Messages in this thread | | | From | Linus Torvalds <> | Date | Thu, 23 May 2019 08:10:44 -0700 | Subject | Re: [RFC][PATCH] kernel.h: Add generic roundup_64() macro |
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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
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