Why 16Mib static array size limit?

[Yuxuan Shui via Digitalmars-d]

On Tuesday, 16 August 2016 at 17:51:13 UTC, Johan Engelen wrote:
> On Tuesday, 16 August 2016 at 01:28:05 UTC, Ali Çehreli wrote:
>> 
>> With ldc2, the best option is to go with a dynamic array ONLY 
>> IF you access the elements through the .ptr property. As seen 
>> in the last result, using the [] operator on the array is 
>> about 4 times slower than that.
>
> As Yuxuan Shui mentioned the difference is in vectorization. 
> The non-POINTER version is not vectorized because the semantics 
> of the code is not the same as the POINTER version. Indexing 
> `arr`, and writing to that address could change `arr.ptr`, and 
> so the loop would do something different when "caching" 
> `arr.ptr` in `p` (POINTER version) versus the case without 
> caching (non-POINTER version).
>
> Evil code demonstrating the problem:
> ```
> ubyte evil;
> ubyte[] arr;
>
> void doEvil() {
>     // TODO: use this in the obfuscated-D contest
>     arr = (&evil)[0..50];
> }
> ```
>
> The compiler somehow has to prove that `arr[i]` will never 
> point to `arr.ptr` (it's called Alias Analysis in LLVM).
>
> Perhaps it is UB in D to have `arr[i]` ever point into `arr` 
> itself, I don't know. If so, the code is vectorizable and we 
> can try to make it so.
>
> -Johan

Wait, doesn't D have strict aliasing rules? ubyte* (&evil) should 
not be allowed to alias with ubyte** (&arr.ptr).