Commit 9454511b authored by Committed by Marge BotBrowse files
Optimisation in Unique.Supply
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.