[swift-users] How to dispatch on the size of Int?

[Hooman Mehr]

I agree, there should be a platform condition that just indicates native word size, such as arch(32bit) and arch(64bit).

> On Nov 23, 2016, at 2:18 PM, Martin R <martinr448 at gmail.com> wrote:
> 
> Yes, I had forgotten about that, thank you! That would satisfy all criteria. And with 
> 
>    func foo(value: Int) {
>    #if arch(i386) || arch(arm) 
>        foo_impl(value: Int32(value))
>    #elseif arch(x86_64) || arch(arm64) 
>        foo_impl(value: Int64(value))
>    #endif
>    }
> 
> it should be „future-safe“, i.e. fail to compile on a new platform. (I would still prefer a solution which does not require me to know about all possible processor architectures.)
> 
>> Am 23.11.2016 um 23:06 schrieb Hooman Mehr <hooman at mac.com>:
>> 
>> func foo_impl(value: Int32) { /* ... */ }
>> func foo_impl(value: Int64) { /* ... */ }
>> 
>> func foo(value: Int) {
>>    #if arch(i386) || arch(arm)
>>        foo_impl(value: Int32(value))
>>    #else
>>        foo_impl(value: Int64(value))
>>    #endif
>> }
>> 
>> 
>>> On Nov 23, 2016, at 1:31 PM, Martin R via swift-users <swift-users at swift.org> wrote:
>>> 
>>> I wonder what the best way would be to call a specialized function dependent on the size of `Int`. Let's say that I have two implementations
>>> 
>>>   func foo_impl(value: Int32) { /* ... */ }
>>>   func foo_impl(value: Int64) { /* ... */ }
>>> 
>>> and I want 
>>> 
>>>   func foo(value: Int)
>>> 
>>> to call the "right one" of them, according to the architecture (32-bit or 64-bit).
>>> 
>>>   func foo(value: Int) { foo_impl(value: value) }
>>> 
>>> does not compile. (I assume that is because `Int` is not a type alias to `Int32` or `Int64` but an independent type.)
>>> 
>>> This works: 
>>> 
>>>   func foo1(value: Int) {
>>>       if MemoryLayout<Int>.size == 4 {
>>>           foo_impl(value: Int32(value))
>>>       } else {
>>>           foo_impl(value: Int64(value))
>>>       }
>>>   }
>>> 
>>> or
>>> 
>>>   func foo2(value: Int) {
>>>       switch MemoryLayout<Int>.size {
>>>       case 4: foo_impl(value: Int32(value))
>>>       case 8: foo_impl(value: Int64(value))
>>>       default:
>>>           abort()
>>>       }
>>>   }
>>> 
>>> But a typo in the constants would go unnoticed and just call the wrong function, or cause a runtime error instead of a compile-time error. And perhaps `Int` can be an 128-bit integer in the future? The compiler would not warn that the code needs to be updated.
>>> 
>>> This seems to be more promising:
>>> 
>>>   func foo3(value: Int) {
>>>       switch (__WORDSIZE) {
>>>       case 32: foo_impl(value: Int32(value)) // Warning: Will never be executed
>>>       case 64: foo_impl(value: Int64(value))
>>>       }
>>>   }
>>> 
>>> Apparently the compiler "knows" which case will be executed, `foo3` does not compile if there is no case matching actual integer size. But there is always an annoying warning for the unused case. And I am not sure if it is guaranteed that __WORDSIZE is the number of bits in an `Int`.
>>> 
>>> So my question is: What would be the best way to dispatch dependent on the size of `Int`, such that
>>> 
>>> - The compiler checks the correctness.
>>> - The compiler optimizes the code so that no runtime check is done.
>>> - No warnings are produced.
>>> 
>>> If `Int` had a `native` property like `CGFloat` then I could simply call
>>> 
>>>   func foo(value: Int) { foo_impl(value: value.native) }
>>> 
>>> but I could not find such a property. (Would that be a reasonable thing to ask on swift-evolution?)
>>> 
>>> Background: I am asking this just out of curiosity, but the question came up when looking at the `hash_combine` function in the Boost library:
>>> 
>>> http://www.boost.org/doc/libs/1_62_0/boost/functional/hash/hash.hpp 
>>> 
>>> with quite different implementations 
>>> 
>>>   inline void hash_combine_impl(boost::uint32_t& h1, boost::uint32_t k1)
>>>   inline void hash_combine_impl(boost::uint64_t& h, boost::uint64_t k)
>>> 
>>> and I wondered how this would be done in Swift.
>>> 
>>> Regards, 
>>> Martin
>>> 
>>> 
>>> _______________________________________________
>>> swift-users mailing list
>>> swift-users at swift.org
>>> https://lists.swift.org/mailman/listinfo/swift-users
>> 
>