Getting rid of Unaligned Accesses (UA)

[Jay Estabrook]

Hi, Alpha folk,

I hope there's someone(s) out there that might have comments to offer
on how best to coerce the GCC compiler into generating the code WE want,
rather than what GCC wants.

To fix an unaligned access (UA) problem in "dbus",  a union pointer
address which was not always aligned (gcc assumes they are, by default,
it seems), I changed something like the following, which appears in the
file: dbus-marshal-basic.c:dbus_marshal_read_basic():

    vp->byt = _dbus_get_byte(str, pos);
to:
    *((char*)&vp->byt) = _dbus_get_byte(str, pos);

Under GCC-4.1.2-23, this generated code that did the appropriate things
to prevent a UA.

HOWEVER, when I built it under GCC-4.1.2-37, it reverted to the original
bad code (always assuming alignment). I had to change it yet again, to
the following:

    *((volatile char*)&vp->byt) = _dbus_get_byte(str, pos);

to get it to produce the good code again.

So, my question: is the final version (using "volatile") the only real
way to guarantee good code in this (and similar) instance?

Or is there another more (or less) obvious way to do so?

I certainly didn't expect the behavior to change like this between
releases and not major/minor versions... :-\

Thanks for any insights,

--Jay++