netdev - Re: [PATCH net] sparc: bpf_jit: fix loads from negative offsets

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Message-ID: <CAMEtUuz1oxsFjQF6aJpcYEEAte=7-yoKorC+4ybZHyWxcHsUAg@mail.gmail.com>
Date:	Wed, 24 Sep 2014 23:30:32 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	David Miller <davem@...emloft.net>
Cc:	Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net] sparc: bpf_jit: fix loads from negative offsets

On Wed, Sep 24, 2014 at 12:08 PM, David Miller <davem@...emloft.net> wrote:
>
> BTW, there is a trick to load arbitrary negative 32-bit constants sign
> extended to 64-bit, in just two instructions:
>
>         sethi   %(~value), reg
>         xor     reg, ~(0x400 | (value & 0x3ff)), reg
>
> Or something like that, it's been a while.  GCC knows how to emit
> that sequence too.

nice, that's a neat trick.
gcc computes it as:
  sethi   %(~value), reg
  xor     reg, -0x400 | (value & 0x3ff), reg

and llvm as:
  sethi   %(~value), reg
  xor     reg, ~(~value & 0x3ff), reg

both are equivalent. I'll try to teach JIT to emit this.

Thanks!
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