Opportunity to improve case-of-case

Consider this code

f g x = (case g x of True  -> not
                     False -> id) `seq` True

With GHC 8 (and -O) we get this Core

f =
  \ (@ t_a1rk) (g_aqD :: t -> Bool) (x_aqE :: t) ->
    case case g_aqD x_aqE of {
           False -> id @ Bool;
           True -> not
         }
    of
    { __DEFAULT ->
    GHC.Types.True
    }

This is obviously bad.

The reasoning is in the long, complicated Note [Single-alternative cases] in Simplify.hs.

Happily with join points the entire Note becomes unnecessary; we can delete the special case that this Note describes, and the Note itself. Result: simpler compiler, and more optimal code.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import labels

added join points label

It seems to be fixed, namely, GHC-8.2.1 with "-ddump-simpl -O" gives

f = \ (@ t_a1TX) (g_ap0 :: t_a1TX -> Bool) (x_ap1 :: t_a1TX) ->
      case g_ap0 x_ap1 of { __DEFAULT -> GHC.Types.True }

I will try to add test.

added Pnormal label

I will add a test.

assigned to @mpickering

I will add a test.

Thank you. I see that Note [Single-alternative cases] in Simplify.hs is long gone -- I can't even see it in ghc-8.10.

mentioned in commit ff720479

mentioned in merge request !6104 (closed)

MR: !6104 (closed)

mentioned in commit 2084f360

mentioned in commit 9ff8cdb4

mentioned in commit 5e038b3f

mentioned in commit 1ee9ff4e

mentioned in commit 93d56291

closed with commit 4beb12db

mentioned in commit 4beb12db