Re: simd branch ready for review

On 02/04/2013 10:12 PM, Johan Tibell wrote:

On Mon, Feb 4, 2013 at 2:09 PM, Geoffrey Mainland wrote:

I'd like to figure out how to properly support having the set of available primops depend on the dynamic flags before adding too much more. I'll be speaking to Simon PJ about it tomorrow.

Could we use a fallback, like we did for e.g. popcount? I don't think have conditionally defined primops is a good idea. How would you use them in programs? You'd have to do something like:

#ifdef ??? -- use primops #else -- use fallback #endif

and everyone would write their own fallback. It would be better if GHC fell back to some generic implementation.

-- Johan

What would a sensible fallback be for AVX instructions? What should we fall back on when the LLVM backend is not being used? Maybe we could desugar AVX instructions to SSE instructions on platforms that support SSE but not AVX, but in practice people would then #ifdef anyway and just use SSE if AVX weren't available. The current idea is to hide the #ifdefs in a library. Clients of the library would then get the "best" short-vector implementation available for their platform by using this library. Right now this library is a modified version of primitive, and I have modified versions of vector and DPH that use this version of the primitive library to generate SSE code. I am certainly open to alternative designs. Geoff