On Thu, 2019-12-05 at 16:30 -0600, Bjorn Helgaas wrote:

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

Yes, sorry about that, I tend to re-read what I meant to say instead of what
it's actually written.

On Tue, Dec 03, 2019 at 12:47:40PM +0100, Nicolas Saenz Julienne wrote:

quoted

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/

Noted

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).

I'll update the commit message to be a more explicit.

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.

So, the commit that introduced it (312a0c170945b) meant to use the '__' variant
as a helper, but, due to the fact this is a header file, some found it and made
use of it. I went over some if the commits introducing '__' usages and none of
them seem to acknowledge its use as opposed to the macro version. I think it's
fair to say it's a case of cargo-culting.

quoted

-#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?

It should be 1ULL on either case.

Regards,
Nicolas