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From: Willy Tarreau <w@1wt.eu>
To: Eric Dumazet <edumazet@google.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>,
	Alexei Starovoitov <ast@kernel.org>,
	netdev <netdev@vger.kernel.org>,
	"David S . Miller" <davem@davemloft.net>
Subject: Re: [PATCH stable 4.9 1/8] x86: bpf_jit: small optimization in emit_bpf_tail_call()
Date: Mon, 29 Jan 2018 15:06:16 +0100	[thread overview]
Message-ID: <20180129140616.GA23326@1wt.eu> (raw)
In-Reply-To: <CANn89iKbXNBJcteAtZpkM=u5dwcTGALNWc9rektdz1BHVuGb+g@mail.gmail.com>

Hi Eric,

On Mon, Jan 29, 2018 at 06:04:30AM -0800, Eric Dumazet wrote:
> > If these 4 bytes matter, why not use
> > cmpq with an immediate value instead, which saves 2 extra bytes ? :
> >
> >   - the mov above is 11 bytes total :
> >
> >    0:   48 8b 84 d6 78 56 34    mov    0x12345678(%rsi,%rdx,8),%rax
> >    7:   12
> >    8:   48 85 c0                test   %rax,%rax
> >
> >   - the equivalent cmp is only 9 bytes :
> >
> >    0:   48 83 bc d6 78 56 34    cmpq   $0x0,0x12345678(%rsi,%rdx,8)
> >    7:   12 00
> >
> > And as a bonus, it doesn't even clobber rax.
> >
> > Just my two cents,
> 
> 
> Hi Willy
> 
> Please look more closely at following instructions.
> 
> We need the value later, not only testing it being zero :)

Ah OK that makes total sense then ;-)

Thanks,
willy

  reply	other threads:[~2018-01-29 14:06 UTC|newest]

Thread overview: 21+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-01-29  1:48 [PATCH stable 4.9 0/8] BPF stable patches Daniel Borkmann
2018-01-29  1:48 ` [PATCH stable 4.9 1/8] x86: bpf_jit: small optimization in emit_bpf_tail_call() Daniel Borkmann
2018-01-29  6:39   ` Willy Tarreau
2018-01-29 14:04     ` Eric Dumazet
2018-01-29 14:06       ` Willy Tarreau [this message]
2018-01-29 12:35   ` Patch "x86: bpf_jit: small optimization in emit_bpf_tail_call()" has been added to the 4.9-stable tree gregkh
2018-01-29  1:48 ` [PATCH stable 4.9 2/8] bpf: fix bpf_tail_call() x64 JIT Daniel Borkmann
2018-01-29 12:35   ` Patch "bpf: fix bpf_tail_call() x64 JIT" has been added to the 4.9-stable tree gregkh
2018-01-29  1:48 ` [PATCH stable 4.9 3/8] bpf: introduce BPF_JIT_ALWAYS_ON config Daniel Borkmann
2018-01-29 12:35   ` Patch "bpf: introduce BPF_JIT_ALWAYS_ON config" has been added to the 4.9-stable tree gregkh
2018-01-29  1:48 ` [PATCH stable 4.9 4/8] bpf: arsh is not supported in 32 bit alu thus reject it Daniel Borkmann
2018-01-29 12:34   ` Patch "bpf: arsh is not supported in 32 bit alu thus reject it" has been added to the 4.9-stable tree gregkh
2018-01-29  1:48 ` [PATCH stable 4.9 5/8] bpf: avoid false sharing of map refcount with max_entries Daniel Borkmann
2018-01-29 12:34   ` Patch "bpf: avoid false sharing of map refcount with max_entries" has been added to the 4.9-stable tree gregkh
2018-01-29  1:48 ` [PATCH stable 4.9 6/8] bpf: fix divides by zero Daniel Borkmann
2018-01-29 12:35   ` Patch "bpf: fix divides by zero" has been added to the 4.9-stable tree gregkh
2018-01-29  1:49 ` [PATCH stable 4.9 7/8] bpf: fix 32-bit divide by zero Daniel Borkmann
2018-01-29 12:34   ` Patch "bpf: fix 32-bit divide by zero" has been added to the 4.9-stable tree gregkh
2018-01-29  1:49 ` [PATCH stable 4.9 8/8] bpf: reject stores into ctx via st and xadd Daniel Borkmann
2018-01-29 12:35   ` Patch "bpf: reject stores into ctx via st and xadd" has been added to the 4.9-stable tree gregkh
2018-01-29 12:30 ` [PATCH stable 4.9 0/8] BPF stable patches Greg KH

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