linux-kernel - Re: [PATCH] x86: Use __builtin_memset and __builtin

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Date:	Fri, 02 Oct 2009 21:19:18 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, tglx@...x.de,
	hpa@...or.com, torvalds@...ux-foundation.org
Subject: Re: [PATCH] x86: Use __builtin_memset and __builtin_memcpy for memset/memcpy

Arjan van de Ven <arjan@...radead.org> writes:

> From ebb81aab0c3df19771ebc0eec1261ae314ddc0af Mon Sep 17 00:00:00 2001
> From: Arjan van de Ven <arjan@...ux.intel.com>
> Date: Mon, 28 Sep 2009 11:21:32 +0200
> Subject: [PATCH] x86: Use __builtin_memset and __builtin_memcpy for memset/memcpy
>
> GCC provides reasonable memset/memcpy functions itself, with __builtin_memset
> and __builtin_memcpy. For the "unknown" cases, it'll fall back to our
> current existing functions, but for fixed size versions it'll inline
> something smart. Quite often that will be the same as we have now,
> but sometimes it can do something smarter (for example, if the code
> then sets the first member of a struct, it can do a shorter memset).
>
> In addition, and this is more important, gcc knows which registers and
> such are not clobbered (while for our asm version it pretty much
> acts like a compiler barrier), so for various cases it can avoid reloading
> values.
>
> The effect on codesize is shown below on my typical laptop .config:
>
>    text	   data	    bss	    dec	    hex	filename
> 5605675	2041100	6525148	14171923	 d83f13	vmlinux.before
> 5595849	2041668	6525148	14162665	 d81ae9	vmlinux.after

I tried this some time ago, but it it generates bad code on some 
gcc 3 versions.

You really need to test such kind of changes on a wide variety
of compilers, not assuming everyone uses the same version as you.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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