Re: [PATCH] libbpf: Fix is_pow_of_2

[Andrii Nakryiko]

On Thu, Jun 2, 2022 at 9:31 PM Zvi Effron <zeffron@xxxxxxxxxxxxx> wrote:
>
> On Thu, Jun 2, 2022 at 9:17 PM Ian Rogers <irogers@xxxxxxxxxx> wrote:
> >
> > From: Yuze Chi <chiyuze@xxxxxxxxxx>
> >
> > There is a missing not. Consider a power of 2 number like 4096:
> >
> > x && (x & (x - 1))
> > 4096 && (4096 & (4096 - 1))
> > 4096 && (4096 & 4095)
> > 4096 && 0
> > 0
> >
> > with the not this is:
> > x && !(x & (x - 1))
> > 4096 && !(4096 & (4096 - 1))
> > 4096 && !(4096 & 4095)
> > 4096 && !0
> > 4096 && 1
> > 1
> >
> > Reported-by: Yuze Chi <chiyuze@xxxxxxxxxx>
> > Signed-off-by: Yuze Chi <chiyuze@xxxxxxxxxx>
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> >  tools/lib/bpf/libbpf.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> > index 3f4f18684bd3..fd0414ea00df 100644
> > --- a/tools/lib/bpf/libbpf.c
> > +++ b/tools/lib/bpf/libbpf.c
> > @@ -4956,7 +4956,7 @@ static void bpf_map__destroy(struct bpf_map *map);
> >
> >  static bool is_pow_of_2(size_t x)
> >  {
> > -       return x && (x & (x - 1));
> > +       return x && !(x & (x - 1));

ugh... *facepalm*

>
> No idea if anyone cares about the consistency, but in linker.c (same directory)
> the same static function is defined using == 0 at the end instead of using the
> not operator.
>
> Aside from the consistency issue, personally I find the == 0 version a little
> bit easier to read and understand because it's a bit less dense (and a "!" next
> to a "(" is an easy character to overlook).
>

I agree, even more so, logical not used with arbitrary integer (not a
pointer or bool) is a mental stumbling block for me, so much so that I
avoid doing !strcmp(), for example.

But in this case, I'm not sure why I copy/pasted is_pow_of_2() instead
of moving the one from linker.c into libbpf_internal.h as static
inline. Let's do that instead?

> >  }
> >
> >  static size_t adjust_ringbuf_sz(size_t sz)
> > --
> > 2.36.1.255.ge46751e96f-goog
> >