How to disable the simple optimizer

[Simon Peyton Jones]

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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