Messages in this thread | | | Date | Mon, 12 Dec 2011 22:01:31 -0800 | From | Dmitry Torokhov <> | Subject | Re: [PATCH] include/log2.h: Fix rounddown_pow_of_two(1) |
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On Mon, Dec 12, 2011 at 03:50:11PM -0800, Linus Torvalds wrote: > On Mon, Dec 12, 2011 at 10:02 AM, Dmitry Torokhov <dtor@vmware.com> wrote: > > From: Andrei Warkentin <andreiw@vmware.com> > > > > 1 is a power of two, therefore rounddown_pow_of_two(1) should return 1. > > It does in case the argument is a variable but in case it's a constant > > it behaves incorrectly and returns 0. Probably nobody ever did it so > > this was never noticed, however drivers/net/vmxnet3 with latest GCC does > > and breaks on unicpu systems. > > > > This is similar to Rolf's patch to roundup_pow_of_two(1). > > Umm. I already applied this patch, but then I started looking at it > more, and asked myself: > > - Why is that "n == 1" test there AT ALL? > > Afaik, that whole test is just plain stupid. It seems to have been > copied from the "roundup()" case (where it exists due to the "-1/+1" > hackery that breaks ilog2()) without any thought about the actual math > of the function at all. > > I think the *real* fix is to just remove that incorrect line, no?
Yes, you are right, special-casing for 1 is not necessary in rounddown case.
Thanks, Dmitry
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