linux-kernel - Re: [PATCH] mm/filemap.c: minor optimization in write

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Message-ID: <20181118121318.GC7861@bombadil.infradead.org>
Date:   Sun, 18 Nov 2018 04:13:18 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Yafang Shao <laoar.shao@...il.com>
Cc:     akpm@...ux-foundation.org, darrick.wong@...cle.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/filemap.c: minor optimization in write_iter file
 operation

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?

$ 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.