linux-kernel - Re: [PATCH 2/2] fs: optimize mpage_bio

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Message-ID: <AANLkTinwjNpAg6y3n1ZzAtPIv-sJE_P7WjbzzrRRCBVD@mail.gmail.com>
Date:	Sat, 29 May 2010 21:33:02 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	Borislav Petkov <bp@...en8.de>, Changli Gao <xiaosuo@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] fs: optimize mpage_bio_submit()

On Sat, May 29, 2010 at 8:47 PM, Borislav Petkov <bp@...en8.de> wrote:
>
> 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.
>

Oh, I didn't check the asm code. Compiler is smarter than I thought.
Thanks for your explaining.


-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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