netdev - Re: [PATCH] x86: bpf_jit_comp: Reduce is

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Date:	Thu, 4 Dec 2014 15:12:11 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Joe Perches <joe@...ches.com>,
	Daniel Borkmann <dborkman@...hat.com>
Cc:	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 3:00 PM, Joe Perches <joe@...ches.com> wrote:
> Use the (1 << reg) & mask trick to reduce code size.
>
> x86-64 size difference -O2 without profiling for various
> gcc versions:
>
> $ size arch/x86/net/bpf_jit_comp.o*
>    text    data     bss     dec     hex filename
>    9266       4       0    9270    2436 arch/x86/net/bpf_jit_comp.o.4.4.new
>   10042       4       0   10046    273e arch/x86/net/bpf_jit_comp.o.4.4.old
>    9109       4       0    9113    2399 arch/x86/net/bpf_jit_comp.o.4.6.new
>    9717       4       0    9721    25f9 arch/x86/net/bpf_jit_comp.o.4.6.old
>    8789       4       0    8793    2259 arch/x86/net/bpf_jit_comp.o.4.7.new
>   10245       4       0   10249    2809 arch/x86/net/bpf_jit_comp.o.4.7.old
>    9671       4       0    9675    25cb arch/x86/net/bpf_jit_comp.o.4.9.new
>   10679       4       0   10683    29bb arch/x86/net/bpf_jit_comp.o.4.9.old
>
> Signed-off-by: Joe Perches <joe@...ches.com>

probably it was worth noting in comment that
reg is 4-bit value and AUX_REG==12, so it won't overflow.

Dave, it's for net-next of course.

Suggested-by: Alexei Starovoitov <ast@...mgrid.com>
Tested-by: Alexei Starovoitov <ast@...mgrid.com>
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