On CI

[Andreas Klebinger]

After the idea of letting marge accept unexpected perf improvements and
looking at https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4759
which failed because of a single test, for a single build flavour
crossing the
improvement threshold where CI fails after rebasing I wondered.

When would accepting a unexpected perf improvement ever backfire?

In practice I either have a patch that I expect to improve performance
for some things
so I want to accept whatever gains I get. Or I don't expect improvements
so it's *maybe*
worth failing CI for in case I optimized away some code I shouldn't or
something of that
sort.

How could this be actionable? Perhaps having a set of indicator for CI of
"Accept allocation decreases"
"Accept residency decreases"

Would be saner. I have personally *never* gotten value out of the
requirement
to list the indivial tests that improve. Usually a whole lot of them do.
Some cross
the threshold so I add them. If I'm unlucky I have to rebase and a new
one might
make it across the threshold.

Being able to accept improvements (but not regressions) wholesale might be a
reasonable alternative.

Opinions?