Messages in this thread | | | Date | Wed, 31 Jan 2007 05:47:13 -0500 (EST) | From | "Robert P. J. Day" <> | Subject | Re: [PATCH] Add "is_power_of_2" checking to log2.h. |
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On Wed, 31 Jan 2007, Vegard Nossum wrote:
> On Tue, January 30, 2007 3:12 pm, Jan Engelhardt wrote: > > > >>> Why the qualifier? Zero *is* not a power of 2, is it? > > > > No, it is not: > > > > In[1]:= Solve[2^n == 0, n] > > > > Out[1]= {} > > > > So says Mathematica5. > > As a side note, I would just like to point out that Mathematica does > not deal with modular arithmetic by default (which programmers very > much do). In fact, in modular arithmetic, zero IS a power of two. > > 2^n = 0 (mod 2^n) > > To see if it holds for bytes, substitute n = 8, and you get 2^8 = 0 > (mod 256). In other words: Zero is the eighth power of two modulo > 256.
that's a bit esoteric but, yes, you make a good point.
> Modular arithmetic is, however, very often a source of errors in > programming (unchecked-for overflows and underflows), and it is > questionable whether the programmer would really want 0 reported as > a power of two.
precisely. given the definition of "is_power_of_2()" that's been published, some people will (quite correctly) point out that using that macro in place of the test "(n & (n - 1))" isn't quite the same thing. and they would be correct.
but one wonders how many programmers have been using that very test "((n & (n - 1))" without realizing that it would accept zero, and that it has been accepting zero all this time, perhaps leading to weird and inexplicable errors.
rewriting that test as "is_power_of_2()" may, in fact, cause some things to suddenly break, but perhaps those are things that should have been *forced* to break anyway, to identify where the condition check was incorrect all this time, and someone's just been lucky, or something like that.
***NOTE***: and that's why i suggested that the individual maintainers might want to make this substitution themselves, to make sure everything continues to work. because if you leave it as a project for the kernel janitors, they might not realize what you had in mind. so it's in your best interest to start cleaning this up on your own.
just my $0.02 (Cdn.)
rday
-- ======================================================================== Robert P. J. Day Linux Consulting, Training and Annoying Kernel Pedantry Waterloo, Ontario, CANADA
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