linux-kernel - Re: [PATCH] x86: bpf_jit_comp: Reduce is

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Message-ID: <CAADnVQ+3jiYOYReRLPUsFeG1p66ezo-TXBLX3BGQQh7V8QX0tQ@mail.gmail.com>
Date:	Thu, 4 Dec 2014 15:31:00 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Joe Perches <joe@...ches.com>,
	Quentin Lambert <lambert.quentin@...il.com>,
	"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: Reduce is_ereg() code size

On Thu, Dec 4, 2014 at 3:14 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Thu, 2014-12-04 at 15:00 -0800, Joe Perches wrote:
>> Use the (1 << reg) & mask trick to reduce code size.
>>
>> x86-64 size difference -O2 without profiling for various
>> gcc versions:
>
>>
>> Signed-off-by: Joe Perches <joe@...ches.com>
>> ---
>>
>> compiled, untested by me, but per Alexei Starovoitov this passes
>> the test_bpf suite
>
> Really, the root cause of this is the 'inline' abuse in non fast paths
> for non trivial functions.

well, it is a trivial function even from compiler point of view.
Dropping inline keyword doesn't help. gcc still inlines them.
Changing all 3 functions to _noinline_ doesn't help either.
So I think this patch is actually quite helpful to reduce code size.
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