linux-kernel - Re: [PATCH] x86: bpf_jit_comp: simplify trivial boolean return

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Message-ID: <547736EB.2090006@gmail.com>
Date:	Thu, 27 Nov 2014 15:36:27 +0100
From:	Quentin Lambert <lambert.quentin@...il.com>
To:	David Laight <David.Laight@...LAB.COM>,
	'Joe Perches' <joe@...ches.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: bpf_jit_comp: simplify trivial boolean return

On 27/11/2014 13:25, David Laight wrote:
> From: Joe Perches
>> On Wed, 2014-11-26 at 10:34 -0800, Alexei Starovoitov wrote:
>>> On Wed, Nov 26, 2014 at 10:02 AM, Joe Perches <joe@...ches.com> wrote:
>>>> On Wed, 2014-11-26 at 09:23 -0800, Alexei Starovoitov wrote:
>>>>> On Wed, Nov 26, 2014 at 8:58 AM, Joe Perches <joe@...ches.com> wrote:
>>>>>> Is there any value in reordering these tests for frequency
>>>>>> or maybe using | instead of || to avoid multiple jumps?
>>>>> probably not. It's not a critical path.
>>>>> compiler may fuse conditions depending on values anyway.
>>>>> If it was a critical path, we could have used
>>>>> (1 << reg) & mask trick.
>>>>> I picked explicit 'return true' else 'return false' here,
>>>>> because it felt easier to read. Just a matter of taste.
>>>> There is a size difference though: (allyesconfig)
>>>>
>>>> $ size arch/x86/net/built-in.o*
>>>>     text    data     bss     dec     hex filename
>>>>    12999    1012    4336   18347    47ab arch/x86/net/built-in.o.new
>>>>    13177    1076    4592   18845    499d arch/x86/net/built-in.o.old
>>> interesting. Compiler obviously thinks that 178 byte increase
>>> with -O2 is the right trade off. Which I agree with :)
>>>
>>> If I think dropping 'inline' and using -Os will give bigger savings...
>> This was allyesconfig which already uses -Os
>>
>> Using -O2, there is no difference using inline
>> or not, but the size delta with the bitmask is
>> much larger
>>
>> $ size arch/x86/net/built-in.o* (allyesconfig, but not -Os)
>>     text	   data	    bss	    dec	    hex	filename
>>    13410	    820	   3624	  17854	   45be	arch/x86/net/built-in.o.new
>>    16130	    884	   4200	  21214	   52de	arch/x86/net/built-in.o.old
>>    16130	    884	   4200	  21214	   52de	arch/x86/net/built-in.o.static
> That is quite a big % change in the code size.
> Why the change in data?
>
> 	David
>
>
>
Do you want me to propose a second version, or should I just
drop it all together ?

I am a new contributor so I have no experience in that sort of thing.

Quentin
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