linux-kernel - Re: [PATCH] fs: dcache: Use bool return value instead of int

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Message-ID: <56A692AA.20902@emindsoft.com.cn>
Date:	Tue, 26 Jan 2016 05:24:58 +0800
From:	Chen Gang <chengang@...ndsoft.com.cn>
To:	Al Viro <viro@...IV.linux.org.uk>
CC:	dhowells@...hat.com, akpm@...ux-foundation.org,
	nicolas.iooss_linux@....org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] fs: dcache: Use bool return value instead of int


On 1/25/16 05:27, Al Viro wrote:
> 
> Again, do you have _any_ evidence of improved code generation with that
> patch?  Because if you do, I would really like to see it, so I could file
> bugs against gcc optimizer.
> 
> Your impression of what _Bool is and what semantics does it have appears
> to be rather different from that described in C99, but that's a secondary
> issue - first and foremost, on which .config and with which gcc version
> do you see improvements from that change?
> 

For our case, the check_mount function have smaller size under x86_64
(movl for int, movb for bool, movl is longer than movb).

The related objdump is below, welcome any ideas, suggestions, and
discussions for it.

  origin (for int):

00000000 <check_mount>:
       0:       8b 12                   mov    (%edx),%edx
       2:       81 e2 00 00 01 00       and    $0x10000,%edx
       8:       74 16                   je     20 <check_mount+0x20>
       a:       c7 00 01 00 00 00       movl   $0x1,(%eax)
      10:       b8 01 00 00 00          mov    $0x1,%eax
      15:       c3                      ret
      16:       8d 76 00                lea    0x0(%esi),%esi
      19:       8d bc 27 00 00 00 00    lea    0x0(%edi,%eiz,1),%edi
      20:       31 c0                   xor    %eax,%eax
      22:       c3                      ret
      23:       8d b6 00 00 00 00       lea    0x0(%esi),%esi
      29:       8d bc 27 00 00 00 00    lea    0x0(%edi,%eiz,1),%edi

  new (for bool):

00000000 <check_mount>:
       0:       8b 12                   mov    (%edx),%edx
       2:       81 e2 00 00 01 00       and    $0x10000,%edx
       8:       74 0e                   je     18 <check_mount+0x18>
       a:       c6 00 01                movb   $0x1,(%eax)
       d:       b8 01 00 00 00          mov    $0x1,%eax
      12:       c3                      ret
      13:       90                      nop
      14:       8d 74 26 00             lea    0x0(%esi,%eiz,1),%esi
      18:       31 c0                   xor    %eax,%eax
      1a:       c3                      ret
      1b:       90                      nop
      1c:       8d 74 26 00             lea    0x0(%esi,%eiz,1),%esi

[root@...alhost fs]# gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/x86_64-pc-linux-gnu/6.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-ana/configure
Thread model: posix
gcc version 6.0.0 20151121 (experimental) (GCC) 


-- 
Chen Gang (陈刚)

Open, share, and attitude like air, water, and life which God blessed