[project @ 2001-10-17 14:24:52 by simonmar]

Comment out a bogus assertion, and add a comment to describe why it
isn't necessarily true (see the comment for details).

diff --git a/ghc/compiler/codeGen/CgExpr.lhs b/ghc/compiler/codeGen/CgExpr.lhs
index 6905285..6297949 100644 (file)
--- a/ghc/compiler/codeGen/CgExpr.lhs
+++ b/ghc/compiler/codeGen/CgExpr.lhs
@@ -1,7 +1,7 @@
 %
 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
 %
-% $Id: CgExpr.lhs,v 1.44 2001/09/26 15:11:50 simonpj Exp $
+% $Id: CgExpr.lhs,v 1.45 2001/10/17 14:24:52 simonmar Exp $
 %
 %********************************************************
 %*                                                     *
@@ -318,21 +318,27 @@ mkRhsClosure      bndr cc bi srt
                         [(con, params, use_mask,
                            (StgApp selectee [{-no args-}]))]
                         StgNoDefault))
-  |  the_fv == scrutinee                       -- Scrutinee is the only free variable
-  && maybeToBool maybe_offset                  -- Selectee is a component of the tuple
+  |  the_fv == scrutinee               -- Scrutinee is the only free variable
+  && maybeToBool maybe_offset          -- Selectee is a component of the tuple
   && offset_into_int <= mAX_SPEC_SELECTEE_SIZE -- Offset is small enough
-  = ASSERT(is_single_constructor)
+  = -- NOT TRUE: ASSERT(is_single_constructor)
+    -- The simplifier may have statically determined that the single alternative
+    -- is the only possible case and eliminated the others, even if there are
+    -- other constructors in the datatype.  It's still ok to make a selector
+    -- thunk in this case, because we *know* which constructor the scrutinee
+    -- will evaluate to.
     cgStdRhsClosure bndr cc bi [the_fv] [] body lf_info [StgVarArg the_fv]
   where
     lf_info              = mkSelectorLFInfo (idType bndr) offset_into_int 
                                                (isUpdatable upd_flag)
-    (_, params_w_offsets) = layOutDynConstr bogus_name con idPrimRep params    -- Just want the layout
+    (_, params_w_offsets) = layOutDynConstr bogus_name con idPrimRep params
+                               -- Just want the layout
     maybe_offset         = assocMaybe params_w_offsets selectee
     Just the_offset      = maybe_offset
     offset_into_int       = the_offset - fixedHdrSize
-    is_single_constructor = maybeToBool (maybeTyConSingleCon tycon)
     bogus_name           = panic "mkRhsClosure"
 \end{code}
+
 Ap thunks
 ~~~~~~~~~