[swift-evolution] [Planning][Request] "constexpr" for Swift 5

[John McCall]

> On Aug 2, 2017, at 5:56 PM, Karl Wagner <razielim at gmail.com> wrote:
>> On 31. Jul 2017, at 21:09, John McCall via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>>> On Jul 31, 2017, at 3:15 AM, Gor Gyolchanyan <gor.f.gyolchanyan at icloud.com <mailto:gor.f.gyolchanyan at icloud.com>> wrote:
>>>> On Jul 31, 2017, at 7:10 AM, John McCall via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>>>> 
>>>>> On Jul 30, 2017, at 11:43 PM, Daryle Walker <darylew at mac.com <mailto:darylew at mac.com>> wrote:
>>>>> The parameters for a fixed-size array type determine the type's size/stride, so how could the bounds not be needed during compile-time? The compiler can't layout objects otherwise. 
>>>> 
>>>> Swift is not C; it is perfectly capable of laying out objects at run time.  It already has to do that for generic types and types with resilient members.  That does, of course, have performance consequences, and those performance consequences might be unacceptable to you; but the fact that we can handle it means that we don't ultimately require a semantic concept of a constant expression, except inasmuch as we want to allow users to explicitly request guarantees about static layout.
>>> 
>>> Doesn't this defeat the purpose of generic value parameters? We might as well use a regular parameter if there's no compile-time evaluation involved. In that case, fixed-sized arrays will be useless, because they'll be normal arrays with resizing disabled.
>> 
>> You're making huge leaps here.  The primary purpose of a fixed-size array feature is to allow the array to be allocated "inline" in its context instead of "out-of-line" using heap-allocated copy-on-write buffers.  There is no reason that that representation would not be supportable just because the array's bound is not statically known; the only thing that matters is whether the bound is consistent for all instances of the container.
>> 
>> That is, it would not be okay to have a type like:
>>  struct Widget {
>>    let length: Int
>>    var array: [length x Int]
>>  }
>> because the value of the bound cannot be computed independently of a specific value.
>> 
>> But it is absolutely okay to have a type like:
>>  struct Widget {
>>    var array: [(isRunningOnIOS15() ? 20 : 10) x Int]
>>  }
>> It just means that the bound would get computed at runtime and, presumably, cached.  The fact that this type's size isn't known statically does mean that the compiler has to be more pessimistic, but its values would still get allocated inline into their containers and even on the stack, using pretty much the same techniques as C99 VLAs.
> 
> Do we really want to make that guarantee about heap/stack allocation? C99’s VLAs are not very loop-friendly:
> 
> echo "int main() { 
>         for(int i = 0; i<1000000; i++) {
>           int myArray[i * 1000]; myArray[0] = 32;
>         }
>         return 0;
>       }" | clang -x c - && ./a.out
> 
> Segmentation Fault: 11
> 
> C compilers also do not inline code with VLAs by default. If you force it, you expose yourself to possible stack overflows:
> 
> echo "static inline void doSomething(int i) {
>         int myArray[i * 1000]; myArray[0] = 32;
>       }
>       int main() {
>         for(int i = 0; i<1000000; i++) {
>           doSomething(i);
>         }
>       return 0;
>       }" | clang -x c - && ./a.out
> 
> Segmentation Fault: 11
> 
> I wouldn’t like us to import these kinds of issues in to Swift

We probably would not make an absolute guarantee of stack allocation, no.

John.
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