Remove unused imports

[ghc-hetmet.git] / compiler / specialise / SpecConstr.lhs

diff --git a/compiler/specialise/SpecConstr.lhs b/compiler/specialise/SpecConstr.lhs
index 23127f4..5357a53 100644 (file)
--- a/compiler/specialise/SpecConstr.lhs
+++ b/compiler/specialise/SpecConstr.lhs
@@ -27,11 +27,11 @@ import Coercion
 import Rules
 import Type            hiding( substTy )
 import Id
+import MkId            ( mkImpossibleExpr )
 import Var
 import VarEnv
 import VarSet
 import Name
-import OccName         ( mkSpecOcc )
 import DynFlags                ( DynFlags(..) )
 import StaticFlags     ( opt_PprStyle_Debug )
 import StaticFlags     ( opt_SpecInlineJoinPoints )
@@ -365,6 +365,19 @@ specialising the loops arising from stream fusion, for example in NDP where
 we were getting literally hundreds of (mostly unused) specialisations of
 a local function.
 
+Note [Do not specialise diverging functions]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Specialising a function that just diverges is a waste of code.
+Furthermore, it broke GHC (simpl014) thus:
+   {-# STR Sb #-}
+   f = \x. case x of (a,b) -> f x
+If we specialise f we get
+   f = \x. case x of (a,b) -> fspec a b
+But fspec doesn't have decent strictnes info.  As it happened,
+(f x) :: IO t, so the state hack applied and we eta expanded fspec,
+and hence f.  But now f's strictness is less than its arity, which
+breaks an invariant.
+
 -----------------------------------------------------
                Stuff not yet handled
 -----------------------------------------------------
@@ -765,7 +778,8 @@ scExpr' env (Case scrut b ty alts)
   where
     sc_con_app con args scrut'         -- Known constructor; simplify
        = do { let (_, bs, rhs) = findAlt con alts
-                  alt_env' = extendScSubstList env ((b,scrut') : bs `zip` trimConArgs con args)
+                                 `orElse` (DEFAULT, [], mkImpossibleExpr (coreAltsType alts))
+                  alt_env'  = extendScSubstList env ((b,scrut') : bs `zip` trimConArgs con args)
             ; scExpr alt_env' rhs }
                                
     sc_vanilla scrut_usg scrut'        -- Normal case
@@ -1006,8 +1020,9 @@ specialise
 
 specialise env bind_calls (fn, arg_bndrs, body, arg_occs) 
                          spec_info@(SI specs spec_count mb_unspec)
-  | notNull arg_bndrs, -- Only specialise functions
-    Just all_calls <- lookupVarEnv bind_calls fn
+  | not (isBottomingId fn)      -- Note [Do not specialise diverging functions]
+  , notNull arg_bndrs          -- Only specialise functions
+  , Just all_calls <- lookupVarEnv bind_calls fn
   = do { (boring_call, pats) <- callsToPats env specs arg_occs all_calls
 --     ; pprTrace "specialise" (vcat [ppr fn <+> ppr arg_occs,
 --                                     text "calls" <+> ppr all_calls,