+mkDupableCont env cont@(Select _ case_bndr [(_,bs,rhs)] se case_cont)
+-- | not (exprIsDupable rhs && contIsDupable case_cont) -- See notes below
+-- | not (isDeadBinder case_bndr)
+ | all isDeadBinder bs
+ = returnSmpl (emptyFloats env, (mkBoringStop scrut_ty, cont))
+ where
+ scrut_ty = substTy se (idType case_bndr)
+
+{- Note [Single-alternative cases]
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+This case is just like the ArgOf case. Here's an example:
+ data T a = MkT !a
+ ...(MkT (abs x))...
+Then we get
+ case (case x of I# x' ->
+ case x' <# 0# of
+ True -> I# (negate# x')
+ False -> I# x') of y {
+ DEFAULT -> MkT y
+Because the (case x) has only one alternative, we'll transform to
+ case x of I# x' ->
+ case (case x' <# 0# of
+ True -> I# (negate# x')
+ False -> I# x') of y {
+ DEFAULT -> MkT y
+But now we do *NOT* want to make a join point etc, giving
+ case x of I# x' ->
+ let $j = \y -> MkT y
+ in case x' <# 0# of
+ True -> $j (I# (negate# x'))
+ False -> $j (I# x')
+In this case the $j will inline again, but suppose there was a big
+strict computation enclosing the orginal call to MkT. Then, it won't
+"see" the MkT any more, because it's big and won't get duplicated.
+And, what is worse, nothing was gained by the case-of-case transform.
+
+When should use this case of mkDupableCont?
+However, matching on *any* single-alternative case is a *disaster*;
+ e.g. case (case ....) of (a,b) -> (# a,b #)
+ We must push the outer case into the inner one!
+Other choices:
+
+ * Match [(DEFAULT,_,_)], but in the common case of Int,
+ the alternative-filling-in code turned the outer case into
+ case (...) of y { I# _ -> MkT y }
+
+ * Match on single alternative plus (not (isDeadBinder case_bndr))
+ Rationale: pushing the case inwards won't eliminate the construction.
+ But there's a risk of
+ case (...) of y { (a,b) -> let z=(a,b) in ... }
+ Now y looks dead, but it'll come alive again. Still, this
+ seems like the best option at the moment.
+
+ * Match on single alternative plus (all (isDeadBinder bndrs))
+ Rationale: this is essentially seq.
+
+ * Match when the rhs is *not* duplicable, and hence would lead to a
+ join point. This catches the disaster-case above. We can test
+ the *un-simplified* rhs, which is fine. It might get bigger or
+ smaller after simplification; if it gets smaller, this case might
+ fire next time round. NB also that we must test contIsDupable
+ case_cont *btoo, because case_cont might be big!
+
+ HOWEVER: I found that this version doesn't work well, because
+ we can get let x = case (...) of { small } in ...case x...
+ When x is inlined into its full context, we find that it was a bad
+ idea to have pushed the outer case inside the (...) case.
+-}
+
projects / ghc-hetmet.git / blobdiff