Messages in this thread | | | Date | Fri, 8 Mar 2019 08:41:58 -0400 | From | Jason Gunthorpe <> | Subject | Re: [PATCH] Avoid that check_shl_overflow() triggers a compiler warning when building with W=1 |
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On Fri, Mar 08, 2019 at 08:58:21AM +0100, Rasmus Villemoes wrote: > On 08/03/2019 01.08, Bart Van Assche wrote: > > On Thu, 2019-03-07 at 08:18 +0100, Rasmus Villemoes wrote: > >> On 07/03/2019 03.14, Bart Van Assche wrote: > >>> On 3/6/19 5:24 PM, Jason Gunthorpe wrote: > >>>>> > >>>>> diff --git a/include/linux/overflow.h b/include/linux/overflow.h > >>>>> index 40b48e2133cb..8afe0c0ada6f 100644 > >>>>> +++ b/include/linux/overflow.h > >>>>> @@ -202,6 +202,24 @@ > >>>>> #endif /* COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW */ > >>>>> +/* > >>>>> + * Evaluate a >= 0 without triggering a compiler warning if the type > >>>>> of a > >>>>> + * is an unsigned type. > >>>>> + */ > >>>>> +#define is_positive(a) ({ \ > >> > >> is_non_negative, please! positive means > 0. And perhaps it's better to > >> move these utility macros closer to the top of the file, together with > >> the other type/range helpers. > > > > Hi Rasmus, > > > > Thank you for the feedback. But according to what I found online opinions > > about whether or not zero is a positive number seem to vary. From > > https://en.wikipedia.org/wiki/Sign_(mathematics): > > Yes, I'm a mathematician, I'm aware of that. You can also find people > who use "less than" in the "<=" sense, and then say "strictly less than" > when they mean "<". > > > Terminology for signs > > > > When 0 is said to be neither positive nor negative, the following phrases > > may be used to refer to the sign of a number: > > * A number is positive if it is greater than zero. > > * A number is negative if it is less than zero. > > * A number is non-negative if it is greater than or equal to zero. > > * A number is non-positive if it is less than or equal to zero. > > > > When 0 is said to be both positive and negative, modified phrases are used > > to refer to the sign of a number: > > * A number is strictly positive if it is greater than zero. > > * A number is strictly negative if it is less than zero. > > * A number is positive if it is greater than or equal to zero. > > * A number is negative if it is less than or equal to zero. > > Right, but in no way does it ever make sense to mix these conventions. > So the options for describing ">= 0, < 0" are "non_negative, negative" > or "positive, strictly_negative". > > In the context of the C language, the first convention is used. While > not explicitly stated, it can be inferred from usage of the terms. > First, the word nonnegative is used (e.g. in defining argc). Second, "If > the value of the right operand [in a shift expression] is negative [...] > the behaviour is undefined.", so certainly negative cannot include 0. > Third, E* constants are required to be positive, and "[errno] is never > set to zero by any library function". Etc. etc.
Lets use is_unsigned() or is_unsigned_value() then for the name of the test, that is pretty unambiguous and alot nicer to read than is_not_negative()
FWIW, in computer science I generally see the terms used as:
negatve: < 0 positive: >= 0 natural: > 0
This language naturally follows the twos complement construction where it is very logical to say a number with the sign bit set is 'negative' and a number with it clear is 'positive', which means 0 is positive.
Which is probably enraging to mathematicians.. But has a certain logic.
.. and most CS places don't actually care about the difference between > 0 and >= 0 , while < 0 is usually highly interesting.
Jason
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