Yes, only changing the rule did indeed cause regressions.
Whichwhen not including the string changes. I don't think it's worth
having one without the other.

But it seems you already backported this?
See https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5263

Cheers
Andreas

Am 22/03/2021 um 07:02 schrieb Moritz Angermann:

The commit message from https://gitlab.haskell.org/ghc/ghc/-/commit/f10d11fa49fa9a7a506c4fdbdf86521c2a8d3495,

makes the changes to string seem required. Applying the commit on its own doesn't apply cleanly and pulls in quite a

bit of extra dependent commits. Just applying the elem rules appears rather risky. Thus will I agree that having that

would be a nice fix to have, the amount of necessary code changes makes me rather uncomfortable for a minor release :-/

On Mon, Mar 22, 2021 at 1:58 PM Gergő Érdi <gergo@erdi.hu> wrote:

Thanks, that makes it less appealing. In the original thread, I got no further replies after my email announcing my "discovery" of that commit, so I thought that was the whole story.

On Mon, Mar 22, 2021, 13:53 Viktor Dukhovni <ietf-dane@dukhovni.org> wrote:

On Mon, Mar 22, 2021 at 12:39:28PM +0800, Gergő Érdi wrote:

> I'd love to have this in a GHC 8.10 release:
> https://mail.haskell.org/pipermail/ghc-devs/2021-March/019629.html

This is already in 9.0, 9.2 and master, but it is a rather non-trivial
change, given all the new work that went into the String case.  So I am
not sure it is small/simple enough to make for a compelling backport.

There's a lot of recent activity in this space.  See also
<https://gitlab.haskell.org/ghc/ghc/-/merge_requests/5259>, which is not
yet merged into master, and might still be eta-reduced one more step).

I don't know whether such optimisation tweaks (not a bugfix) are in
scope for backporting, we certainly need to be confident they'll not
cause any new problems.  FWIW, 5259 is dramatically simpler...

Of course we also have
<https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4890> in much the
same territory, but there we're still blocked on someone figuring out
what's going on with the 20% compile-time hit with T13056, and whether
that's acceptable or not...

--
    Viktor.
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