Re: [RFC][PATCH] kernel.h: Add generic roundup_64() macro

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, May 23, 2019 at 7:00 AM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> +# define roundup_64(x, y) (                            \
> +{                                                      \
> +       typeof(y) __y = y;                              \
> +       typeof(x) __x = (x) + (__y - 1);                \
> +       do_div(__x, __y);                               \
> +       __x * __y;                                      \
> +}                                                      \

The thing about this is that it absolutely sucks for power-of-two arguments.

The regular roundup() that uses division has the compiler at least
optimize them to shifts - at least for constant cases. But do_div() is
meant for "we already know it's not a power of two", and the compiler
doesn't have any understanding of the internals.

And it looks to me like the use case you want this for is very much
probably a power of two. In which case division is all kinds of just
stupid.

And we already have a power-of-two round up function that works on
u64. It's called "round_up()".

I wish we had a better visual warning about the differences between
"round_up()" (limited to powers-of-two, but efficient, and works with
any size) and "roundup()" (generic, potentially horribly slow, and
doesn't work for 64-bit on 32-bit).

Side note: "round_up()" has the problem that it uses "x" twice.

End result: somebody should look at this, but I really don't like the
"force division" case that is likely horribly slow and nasty.

                  Linus