Re: [PATCH v7 1/4] bitops: Introduce the the for_each_set_clump macro

Syed Nayyar Waris
2020-06-01T00:31:56Z

On Mon, Jun 1, 2020 at 4:07 AM Rikard Falkeborn
<rikard.falkeborn@gmail.com> wrote:
>
> + Emil who was working on a patch for this
>
> On Sun, May 31, 2020 at 02:00:45PM +0300, Andy Shevchenko wrote:
> > On Sun, May 31, 2020 at 4:11 AM Syed Nayyar Waris <syednwaris@gmail.com> wrote:
> > > On Sat, May 30, 2020 at 2:50 PM Andy Shevchenko
> > > <andy.shevchenko@gmail.com> wrote:
> > > > On Sat, May 30, 2020 at 11:45 AM Syed Nayyar Waris <syednwaris@gmail.com> wrote:
> > > > > On Sat, May 30, 2020 at 3:49 AM Andy Shevchenko
> > > > > <andy.shevchenko@gmail.com> wrote:
> >
> > ...
> >
> Sorry about that, it seems it's only triggered by gcc-9, that's why I
> missed it.
>
> > > #if (l) == 0
> > > #define GENMASK_INPUT_CHECK(h, l) 0
> > > #elif
> > > #define GENMASK_INPUT_CHECK(h, l) \
> > > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > > __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> > > #endif
> > >
> > > I have verified that this works. Basically this just avoids the sanity
> > > check when the 'lower' bound 'l' is zero. Let me know if it looks
> > > fine.
>
> I don't understand how you mean this? You can't use l before you have
> defined GENMASK_INPUT_CHECK to take l as input? Am I missing something?

Excuse me for the incorrect code snippet that I shared (above). Kindly
ignore it. I realise the error in it.

>
> How about the following (with an added comment about why the casts are
> necessary):
>
> diff --git a/include/linux/bits.h b/include/linux/bits.h
> index 4671fbf28842..5fdb9909fbff 100644
> --- a/include/linux/bits.h
> +++ b/include/linux/bits.h
> @@ -23,7 +23,7 @@
> #include <linux/build_bug.h>
> #define GENMASK_INPUT_CHECK(h, l) \
> (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> - __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> + __builtin_constant_p((int)(l) > (int)(h)), (int)(l) > (int)(h), 0)))
> #else
> /*
> * BUILD_BUG_ON_ZERO is not available in h files included from asm files,

Yes, the above fix is working fine. Now the compilation warning is not
getting reported.

Regards
Syed Nayyar Waris

>
> I can send a proper patch if this is ok.
> >
> > Unfortunately, it's not enough. We need to take care about the following cases
>
> The __GENMASK macro is only valid for values of h and l between 0 and 63
> (or 31, if unsigned long is 32 bits). Negative values or values >=
> sizeof(unsigned long) (or unsigned long long for GENMASK_ULL) result in
> compiler warnings (-Wshift-count-negative or -Wshift-count-overflow). So
> when I wrote the GENMASK_INPUT_CHECK macro, the intention was to catch
> cases where l and h were swapped and let the existing compiler warnings
> catch negative or too large values.
>
> > 1) h or l negative;
>
> Any of these cases will trigger a compiler warning (h negative triggers
> Wshift-count-overflow, l negative triggers Wshift-count-negative).
>
> > 2) h == 0, if l == 0, I dunno what is this. it's basically either 0 or warning;
>
> h == l == 0 is a complicated way of saying 1 (or BIT(0)). l negative
> triggers compiler warning.
>
> > 3) l == 0;
>
> if h is negative, compiler warning (see 1). If h == 0, see 2. If h is
> positive, there is no error in GENMASK_INPUT_CHECK.
>
> > 4) h and l > 0.
>
> The comparisson works as intended.
>
> >
> > Now, on top of that (since it's a macro) we have to keep in mind that
> > h and l can be signed and / or unsigned types.
> > And macro shall work for all 4 cases (by type signedess).
>
> If we cast to int, we don't need to worry about the signedness. If
> someone enters a value that can't be cast to int, there will still
> be a compiler warning about shift out of range.
>
> Rikard
>
> > > Regarding min, max macro that you suggested I am also looking further into it.
> >
> > Since this has been introduced in v5.7 and not only your code is
> > affected by this I think we need to ping original author either to fix
> > or revert.
> >
> > So, I Cc'ed to the author and reviewers, because they probably know
> > better why that had been done in the first place and breaking existing
> > code.
> >
> > --
> > With Best Regards,
> > Andy Shevchenko