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

Rikard Falkeborn
2020-06-02T19:01:46Z

On Mon, Jun 01, 2020 at 11:33:30AM +0300, Andy Shevchenko wrote:
> On Mon, Jun 01, 2020 at 12:37:16AM +0200, Rikard Falkeborn 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.
>
> I guess every compiler (more or less recent) will warn here.
> (Sorry, there is a cut in the thread, the problem is with comparison unsigned
> type(s) to 0).
>
> > > > #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?
> >
> > 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,
> >
> > 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.
>
> GENAMSK sometimes is used with non-constant arguments that's why your check
> made a regression.
>
> What I described below are the cases to consider w/o what should we do. What
> you answered is the same what I implied. So, we are on the same page here.
>
> > > 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.
>
> Oh, yes GENMASK(h, l), when h==l==0 should be equivalent to BIT(0) with no
> warning given.
>
> > > 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.
>
> If the argument unsigned long long will it be the warning (it should not)?

No, there should be no warning there.

The inputs to GENMASK() needs to be between 0 and 31 (or 63 depending on the
size of unsigned long). For any other values, there will be undefined behaviour,
since the operands to the shifts in __GENMASK will be too large (or negative).

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.
>
> Please, when you do something there, add a test case to test_bitops.c.
>
> --
> With Best Regards,
> Andy Shevchenko
>
>