On 12/09/14 00:25, haynberg@xxxxxxxxx wrote:
>>> msg *p = reinterpret_cast<msg*>(get_bytes());
>>
>> Why are you doing this?
>
> For efficiency, by preventing a copy (imagine get_bytes() is getting
> bytes out of a socket buffer).
Firstly, char types alias everything.
Secondly, even if you call memcpy(), a compiler doesn't have to do any
copies if it can prove that the union you're reading into doesn't
escape.
Look at this:
double kludge(void *p) {
union {
char bytes[sizeof (double)];
double d;
} u;
memcpy(u.bytes, p, sizeof u.bytes);
return u.d;
}
which generates
kludge:
ldr d0, [x0]
ret
and is completely portable, with no undefined behaviour.
> Putting alignment/padding concerns aside, it would be nice if there
> was a way to explicitly tell the compiler, I want to do this and,
> please don’t reorder stores and loads, or perform other
> strict-aliasing optimizations, on the memory pointed to by this
> pointer (similar to the effect of a memcpy). I believe the only way
> to do this is with the GCC may_alias attribute, or a more
> heavy-handed memory clobber. I think the OP wanted to ask the GCC
> folks if there was another, possibly more portable, way; for
> example, placement new, but that turned out not to be an option.
Well, there isn't a more portable way, and we can't ignore alignment.
All that GCC can do is provide a way to do it; we can't make anyone
else comply.
> Another related, maybe more important, question is if GCC sees a
> reinterpet_cast like this (without a may_alias type), is it free to
> discard code or otherwise drastically change it due to the fact that
> it’s undefined by the standard?
Yes. It may, and it does.
Andrew.
