netdev - Re: [PATCH bpf-next 1/2] bpf, x86: Small optimization in comparing against imm0

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Date:   Fri, 4 Oct 2019 12:36:07 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Song Liu <liu.song.a23@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 1/2] bpf, x86: Small optimization in comparing
 against imm0

On Thu, Oct 3, 2019 at 2:08 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> Song Liu wrote:
> > On Wed, Oct 2, 2019 at 5:30 PM Daniel Borkmann <daniel@...earbox.net> wrote:
> > >
> > > Replace 'cmp reg, 0' with 'test reg, reg' for comparisons against
> > > zero. Saves 1 byte of instruction encoding per occurrence. The flag
> > > results of test 'reg, reg' are identical to 'cmp reg, 0' in all
> > > cases except for AF which we don't use/care about. In terms of
> > > macro-fusibility in combination with a subsequent conditional jump
> > > instruction, both have the same properties for the jumps used in
> > > the JIT translation. For example, same JITed Cilium program can
> > > shrink a bit from e.g. 12,455 to 12,317 bytes as tests with 0 are
> > > used quite frequently.
> > >
> > > Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> >
> > Acked-by: Song Liu <songliubraving@...com>
>
> Bonus points for causing me to spend the morning remembering the
> differences between cmd, and, or, and test.
>
> Also wonder if at some point we should clean up the jit a bit and
> add some defines/helpers for all the open coded opcodes and such.
>
> Acked-by: John Fastabend <john.fastabend@...il.com>

Applied both. Thanks