linux-kernel - Re: [PATCH] [9/58] x86_64: Always use builtin memcpy on gcc 4.3

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Message-Id: <200707220127.35975.ak@suse.de>
Date:	Sun, 22 Jul 2007 01:27:35 +0200
From:	Andi Kleen <ak@...e.de>
To:	Oleg Verych <olecom@...wer.upol.cz>
Cc:	jh@...e.cz, patches@...-64.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [9/58] x86_64: Always use builtin memcpy on gcc 4.3

On Sunday 22 July 2007 01:16:42 Oleg Verych wrote:
> * From: Andi Kleen <ak@...e.de>
> * Date: Thu, 19 Jul 2007 11:54:53 +0200 (CEST)
> >
> > Jan asked to always use the builtin memcpy on gcc 4.3 mainline because
> > it should generate better code than the old macro. Let's try it.
> 
> Unfortunately such info is hard to find. The discuss@...-64 list is
> empty. So, let me ask how this memcpy relates to recently submitted
> for glibc one [0]?

It doesn't relate at all. The kernel still uses its own memcpy.

Note that a lot of the traditional memcpy optimizations (like WC copies) 
are pointless in kernel space because the kernel rarely deals with continuous 
memory areas larger than a 4K page.

The only difference from the patch is that instead of using an own
heuristic when to use an out of line memcpy trust gcc's heuristic.

-Andi

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