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Date: Fri, 21 Dec 2018 23:46:16 +0300
From: Cyrill Gorcunov <gorcunov@...il.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Igor Stoppa <igor.stoppa@...il.com>,
	Andy Lutomirski <luto@...capital.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
	igor.stoppa@...wei.com, Nadav Amit <nadav.amit@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Ahmed Soliman <ahmedsoliman@...a.vt.edu>,
	linux-integrity@...r.kernel.org,
	kernel-hardening@...ts.openwall.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 01/12] x86_64: memset_user()

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.

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