@primOpIsCheap@, as used in \tr{SimplUtils.lhs}. For now (HACK
WARNING), we just borrow some other predicates for a
what-should-be-good-enough test. "Cheap" means willing to call it more
-than once. Evaluation order is unaffected.
+than once, and/or push it inside a lambda. The latter could change the
+behaviour of 'seq' for primops that can fail, so we don't treat them as cheap.
\begin{code}
primOpIsCheap :: PrimOp -> Bool
-primOpIsCheap op = False
- -- March 2001: be less eager to inline PrimOps
- -- Was: not (primOpHasSideEffects op || primOpOutOfLine op)
+primOpIsCheap op = primOpOkForSpeculation op
+-- In March 2001, we changed this to
+-- primOpIsCheap op = False
+-- thereby making *no* primops seem cheap. But this killed eta
+-- expansion on case (x ==# y) of True -> \s -> ...
+-- which is bad. In particular a loop like
+-- doLoop n = loop 0
+-- where
+-- loop i | i == n = return ()
+-- | otherwise = bar i >> loop (i+1)
+-- allocated a closure every time round because it doesn't eta expand.
+--
+-- The problem that originally gave rise to the change was
+-- let x = a +# b *# c in x +# x
+-- were we don't want to inline x. But primopIsCheap doesn't control
+-- that (it's exprIsDupable that does) so the problem doesn't occur
+-- even if primOpIsCheap sometimes says 'True'.
\end{code}
primOpIsDupable