Re: [PATCH v3 1/7] linux/log2.h: Add roundup/rounddown_pow_two64() family of functions

[Leon Romanovsky <leon@kernel.org>]

On Wed, Nov 27, 2019 at 07:06:12PM +0000, Robin Murphy wrote:
> On 27/11/2019 6:24 pm, Nicolas Saenz Julienne wrote:
> > On Wed, 2019-11-27 at 18:06 +0000, Robin Murphy wrote:
> > > On 26/11/2019 12:51 pm, Leon Romanovsky wrote:
> > > > On Tue, Nov 26, 2019 at 10:19:39AM +0100, Nicolas Saenz Julienne wrote:
> > > > > Some users need to make sure their rounding function accepts and returns
> > > > > 64bit long variables regardless of the architecture. Sadly
> > > > > roundup/rounddown_pow_two() takes and returns unsigned longs. Create a
> > > > > new generic 64bit variant of the function and cleanup rougue custom
> > > > > implementations.
> > > >
> > > > Is it possible to create general roundup/rounddown_pow_two() which will
> > > > work correctly for any type of variables, instead of creating special
> > > > variant for every type?
> > >
> > > In fact, that is sort of the case already - roundup_pow_of_two() itself
> > > wraps ilog2() such that the constant case *is* type-independent. And
> > > since ilog2() handles non-constant values anyway, might it be reasonable
> > > to just take the strongly-typed __roundup_pow_of_two() helper out of the
> > > loop as below?
> > >
> > > Robin
> > >
> >
> > That looks way better that's for sure. Some questions.
> >
> > > ----->8-----
> > > diff --git a/include/linux/log2.h b/include/linux/log2.h
> > > index 83a4a3ca3e8a..e825f8a6e8b5 100644
> > > --- a/include/linux/log2.h
> > > +++ b/include/linux/log2.h
> > > @@ -172,11 +172,8 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
> > >     */
> > >    #define roundup_pow_of_two(n)			\
> > >    (						\
> > > -	__builtin_constant_p(n) ? (		\
> > > -		(n == 1) ? 1 :			\
> > > -		(1UL << (ilog2((n) - 1) + 1))	\
> > > -				   ) :		\
> > > -	__roundup_pow_of_two(n)			\
> > > +	(__builtin_constant_p(n) && (n == 1)) ?	\
> > > +	1 : (1UL << (ilog2((n) - 1) + 1))	\
> >
> > Then here you'd have to use ULL instead of UL, right? I want my 64bit value
> > everywhere regardless of the CPU arch. The downside is that would affect
> > performance to some extent (i.e. returning a 64bit value where you used to have
> > a 32bit one)?
>
> True, although it's possible that 1ULL might result in the same codegen if
> the compiler can see that the result is immediately truncated back to long
> anyway. Or at worst, I suppose "(typeof(n))1" could suffice, however ugly.
> Either way, this diff was only an illustration rather than a concrete
> proposal, but it might be an interesting diversion to investigate.
>
> On that note, though, you should probably be using ULL in your current patch
> too.
>
> > Also, what about callers to this function on platforms with 32bit 'unsigned
> > longs' that happen to input a 64bit value into this. IIUC we'd have a change of
> > behaviour.
>
> Indeed, although the change in such a case would be "start getting the
> expected value instead of nonsense", so it might very well be welcome ;)

Agree, if code overflowed with 32 bits before this change, the code was already
broken. At least now, it won't overflow.

>
> Robin.
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