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

[Al Viro <viro@ftp.linux.org.uk>]

On Sun, Sep 18, 2005 at 10:31:36AM -0700, Linus Torvalds 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...
 
> (For _small_ structures it's wonderful. As far as I can tell, gcc does a
> pretty good job on structs that are just a single long-word in size).