linux-kernel - Re: [PATCH] mm: migrate: Use bool instead of int for the return value of PageMovable

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Message-ID: <20160712074841.GE14586@dhcp22.suse.cz>
Date:	Tue, 12 Jul 2016 09:48:41 +0200
From:	Michal Hocko <mhocko@...nel.org>
To:	Chen Gang <chengang@...ndsoft.com.cn>
Cc:	Minchan Kim <minchan@...nel.org>, akpm@...ux-foundation.org,
	vbabka@...e.cz, mgorman@...hsingularity.net,
	gi-oh.kim@...fitbricks.com, iamjoonsoo.kim@....com,
	hillf.zj@...baba-inc.com, rientjes@...gle.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Chen Gang <gang.chen.5i5j@...il.com>
Subject: Re: [PATCH] mm: migrate: Use bool instead of int for the return
 value of PageMovable

On Tue 12-07-16 03:47:42, Chen Gang wrote:
[...]
> In our case, the 2 output size are same, but under x86_64, the insns are
> different. After uses bool, it uses push/pop instead of branch, for me,
> it should be a little better for catching.

The code generated for bool version looks much worse. Look at the fast
path. Gcc tries to reuse the retq from the fast path in the bool case
and so it has to push rbp and rbx on the stack.

That being said, gcc doesn't seem to generate a better code for bool so
I do not think this is really worth it.

> The orig:
> 
>   0000000000001290 <PageMovable>:
>       1290:       48 8b 47 08             mov    0x8(%rdi),%rax
>       1294:       83 e0 03                and    $0x3,%eax
>       1297:       48 83 f8 02             cmp    $0x2,%rax
>       129b:       74 03                   je     12a0 <__SetPageMovable+0x12a0>
>       129d:       31 c0                   xor    %eax,%eax
>       129f:       c3                      retq
>       12a0:       55                      push   %rbp
>       12a1:       48 89 e5                mov    %rsp,%rbp
>       12a4:       e8 00 00 00 00          callq  12a9 <__SetPageMovable+0x12a9>
>       12a9:       48 85 c0                test   %rax,%rax
>       12ac:       74 17                   je     12c5 <__SetPageMovable+0x12c5>
>       12ae:       48 8b 50 68             mov    0x68(%rax),%rdx
>       12b2:       48 85 d2                test   %rdx,%rdx
>       12b5:       74 0e                   je     12c5 <__SetPageMovable+0x12c5>
>       12b7:       48 83 7a 68 00          cmpq   $0x0,0x68(%rdx)
>       12bc:       b8 01 00 00 00          mov    $0x1,%eax
>       12c1:       74 02                   je     12c5 <__SetPageMovable+0x12c5>
>       12c3:       5d                      pop    %rbp
>       12c4:       c3                      retq
>       12c5:       31 c0                   xor    %eax,%eax
>       12c7:       5d                      pop    %rbp
>       12c8:       c3                      retq
>       12c9:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)
> 
> The new:
> 
>   0000000000001290 <PageMovable>:
>       1290:       48 8b 47 08             mov    0x8(%rdi),%rax
>       1294:       55                      push   %rbp
>       1295:       48 89 e5                mov    %rsp,%rbp
>       1298:       53                      push   %rbx
>       1299:       31 db                   xor    %ebx,%ebx
>       129b:       83 e0 03                and    $0x3,%eax
>       129e:       48 83 f8 02             cmp    $0x2,%rax
>       12a2:       74 05                   je     12a9 <__SetPageMovable+0x12a9>
>       12a4:       89 d8                   mov    %ebx,%eax
>       12a6:       5b                      pop    %rbx
>       12a7:       5d                      pop    %rbp
>       12a8:       c3                      retq
>       12a9:       e8 00 00 00 00          callq  12ae <__SetPageMovable+0x12ae>
>       12ae:       48 85 c0                test   %rax,%rax
>       12b1:       74 f1                   je     12a4 <__SetPageMovable+0x12a4>
>       12b3:       48 8b 40 68             mov    0x68(%rax),%rax
>       12b7:       48 85 c0                test   %rax,%rax
>       12ba:       74 e8                   je     12a4 <__SetPageMovable+0x12a4>
>       12bc:       48 83 78 68 00          cmpq   $0x0,0x68(%rax)
>       12c1:       0f 95 c3                setne  %bl
>       12c4:       89 d8                   mov    %ebx,%eax
>       12c6:       5b                      pop    %rbx
>       12c7:       5d                      pop    %rbp
>       12c8:       c3                      retq
>       12c9:       0f 1f 80 00 00 00 00    nopl   0x0(%rax)
> 
> Thanks.
> -- 
> Chen Gang (陈刚)
> 
> Managing Natural Environments is the Duty of Human Beings.

-- 
Michal Hocko
SUSE Labs