linux-kernel - Re: [PATCH v4 7/8] linux/log2.h: Fix 64bit calculations in roundup/down_pow

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Date:   Thu, 5 Dec 2019 16:30:44 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Nicolas Saenz Julienne <nsaenzjulienne@...e.de>
Cc:     andrew.murray@....com, maz@...nel.org,
        linux-kernel@...r.kernel.org,
        Michael Turquette <mturquette@...libre.com>,
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Subject: Re: [PATCH v4 7/8] linux/log2.h: Fix 64bit calculations in
 roundup/down_pow_two()

You got the "n" on "down" in the subject, but still missing "of" ;)

On Tue, Dec 03, 2019 at 12:47:40PM +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. It turns
> out ilog2() already handles 32/64bit calculations properly, and being
> the building block to the round functions we can rework them as a
> wrapper around it.

Missing "of" in the function names here.
s/a wrapper/wrappers/

IIUC the point of this is that roundup_pow_of_two() returned
"unsigned long", which can be either 32 or 64 bits (worth pointing
out, I think), and many callers need something that returns
"unsigned long long" (always 64 bits).

It's a nice simplification to remove the "__" variants.  Just as a
casual reader of this commit message, I'd like to know why we had both
the roundup and the __roundup versions in the first place, and why we
no longer need both.

> -#define roundup_pow_of_two(n)			\
> -(						\
> -	__builtin_constant_p(n) ? (		\
> -		(n == 1) ? 1 :			\
> -		(1UL << (ilog2((n) - 1) + 1))	\
> -				   ) :		\
> -	__roundup_pow_of_two(n)			\
> - )
> +#define roundup_pow_of_two(n)			  \
> +(						  \
> +	(__builtin_constant_p(n) && ((n) == 1)) ? \
> +	1 : (1ULL << (ilog2((n) - 1) + 1))        \
> +)

Should the resulting type of this expression always be a ULL, even
when n==1, i.e., should it be this?

  1ULL : (1ULL << (ilog2((n) - 1) + 1))        \

Or maybe there's no case where that makes a difference?

Bjorn