Simon Peyton Jones authored May 23, 2020 and Marge Bot committed Jun 10, 2020

This patch switches on -fno-state-hack in GHC.Types.Unique.Supply.

It turned out that my fixes for #18078 (coercion floating) changed the
optimisation pathway for mkSplitUniqSupply in such a way that we had
an extra allocation inside the inner loop.  Adding -fno-state-hack
fixed that -- and indeed the loop in mkSplitUniqSupply is a classic
example of the way in which -fno-state-hack can be bad; see #18238.

Moreover, the new code is better than the old.  They allocate
the same, but the old code ends up with a partial application.
The net effect is that the test
    perf/should_run/UniqLoop
runs 20% faster!   From 2.5s down to 2.0s.  The allocation numbers
are the same -- but elapsed time falls. Good!

The bad thing about this is that it's terribly delicate.  But
at least it's a good example of such delicacy in action.

There is a long Note [Optimising the unique supply] which now
explains all this.