Re: [PATCH 01/12] x86_64: memset_user()

Cyrill Gorcunov
2018-12-21T20:46:19Z

On Fri, Dec 21, 2018 at 12:29:46PM -0800, Matthew Wilcox wrote:
> > >
> > > That's inefficient.
> > >
> > > pattern = (unsigned char)c;
> > > pattern |= pattern << 8;
> > > pattern |= pattern << 16;
> > > pattern |= pattern << 32;
> >
> > Won't
> >
> > pattern = 0x0101010101010101 * c;
> >
> > do the same but faster?
>
> Depends on your CPU. Some yes, some no.
>
> (Also you need to cast 'c' to unsigned char to avoid someone passing in
> 0x1234 and getting 0x4646464646464634 instead of 0x3434343434343434)

Cast to unsigned char is needed in any case. And as far as I remember
we've been using this multiplication trick for a really long time
in x86 land. I'm out of sources right now but it should be somewhere
in assembly libs.