Commit e1c110af authored by Simon Marlow's avatar Simon Marlow
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Fix an example where we weren't doing case-of-case when we should

That's 1 line of new code and 38 lines of new comments
parent 7e09fadd
......@@ -1827,11 +1827,13 @@ mkDupableCont env (ApplyTo _ arg se cont)
; let app_cont = ApplyTo OkToDup arg'' (zapSubstEnv env') dup_cont
; return (env'', app_cont, nodup_cont) }
mkDupableCont env cont@(Select _ _ [(_, bs, _rhs)] _ _)
mkDupableCont env cont@(Select _ case_bndr [(_, bs, _rhs)] _ _)
-- See Note [Single-alternative case]
-- | not (exprIsDupable rhs && contIsDupable case_cont)
-- | not (isDeadBinder case_bndr)
| all isDeadBinder bs -- InIds
| all isDeadBinder bs -- InIds
&& not (isUnLiftedType (idType case_bndr))
-- Note [Single-alternative-unlifted]
= return (env, mkBoringStop, cont)
mkDupableCont env (Select _ case_bndr alts se cont)
......@@ -2080,3 +2082,37 @@ Other choices:
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.
Note [Single-alternative-unlifted]
Here's another single-alternative where we really want to do case-of-case:
data Mk1 = Mk1 Int#
data Mk1 = Mk2 Int#
M1.f =
\r [x_s74 y_s6X]
case y_s6X of tpl_s7m {
M1.Mk1 ipv_s70 -> ipv_s70;
M1.Mk2 ipv_s72 -> ipv_s72;
{ __DEFAULT ->
case x_s74 of tpl_s7n {
M1.Mk1 ipv_s77 -> ipv_s77;
M1.Mk2 ipv_s79 -> ipv_s79;
{ __DEFAULT -> ==# [wild1_s7b wild_s7c];
So the outer case is doing *nothing at all*, other than serving as a
join-point. In this case we really want to do case-of-case and decide
whether to use a real join point or just duplicate the continuation.
Hence: check whether the case binder's type is unlifted, because then
the outer case is *not* a seq.
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