Date: Sat, 22 Dec 2018 00:17:39 +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 01:07:21PM -0800, Matthew Wilcox wrote: > On Fri, Dec 21, 2018 at 11:46:16PM +0300, Cyrill Gorcunov wrote: > > 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. > > x86 isn't the only CPU. Some CPUs have slow multiplies but fast shifts. This is x86-64 patch, not some generic code. > Also loading 0x0101010101010101 into a register may be inefficient on > some CPUs. It is pretty efficient on x86-64. Moreover the self dependents as a |= a << b is a source for data hazards inside cpu engine. Anyway i'm not going to insist, just wanted to remind about such trick. Up to you what to choose.
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