auto vectorization of interleaves

[Bruce Carneal]

On Monday, 10 January 2022 at 19:21:06 UTC, Johan wrote:
> On Monday, 10 January 2022 at 03:04:22 UTC, Bruce Carneal wrote:
>> ...
>
> The compiler complains about aliasing when optimizing.
> https://d.godbolt.org/z/hnGj3G3zo
>
> For example, the write to `dst[0]` may alias with `s1[i]` so 
> `s1[i]` needs to be reloaded. I think the problem gets worse 
> with 16bit numbers because they may partially overlap? (8bits 
> of dst[0] overlap with s1[i]) Just a guess of why the lookup 
> tables `.LCPI0_x` are generated...
>
> -Johan

Thanks for the clarification Johan.  For some reason I was 
thinking FORTRAN rather than C wrt aliasing assumptions.  LDC's 
@restrict attribute looks useful.

That said, for my current work the performance predictability of 
explicit __vector forms has come to outweigh the convenience of 
auto-vectorization.  The unittesting is more laborious but the 
performance is easier to understand.

Thanks again for your answers and patience.  I'm pretty sure I'll 
be using @restrict in other projects.