Converting unboxed sum types in StgCmm

[Johan Tibell]

Yup, I think I have it figured out. Will just need to find the time to
write the remaining code.

On Tue, Sep 22, 2015 at 10:13 AM, Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> Johan
>
>
>
> Sorry I’ve been buried.  Let’s fix a time for a Skype call if you’d like
> to chat about this stuff.
>
>
>
> Quick response to the below. I think that afterwards we want it to look
> like this:
>
>
>
> post-unarise
>
>
>
> f = \r [ ds1::Int#  ds2::Ptr ]
>
>       case ds1 of
>
>           0#  ->  <rhs with ds2 in place of x_svo>
>
>          1#  -> <rhs>
>
>
>
> ds2 is the thing that contains either an Int or a char; ds1 is the tag
> that distinguishes htem.
>
>
>
> Simon
>
>
>
> *From:* Johan Tibell [mailto:johan.tibell at gmail.com]
> *Sent:* 14 September 2015 17:03
> *To:* Ryan Newton; Simon Peyton Jones
> *Cc:* Simon Marlow; ghc-devs at haskell.org
> *Subject:* Re: Converting unboxed sum types in StgCmm
>
>
>
> I've given this a yet some more thought. Given this simple core program:
>
>
>
> f [InlPrag=NOINLINE] :: (#|#) Int Char -> Int
>
> [GblId, Arity=1, Str=DmdType]
>
> f =
>
>   \ (ds_dmE :: (#|#) Int Char) ->
>
>     case ds_dmE of _ [Occ=Dead] {
>
>       (#_|#) x_amy -> x_amy;
>
>       (#|_#) ipv_smK -> patError @ Int "UnboxedSum.hs:5:1-15|function f"#
>
>     }
>
>
>
> We will get this stg pre-unarise:
>
>
>
> unarise
>
>   [f [InlPrag=NOINLINE] :: (#|#) Int Char -> Int
>
>    [GblId, Arity=1, Str=DmdType, Unf=OtherCon []] =
>
>        \r srt:SRT:[0e :-> patError] [ds_svm]
>
>            case ds_svm of _ [Occ=Dead] {
>
>              (#_|#) x_svo [Occ=Once] -> x_svo;
>
>              (#|_#) _ [Occ=Dead] -> patError
> "UnboxedSum.hs:5:1-15|function f"#;
>
>            };]
>
>
>
> What do we want it to look like afterwards? I currently, have this,
> modeled after unboxed tuples:
>
>
>
> post-unarise:
>
>   [f [InlPrag=NOINLINE] :: (#|#) Int Char -> Int
>
>    [GblId, Arity=1, Str=DmdType, Unf=OtherCon []] =
>
>        \r srt:SRT:[0e :-> patError] [ds_gvu ds_gvv]
>
>            case (#_|#) [ds_gvu ds_gvv] of _ [Occ=Dead] {  -- <-- WHAT
> SHOULD GO HERE?
>
>              (#_|#) x_svo [Occ=Once] -> x_svo;
>
>              (#|_#) _ [Occ=Dead] -> patError
> "UnboxedSum.hs:5:1-15|function f"#;
>
>            };]
>
>
>
> Here I have performed the same rewriting of the scrutinee in the case
> statement as for unboxed tuples, but note that this doesn't quite work, as
> we don't know which data constructor to apply in "..." in case ... of. In
> the case of tuples it's easy; there is only one.
>
>
>
> It seems to me that we just want to rewrite the case altogether into
> something that looks at the tag field of the data constructor. Also, in stg
> we use the same DataCon as in core, but after stg the unboxed sum case
> really only has one constructor (one with the union of all the fields),
> which makes it awkward to reuse the original DataCon.
>
>
>
>
>
>
>
> On Mon, Sep 14, 2015 at 7:27 AM, Ryan Newton <rrnewton at gmail.com> wrote:
>
>
>    -
>    data RepType = UbxTupleRep [UnaryType]
>        | UbxSumRep [UnaryType]
>        | UnaryRep UnaryType
>
> Not, fully following, but ... this reptype business is orthogonal to
> whether you add a normal type to the STG level that models anonymous,
> untagged unions, right?
>
>
>
> That is, when using Any for pointer types, they could use indicative
> phantom types, like "Any (Union Bool Char)", even if there's not full
> support for doing anything useful with (Union Bool Char) by itself.  Maybe
> the casting machinery could greenlight a cast from Any (Union Bool Char) to
> Bool at least?
>
>
>
> There's already the unboxed union itself, (|# #|) , but that's different
> than a pointer to a union of types...
>
>
>
>
>
>
>
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