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SubjectRe: [PATCH 2/2] fs: optimize mpage_bio_submit()
From: Changli Gao <xiaosuo@gmail.com>
Date: Sat, May 29, 2010 at 09:19:26AM +0800

> optimize mpage_bio_submit()
>
> check rw, then initialize bio->bi_end_io.
>
> Signed-off-by: Changli Gao <xiaosuo@gmail.com>
> ----
> fs/mpage.c | 4 +---
> 1 file changed, 1 insertion(+), 3 deletions(-)
> diff --git a/fs/mpage.c b/fs/mpage.c
> index 94ff0d1..632681c 100644
> --- a/fs/mpage.c
> +++ b/fs/mpage.c
> @@ -85,9 +85,7 @@ static void mpage_end_io_write(struct bio *bio, int err)
>
> static struct bio *mpage_bio_submit(int rw, struct bio *bio)
> {
> - bio->bi_end_io = mpage_end_io_read;
> - if (rw == WRITE)
> - bio->bi_end_io = mpage_end_io_write;
> + bio->bi_end_io = rw != WRITE ? mpage_end_io_read : mpage_end_io_write;
> submit_bio(rw, bio);
> return NULL;

ok, can you sincerely tell me that your change makes the code more
readable? And have you checked to see what the compiler actually
"optimizes"?

original code:

movq $mpage_end_io_read, %rdx #, tmp65
movq $mpage_end_io_write, %rax #, tmp64
cmpl $1, %edi #, rw
cmovne %rdx, %rax # tmp65,, tmp64
movq %rax, 80(%rsi) # tmp64, <variable>.bi_end_io
.loc 1 91 0
call submit_bio #

your change:

movq $mpage_end_io_read, %rdx #, tmp63
movq $mpage_end_io_write, %rax #, iftmp.561
cmpl $1, %edi #, rw
cmovne %rdx, %rax # tmp63,, iftmp.561
movq %rax, 80(%rsi) # iftmp.561, <variable>.bi_end_io
.loc 1 90 0
call submit_bio #

so that change does nothing except obfuscating the source a bit more so
that you get more headaches whenever you look at it.

--
Regards/Gruss,
Boris.


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