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

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Date:	Tue, 29 Sep 2009 14:44:06 +0200
From:	Arnd Bergmann <arnd@...db.de>
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

On Monday 28 September 2009, Arjan van de Ven wrote:
> 
> 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
> 

The patch looks good, but is there a reason to keep it architecture
specific? I would guess that the same logic applies to all architectures
with gcc-4.x and could be put into include/linux/compiler-gcc4.h.

	Arnd <><
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