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

[Cyrill Gorcunov]

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.