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

Nicolas Saenz Julienne
2019-11-27T18:24:21Z

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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)?

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.

Regards,
Nicolas

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