The stricteness analyser used to have a HACK which ensured that NOINLNE things
were not strictness-analysed. The reason was unsafePerformIO. Left to itself,
the strictness analyser would discover this strictness for unsafePerformIO:
unsafePerformIO: C(U(AV))
But then consider this sub-expression
unsafePerformIO (\s -> let r = f x in
case writeIORef v r s of (# s1, _ #) ->
(# s1, r #)
The strictness analyser will now find that r is sure to be eval'd,
and may then hoist it out. This makes tests/lib/should_run/memo002
deadlock.
Solving this by making all NOINLINE things have no strictness info is overkill.
In particular, it's overkill for runST, which is perfectly respectable.
Consider
f x = runST (return x)
This should be strict in x.
So the new plan is to define unsafePerformIO using the 'lazy' combinator:
unsafePerformIO (IO m) = lazy (case m realWorld# of (# _, r #) -> r)
Remember, 'lazy' is a wired-in identity-function Id, of type a->a, which is
magically NON-STRICT, and is inlined after strictness analysis. So
unsafePerformIO will look non-strict, and that's what we want.
Now we don't need the hack in the strictness analyser.