Ben I have pushed a new fix to the 7.10 branch, onto branch wip/T10572-2. (Don't apply this to HEAD.) Can you try it. I get validate failures as below, but I am hard pressed to see how they could be related. (The perf changes may be.) Simon Unexpected failures: codeGen/should_run T10414 [exit code non-0] (threaded2) driver T8959a [bad stderr] (normal) ghci/scripts T9878b [bad stderr] (ghci) simplCore/should_compile T4945 [bad stdout] (normal) th T10279 [stderr mismatch] (normal) Unexpected stat failures: perf/compiler T9961 [stat too good] (normal) perf/should_run T4830 [stat not good enough] (normal) perf/should_run T7436 [stat not good enough] (normal)