[dmd-internals] Shift support for vector types (or: is vector type a first class type?)

[Walter Bright]

On 3/31/2013 6:56 PM, Kai Nacke wrote:
> Hi!
>
> I try to write a generic vectorized version of SHA1. During that I noticed 
> that only some operations are allowed on vector types.
>
> For the SHA1 algorithm I need to implement a 'rotate left'. I like to write 
> something like this
>
>     uint4 w = ...;
>     uint4 v = (w << 1) | (w >> 31);
>
> which is not allowed by DMD.
>
> Is this by design or simply not implemented because the backend is not capable 
> of generating code for it? The TDPL says nothing about vector types. My 
> understanding of the language reference on the web 
> (http://dlang.org/simd.html) is that the supported operators are CPU 
> architecture dependent.
>
> I really like to see more support for vector operations in the language, e.g. 
> for shifting. What is the view of the language designers? Is the vector type a 
> first class type or just an architecture (maybe vendor) dependent type with 
> limited usability?
>
> Because LLVM treats the vector type as a first class type supporting more 
> operators is easy with LDC. See my pull request for shift operators here: 
> https://github.com/ldc-developers/ldc/pull/321

The idea is if a vector operation is not supported by the underlying hardware, 
then dmd won't allow it. It specifically does not generate "workaround" code 
like gcc does. The reason for this is the workaround code is terribly, terribly 
slow (because moving code between the ALU and the SIMD unit is awful), and 
having the compiler silently insert it leaves the programmer mystified why he is 
getting execrable performance.

The vector design philosophy in D is if you write SIMD code, and it compiles, 
you can be confident it will execute in the SIMD unit of your particular target 
processor. You won't have to dump the assembler output to be sure.