On 8/16/16 4:11 PM, Yuxuan Shui wrote:
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).
Even if it did, I believe the wildcard is ubyte*. Just like in C, char*
can point at anything, ubyte is D's equivalent.
-Steve
