Late CC profiling

[Matthew Pickering]

Hi Simon,

It does seem strange (at first glance) that these implicit things happen in
two different places.

However, I think you should do this refactoring in a separate MR to !10479,
it seems unrelated to the purpose of that patch and affects other things
like you have mentioned.

Why do you think the change in callstack is fine? It seems the difference
is `callstack001` is whether there an entry for `>>=` or not, which seems
like quite an important thing to place a cost centre on? Andreas has a
better intuition for this than me. I would quite like to know if there is
an unoptimised call to >>= in my program. You have probably thought about
this a little, so interested to hear your opinion.

Cheers,

Matt


On Fri, Apr 4, 2025 at 10:44 PM Simon Peyton Jones <
simon.peytonjones at gmail.com> wrote:

> Hi Matthew, Andreas, Ben
>
> In GHC today there is a strange inconsistency:
>
>    - Bindings for data constructor wrappers and class method selectors
>    are added at the end of CoreTidy.
>    - Bindings for data constructor workers are added at the start of
>    CorePrep
>
> This is deeply strange.  In !10479 (unary class patch)
> <https://gitlab.haskell.org/ghc/ghc/-/merge_requests/10479> I had to make
> some changes in this area and after being baffled I moved all these
> implicit bindings to one place at the start of CorePrep.  That is simpler
> and neater.  There is no reason for them to be separate.
>
> BUT I find that one thing happens between Tidy and Prep, namely
> late-cost-centre injection.  That means we will no longer get
> late-cost-centres for data constructor wrappers and class method
> selectors.  I think that this is what is causing diffs in the output for
> callstack001 and friends in !10479.
>
> I think that this is perfectly fine, and I propose to accept the changes.
>
> Another alternative would be to inject all of these bindings at the end of
> Tidy (instead of the start of Prep),  But it's much nicer at the start of
> Prep -- we don't want any of these bindings to appear  in interface files.
>
> Can you confirm that the change is ok?  Thanks
>
> Simon
>
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