Re: [RFC-UGLYPATCH] ata: small optimization in linux/libata.h

[Tejun Heo <htejun@gmail.com>]

Harvey Harrison wrote:
> This patch may be too ugly to live, it suppresses a lot of
> sparse warnings in the libata build and produces slightly
> tighter code. (4 instructions vs 5 and a few bytes saved).
> 
> include/linux/libata.h:1214:13: warning: potentially expensive pointer subtraction
> 
> Original:
> 	if (++link - ap->pmp_link < ap->nr_pmp_links)
> 		return link;
> 
>      52b:	89 d8                	mov    %ebx,%eax
>      52d:	2b 82 60 26 00 00    	sub    0x2660(%edx),%eax
>      533:	c1 f8 02             	sar    $0x2,%eax
>      536:	69 c0 dd 3d c8 44    	imul   $0x44c83ddd,%eax,%eax
>      53c:	3b 82 5c 26 00 00    	cmp    0x265c(%edx),%eax
>      542:	7d 04                	jge    548 <sata_pmp_detach+0xbe>
> 
> Next:
> 	if ((char*)++link - (char *)ap->pmp_link < ap->nr_pmp_links * sizeof(*link))
> 		return link;
> 
>      52b:	69 81 5c 26 00 00 d4 	imul   $0x9d4,0x265c(%ecx),%eax
>      532:	09 00 00
>      535:	89 da                	mov    %ebx,%edx
>      537:	2b 91 60 26 00 00    	sub    0x2660(%ecx),%edx
>      53d:	39 c2                	cmp    %eax,%edx
>      53f:	73 04                	jae    545 <sata_pmp_detach+0xbb>
> 
> Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
> ---
> I know it's ugly, but I had it done anyways.  The one real problem I have
> with it is that if link and ap->pmp_link ever get changed to different types
> the compiler will not even warn as we cast away to (char *).  To make it
> a bit more robust, a BUILD_BUG_ON checking the pointer types may be a
> good idea.

Sorry, but Nacked-by: Tejun Heo <htejun@gmail.com>

-- 
tejun