How to disable the simple optimizer

[Facundo Domínguez]

> Back then we installed a custom desugarer that did not inline simple bindings.

Ah, it is interesting how the problems repeat. I think I could compose
a custom desugarer by copying bits of the GHC API as a workaround.

> Before you go much further, do open a ticket.

Thanks Simon. I created a ticket now [1].

Facundo

[1] https://gitlab.haskell.org/ghc/ghc/-/issues/24386

On Fri, Jan 26, 2024 at 10:11 AM Simon Peyton Jones
<simon.peytonjones at gmail.com> wrote:
>
> Hi Facundo
>
> Currently there is no way to disable the simple optimiser, but it would be easy to add a flag to do so.
>
> I'm not sure if that is really what you want -- you'll be left with a program with a lot of "junk" like (let x = y in ...).   But maybe LH doesn't mind the junk!
>
> If you decide that is what you want, I'm sure we (or you) can add a flag to control it.  The relevant code is in GHC.HsToCore, below
>
> Before you go much further, do open a ticket.
>
> Simon
>
> I think this the code you want to disable with a flag:
>
>         ; let simpl_opts = initSimpleOpts dflags
>         ; let (ds_binds, ds_rules_for_imps, occ_anald_binds)
>                 = simpleOptPgm simpl_opts mod final_pgm rules_for_imps
>                          -- The simpleOptPgm gets rid of type
>                          -- bindings plus any stupid dead code
>         ; putDumpFileMaybe logger Opt_D_dump_occur_anal "Occurrence analysis"
>             FormatCore (pprCoreBindings occ_anald_binds $$ pprRules ds_rules_for_imps )
>
>         ; endPassHscEnvIO hsc_env name_ppr_ctx CoreDesugarOpt ds_binds ds_rules_for_imps
>
> On Fri, 26 Jan 2024 at 12:29, Facundo Domínguez <facundo.dominguez at tweag.io> wrote:
>>
>> Dear devs,
>>
>> I noticed that GHC sometimes inlines bindings during desugaring, e.g
>>
>>     foo = z
>>       where
>>             z  = z1 + z2
>>             z1 = 42
>>             z2 = 1
>>
>> is desugared to
>>
>>      foo = 42 + 1
>>
>> This inlining is problematic when using a plugin like Liquid Haskell,
>> which wants to analyse in Core the local bindings provided by the
>> user.
>>
>> Until ghc-9.6 LH didn't experience the inlining when using the GHC
>> API. And this is despite the fact that  using ghc from the command
>> line would seem to always inline during desugaring, even when using
>> -O0.
>>
>> When using ghc-9.8.1 though, even the output of the GHC API started
>> having the bindings inlined. Some debugging with -ddump-ds and
>> -ddump-ds-preopt shows that inlining must be happening in the simple
>> optimizer.
>>
>> Is there anything one could do to disable the optimizations?
>>
>> Thanks in advance,
>> Facundo
>>
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