Re: [PATCH 11/12] tools: sync lib/find_bit implementation

Arnd Bergmann
2021-05-12T07:50:01Z

On Tue, May 11, 2021 at 10:39 PM Rikard Falkeborn
<rikard.falkeborn@gmail.com> wrote:
> On Tue, May 11, 2021 at 08:53:53PM +0900, Tetsuo Handa wrote:

> > #define GENMASK_INPUT_CHECK(h, l) \
> > (BUILD_BUG_ON_ZERO(__builtin_choose_expr( \
> > __builtin_constant_p((l) > (h)), (l) > (h), 0)))
> >
> > __GENMASK() does not need "h" and "l" being a constant.
> >
> > Yes, small_const_nbits(size) in find_next_bit() can guarantee that "size" is a
> > constant and hence "h" argument in GENMASK_INPUT_CHECK() call is also a constant.
> > But nothing can guarantee that "offset" is a constant, and hence nothing can
> > guarantee that "l" argument in GENMASK_INPUT_CHECK() call is also a constant.
> >
> > Then, how can (l) > (h) in __builtin_constant_p((l) > (h)) be evaluated at build time
> > if either l or h (i.e. "offset" and "size - 1" in find_next_bit()) lacks a guarantee of
> > being a constant?
> >
>
> So the idea is that if (l > h) is constant, __builtin_constant_p should
> evaluate that, and if it is not it should use zero instead as input to
> __builtin_chose_expr(). This works with non-const inputs in many other
> places in the kernel, but apparently in this case with a certain
> compiler, it doesn't so I guess we need to work around it.

I have a vague memory that __builtin_constant_p() inside of
__builtin_choose_expr()
always evaluates to false because of the order in which the compiler processes
those: If constant-folding only happens after __builtin_choose_expr(), then
__builtin_constant_p() has to be false.

Arnd