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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Tue, 1 Dec 2020 21:55:22 -0800
Message-ID: <CAADnVQLYMKhC4D9AzcOEXM9s9LfdFo4sEL3hsU=UAzBOXGwb-A@mail.gmail.com>
Subject: Re: [PATCH v2 bpf-next 10/13] bpf: Add instructions for atomic[64]_[fetch_]sub
To:     Brendan Jackman <jackmanb@google.com>
Cc:     Yonghong Song <yhs@fb.com>, bpf <bpf@vger.kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        KP Singh <kpsingh@chromium.org>,
        Florent Revest <revest@chromium.org>,
        LKML <linux-kernel@vger.kernel.org>, Jann Horn <jannh@google.com>
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On Tue, Dec 1, 2020 at 4:38 AM Brendan Jackman <jackmanb@google.com> wrote:
>
> I guess it's also worth remembering other archs might have an atomic
> subtract.

which one?
arm64 LSE implements atomic_fetch_sub as neg+ldadd.
imo x64 and arm64 example outweighs choices by other archs if there are such.
Even without LSE it will be neg+llsc loop.
The reason I proposed bpf xsub insn earlier is that I thought that llvm
won't be able to emit it so easily and JIT/verifier would struggle.