diff options
| -rw-r--r-- | compiler/specialise/SpecConstr.lhs | 18 |
1 files changed, 16 insertions, 2 deletions
diff --git a/compiler/specialise/SpecConstr.lhs b/compiler/specialise/SpecConstr.lhs index 23127f440f..3dfda94ca3 100644 --- a/compiler/specialise/SpecConstr.lhs +++ b/compiler/specialise/SpecConstr.lhs @@ -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 ----------------------------------------------------- @@ -1006,8 +1019,9 @@ 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, |