linux-kernel - Re: [PATCH] block: convert ata_port_for_each_link to use a while loop to reduce code size

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Message-Id: <1224656226.15955.1280589369@webmail.messagingengine.com>
Date:	Wed, 22 Oct 2008 08:17:06 +0200
From:	"Alexander van Heukelum" <heukelum@...tmail.fm>
To:	"Richard Kennedy" <richard@....demon.co.uk>, jgarzik@...ox.com
Cc:	"lkml" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: convert ata_port_for_each_link to use a while loop
 to reduce code size

On Tue, 21 Oct 2008 21:24:32 +0100, "Richard Kennedy"
<richard@....demon.co.uk> said:
> convert a for loop to a while loop in the ata_port_for_each_link macros
> to reduce code size.
> 
> on x86_64 size reports reduction of text size in
> 	libata.ko : -272 bytes
> 	ahci.ko   : -44 bytes
> 
> Signed-off-by: Richard Kennedy <richard@....demon.co.uk>
> 
> ----
> This patch is against 2.6.27 git head.
> 
> I've been running this patch on my AMD64 desktop machine for several
> days & have not seen any problems.
> regards
> Richard
> 
> 
> diff --git a/include/linux/libata.h b/include/linux/libata.h
> index 947cf84..bbe3b9d 100644
> --- a/include/linux/libata.h
> +++ b/include/linux/libata.h
> @@ -1283,12 +1283,12 @@ extern struct ata_link
> *__ata_port_next_link(struct ata_port *ap,
>  					     bool dev_only);
>  
>  #define __ata_port_for_each_link(link, ap) \
> -	for ((link) = __ata_port_next_link((ap), NULL, false); (link); \
> -	     (link) = __ata_port_next_link((ap), (link), false))
> +	(link) = NULL; \
> +	while (((link) = __ata_port_next_link((ap), (link), false)))

Hi Richard,

Please imagine how this expands...

if (some_condition)
        __ata_port_for_each_link(link, ap)
                do_something(...)

But it is probably possible to get the same size reduction by
changing the for-loop like this (safety parentheses left out):

        for (link = NULL;
                link = __ata_port_next_link(ap, link, false), link; )

Some programmers think this is abuse of the for-construct, though ;).

Greetings,
    Alexander

>  #define ata_port_for_each_link(link, ap) \
> -	for ((link) = __ata_port_next_link((ap), NULL, true); (link); \
> -	     (link) = __ata_port_next_link((ap), (link), true))
> +	(link) = NULL; \
> +	while (((link) = __ata_port_next_link((ap), (link), true)))
>  
>  #define ata_link_for_each_dev(dev, link) \
>  	for ((dev) = (link)->device; \
-- 
  Alexander van Heukelum
  heukelum@...tmail.fm

-- 
http://www.fastmail.fm - mmm... Fastmail...

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