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

Andy Shevchenko
2020-05-29T22:08:10Z

On Sat, May 30, 2020 at 12:42 AM Andy Shevchenko
<andy.shevchenko@gmail.com> wrote:
> On Fri, May 29, 2020 at 11:07 PM Syed Nayyar Waris <syednwaris@gmail.com> wrote:
> > On Sat, May 30, 2020 at 12:08 AM Andy Shevchenko
> > <andriy.shevchenko@linux.intel.com> wrote:

...

> > Explanation:
> > Actually when nbits (clump size) is 64 (BITS_PER_LONG is 64 on my computer),
> > (BIT(nbits) - 1)
> > gives a value of zero and when this zero is ANDed with any value, it
> > makes it full zero. This is unexpected and incorrect calculation happening.
> >
> > What actually happens is in the macro expansion of BIT(64), that is 1
> > << 64, the '1' overflows from leftmost bit position (most significant
> > bit) and re-enters at the rightmost bit position (least significant
> > bit), therefore 1 << 64 becomes '0x1', and when another '1' is
> > subtracted from this, the final result becomes 0.
> >
> > Since this macro is being used in both bitmap_get_value and
> > bitmap_set_value functions, it will give unexpected results when nbits or clump
> > size is BITS_PER_LONG (32 or 64 depending on arch).
>
> I see, something like
> https://elixir.bootlin.com/linux/latest/source/include/linux/dma-mapping.h#L139
> should be done.
> But yes, let's try to fix GENMASK().
>
> So, if we modify the following
>
> #define GENMASK_INPUT_CHECK(h, l) \
> (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> __builtin_constant_p((l) > (h)), (l) > (h), 0)))
>
> to be
>
> #define GENMASK_INPUT_CHECK(h, l) \
> (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> __builtin_constant_p((l) > (h)), (l) ? (l) > (h) : 0, 0)))
>
> would it work?

Actually it needs an amendment for signed types...

(l) ? (l) > (h) : !((h) > 0)

...but today is Friday night, so, mistakes are warranted :-)

--
With Best Regards,
Andy Shevchenko