Re: p = kmalloc(sizeof(*p), )

Roman Zippel
2005-09-18T20:35:04Z

Hi,

On Sun, 18 Sep 2005, Al Viro wrote:

> > On Sun, 18 Sep 2005, Al Viro wrote:
> > >
> > > That's why you do
> > > *p = (struct foo){....};
> > > instead of
> > > memset(p, 0, sizeof...);
> > > p->... =...;
> >
> > Actually, some day that migth be a good idea, but at least historically,
> > gcc has really really messed that kind of code up.
> >
> > Last I looked, depending on what the initializer was, gcc would create a
> > temporary struct on the stack first, and then do a "memcpy()" of the
> > result. Not only does that obviously generate a lot of extra code, it also
> > blows your kernel stack to kingdom come.
>
> Ewwwww... I'd say that it qualifies as one hell of a bug (and yes, at least
> 3.3 and 4.0.1 are still doing that). What a mess...

It's not a bug, it's exactly what you're asking for, e.g. "*p1 = *p2"
translates to memcpy. gcc also can't simply initialize that structure in
place, e.g. you could do something like this (not necessarily useful but
still valid): "*p = (struct foo){..., bar(p),...};".
In the end it all depends on how good gcc can optimize away the memcpy,
but initially there is always a memcpy.

bye, Roman