Case-of-identical-alts optimisation fails abjectly

Herbert Valerio Riedel writes: I've been trying to improve/fix a minor optimization sub-optimality w.r.t to the following code (code like that results from the generics-based NFData deriver[1]):

    data Foo = Foo1 | Foo2 | Foo3 !Int
    
    rnf1 :: Foo -> ()
    rnf1 x = case x of
               Foo1 -> ()
               Foo2 -> ()
               Foo3 {} -> ()

which the current GHC 7.6.1 translates to the following core expression:

    NFDataTest2.rnf1 =
      \ x_aeG ->
        case x_aeG of _ {
          __DEFAULT -> GHC.Tuple.();
          NFDataTest2.Foo3 ds_deT -> GHC.Tuple.()
        }

1. ..whereas I'd have expected it to to compile it to a collapsed __DEFAULT-only case, i.e.

    NFDataTest2.rnf1 =
      \ x_aeG -> case x_aeG of _ { __DEFAULT -> GHC.Tuple.() }

Now I've been hunting it down to the function SimplUtils.mkCase1 [2], which according to the source-code comments, is supposed to merge identical alternatives, i.e.:

| 3.  Merge identical alternatives.
|     If several alternatives are identical, merge them into
|     a single DEFAULT alternative.  I've occasionally seen this
|     making a big difference:
| 
|         case e of               =====>     case e of
|           C _ -> f x                         D v -> ....v....
|           D v -> ....v....                   DEFAULT -> f x
|           DEFAULT -> f x

1. ..and the mkCase1 function itself reads as follows:

    mkCase1 dflags scrut case_bndr alts_ty ((_con1,bndrs1,rhs1) : con_alts)
      | all isDeadBinder bndrs1                     -- Remember the default
      , length filtered_alts < length con_alts      -- alternative comes first
            -- Also Note [Dead binders]
      = do  { tick (AltMerge case_bndr)
            ; mkCase2 dflags scrut case_bndr alts_ty alts' }
      where
        alts' = (DEFAULT, [], rhs1) : filtered_alts
        filtered_alts         = filter keep con_alts
        keep (_con,bndrs,rhs) = not (all isDeadBinder bndrs && rhs `cheapEqExpr` rhs1)

Now the problem seems to be, that isDeadBinder returns False; so I hacked up mkCase1 and inserted a occurAnalyseExpr on artificially constructed single-alternative 'Case' values before applying isDeadBinder, and then it would return True and simplify the case expression as expected.

So now my question is, why isn't the occurrence information available for the case-alternative's binders (the occInfo is set to NoOccInfo) at mkCase1-time? When is occurence analysis performed relative to the simplifier?

• [1]: http://hackage.haskell.org/package/deepseq-generics
• [2]: http://www.haskell.org/ghc/docs/7.6.1/html/libraries/ghc-7.6.1/src/SimplUtils.html#mkCase1

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
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CC	hvr@gnu.org
Operating system
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