From: Joe Perches <joe@perches•com>
To: Alexei Starovoitov <alexei.starovoitov@gmail•com>
Cc: Quentin Lambert <lambert.quentin@gmail•com>,
"David S. Miller" <davem@davemloft•net>,
Alexey Kuznetsov <kuznet@ms2•inr.ac.ru>,
James Morris <jmorris@namei•org>,
Hideaki YOSHIFUJI <yoshfuji@linux-ipv6•org>,
Patrick McHardy <kaber@trash•net>,
Thomas Gleixner <tglx@linutronix•de>,
Ingo Molnar <mingo@redhat•com>, "H. Peter Anvin" <hpa@zytor•com>,
x86@kernel•org, "netdev@vger•kernel.org" <netdev@vger•kernel.org>,
"linux-kernel@vger•kernel.org" <linux-kernel@vger•kernel.org>
Subject: Re: [PATCH] x86: bpf_jit_comp: simplify trivial boolean return
Date: Wed, 26 Nov 2014 10:41:31 -0800 [thread overview]
Message-ID: <1417027291.19695.12.camel@perches.com> (raw)
In-Reply-To: <CAADnVQK3iui8cAmhi28=65p2CJbTu45fR98ri8Uh8xLRb=cwnA@mail.gmail.com>
On Wed, 2014-11-26 at 10:34 -0800, Alexei Starovoitov wrote:
> On Wed, Nov 26, 2014 at 10:02 AM, Joe Perches <joe@perches•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@perches•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 :)
498 overall.
> If I think dropping 'inline' and using -Os will give bigger savings...
> but I suspect 'tinification' folks will compile JIT out anyway...
Smaller is generally better/faster in any case.
> thanks for giving it a shot :)
> That's exactly what I had in mind.
> imo it's less readable, but we probably not going
> to mess much with this piece of code anyway.
> Though to be safe in the future, we'd need to
> add BUILD_BUG_ON that largest value (AUX_REG)
> fits in 32bit (or 64bit) and add a comment that
> verifier goes before the JIT and checks that
> insn->src_reg, insn->dst_reg are less than MAX_BPF_REG,
> so argument 'reg' also doesn't trigger too large shift.
> Perfectionists r us. :)
> ... or just leave it as-is ;)
16 registers max anyway as it's stored
in a :4
No worries, it was just playtime anyway.
cheers, Joe
next prev parent reply other threads:[~2014-11-26 18:41 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-11-26 18:34 [PATCH] x86: bpf_jit_comp: simplify trivial boolean return Alexei Starovoitov
2014-11-26 18:41 ` Joe Perches [this message]
2014-11-26 20:00 ` Joe Perches
2014-11-27 12:25 ` David Laight
2014-11-27 14:36 ` Quentin Lambert
2014-11-27 18:35 ` Joe Perches
2014-11-27 18:49 ` Joe Perches
2014-12-04 9:26 ` Joe Perches
2014-12-04 15:56 ` Alexei Starovoitov
2014-12-04 18:05 ` Joe Perches
-- strict thread matches above, loose matches on Subject: below --
2014-12-04 22:44 Alexei Starovoitov
2014-11-26 17:23 Alexei Starovoitov
2014-11-26 18:02 ` Joe Perches
2014-11-26 16:42 Alexei Starovoitov
2014-11-26 16:58 ` Joe Perches
2014-11-26 17:55 ` Daniel Borkmann
2014-11-26 9:18 Quentin Lambert
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