netdev - Re: [PATCH stable 4.9 1/8] x86: bpf_jit: small optimization in emit_bpf_tail

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Date:   Mon, 29 Jan 2018 07:39:19 +0100
From:   Willy Tarreau <w@....eu>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     ast@...nel.org, netdev@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH stable 4.9 1/8] x86: bpf_jit: small optimization in
 emit_bpf_tail_call()

Hi,

[ replaced stable@ and greg@ by netdev@ as my question below is not
  relevant to stable ]

On Mon, Jan 29, 2018 at 02:48:54AM +0100, Daniel Borkmann wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> [ upstream commit 84ccac6e7854ebbfb56d2fc6d5bef9be49bb304c ]
> 
> Saves 4 bytes replacing following instructions :
> 
> lea rax, [rsi + rdx * 8 + offsetof(...)]
> mov rax, qword ptr [rax]
> cmp rax, 0
> 
> by :
> 
> mov rax, [rsi + rdx * 8 + offsetof(...)]
> test rax, rax

I've just noticed this on stable@. 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,
Willy