[rust-dev] Integer overflow, round -2147483648

Gábor Lehel glaebhoerl at gmail.com
Fri Jun 20 14:07:58 PDT 2014

On Thu, Jun 19, 2014 at 9:05 AM, Jerry Morrison <jhm456 at gmail.com> wrote:

> Nice analysis!
> Over what scope should programmers pick between Gábor's 3 categories?
> The "wraparound is desired" category should only occur in narrow parts of
> code, like computing a hash. That suits a wraparound-operator better than a
> wraparound-type, and certainly better than a compiler switch. And it
> doesn't make sense for a type like 'int' that doesn't have a fixed size.

I thought hash algorithms were precisely the kind of case where you might
opt for types which were clearly defined as wrapping. Why do you think
using different operators instead would be preferred?

> The "wraparound is undesired but performance is critical" category occurs
> in the most performance critical bits of code [I'm doubting that all parts
> of all Rust programs are performance critical], and programmers need to
> make the trade-off over that limited scope. Maybe via operators or types,
> but not by a compiler switch over whole compilation units.
> That leaves "wraparound is undesired and performance is not critical"
> category for everything else. The choice between checked vs. unbounded
> sounds like typing.
> BigUint is weird: It can underflow but not overflow. When you use its
> value in a more bounded way you'll need to bounds-check it then, whether it
> can go negative or not. Wouldn't it be easier to discard it than squeeze it
> into the wraparound or checked models?

Making the unbounded integer types implement the Checking/Wrapping traits
is more for completeness than anything else, I'm not sure whether it has
practical value.

A BigUint/Natural type is not as important as BigInt/Integer, but it can be
nice to have. Haskell only has Integer in the Prelude, but an external
package provides Natural, and there've been proposals to mainline it. It's
useful for function inputs where only nonnegative values make sense. You
could write asserts manually, but you might as well factor them out. And
types are documentation.

The Haskell implementation of Natural is just a newtype over Integer with
added checks, and the same thing might make sense for Rust.

On Wed, Jun 18, 2014 at 11:21 AM, Brian Anderson <banderson at mozilla.com>

> On 06/18/2014 10:08 AM, Gábor Lehel wrote:
>> # Checked math
>> For (2), the standard library offers checked math in the `CheckedAdd`,
>> `CheckedMul` etc. traits, as well as integer types of unbounded size:
>> `BigInt` and `BigUint`. This is good, but it's not enough. The acid test
>> has to be whether for non-performance-critical code, people are actually
>> *using* checked math. If they're not, then we've failed.
>> `CheckedAdd` and co. are important to have for flexibility, but they're
>> far too unwieldy for general use. People aren't going to write
>> `.checked_add(2).unwrap()` when they can write `+ 2`. A more adequate
>> design might be something like this:
>>  * Have traits for all the arithmetic operations for both checking on
>> overflow and for wrapping around on overflow, e.g. `CheckedAdd` (as now),
>> `WrappingAdd`, `CheckedSub`, `WrappingSub`, and so on.
>>  * Offer convenience methods for the Checked traits which perform
>> `unwrap()` automatically.
>>  * Have separate sets of integer types which check for overflow and which
>> wrap around on overflow. Whatever they're called: `CheckedU8`,
>> `checked::u8`, `uc8`, ...
>>  * Both sets of types implement all of the Checked* and Wrapping* traits.
>> You can use explicit calls to get either behavior with either types.
>>  * The checked types use the Checked traits to implement the operator
>> overloads (`Add`, Mul`, etc.), while the wrapping types use the Wrapping
>> traits to implement them. In other words, the difference between the types
>> is (probably only) in the behavior of the operators.
>>  * `BigInt` also implements all of the Wrapping and Checked traits:
>> because overflow never happens, it can claim to do anything if it "does
>> happen". `BigUint` implements all of them except for the Wrapping traits
>> which may underflow, such as `WrappingSub`, because it has nowhere to wrap
>> around to.
>> Another option would be to have just one set of types but two sets of
>> operators, like Swift does. I think that would work as well, or even
>> better, but we've been avoiding having any operators which aren't familiar
>> from C.
> The general flavor of this proposal w/r/t checked arithmetic sounds pretty
> reasonable to me, and we can probably make progress on this now. I
> particularly think that having checked types that use operator overloading
> is important for ergonomics.
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