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2020-08-05

sexp-grammar

Today I learned that the cabal.project file can be used for detailed control of how to handle dependencies. For instance, in my futhark-linear-scan project, I'd like to start experimenting with using the sexp branch to do my testing. But the changes in that branch are not ready to be included in a released version of Futhark yet. Instead, we can specify an override in cabal.project, like so: 

source-repository-package
    type: git
    location: https://github.com/diku-dk/futhark.git
    tag: 454000219f3cd2c296e968f8076a68f8ad7ecc12

This created another problem: sexp-grammar relies on recursion-schemes for… something. I'm not sure. Anyway, since sexp-grammar 2.1.0 was released, recursion-schemes has made breaking changes, and the dependency specification in sexp-grammar explicitly allows breaking changes in recursion-schemes… Sigh… But, thanks to cabal.project, we can just set an additional constraint, that recursion-schemes should not use the newer version: 

constraints: recursion-schemes < 5.2

And thus, futhark-linear-scan builds!

By the way, futhark-linear-scan is not a very descriptive name any more. In the end, we're going to use an interference graph and some graph colouring algorithms to do our optimisation. It doesn't really matter though; the module inside is called ReuseAllocations, which is what's going to be transferred into Futhark.

Now, how far are we with our isomorphisms? As of 1d374da, we support FunDef and Prog. If the goal is to be able to rewrite the psum.fut test from futhark-linear-scan, that would seem to indicate that we're not far off. Next up is to make an isomorphism for KernelsMem. Although, perhaps I should start with SOACs?

Yes! After quite a bit of fiddling, I got the SOACS lore to work: e99b92.

Now, I can do stuff like this: 

1: λ> encode $ (BasicOp $ Rotate [] (VName (nameFromString "mem") 42) :: ExpT SOACS)
2: Right "(basicop (rotate () (vname mem 42)))"

Finally, I also managed to add a --sexp flag to futhark dev! I had to disable some warning/error flags in futhark.cabal for now, but I can print sexps for large functions now! The real test is to see if I can also read sexps back in, but that'll have to wait until tomorrow.