vectorisation code?

[Richard Eisenberg]

Hi devs,

There's a sizable number of modules in the `vectorise` subdirectory of GHC. I'm sure these do all sorts of wonderful things. But what, exactly? And, does anyone make use of these wonderful things?

A quick poking through the code shows a tiny link between the vectorise code and the rest of GHC -- the function `vectorise` exported from the module `Vectorise`, which is named in exactly one place from SimplCore. From what I can tell, the function will be called only when `-fvectorise` is specified, and then it seems to interact with a {-# VECTORISE #-} pragma. However, `{-# VECTORISE #-}` doesn't appear in the manual at all, and `-fvectorise` is given only a cursory explanation. It seems these work with DPH... which has been disabled, no? Searching online finds several hits, but nothing more recent than 2012.

I hope this question doesn't offend -- it seems that vectorisation probably has amazing performance gains. Yet, the feature also seems unloved. In the meantime, compiling (and recompiling, and recompiling...) the modules takes time, as does going through them to propagate changes from elsewhere. If this feature is truly orphaned, unloved, and unused at the moment, is it reasonable to consider putting it on furlough?

Thanks,
Richard