X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=compiler%2Fspecialise%2FSpecConstr.lhs;h=5357a5353958c0b094ca3928e11985082ac2849e;hp=02802555104c2f391e21103cf2c48904a1ac4cc5;hb=703ca1542c8e0983cc9d8eebce6e9f3dd3fd71e2;hpb=3f44fb8231db3277a584470cbe7397bec801cd0e

diff --git a/compiler/specialise/SpecConstr.lhs b/compiler/specialise/SpecConstr.lhs
index 0280255..5357a53 100644
--- a/compiler/specialise/SpecConstr.lhs
+++ b/compiler/specialise/SpecConstr.lhs
@@ -20,7 +20,6 @@ import CoreSyn
 import CoreSubst
 import CoreUtils
 import CoreUnfold	( couldBeSmallEnoughToInline )
-import CoreLint		( showPass, endPass )
 import CoreFVs 		( exprsFreeVars )
 import WwLib		( mkWorkerArgs )
 import DataCon		( dataConRepArity, dataConUnivTyVars )
@@ -28,13 +27,12 @@ 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 ErrUtils		( dumpIfSet_dyn )
-import DynFlags		( DynFlags(..), DynFlag(..) )
+import DynFlags		( DynFlags(..) )
 import StaticFlags	( opt_PprStyle_Debug )
 import StaticFlags	( opt_SpecInlineJoinPoints )
 import BasicTypes	( Activation(..) )
@@ -367,6 +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
 -----------------------------------------------------
@@ -451,19 +462,8 @@ unbox the strict fields, becuase T is polymorphic!)
 %************************************************************************
 
 \begin{code}
-specConstrProgram :: DynFlags -> UniqSupply -> [CoreBind] -> IO [CoreBind]
-specConstrProgram dflags us binds
-  = do
-	showPass dflags "SpecConstr"
-
-	let (binds', _) = initUs us (go (initScEnv dflags) binds)
-
-	endPass dflags "SpecConstr" Opt_D_dump_spec binds'
-
-	dumpIfSet_dyn dflags Opt_D_dump_rules "Top-level specialisations"
-		      (pprRulesForUser (rulesOfBinds binds'))
-
-	return binds'
+specConstrProgram :: DynFlags -> UniqSupply -> [CoreBind] -> [CoreBind]
+specConstrProgram dflags us binds = fst $ initUs us (go (initScEnv dflags) binds)
   where
     go _   []	        = return []
     go env (bind:binds) = do (env', bind') <- scTopBind env bind
@@ -778,7 +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
@@ -1019,8 +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,