linux-kernel - Re: [PATCH] fs/fat: calculate checksum in a loop instead of directly calculating

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Date:	Tue, 24 Feb 2015 11:42:39 +0900
From:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
To:	Heinrich Schuchardt <xypron.glpk@....de>
Cc:	Alexander Kuleshov <kuleshovmail@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs/fat: calculate checksum in a loop instead of directly calculating

Heinrich Schuchardt <xypron.glpk@....de> writes:

> You have to put
> __attribute__((optimize("unroll-loops")))
> here to unroll the loop inside the function:
>
>
> static __attribute__ ((noinline))
> __attribute__((optimize("unroll-loops")))
> int test(unsigned char *name)
>
> $ time ./c.inline aaaaaaaaaaa
> 14
>
> real    0m0.743s
> user    0m0.740s
> sys     0m0.000s
>
> Without __attribute__((optimize("unroll-loops"))) :
> $ time ./c.loop aaaaaaaaaaa
> 14
>
> real    0m1.482s
> user    0m1.472s
> sys     0m0.004s
>
> With __attribute__((optimize("unroll-loops"))) :
>
> $ time ./c.loop aaaaaaaaaaa
> 14
>
> real    0m0.742s
> user    0m0.740s
> sys     0m0.000s

This attribute has to be added to the caller, not fat_checksum()?  I.e.,
we has to add it to all callers of fat_checksum()?

Well, this is interesting gcc optimize option though, maybe not worth to
introduce this to kernel only for fatfs.

Thanks.
-- 
OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
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