netdev - Re: [PATCH bpf] bpf: Improve bucket

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Message-ID: <CAHk-=wgdCXgQyBSSx-ovfiZ7WFR6fStOZ_2R9mxkX3a+R5MkxQ@mail.gmail.com>
Date:   Fri, 7 Feb 2020 13:09:32 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Martin KaFai Lau <kafai@...com>, bpf@...r.kernel.org,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>, kernel-team@...com,
        Linux-Sparse <linux-sparse@...r.kernel.org>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Netdev <netdev@...r.kernel.org>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH bpf] bpf: Improve bucket_log calculation logic

On Fri, Feb 7, 2020 at 12:13 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> In addition to this patch I've tried:
> +#define __const_ilog2(n, unique_n) ({                  \
> +       typeof(n) unique_n = (n);                       \

Yeah, as you found out, this doesn't work for the case of having
global initializers or things like array sizes.

Which people do do - often nor directly, but through various size macros.

It's annoying, but one of the failures of C is having a nice form of
compile-time constant handling where you can do slightly smarter
arithmetic. The definition of a "const expression" is very very
limited, and hurts us exactly for the array declaration and constant
initializer case.

Oh well.

          Linus