Avoiding full laziness xform / floating-out (Re: What's the benefit of taking "do" blocks apart? Is there a way to turn that off?)

Thu Dec 30 09:03:37 UTC 2021

PUBLIC

Hi Joachim,

Thanks for the hints!

> Hi Gergo,
> 
> Am Dienstag, dem 28.12.2021 um 15:57 +0000 schrieb Erdi, Gergo via ghc-
> devs:
> > PUBLIC
> 
> phew

Yeah obviously I'm sitting here not only adding these tags, but also
coming up with the automated systems and also the company policies
forcing the usage of said systems ;)

> didn't investigate deeper (maybe if you provide a small example I would), but just from looking at this:

Unfortunately, it's hard to make a small example because this all
lives in bizarro world where "IO" isn't IO, "String" isn't [Char] etc,
so I can't just take the input program and pass it to vanilla GHC.

> 
>  * It is generally preferable to turn local lambda expressions
>    into top-level functions. This way, instead of dynamically
>    allocating a FUN heap object, it's just a static function.
> 
>  * sat_sKv is an IO expression? Then it is actually a function in a way
>    (taking the "State token" as an argument). So the above applies.

This is "IO" but not GHC's IO, just another type with an opaque
definition and a Monad instance. There's no reason for GHC to think
that sat_sKv would be a function.

Of course, sat_sKw is necessarily a function since it is the second
argument to bind. But only now I am noticing that even sat_sKw's
definition includes *yet another* floated-out variable:

sat_sKv = some complicated expression 1
sat_sKu = some complicated expression 2
sat_sKw = \_ -> sat_sKu
main = bindIO sat_sKv sat_sKw

Here, I don't see why GHC should think that sat_sKv and sat_sKu are
functions. For example, here's the definition of sat_sKu:

sat_sKu :: IO ()
[LclId]
sat_sKu
  = let {
      sat_sRy [Occ=Once1] :: String
      [LclId]
      sat_sRy
        = let {
            sat_sRx [Occ=Once1] :: String
            [LclId]
            sat_sRx = unpackCStringUtf8# "\n"# } in
          let {
            sat_sRw [Occ=Once1] :: String
            [LclId]
            sat_sRw
              = case foobar True of {
                  False -> unpackCStringUtf8# "False"#;
                  True -> unpackCStringUtf8# "True"#
                } } in
          sApp# sat_sRw sat_sRx } in
    putStrOut sat_sRy

Here, putStrOut is so opaque that it doesn't even have a definition,
it is merely a name registered into GHC's name tables. So I really
don't see why this looks "function-y" to GHC.

The reason this is all a problem for me is because I would like to
then interpret all these let-bindings, including the toplevel ones, as
eagerly defined variables. But then there is of course a difference between

main =
  let a1 = someIOAction1
      a2 = someOtherIOAction2
      a2' = \_ -> a2
  in bindIO a1 a2'

and

main =
  let a1 = someIOAction1
      a2 = \_ -> someIOAction2
  in bindIO a1 a2

because if the *definition* of "a2" throws (not its action), then the
first version will throw immediately whereas the second will only
throw when "main" is *run*, after running "a1".

>  * I think this is the FloatOut pass. You can turn it out using
>    -fno-full-laziness. Not sure if some others passes might
>    do similar things, though.

I tried turning off Opt_FullLaziness, but unfortunately I am still
getting the same result. I was also hopeful when I saw there's an
Opt_FloatIn flag, but alas that doesn't change this behaviour either
(and it's turned on by default anyway...)

I'll try to make a self-contained minimal example next week.

Thanks,
        Gergo

-----Original Message-----
From: ghc-devs <ghc-devs-bounces at haskell.org> On Behalf Of Joachim Breitner
Sent: Wednesday, December 29, 2021 8:39 PM
To: ghc-devs at haskell.org
Subject: [External] Re: What's the benefit of taking "do" blocks apart? Is there a way to turn that off?

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Hi Gergo,

Am Dienstag, dem 28.12.2021 um 15:57 +0000 schrieb Erdi, Gergo via ghc-
devs:
> PUBLIC

phew

> I’m seeing ‘do’ blocks getting taking apart into top-level 
> definitions, so e.g.
>
> main = do
>   some complicated expression 1
>   some complicated expression 2
>
> is compiled into
>
> sat_sKv = some complicated expression 1 sat_sKw = \_ -> some 
> complicated expression 2 main = bindIO sat_sKv sat_sKw
>
> This seems to happen regardless of any common subexpressions, i.e. it 
> is not the case that sat_sKv or sat_sKw are used anywhere else.
>
> What is the intended benefit of this floating-out? Is there a 
> particular Core-to-Core pass that causes this? Is it possible to turn 
> it off?

didn’t investigate deeper (maybe if you provide a small example I would), but just from looking at this:

 * It is generally preferable to turn local lambda expressions
   into top-level functions. This way, instead of dynamically
   allocating a FUN heap object, it’s just a static function.

 * sat_sKv is an IO expression? Then it is actually a function in a way
   (taking the “State token” as an argument). So the above applies.

 * I think this is the FloatOut pass. You can turn it out using
   -fno-full-laziness. Not sure if some others passes might
   do similar things, though.

Cheers,
Joachim

--
Joachim Breitner
  mail at joachim-breitner.de
  https://clicktime.symantec.com/33oTTj3Bye1kkqsmxZWi3QZ6xU?u=http%3A%2F%2Fwww.joachim-breitner.de%2F

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