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From:   Yafang Shao <laoar.shao@gmail.com>
Date:   Sun, 18 Nov 2018 23:02:19 +0800
Message-ID: <CALOAHbAfWkAYJPTRfyPmHKSmg7UEhtnamzUVx9xd4oYkqi_x8g@mail.gmail.com>
Subject: Re: [PATCH] mm/filemap.c: minor optimization in write_iter file operation
To:     willy@infradead.org
Cc:     Andrew Morton <akpm@linux-foundation.org>, darrick.wong@oracle.com,
        Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Sun, Nov 18, 2018 at 8:13 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Sun, Nov 18, 2018 at 08:02:18PM +0800, Yafang Shao wrote:
> > This little adjustment on bitwise operation could make the code a little
> > faster.
> > As write_iter is used in lots of critical path, so this code change is
> > useful for performance.
>
> Did you check the before/after code generation with this patch applied?
>

Yes, I did.
My oompiler is gcc-4.8.5, a litte old, and with CONFIG_CC_OPTIMIZE_FOR_SIZE on.
The output file is differrent.

> $ diff -u before.S after.S
> --- before.S    2018-11-18 07:11:48.031096768 -0500
> +++ after.S     2018-11-18 07:11:36.883069103 -0500
> @@ -1,5 +1,5 @@
>
> -before.o:     file format elf32-i386
> +after.o:     file format elf32-i386
>
>
>  Disassembly of section .text:
>
> with gcc 8.2.0, I see no difference, indicating that the compiler already
> makes this optimisation.

Could pls. try set CONFIG_CC_OPTIMIZE_FOR_SIZE on and then compare them again ?

Thanks
Yafang