Specialisation doesn't kick in (RE: Instantiation of overloaded definition in Core)

Wed Oct 6 08:24:06 UTC 2021

I think you need to run at least one simplifier pass as the
specialisations are applied via rules (created by specProgram).

On Wed, Oct 6, 2021 at 3:10 AM Erdi, Gergo via ghc-devs
<ghc-devs at haskell.org> wrote:
>
> PUBLIC
>
>
> PUBLIC
>
>
>
> Hi,
>
>
>
> Thanks! Originally I was going to reply to this saying that my transformation isn’t running in CoreM so where do I get that environment from, but then I realized I can just build it from the md_insts field of ModDetails. However, after thinking more about it, I also realized that I shouldn’t ever really need to conjure up dictionaries from thin air: the whole reason I am making a specific specialization of an overloaded function is because I found somewhere a call at that type. But then, that call also gives me the dictionary!
>
>
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> Of course at this point, this sounds exactly like what GHC already does in `specProgram`. So maybe I should be able to just use that?
>
>
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> Unfortunately, my initial testing seems to show that even if I run `specBind` manually on my whole-program collected CoreProgram, it doesn’t do the work I would expect from it!
>
>
>
> In the following example, I have only kept the definitions that are relevant. Before specialisation, I have the following whole-program Core:
>
>
>
> (>>=)
>
>   :: forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
>
> [GblId[ClassOp], Arity=1, Caf=NoCafRefs, Str=<S(LSL),U(A,U,A)>]
>
> (>>=)
>
>   = \ (@(m :: * -> *)) (v_sGm [Occ=Once1!] :: Monad m) ->
>
>       case v_sGm of
>
>       { C:Monad _ [Occ=Dead] v_sGp [Occ=Once1] _ [Occ=Dead] ->
>
>       v_sGp
>
>       }
>
> $dm>> :: forall (m :: * -> *) a b. Monad m => m a -> m b -> m b
>
> [GblId, Arity=3, Unf=OtherCon []]
>
> $dm>>
>
>   = \ (@(m :: * -> *))
>
>       ($dMonad [Occ=Once1] :: Monad m)
>
>      (@a)
>
>       (@b)
>
>       (ma [Occ=Once1] :: m a)
>
>       (mb [Occ=OnceL1] :: m b) ->
>
>       let {
>
>         sat_sGQ [Occ=Once1] :: a -> m b
>
>         [LclId]
>
>         sat_sGQ = \ _ [Occ=Dead] -> mb } in
>
>       >>= @m $dMonad @a @b ma sat_sGQ
>
> C:Monad [InlPrag=NOUSERINLINE CONLIKE]
>
>   :: forall (m :: * -> *).
>
>      Applicative m
>
>      -> (forall a b. m a -> (a -> m b) -> m b)
>
>      -> (forall a b. m a -> m b -> m b)
>
>      -> Monad m
>
> [GblId[DataCon], Arity=3, Caf=NoCafRefs, Cpr=m1, Unf=OtherCon []]
>
> C:Monad
>
>   = \ (@(m :: * -> *))
>
>       (eta_B0 [Occ=Once1] :: Applicative m)
>
>       (eta_B1 [Occ=Once1] :: forall a b. m a -> (a -> m b) -> m b)
>
>       (eta_B2 [Occ=Once1] :: forall a b. m a -> m b -> m b) ->
>
>       C:Monad @m eta_B0 eta_B1 eta_B2
>
> $fMonadIO [InlPrag=NOUSERINLINE CONLIKE] :: Monad IO
>
> [GblId[DFunId]]
>
> $fMonadIO = C:Monad @IO $fApplicativeIO bindIO $fMonadIO_$c>>;
>
> $fMonadIO_$c>> [Occ=LoopBreaker]
>
>   :: forall a b. IO a -> IO b -> IO b
>
> [GblId]
>
> $fMonadIO_$c>> = \ (@a) (@b) -> $dm>> @IO $fMonadIO @a @b;
>
> sat_sHr :: IO ()
>
> [LclId]
>
> sat_sHr = returnIO @() ()
>
> sat_sHq :: IO ()
>
> [LclId]
>
> sat_sHq = returnIO @() ()
>
> main :: IO ()
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> [GblId]
>
> main = $fMonadIO_$c>> @() @() sat_sHq sat_sHr
>
>
>
>
>
> Now I pass this to GHC’s `specBind`, but the output is exactly the same as the input! (or it’s close enough that I can’t spot the difference).
>
>
>
> (>>=)
>
>   :: forall (m :: * -> *) a b. Monad m => m a -> (a -> m b) -> m b
>
> [GblId[ClassOp], Arity=1, Caf=NoCafRefs, Str=<S(LSL),U(A,U,A)>]
>
> (>>=)
>
>   = \ (@(m :: * -> *)) (v_sGm [Occ=Once1!] :: Monad m) ->
>
>       case v_sGm of
>
>       { C:Monad _ [Occ=Dead] v_sGp [Occ=Once1] _ [Occ=Dead] ->
>
>       v_sGp
>
>       }
>
> $dm>> :: forall (m :: * -> *) a b. Monad m => m a -> m b -> m b
>
> [GblId, Arity=3, Unf=OtherCon []]
>
> $dm>>
>
>   = \ (@(m :: * -> *))
>
>       ($dMonad [Occ=Once1] :: Monad m)
>
>       (@a)
>
>       (@b)
>
>       (ma [Occ=Once1] :: m a)
>
>       (mb [Occ=OnceL1] :: m b) ->
>
>       let {
>
>         sat_MHt [Occ=Once1] :: a -> m b
>
>         [LclId]
>
>         sat_MHt = \ _ [Occ=Dead] -> mb } in
>
>       >>= @m $dMonad @a @b ma sat_MHt
>
> C:Monad [InlPrag=NOUSERINLINE CONLIKE]
>
>   :: forall (m :: * -> *).
>
>      Applicative m
>
>      -> (forall a b. m a -> (a -> m b) -> m b)
>
>      -> (forall a b. m a -> m b -> m b)
>
>      -> Monad m
>
> [GblId[DataCon], Arity=3, Caf=NoCafRefs, Cpr=m1, Unf=OtherCon []]
>
> C:Monad
>
>   = \ (@(m :: * -> *))
>
>       (eta_B0 [Occ=Once1] :: Applicative m)
>
>       (eta_B1 [Occ=Once1] :: forall a b. m a -> (a -> m b) -> m b)
>
>       (eta_B2 [Occ=Once1] :: forall a b. m a -> m b -> m b) ->
>
>       C:Monad @m eta_B0 eta_B1 eta_B2
>
> $fMonadIO [InlPrag=NOUSERINLINE CONLIKE] :: Monad IO
>
> [GblId[DFunId]]
>
> $fMonadIO = C:Monad @IO $fApplicativeIO bindIO $fMonadIO_$c>>;
>
> $fMonadIO_$c>> [Occ=LoopBreaker]
>
>   :: forall a b. IO a -> IO b -> IO b
>
> [GblId]
>
> $fMonadIO_$c>> = \ (@a) (@b) -> $dm>> @IO $fMonadIO @a @b;
>
> sat_sHr :: IO ()
>
> [LclId]
>
> sat_sHr = returnIO @() ()
>
> sat_sHq :: IO ()
>
> [LclId]
>
> sat_sHq = returnIO @() ()
>
> main :: IO ()
>
> [GblId]
>
> main = $fMonadIO_$c>> @() @() sat_sHq sat_sHr
>
>
>
>
>
> Why is that? I would have expected that the call chain main >-> $fMonadIO_$c>>  >-> $dm>> would have resulted in a specialization along the lines of:
>
>
>
> $dm>>_IO :: forall a b. IO a -> IO b -> IO b
>
> >>=_IO :: forall a b. IO a -> (a -> IO b) -> IO b
>
>
>
> With appropriate definitions that can then be simplified away.
>
>
>
> But none of this seems to happen -- $dm>> doesn’t get an IO-specific version, and so $fMonadIO_$c>> still ends up with a dictionary-passing call to $dm>>. Isn’t this exactly the situation that the specialiser is supposed to eliminate?
>
>
>
> Thanks,
>
>             Gergo
>
>
>
> From: Simon Peyton Jones <simonpj at microsoft.com>
> Sent: Monday, October 4, 2021 7:29 PM
> To: Erdi, Gergo <Gergo.Erdi at sc.com>
> Cc: Montelatici, Raphael Laurent <Raphael.Montelatici at sc.com>; GHC <ghc-devs at haskell.org>
> Subject: [External] RE: Instantiation of overloaded definition *in Core*
>
>
>
> PUBLIC
>
> You can look it up in the class instance environment, which the Simplifier does have access to it.  That’s relatively easy when you have a simple dictionary like (Monad IO).  But if you want (Eq [Int]) you first of all have to look up the (Eq [a]) dictionary, then the Eq Int dictionary, and apply the former to the latter.  We don’t (yet) have a simple API to do that, although it would not be hard to create one.
>
>
>
> Simon
>
>
>
> PS: I am leaving Microsoft at the end of November 2021, at which point simonpj at microsoft.com will cease to work.  Use simon.peytonjones at gmail.com instead.  (For now, it just forwards to simonpj at microsoft.com.)
>
>
>
> From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Erdi, Gergo via ghc-devs
> Sent: 04 October 2021 10:30
> To: 'GHC' <ghc-devs at haskell.org>
> Cc: Montelatici, Raphael Laurent <Raphael.Montelatici at sc.com>
> Subject: Instantiation of overloaded definition *in Core*
>
>
>
> PUBLIC
>
>
>
> Hi,
>
>
>
> I’d like to instantiate Core definitions. For example, suppose I have the following Core definition:
>
>
>
> foo :: forall m a b. Monad m => m a -> m b -> m b
>
> foo = \ @m ($d :: Monad m) @a @b (ma :: m a) (mb :: m b) -> ...
>
>
>
> Now let’s say I’d like to instantiate it for m ~ IO. It is quite straightforward to go from the above to:
>
>
>
> foo_IO_0 :: forall a b. Monad IO => IO a -> IO b -> IO b
>
> foo_IO_0 = \ ($d :: Monad IO) @a @b (ma :: IO a) (mb :: IO b) -> ...
>
>
>
> However, I would like to go all the way to:
>
>
>
> foo_IO :: forall a b. IO a -> IO b -> IO b
>
> foo_IO = \ @a @b (ma :: IO a) (mb :: IO b) -> ...
>
>
>
> Because instances are coherent, it should be sound to replace all occurrences of $d with “the” dictionary for Monad IO. However, the places I’ve found for this kind of query seem to live in the typechecker. How do I access this information while working with Core?
>
>
>
> Thanks,
>
>             Gergo
>
>
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Specialisation doesn't kick in (RE: Instantiation of overloaded definition *in Core*)

Specialisation doesn't kick in (RE: Instantiation of overloaded definition in Core)