Buy-in for technical proposal 47 which affect GHC devs

[Phyx]

Hi,

Though I'm no longer a very active GHC developer I do have some questions.

Overall I'm in support of this but I think I'd rather see an outright split
from the start rather than first having a forwarder library.

The reason for this is that I'm afraid of the performance impact of having
this intermediate layer.

For statically linked programs this means at least an additional load and
branch on every call to a standard library. This would for instance affect
Windows quite heavily. I'm not sure the impact is mitigated by branch
prediction and prefetching. At the least it'll polute your L2 cache much
more than before.

For dynamically linked we could potentially use symbol preemption to remove
the forwarding or on Windows redirect using import libraries.

Now maybe I'm overestimating the impact this would have, but I'd very much
like to see some numbers on a small-ish experiment to see what impact (if
any) there are and what mitigation we can do.

Typically it's quite hard to optimize after the fact. Maybe I've missed it
in there. Proposal, but can the compiler remove the forwarding? i.e. Can
the calls be specialized directly to the definition one? If so it'll break
having alternative standard libs at runtime?

Kind regards,
Tamar

On Fri, Mar 24, 2023, 04:47 Ben Gamari <ben at smart-cactus.org> wrote:

> "Adam Sandberg Eriksson" <adam at sandbergericsson.se> writes:
>
> > I switched all the HADDOCK hide to not-home in base a couple of years
> > ago, but I see a couple of new ones have snuck in in the meantime. I
> > would suggest adding a lint against hiding in GHC CI.
> >
> Sounds reasonable to me.
> Perhaps someone could open a ticket to track this?
>
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