[llvm-dev] [RFC] Coding Standards: "prefer `int` for regular arithmetic, use `unsigned` only for bitmask and when you intend to rely on wrapping behavior."

Jake Ehrlich via llvm-dev
Tue, 11 Jun 2019 20:22:47 -0000

This whole debate seems kind of odd to me. I don't know that cases where it
isn't clear what type to use come up that often. If a value can truly never
be negative you should use an unsigned value. If a value can be negative,
you should use a signed value. Anecdotal evidence in my case is that the
vast majority of values are unsigned by this rule.

Is there a reason to use a signed value when you know a value will never be
negative? Trapping on overflow doesn't seem motivated to me to me since
I'm
not aware of anything that does that. UBSan also checks for overflow in
unsigned types by default as well so you can still check for that issue.

I'm not going to go watch the YouTube videos but the ES.102 lacks merit. On
systems I work with the bug they mention wouldn't be caught the way they
say. They also use subtraction (a rare operation IMO) as a motivating
example and arbitrarily declare large values to be less obvious bugs than
negative values without evidence to this.

ES.101 is valid but is not a reason to prefer signed to unsigned values in
any context. I've also run into a number of instances of signed shifts
being used and the interplay between negation and bitwise operators being
used. Not that those are common but it's just to say that exceptions exist
even to that rule.

On Tue, Jun 11, 2019, 12:59 PM Zachary Turner via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On Tue, Jun 11, 2019 at 12:24 PM Mehdi AMINI <joker.eph at gmail.com>
wrote:
>
>> I agree that readability, maintainability, and ability to debug/find
>> issues are key.
>> I haven't found myself in a situation where unsigned was helping my
>> readability: on the opposite actually I am always wondering where is
the
>> expecting wrap-around behavior and that is one more thing I have to
keep in
>> mind when I read code that manipulate unsigned. So YMMV but using
unsigned
>> *increases* my mental load when reading code.
>>
> I'm on the other end.  I'm always reading the code wondering
"is this
> going to warn?"  "Why could a container ever have a negative
number of
> elements?"  "The maximum value representable by the return type
(unsigned)
> is larger than that of the value i'm storing it in (signed), so an
overflow
> could happen even if there were no error.  What then?"
>
>
> On Tue, Jun 11, 2019 at 12:26 PM Michael Kruse <llvmdev at
meinersbur.de>
> wrote:
>
>> Am Di., 11. Juni 2019 um 11:45 Uhr schrieb Zachary Turner via llvm-dev
>> <llvm-dev at lists.llvm.org>:
>> >
>> > I'm personally against changing everything to signed integers.
To me,
>> this is an example of making code strictly less readable and more
confusing
>> in order to fight deficiencies in the language standard.  I get the
problem
>> that it's solving, but I view this as mostly a theoretical problem,
whereas
>> being able to read the code and have it make sense is a practical
problem
>> that we must face on a daily basis.  If you change everything to signed
>> integers, you may catch a real problem with it a couple of times a
year.
>> And by "real problem" here, I'm talking about a
miscompile or an actual bug
>> that surfaces in production somewhere, rather than a "yes, it
seems
>> theoretically possible for this to overflow".
>>
>> Doesn't it make it already worth it?
>>
> vector.size() returns a size_t, which on 64-bit platforms can represent
> types values larger than those that can fit into an int64_t.  So to turn
> your argument around, since it's theoretically possible to have a
vector
> with more items than an int64_t can represent, isn't it already worth
it to
> use size_t, which is an unsigned type?
>
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