Not auto-vectorization

[Martin Nowak]

> GCC is good, it knows many tricks, it contains a lot of pattern matching  
> code and other code to allow such vectorizations, and that C code is  
> almost transparent & standard (restrict is standard, and I think  
> __builtin_assume_aligned isn't too much hard to #define away when not  
> available. And something like --fast-math is available on most compilers  
> (despite Walter doesn't like it)). So it's good to optimize legacy C  
> code too.

I was really surprised that all vectorization approaches seem to be  
restricted to loops.
I'd think that loop unrolling + arithmetic vectorization should achieve  
most of a
specialized loop vectorization.

http://forum.dlang.org/post/jf1s30$14mj$1@digitalmars.com
https://github.com/D-Programming-Language/phobos/blob/master/std/numeric.d#L2329