Remove special case case of bool during STG -> C--

Allow removing the no longer needed cgPrimOp, getting rid of a small a
small layer violation too.

Change which made the special case no longer needed was #6135 /
6579a6c7, which dates back to 2013,
making me feel better.

compiler/GHC/StgToCmm/Expr.hs

@@ -301,75 +301,7 @@ data GcPlan
 -------------------------------------
 cgCase :: CgStgExpr -> Id -> AltType -> [CgStgAlt] -> FCode ReturnKind
 
-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. See Note [Dead-binder optimisation]
-       ; 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))
-
 {-
-Note [case on bool]
-~~~~~~~~~~~~~~~~~~~
-This special case handles code like
-
-  case a <# b of
-    True ->
-    False ->
-
--->  case tagToEnum# (a <$# b) of
-        True -> .. ; False -> ...
-
---> case (a <$# b) of r ->
-    case tagToEnum# r of
-        True -> .. ; False -> ...
-
-If we let the ordinary case code handle it, we'll get something like
-
- tmp1 = a < b
- tmp2 = Bool_closure_tbl[tmp1]
- if (tmp2 & 7 != 0) then ... // normal tagged case
-
-but this junk won't optimise away.  What we really want is just an
-inline comparison:
-
- if (a < b) then ...
-
-So we add a special case to generate
-
- tmp1 = a < b
- if (tmp1 == 0) then ...
-
-and later optimisations will further improve this.
-
-Now that #6135 has been resolved it should be possible to remove that
-special case. The idea behind this special case and pre-6135 implementation
-of Bool-returning primops was that tagToEnum# was added implicitly in the
-codegen and then optimized away. Now the call to tagToEnum# is explicit
-in the source code, which allows to optimize it away at the earlier stages
-of compilation (i.e. at the Core level).
-
 Note [Scrutinising VoidRep]
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Suppose we have this STG code:

compiler/GHC/StgToCmm/Prim.hs

@@ -17,8 +17,6 @@
 
 module GHC.StgToCmm.Prim (
    cgOpApp,
-   cgPrimOp, -- internal(ish), used by cgCase to get code for a
-             -- comparison without also turning it into a Bool.
    shouldInlinePrimOp
  ) where
 
@@ -110,7 +108,7 @@ cgOpApp (StgPrimOp primop) args res_ty = do
                 f regs
                 emitReturn (map (CmmReg . CmmLocal) regs)
 
-          | otherwise -> panic "cgPrimop"
+          | otherwise -> panic "cgOpApp"
           where
              result_info = getPrimOpResultInfo primop
 
@@ -134,21 +132,6 @@ cgOpApp (StgPrimCallOp primcall) args _res_ty
 asUnsigned :: Width -> Integer -> Integer
 asUnsigned w n = n .&. (bit (widthInBits w) - 1)
 
----------------------------------------------------
-cgPrimOp   :: [LocalReg]        -- where to put the results
-           -> PrimOp            -- the op
-           -> [StgArg]          -- arguments
-           -> FCode ()
-
-cgPrimOp results op args = do
-  dflags <- getDynFlags
-  arg_exprs <- getNonVoidArgAmodes args
-  case emitPrimOp dflags op arg_exprs of
-    PrimopCmmEmit_External -> panic "External prim op"
-    PrimopCmmEmit_Raw _ -> panic "caller should handle TagToEnum themselves"
-    PrimopCmmEmit_IntoRegs f -> f results
-
-
 ------------------------------------------------------------------------
 --      Emitting code for a primop
 ------------------------------------------------------------------------