[GHC] #14283: Remove the special case for tagToEnum# in the code generator?

[GHC]

#14283: Remove the special case for tagToEnum# in the code generator?
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.2.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 > The only remaining primop (aside from unsafeCoerce# and seq) that
 produces an enumeration type is tagToEnum#. There is some magic in the
 code generator to make garbage collection relating to this work as it has
 historically.

 What/where is the magic you want to remove? I think it might be
 {{{
 cgCase (StgOpApp (StgPrimOp op) args _) bndr (AlgAlt tycon) alts
   | isEnumerationTyCon tycon -- Note [case on bool]
   = do { tag_expr <- do_enum_primop op args

        -- If the binder is not dead, convert the tag to a constructor
        -- and assign it.
        ; unless (isDeadBinder bndr) $ do
             { dflags <- getDynFlags
             ; tmp_reg <- bindArgToReg (NonVoid bndr)
             ; emitAssign (CmmLocal tmp_reg)
                          (tagToClosure dflags tycon tag_expr) }

        ; (mb_deflt, branches) <- cgAlgAltRhss
 (NoGcInAlts,AssignedDirectly)
                                               (NonVoid bndr) alts
                                  -- See Note [GC for conditionals]
        ; emitSwitch tag_expr branches mb_deflt 0 (tyConFamilySize tycon -
 1)
        ; return AssignedDirectly
        }
   where
     do_enum_primop :: PrimOp -> [StgArg] -> FCode CmmExpr
     do_enum_primop TagToEnumOp [arg]  -- No code!
       = getArgAmode (NonVoid arg)
     do_enum_primop primop args
       = do dflags <- getDynFlags
            tmp <- newTemp (bWord dflags)
            cgPrimOp [tmp] primop args
            return (CmmReg (CmmLocal tmp))
 }}}
 in `StgCmmExpr`.  Right?

 If so, what is the effect of just removing it?  Why is it bad to have
 {{{
 case tagToEnum# x of y
   DEFAULT -> f y
 }}}
 ?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14283#comment:2>
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