Why 16Mib static array size limit?

[Johan Engelen via Digitalmars-d]

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