our own module _init (rce_modinit) and module _fini (rce_modfinish) code. Another reason not to use the standard _init/_fini code is that their source code was compiled with -fpic.
-nostdlib also prevents searching of the standard C++ libraries. Instead we'll want to search the symbol table of the RTEMS+newlib executable either at the time the module is produced or at run time.
* -fvisiblity=hidden
A shared object actually has two symbol tables, the normal one and a special "dynamic" table. The latter is the one that is searched in order to resolve inter-object references. In the past pretty much every non-static symbol from every object code file used to make the shared object went into the dynamic table. Since GCC 4.0 the compiler recognizes the -visibility option which controls the default visibility of symbols. Using -fvisibility=hidden makes "hidden" the default. Hidden symbols don't go into the dynamic symbol table so one can dramatically reduce its size. To make sure that the symbols associated with a variable, function or class do make it into the dynamic symbol table you need to mark it with _attribute_ ((visibility ("default"))):
...
-fvisibility=hidden also hides out-of-line code generated for inline functions and other compiler-generated code.
* -mlongcall
By default GCC will compile function calls into branch instructions which use 26-bit PC-relative offsets. In general this won't be adequate for inter-module calls so the compiler must be instructed to make all function calls "long" calls. These use full 32-bit addresses at the cost of some speed.
...