"Codice di posizione indipendente" è sempre necessario se il modulo dell'oggetto verrà utilizzato in una libreria condivisa. È altamente dipendente dalla piattaforma e comporta un sovraccarico.
Il motivo per cui è necessario specificarlo esplicitamente su amd64, ma non su x386, è semplicemente che è il default per x86, ma non per amd64.
nota, anche, la differenza tra "-fPIC" e "-fPIC":
-fpic
Generate position-independent code (PIC) suitable for use in a
shared library, if supported for the target machine. Such code
accesses all constant addresses through a global offset table
(GOT). The dynamic loader resolves the GOT entries when the pro-
gram starts (the dynamic loader is not part of GCC; it is part
of the operating system). If the GOT size for the linked execu-
table exceeds a machine-specific maximum size, you get an error
message from the linker indicating that -fpic does not work; in
that case, recompile with -fPIC instead. (These maximums are 8k
on the SPARC and 32k on the m68k and RS/6000. The 386 has no
such limit.)
Position-independent code requires special support, and there
fore works only on certain machines. For the 386, GCC supports
PIC for System V but not for the Sun 386i. Code generated for
the IBM RS/6000 is always position-independent.
When this flag is set, the macros __pic__ and __PIC__ are defined to 1.
-fPIC
If supported for the target machine, emit position-independent
code, suitable for dynamic linking and avoiding any limit on the
size of the global offset table. This option makes a difference
on the m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore
works only on certain machines.
When this flag is set, the macros __pic__ and __PIC__ are defined to 2.
Anche relativo a questo http: //stackoverflow.com/questions/3146744/difference-in-position-independent-code-x86-vs-x86-64 –