Support 128-bit pointers (protected mode) on E2K

[ivmai]

Default compilation mode is 64-bit (as if -m64 option is passed).

If -m128 option is passed to gcc, the following predefined macros are changed:

#undef __ptr64__
#define __ptr128__ 1
#define __SIZEOF_POINTER__ 16
#define __PROTECTED__ 1

Related issue: #411, #456

[ivmai]

This would probably also require usage of GC_size_t for counters in gc.h (instead of GC_word)

[ivmai]

Additional big task (both for E2K protected mode and CHERI arch extension) would be to organize objects protection and object access right revocation on sweep (i.e. if some object is collected then pointer tag bit should be cleared in all remaining (inaccessible) pointers to it).

[ivmai]

Related issue (in libatomic_ops): ivmai/libatomic_ops#61

[ivmai]

CHERI is a similar protection technology (with double-wide pointer).

[ivmai]

Besides the changes done and to be done in #627, there at least a number of extra code places to care of:

• include/gc/gc.h: define GC_uintptr_t to unsigned __int128
• include/gc/gc.h: GC_REVEAL_POINTER, GC_REVEAL_NZ_POINTER
• include/private/gc_priv.h: CAST_THRU_UINTPTR: do not cast to numeric type
• include/private/gc_priv.h: CPTR_CLEAR_FLAGS: define to ((ptr_t)(p) - (ADDR(p) & (word)(mask))), add comment: The pointer argument should not have side effects. Avoid conversion from a numeric type to a pointer one
• include/private/gc_priv.h: MAKE_CPTR
• include/private/gcconfig.h: PTR_ALIGN_DOWN, PTR_ALIGN_UP (e.g. define to ((ptr_t)(p) + (~ADDR(p) & (word)(mask))))
• include/private/gcconfig.h: CPP_PTRSZ
• include/private/gcconfig.h: FIXUP_POINTER
• mallocx.c: GC_realloc: define cleared_p to p