+\begin{code}
+rhsIsStatic :: CoreExpr -> Bool
+-- This function is called only on *top-level* right-hand sides
+-- Returns True if the RHS can be allocated statically, with
+-- no thunks involved at all.
+--
+-- It's called (i) in TidyPgm.hasCafRefs to decide if the rhs is, or
+-- refers to, CAFs; and (ii) in CoreToStg to decide whether to put an
+-- update flag on it.
+--
+-- The basic idea is that rhsIsStatic returns True only if the RHS is
+-- (a) a value lambda
+-- (b) a saturated constructor application with static args
+--
+-- BUT watch out for
+-- (i) Any cross-DLL references kill static-ness completely
+-- because they must be 'executed' not statically allocated
+--
+-- (ii) We treat partial applications as redexes, because in fact we
+-- make a thunk for them that runs and builds a PAP
+-- at run-time. The only appliations that are treated as
+-- static are *saturated* applications of constructors.
+
+-- We used to try to be clever with nested structures like this:
+-- ys = (:) w ((:) w [])
+-- on the grounds that CorePrep will flatten ANF-ise it later.
+-- But supporting this special case made the function much more
+-- complicated, because the special case only applies if there are no
+-- enclosing type lambdas:
+-- ys = /\ a -> Foo (Baz ([] a))
+-- Here the nested (Baz []) won't float out to top level in CorePrep.
+--
+-- But in fact, even without -O, nested structures at top level are
+-- flattened by the simplifier, so we don't need to be super-clever here.
+--
+-- Examples
+--
+-- f = \x::Int. x+7 TRUE
+-- p = (True,False) TRUE
+--
+-- d = (fst p, False) FALSE because there's a redex inside
+-- (this particular one doesn't happen but...)
+--
+-- h = D# (1.0## /## 2.0##) FALSE (redex again)
+-- n = /\a. Nil a TRUE
+--
+-- t = /\a. (:) (case w a of ...) (Nil a) FALSE (redex)
+--
+--
+-- This is a bit like CoreUtils.exprIsValue, with the following differences:
+-- a) scc "foo" (\x -> ...) is updatable (so we catch the right SCC)
+--
+-- b) (C x xs), where C is a contructors is updatable if the application is
+-- dynamic
+--
+-- c) don't look through unfolding of f in (f x).
+--
+-- When opt_RuntimeTypes is on, we keep type lambdas and treat
+-- them as making the RHS re-entrant (non-updatable).
+
+rhsIsStatic rhs = is_static False rhs
+
+is_static :: Bool -- True <=> in a constructor argument; must be atomic
+ -> CoreExpr -> Bool
+
+is_static False (Lam b e) = isRuntimeVar b || is_static False e
+
+is_static in_arg (Note (SCC _) e) = False
+is_static in_arg (Note _ e) = is_static in_arg e
+
+is_static in_arg (Lit lit)
+ = case lit of
+ MachLabel _ _ -> False
+ other -> True
+ -- A MachLabel (foreign import "&foo") in an argument
+ -- prevents a constructor application from being static. The
+ -- reason is that it might give rise to unresolvable symbols
+ -- in the object file: under Linux, references to "weak"
+ -- symbols from the data segment give rise to "unresolvable
+ -- relocation" errors at link time This might be due to a bug
+ -- in the linker, but we'll work around it here anyway.
+ -- SDM 24/2/2004
+
+is_static in_arg other_expr = go other_expr 0